ASNT NDT Handbook Volume 8 Magnetic Testing

Effect of Flux Leakage on discontinuities depends on the value of
False Indications H(t) and, in turn, on how large a B(t)
value the field intensity causes around the
In a magnetic particle test, it is important discontinuity.
to raise the field intensity and flux
density in the object to a level that Why Particle Indications
produces magnetic flux leakage sufficient Form
for holding particles in place over
discontinuities. On the other hand, Surface-breaking discontinuities best
excessive magnetization causes particles to detected by magnetic particle tests are
stick to minor surface leakages not caused those that expel the optimal magnetic
by discontinuities. flux leakage for the technique. To gain a
clearer insight of this, it is necessary to
If such surface leakage occurs (Fig. 3) understand three sets of variables: (1) how
and attracts large numbers of particles, discontinuity parameters affect the
the result is a false indication and the test external magnetic flux leakage, (2) how
object is said to be over magnetized for magnetic field parameters affect the
this inspection. It may then be necessary external flux leakage field and (3) how the
to verify the test results with another sensor reacts to passing through such
nondestructive testing method. Such false fields.
indications may result from local
permeability changes which are caused by Discontinuity Parameters
local stresses in the test object. In some
cases, the magnetic flux leakage field The discontinuity characteristics that are
might be caused by a subsurface material critical to the formation of magnetic
discontinuity and it may not be possible particle indications include depth, width,
to distinguish the cause of the leakage and angle to the object surface. The
without the use of additional effects of discontinuity width on the
nondestructive testing technology. topography of the magnetic flux leakage
field have been described in what might
One way around this problem of be termed classical approaches6–8 where
excessive magnetization is to localize the discontinuity may be replaced by
magnetizing fields at the object surface. arrays of poles. Higher ambient field
This can be done using alternating current intensities or flux densities are included
fields and the corresponding skin effect. within such models by increasing the pole
As a rule, skin depth (also called standard densities that give rise to the magnetic
depth of penetration) for 60 Hz alternating flux leakage fields. More recently,
current fields in steel is typically about computer models have been developed4,9
1 mm (0.04 in.), depending on the to explain how magnetic flux leakage
permeability and electrical conductivity of fields are related to discontinuity
the test object. The field intensity falls to parameters (Fig. 1 is an example of such
e –1 of its surface value, or 37 percent, at work).
this depth. At two skin depths, field
intensity falls to e –2 or 13 percent of its In cases where the discontinuity is
surface value. Magnetic flux leakage from narrow and surface breaking (seams, laps,
quench cracks and grind tears), the
FIGURE 2. Micrograph of typical arc burn on magnetic flux leakage field near the
surface of steel pipe, caused by direct mouth of the discontinuity is highly
contact magnetization. curved (Fig. 4). The activating field
intensity may be quite small (a few
A amperes per meter) or, after saturating the
test object, inspection can be performed
B with the resulting residual induction.
C
D In the case of subsurface
discontinuities (inclusions and

FIGURE 3. Minor surface flux leakage from
variations in local magnetic permeability
may be source of false test indications.

Legend
A. Air.
B. Molten metal, solidified.
C. Steel, burnt and recrystallized.
D. Steel not burnt.

Magnetic Leakage Field Measurements 141

laminations), the magnetic flux leakage tension forces between the particle vehicle
field at the inspected surface (Fig. 5) is and the object surface (wet method
much less curved. Relatively high values tests).10
of field intensity and flux density within
the object are required for testing. This Some of these forces may in turn vary
lack of leakage field curvature is due to with: discontinuity orientation; the
saturation and greatly reduces the Earth's gravitational field; particle shape
particles’ ability to stick to such and size (in effect, with particle effective
indications. permeability); and with the particles’
containing medium.
Magnetic Field Parameters
The magnetic force Fm (N) that holds a
The properties of the magnetic field that single particle to a magnetic flux leakage
most affect flux leakage include the field field is determined by the vector relation
intensity, local BH properties and the in Eqs. 1a and 1b. Strictly speaking, the
angle to the discontinuity opening. The magnetostatic force Fm acts on a magnetic
leakage field’s ability to attract magnetic dipole with a spatially constant
particles is determined by several moment m (such as a magnetic particle or
additional factors. These include (1) the a current carrying loop with a length
magnetic forces between the magnetic much smaller than the scale over which
flux leakage field and the particle, the field is varying) in a magnetic leakage
(2) image forces between a magnetized field H:
particle and its magnetic image in the (1a) Fm = m ⋅∇H
surface plane of the test object, where H is the ambient leakage field
(3) gravitational forces that may act intensity (A·m–1) and ٌH is the gradient
to pull the particle into or out of the of the field (A·m–2).
magnetic flux leakage field and (4) surface
FIGURE 4. Highly curved magnetic field from In the special circumstance where the
narrow, surface breaking discontinuity. particle’s moment is directly proportional
to the magnetic leakage field, Eq. 1a can
0.5 mm be rewritten:
(0.02 in.)
(1b) Fm = K (H ⋅ ∇H )
FIGURE 5. Effects of induction on flux lines in
presence of discontinuity: (a) compression of where K is a mathematical constant
flux lines at low levels of induction around (N·m3·A–2).
discontinuity, so that no surface flux leakage
occurs; (b) lack of compression at high It can be seen from Eq. 1 that magnetic
induction, showing some broad surface faonrdcehFomwdietpcehnadnsgoesnolvoecralthfieelldeningttehnosiftythHe
magnetic flux leakage. particle ٌH. For surface discontinuities,
(a) ٌH is large (because the field is highly
curved), while H itself need not be large.
(b) Flux leakage For subsurface discontinuities, ٌH is
relatively small and H itself must be raised
to compensate for the small change.
Unfortunately, raising H will also raise
surface noise.

In other forms of magnetic flux leakage
testing, the flux density is raised to a
higher level than is common with
magnetic particle testing and nonrelevant
indications (noise) are in some way
recognized. For example, the signals that
noise induces in flux sensitive detectors
may be filtered out. Magnetic flux leakage
testing is therefore not limited by a
human inability to distinguish real from
apparent discontinuities. It is limited by
an electronic inability to perform the
same function.

142 Magnetic Testing

PART 2. Flux Sensitive Devices

Described below are flux sensitive devices (meters) and ⌬x is the distance between
used in magnetic nondestructive testing. position AA' and CC' (meters). The
The sensors detailed here measure either magnetic flux interrupted by the wire is:
magnetic fields or their gradients.
(3) ∆Φ = B ⋅ nˆdA
Research11 indicates that a lack of
discontinuity detection can be blamed on where Bnˆ is the magnetic flux density dA
the magnetizing method, the particles (tesla), is the unit vector for the area
used, and the capability of the inspector. and ⌬⌽ is the interrupted magnetic flux
The important question that must be (weber). The two equations together give:
answered before beginning any magnetic
particle test is: what is the best possible (4) ∆Φ = LB ⋅ nˆ∆x
combination of magnetizing means, particle
shape, type and size, and operator training to Faraday’s law of induction states that
detect a discontinuity of a specific size every an electromotive force e will be induced
time? Commonly accepted magnetization in the wire and its magnitude is given by
methods may not always be the best. Flux the relation:
measurement devices can help provide
more accurate information about the test (5) e = − dΦ
procedure. dt

Commonly used magnetic flux This is the rate at which the magnetic
sensitive devices include: (1) a long flux is cut. Eliminating the flux between
straight wire passing through a magnetic Eqs. 4 and 5, and taking the component
field, (2) the search coil, (3) search coil
derivatives such as C and E cores, (4) the of B perpendicular to ˆndA gives:
hall element, (5) the magnetodiode,
(6) the ferroprobe and (7) the flux gate (6) e = − BL dx
magnetometer. For sensors in categories dt
1 through 3, the output signal depends
on some form of time variation for the Finally, since dx·dt –1 is actually the
ambient field intensity. Sensors in velocity v of the wire, the induced
categories 4 through 7 are not time electromotive force becomes:
dependent.
(7a) e = − B Lv
A long straight wire passing through
a magnetic field is not used for FIGURE 6. Wire cutting magnetic flux
nondestructive testing, but it is a crucial between A-A′ and C-C′.
concept for understanding the signal
developed in coil sensors as they pass AC
through magnetic flux leakage patterns. Q

Voltage Developed P
between Ends of Straight v = dx·(dt)–1
Wire L

As shown in Fig. 6, two conducting wires R
PQ and RS are placed at right angles to a
magnetic field (shaded area) of constant ∆x S
flux density B directed toward the reader. A′ C′
Let another free wire AA' be moved to
position CC', a distance ⌬x away. The area
swept out by the wire is then:
(2) dA = L∆x
where dA is the area swept out by the
moving wire (square meters), L is the
length of the wire between PQ and RS

Magnetic Leakage Field Measurements 143

As an example calculation, consider a sensitive voltmeter. No current flows if P
truck traveling north at 100 km·h–1. If the and R are not connected. Furthermore, in
length of the truck’s axle is 2 m (6.6 ft) the general case of conductor motion
and the vertical component of the Earth’s through magnetic fields, the variation of
magnetic field intensity is 3 ϫ 10–5 B along the conductor must be known so
Wb·m–2 (0.3 G), then from Eq. 7 the that the integral of Eq. 9 can be
electromotive force between the ends of computed.
the axle is:

( )(7b) e = 3 × 10−5 Wb ⋅ m−2 (2 m) Example of a Straight Wire Signal

× ⎛ 100 000 m ⋅ s−2⎠⎟⎞ The electromotive force generated in the
⎝⎜ 60 × 60 leading edge of the coil shown in Fig. 7 is
deduced from the perpendicular field
= 1.7 × 10−3 V component of a tight crack. The simplest
approximation for this magnetic flux
leakage field is:6,7

This example indicates the magnitude (10) Bt = ⎛ Bg Lg ⎞ y
of voltages induced when metal objects ⎜ ⎟
move in relatively small magnetic fields. ⎝ π ⎠ x2 + y2
As another example, compute the
electromotive force generated between the and:
ends of a 10 mm long wire when moving
at 500 mm·s–1 through a field of (11) B⊥ = ⎛ Bg Lg ⎞ x
1.6 ϫ 10–3 Wb·m–2 (16 kG). ⎝⎜ π ⎠⎟ x2 + y2

( )(7c) e = 1.6 × 10−3 Wb ⋅ m−2 where Bg is the flux density deep within
( )× (0.01 m) 0.5 m ⋅ s−1 the discontinuity (weber per square
meter); Lg is the width of the
= 8 × 10−6 V discontinuity (meter).

It is unusual for B to be at right angles The origin of coordinates is the mouth
to ␯ and under such circumstances a more of the discontinuity. If the length L of the
general form of Eq. 7 is required: wire is parallel to and shorter than the

∫(8) e = (v × B)dl FIGURE 7. Parallel and perpendicular coils cutting magnetic
flux leakage fields from discontinuity at speed v. For
where ␯ ϫ B is the vector cross product of discontinuity fields longer than coil, coil outputs are as given
the wire velocity and the flux density in Legend for E and E⊥.
through which it passes. Numerically, this
is (␯B) sin ␪, where ␪ is the smaller angle Y
between ␯ and B. The integral is taken
along the length of the wire because the Perpendicular coil
local value of B (through which each By2
segment of wire is passing) may vary.
By1 By2 By1 v
In magnetic nondestructive testing, Hc X
wires in the form of coils are moved in a Parallel coil
controlled fashion over a test surface so h
that the value of ␯ is known and Eq. 8 can
then be written as:

∫(9) e = − v B⊥dl Legend

where BЌ is the perpendicular component By = leakage as measured in vertical direction
of the magnetic field, such as the B = magnetic flux leakage (T)
magnetic flux leakage field shown in E = plpifeatrroapflfelenmldceiocausilulaorreudctpotuiolto,cuwetnhpteuerrte,owEf hc∝oerile((BEmy⊥1)∝– B(yB2y)1Lv–
Fig. 1. HE⊥c = By2)Lv
=
The tangential flux density Bt plays no h = constant sensor liftoff (m)
role in the development of the L = coil length into page (m)
electromotive force in the conductor v = speed of coil relative to test object (m·s–1)
because sin ␪ is zero for this field X = horizontal axis
component. Y = vertical axis

The electromotive force developed
between A and A' appears across PR
(Fig. 6) and can be measured with a

144 Magnetic Testing

discontinuity opening, then the Simple Pickup Coils
electromotive force developed between
the ends of the wire is taken from Eqs. 10 Figure 7 shows two commonly used
and 11 as: pickup coils: parallel and perpendicular.
In some cases, the turns of these coils are
(12) e = − ⎛ Bg Lg Lv ⎞ x wound onto small blocks of ferrite to
⎜ ⎟ + (ipnBeЌcrrpeiesanstdehietchufleluavxratlloeuaetkhoaegf etBeЌcsotamsbuoprvofaenceietn)s.taAiirrvcaoluree
⎝ π ⎠ x2 y2 coils are discussed below.

In traditional magnetic flux leakage Perpendicular Coil
testing equipment, the value of y is
maintained at some constant value h With a one-turn coil passing at speed ␯
(liftoff of the sensor). The form of the through the same magnetic flux leakage
electromotive force is shown in Fig. 8 for field as above, the signal electromotive
increasing value of liftoff h. From Eq. 12, force is the difference between the two
the magnitude of the electromotive force electromotive forces developed in the
is shown to depend linearly on (1) the branches:
value of BgLg (the magnetomotive force of
the discontinuity), (2) L (the length of the (14) e = − Bg Lg Lv (x)
wire, provided that the aforementioned π
conditions are met) and (3) ␯ (the relative
velocity between the object and the × ⎡1 − 1⎤
conductor). ⎢ ⎥
⎢⎣ x2 + h12 x2 + h22 ⎥⎦
The dependence on liftoff can be seen
by differentiating Eq. 12 for the turning
points (xo = ±h) and using these values to
compute the swing ⌬e in e. The result is:

(13) ∆e = Bg Lg Lv where h1 and h2 are the liftoffs of the two
πh branches.

In this field approximation, the swing If the coil has N turns and a width of
in voltage as the conductor passes 2b, then h1 = Hc + b and h2 = H – b (the
through the magnetic flux leakage field is liftoff Hc is measured to the center of the
inversely proportional to the liftoff. coil). The electromotive force then
becomes:

Bg Lg LN vx ⎡ 1
π ⎢ Hc + b 2
FIGURE 8. Electromotive force developed ( )(15)e = − ⎢ x2
between ends of conductor passing at ⎣
constant speed through leakage field such +
as Eqs. 11 and 12.
1 ⎤
e ( )− x2 + ⎥
Small 2 ⎥
liftoff Hc − b ⎦

Large liftoff The results of varying h and b are
shown in Fig. 9 where the electromotive
∆e force is similar in form to that of the
straight wire. The turning points in the
electromotive force are given by the
solution to Eq. 16.

2 +b4 − 2
( ) ( )(16)
x02 = 2 H 4 − Hcb Hc2 +b2
c

3

2xo x For example, when b = 0.5 Hc, then the
turning points are xo = ±0.74H and 2xo
–xo xo (the distance between the turning points)
is 1.49Hc. The swing in signal e is difficult
Legend to compute in closed algebraic form.
e = electromotive force signal (V)
x = lateral distance (meter) Parallel Coil

When the coil is oriented so that one set
of wires follows another, then the output
signal is the difference between the

Magnetic Leakage Field Measurements 145

signals developed in the leading and to b indicates that the coil signal is
trailing branches: maximized when b = h. Thus when the
half-width of the coil is equal to the
(17) e = − ⎡⎣B⊥L − B⊥T ⎦⎤ Lv liftoff, the coil output voltage is
maximized with respect to the magnetic
Using Eq. 11 for the leading and flux leakage from the discontinuity. This
trailing edge =fiexld–sb(BaЌnLdaxnTd=BxЌT+),b argument also indicates that this type of
substitute xL for the coil discriminates against relatively long
leading and trailing edge distances from range material surface noise such as might
the center of the coil: be caused by local permeability variations.

(18) e = ⎛ 2bNBg Lg Lv ⎞ Ferrite Cores in Coils
⎝⎜ π ⎠⎟
Ferrites are useful in pickup coils because
× h2 + b2 − x2 they not only provide support for the wire
turns but they also amplify the flux
⎢⎣⎡(x + b)2 + h2 ⎤ ⎡⎣⎢( x − b)2 + h2 ⎤ density through the coil windings by a
⎦⎥ ⎦⎥ value equal to the effective permeability
of the ferrite.
The form of Eq. 18 is shown in Fig. 10.
The dashed lines are voltages induced in For small pieces of ferrite (Fig. 11)
the leading and trailing edges. The solid where the dimensional (length to depth)
line is their difference or the form of the ratio is small, the effective permeability of
electromotive force. The signal consists the ferrite may vary from the low teens to
of a major peak at x = 0 and two smaller the thousands. The advantage of using
side peaks. The roots of Eq. 11 occur at ferrite occurs not only in this
xo = ±(h2 + b2)0.5 so that the distance amplification but also in the fact that
between the points at which e = 0 is ferrites have very low electrical
given: conductivities, minimizing detrimental
eddy current effects in them.
(19) 2xo = 2 h2 + b2
The maximum value of the coil signal Electronic Considerations for Coil
Voltages
occurs at x = 0 and is proportional to
b·(h2 + b2).–1 Differentiation with respect It is essential that pickup coils are used to
generate voltages and not currents. Once
FIGURE 9. Form of voltage signal developed
in perpendicular coil when passing at FIGURE 10. Electromotive force induced in
constant speed through magnetic leakage parallel coil by passing it through magnetic
field.2 flux leakage field such as in Eq. 18. Coil
potential (volts) is difference between
e b = 0.5 Hc leading and trailing edge signals.

e

b = 0.25 Hc Raw signal from
b = 0.15 Hc leading edge of coil
Raw signal from
x·Hc–1 trailing edge of coil
123 4
–4 –3 –2 –1

–xo xo Coil motion
x

Legend Legend
b = half of coil width (m) e = signal (V)
e = signal (V) x = lateral distance (m)
Hc = liftoff (m) measured to center of coil
x = lateral distance (m)

146 Magnetic Testing

a current is allowed to flow in a coil, it supply current to a hall element, to detect
creates its own magnetic field, one that and measure the resulting voltage and to
can interfere with the field under then convert it to the measured field
investigation. The output of such coils is value.
therefore generally fed to an operational
amplifier of high impedance. Theory of Hall Element Operation

Coil Applications and Derivatives Electrically conducting solids are almost
transparent to the flow of conduction
Examples of coils as detectors for electrons because the ions in the element
magnetic flux leakage are presented in the lattice do not deflect conduction electrons
Nondestructive Testing Handbook on as might be expected from a typical
electromagnetic testing. Such coils can billiard ball model. As current is fed into
be connected in series adding, series one end of an element (Fig. 12), electrons
opposing (a figure eight), overlapping and are deflected toward one side of the
many other configurations. element, in accordance with the lorentz
force F:
Search coils are often wound on ferrite
cores to increase the flux through them (20) F = − e (E + v × B)
(Fig. 11 shows two common
configurations). A detailed discussion where B is applied flux density (tesla), E is
of such sensors is given in the electric field intensity (volt per meter) on
electromagnetic testing volume. the particle, e is electronic charge
(coulomb) and ␯ is velocity of the particle
Hall Element Sensors (meter per second).

Hall elements are slices of semiconductor The term ␯ ϫ B is a vector cross
material. When a current is passed product and is itself a vector at right
through them and they are placed in a angles to both ␯ and B. Its direction
magnetic field, then a voltage develops determines which side of the element the
across two of the faces of the element. electrons are deflected toward. The theory
The voltage is proportional to the of solid state physics provides a voltage Vh
magnetic flux density B. across the element:2

A solid state tesla meter is made up of (21) Vh = Rh IBz
the electronic components needed to b

FIGURE 11. Ferrite cored magnetic flux where b is the thickness of the element in
leakage detector coil systems: the direction of the magnetic field
(a) configuration one; (b) configuration two. (meter), Bz is the component of the
(a) applied field at right angles to the current
(webers per square meter), I is the applied
Ferrite current (amperes) and Rh is the hall
Input coefficient (A–1·s–1).
In general, if the element is placed at
fo 2fo an angle to the field B, such tahnagtleBzm=uBst
cos ␪, then the cosine of the
be found. Normally, the crystal is rotated

FIGURE 12. Magnetic flux, drive current and
hall effect voltage relationship (see Eqs. 20
(b) and 21).

fo 2fo 3 to 5 mm Vh Bz

d 2ℓ F
bI
Legend Z
d = depth (meter) YX
f = winding
ℓ = length (meter)

Magnetic Leakage Field Measurements 147

until the maximum tesla meter reading is into small rectangular blocks and have
found. At that point, ␪ = 0 because current and voltage leads attached before
cos ␪ = 1. being encapsulated. Typical sizes are as
small as 0.8 mm (0.03 in.) long by
The value the hall coefficient Rh is 0.4 mm (0.015 in.) wide by 0.5 mm
determined by the interaction of charge (0.02 in.) thick.5
carriers with the crystal lattice. In a metal
element, it would be given by: Vapor deposited hall elements have
been reported for use in the testing of ball
(22) Rh = − 1 bearings by the magnetic flux
ne technique.12 In this application, bismuth
was evaporated onto an alumina
where e is the charge on the electron substrate. A newer development is to
(–1.6 ϫ 10–19 C) and n is the electron combine the hall sensor, its power supply
concentration. and an amplifier on one chip. Figure 13
shows configurations of typical hall
Metals do not make good hall sensors sensors and their specifications.
because their hall coefficients are too low.
As can be seen in Eq. 21, the larger the Applications of Hall Elements
hall coefficient, the larger the hall voltage.
Investigations of hall coefficients for Hall elements are used with tesla meters
many substances have shown that or other devices to detect or measure
combinations of elements from groups III magnetic fields. Typical configurations are
and V of the periodic table give the shown in Fig. 14. In Fig. 14a, the hall
highest hall voltages and have the least sensor is held a fixed distance from a
sensitivity to changes in temperature. current carrying wire and the tesla meter
Also, the charge carrier for these groups is measures the field intensity created by the
more likely to be a hole rather than an current. In the case of pulsed currents, the
electron. peak current can be measured with a peak
reading tesla meter.
Excitation of Hall Elements
In Fig. 14b, a ferrite ring is added to
Where contacts occur between two measure small fields or currents. The high
dissimilar metals such as the current and permeability of the ferrite aids in creating
voltage attachments on the hall crystal, a high B value in the vicinity of the
thermoelectric electromotive forces are sensor’s active area. Figure 14c, 14d and
generated. 14e show combinations of hall elements
and ferrite flux concentrator
If direct current is used to excite the configurations used in magnetic flux
crystal, the voltage read by circuitry leakage testing.
following the voltage contacts is the sum
of the hall voltage and the thermoelectric The level of external field just outside a
voltage. For this reason, hall element partially demagnetized material may best
crystal excitation is usually performed be measured with a hall element meter.
with 25 to 350 mA alternating current. Figure 15 shows an inspector checking the
external field level with a tesla meter after
Manufacture of Hall Elements partial demagnetization of the test object,
a 270 mm (10.75 in.) diameter steel tube.
Bulk hall elements are generally bismuth
doped semiconductors such as indium Crossed hall elements can also be used.
antimonide (InSb). These are produced by Such configurations are used to check
solid state crystal growth technology, cut welds or to reconstruct the total field
from the measured components.13

FIGURE 13. Typical hall element probes: Magnetodiodes
(a) flat; (b) axial.
(a) 2 × 5 mm (0.08 × 0.2 in.) The magnetodiode is a solid state device
whose resistance changes with field
Aluminum holder intensity. The device consists of positive
and negative zones within a
(b) semiconductor, separated by a region of
material that has been modified to create
0.64 × 2 mm (0.025 × 0.08 in.) a recombination zone (Fig. 16). Its
frequency response is flat from direct
current to 3 kHz and the device is stable
without temperature dependence from
–10 to 50 °C (15 to 120 °F).

Brass tube (nonmetallic optional)

148 Magnetic Testing

Applications of Magnetodiodes discontinuities with a depth of only
0.1 mm (0.004 in.).
Magnetodiodes have been used for
detecting magnetic flux leakage from Magnetic particle testing is often used
discontinuities in tubes.3 The magnetic to inspect such tubes but while it is
flux leakage is excited by alternating extremely sensitive to outer surface, tight
current electromagnets arranged to detect discontinuities, its use for inner surface
either internal or external surface discontinuities requires a viewing device.
breaking discontinuities. The system
illustrates the general principles of Ferroprobes
magnetic flux leakage testing.1
Ferroprobes (also called foerster
Sensors are connected differentially to microprobes) take many forms but for the
eliminate signals from the applied field purposes of nondestructive testing they
and from relatively long range variations generally consist of cylindrical or
in surface field intensity. This system and rectangular ferrite upon which one or two
magnetic flux leakage systems like it are coils are wound (Fig. 11).
used to rapidly evaluate the surface
condition of tubes and can detect tight Flux gate magnetometers are used to
FIGURE 14. Hall element configurations: detect small changes in the Earth’s
(a) sensor at fixed distance from wire; magnetic field. As might be used by
(b) ferrite core; (c) free standing flux geophysical prospectors, these devices
concentrator; (d) symmetrically positioned consist of ferrite rings carrying many coil
contacting concentrator; (e) asymmetric configurations.
contacting concentrator.
(a) Both of these devices are based on the
same physical laws as a tape recorder head
I or any other ferrite cored magnetic field
pickup. The difference between the two is
(b) I that ferroprobes are activated at high
frequency.

Typically, one coil is excited with
alternating current at a frequency f. The
voltage induced in a second coil at
frequency 2f is then detected. This
secondary signal carries information
about the scanned magnetic flux leakage
field. Figure 17 is an example of the
tangential magnetic flux leakage field
taken with such a probe over an angle slot
in residual induction at a liftoff of 1 mm
(0.04 in.).8

Ferrite cores might be solid or hollow,
to reduce eddy currents in the ferrite.

FIGURE 15. Checking external field level with
(c) tesla meter after partial demagnetization.

(d)

(e)

Legend
= hall element

I = current (A)

Magnetic Leakage Field Measurements 149

Large Volume Magnetic The magnetic field indicator is also
Field Indicators14 used to determine the existence of a
magnetic field external to a ferromagnetic
Bulk Field Indicators object. To do this, the indicator is
oriented against the object’s surface and
The field measurement systems discussed moved to the position that gives the
above are designed and used for the maximum external field reading.
assessment of magnetic leakage fields
from material discontinuities. In all cases, Bulk Field Indicator Construction
the active sensing area of such a device
is very small. In the case of the hall Many magnetic field indicators are round,
element, which is rectangular in shape, it about 64 mm (2.5 in.) in diameter with
is possible to integrate the field over the thicknesses of 13 to 25 mm (0.5 to 1 in.).
active area of the hall crystal, and so The indicators commonly used for
compensate for it. Then, by taking checking external field levels after
measurements at controlled distances magnetic particle tests have a range of
above a magnetized surface, it is possible about 1 to 2 mT (10 to 20 G) in divisions
to extrapolate the field values to that at of 0.05 to 0.1 mT (0.5 to 1 G). Positive
the surface. Once this is done, the readings are north and negative readings
electromagnetic boundary conditions are south (Fig. 18).
indicate the magnetic field intensity just
inside the surface. A key component of the magnetic field
indicator is a small movable field sensing
The following text relates to the magnet. The magnet is mounted so it is
detection or measurement of magnetic free to rotate. Its angular deflection is
fields over much larger areas, since the shown by the movement of a pointer.
active area of the sensor is larger than that
used for leakage field testing. The A second key component is a fixed
instruments are handheld, moving permanent magnet. Its magnetic field
magnet sensors used to measure bulk intensity limits the useful range of the
external fields at a relatively high liftoff unit by providing a restraining force to
from the test object. They are often used prevent the sensing magnet from rotating
as a practical check on the external freely. With no external magnetic field,
demagnetization state of an object. Their these two magnets stay antiparallel to
use to detect magnetic field intensity each other and the pointer remains in a
within coils should be discouraged,
since the coil field may remagnetize the FIGURE 17. Tangential magnetic flux leakage
moving magnetics. Note that these fields in saturated residual induction over
devices do not measure leakage fields 40 degree slot.8
from discontinuities.
40 degrees
A bulk magnetic field indicator can be
used to measure the value of a uniform 0.4 (4)
magnetic induction field in air. Because
the relative magnetic permeability of air Magnetic flux density Bx, mT (G) 0.2 (2) B
is 1, this reading is also the numerical 0 A
equivalent of the magnetic field intensity
of air.

FIGURE 16. Diagram of magnetodiode, –0.2 (–2) A
showing positive and negative zones in
intrinsic semiconductor material. Changes in
magnetic field intensity H perpendicular to
surface alters resistance in recombination
zone.

Negative zone B

Positive –0.8 (–8)
zone

–4 0 48
(– 0.16) (0.16) (0.32)

Recombination zone Distance X, mm (in.)
H– H+
Legend
A. Experimental data at 1 mm (0.04 in.) liftoff.
B. Model with increased charge on acute face of slot.

150 Magnetic Testing

neutral position, registering a zero reading recalibration of the unit. Also, a strongly
(Fig. 18b). poled sensing magnet, when placed very
close to an unmagnetized object, could
The field indicator is designed so that induce localized poles and cause
the net magnetic field from these two inaccurate readings.
magnets is weak outside the device.
Placing the indicator on an unmagnetized Principles of Field Indicator
object does not induce poles on the object Operation
sufficient for causing inaccurate meter
readings. A fixed magnet inside the device’s
housing sets up a reference magnetic field.
In principle, magnetic field indicators A small movable sensing magnet is
could use a coiled spring instead of a mounted inside this field. If a nearby
calibrated magnet to return the pointer to object also sets up a magnetic field in the
zero once the external field is removed. same area, the field sensing magnet
However, slight changes in the sensing rotates into a direction parallel to the
magnet’s intensity would then require resulting, combined magnetic field.

FIGURE 18. Typical magnetic field indicator: The instrument’s pointer is attached to
(a) photograph; (b) diagram. Distance from the sensing magnet and correspondingly
pivoting point of sensing magnet to closest rotates into a direction perpendicular to
point on edge of casing is about 18 mm the resulting field. If the external field
(0.75 in.). Size, shape, material, magnetic changes polarity, the pointer rotates in
field intensity and relative positions of the opposite direction and the reading’s
sensing magnet and reference magnet vary algebraic sign changes. If the magnetic
with manufacturer. field indicator is rotated through
(a) 180 degrees about its pointer’s zero
direction, there is no algebraic sign
(b) change in the reading (the scale is also
rotated 180 degrees).
Field indicator
However, in practice a reverse of
Plan polarity or rotation of the indicator often
view produces a change in a reading’s
magnitude unless the external field is
S NS N perpendicular to the reference field.

NS Side Calibration and Use of Field
SN view Indicators

Generally, there are two ways to calibrate
magnetic field indicators. These
calibration methods provide two distinct
ways of using the devices.

The most common calibration method
correlates the angular deflection ␪ of the
indicator’s pointer with the magnitude B
of a uniform external field whose
direction is parallel to the zero direction
of the pointer. To measure a uniform field,
the field indicator is positioned so that
the zero direction of the pointer is parallel
to the field.

Used in this manner, the field B from
an external object is perpendicular to the
reference field B* inside the magnetic field
indicator. Or as depicted in Fig. 19:

(23) B = B * tan θ

For a small deflection, B* may be
considered uniform and a large ␪ indicates
a relatively strong B. To keep within a
practical calibrated scale when ␪ is
between +45 and –45 degrees, the
measured field must be weaker than the
reference field (B less than B*).

In many applications, B may not be
perpendicular to B*. For example, the

Magnetic Leakage Field Measurements 151

direction of B may be unknown or the As a consequence of this inequality, the
field could be nonuniform. In a case like scale of the second type of calibration is
this, the magnetic field indicator is generally wider if marked on the same arc
positioned against the object’s surface and inside the same magnetic field indicator.
oriented in such a way that the When using an instrument that is
directional marking on the device’s casing calibrated in the second way, the unit is
is near to and perpendicular to the often rotated to verify that the readings
object’s surface. Because the field is not are maximized. Sometimes this is
necessarily normal to the object’s surface, inconvenient for objects with complicated
the reading can be less or greater than the geometry because a rotation of the device
actual value, depending on whether the may move its sensing magnet away from
field makes an acute or obtuse angle with the area of interest.
the reference field (␣ and ␤ in Fig. 20).
If magnetic field indicators of the first
The alternate way of calibrating type are used as if they had the second
correlates the uniform field value B with type of calibration (maximizing their
the maximum deflection ␾ of the readings by rotation) then the resulting
magnetic field indicator’s pointer. The maximum values are actually greater than
value of ␾ is obtained by orienting the the true field values. Sometimes, in this
instrument inside the field B. As shown in way, an estimate can be made for the size
Fig. 21, the field vector B changes its of a uniform or nearly uniform field, even
direction relative to the field vector B* though its direction is unknown.
and their resulting vector traces out a FIGURE 20. Pointer deflection α and β when
circular path of radius B centered at the B is not perpendicular to B* (α < θ < β).
tip of B*.
Bα B
For B less than B* and for a maximum β
deflection ␾, the resulting field vector is
tangential to this circular path. It follows αθ β
that the field B is actually parallel to the B*
pointer (Fig. 21). When B Ͻ B*, Eq. 24 is
valid.
(24) B = B * sin φ

Note the differences between Eqs. 23
and 24. When measuring very weak fields,
these two calibration methods are about
the same and the pointer’s angular
deflection is approximately linear
with the uniform field’s magnitude
(␪ = B·B* = ␾ for a small B·B*). In general,
for the same uniform field B:
(25) φ Ͼ θ

FIGURE 19. Pointer deflection θ in first FIGURE 21. Pointer deflection φ in second calibration type. B
calibration type. B is perpendicular to B* and and the pointer are parallel and φ is at maximum (B* > B and
tan θ = B·B*–1. φ > θ).

Pointer’s Pointer’s
initial/zero initial/zero
position position

Pointer’s Pointer’s new
new position position parallel to B
due to B

θ Sensing φ Sensing magnet’s
θ magnet’s initial position
initial position θφ Sensing magnet’s
new position
B*

B*

Sensing magnet’s
new position due to B

152 Magnetic Testing

When measuring a nonuniform field, However, the same accuracy is not
the reading of a magnetic field indicator is possible for geometries with sharp corners
at best the average field value over the or for objects that are small compared to
area covered by the sensing magnet. For the size of the indicator. In these
example, assume that the flux lines from instances, the measured field may not be
an external magnetic source are almost uniform (the direction and density of the
parallel with the special directional flux lines vary across a small distance) and
marking on the magnetic field indicator. a tangential component exists. It is
The two ends of the magnetic field known from electromagnetic field theory
indicator’s field sensing magnet may still that the tangential component of
experience deflection forces of different magnetic induction may not be
magnitudes because of different field continuous when crossing the boundary
values and the resulting pointer deflection surface between two media of different
is an average of the two field values. permeabilities. Therefore, the magnetic
induction inside and outside the object
Measuring of Residual Fields may not be the same.

The primary function of a field indicator When the test object has an irregular
is to measure the external magnetic field shape or the residual field readings are
intensity close to an object, but not every large, one way to test external magnetism
kind of residual magnetism can be is to scan the entire surface of the object
detected by these instruments. with a magnetic field indicator. Maximum
readings occur at locations where
In a circularly magnetized object, significant external poles exist.
where residual flux lines are
circumferential and form closed loops At such maximum reading locations,
inside the material, the induction is the field indicator can be used to
significantly different from zero but the determine if the field is normal to the
field may not produce poles outside the object’s surface. The field indicator is
object. Such a circular field does not positioned against the object and oriented
produce significant readings in magnetic with its directional marking normal to the
field indicators. Circularly magnetized surface. The device is rotated through
objects do not attract or deflect magnetic 180 degrees about the directional marking
materials during normal use unless surface (normal to the object’s surface). During
discontinuities occur in the magnetized rotation, variations of the indicator
object, producing strong external poles. reading are noted.

Consider a cylinder with flat ends that If the rotation does not affect the
has been longitudinally magnetized. A indicator reading, then the reading is the
magnetic field indicator is placed against true field value and the true field is
the cylinder’s end surface and the normal to the object’s surface at this
directional marking on the indicator’s location. If the field is not perpendicular
casing is lined up with the length of the to the object’s surface, it is likely that, at a
test object. The indicator is aligned certain time during rotation, the actual
normal to the cylinder’s surface and a field vector will have no projection along
reading is taken. the direction of the indicator’s reference
field, and the reading at that time will be
If a demagnetization procedure has exactly the component of the field
been properly performed, the indicator normal to the object’s surface. In other
reading will be 0.1 mT (1 G) or less. words, the normal component of the field
Similar readings obtained on the side of at this location will be no greater than the
the cylinder (with the directional marking largest value observed in the rotation.
perpendicular to the side surface) should More precisely, it is in between the
be about zero. Sometimes, if residual readings obtained at the beginning and at
magnetism is high, a nonmagnetic spacer the end of the rotation and is no greater
is placed between the object and the than the average of these two values. If
magnetic field indicator and relative the same value appears twice during
readings are obtained. rotation, then it must be the normal
component of the field.
For a cylinder that is large compared to
the indicator’s size, measurements made In most applications, the purpose of
at the center of the end surface are close external residual field measurement is to
to the actual values immediately beneath ensure that the objects are free of
the surface of the object. The reason for magnetic poles that detract from
this accuracy is that the magnetic fluxes serviceability. The exact value of the
immediately inside and outside the external residual magnetism may not be
cylinder’s end are perpendicular to the critical, so long as it is lower than a limit
end surface and the perpendicular predetermined by the user’s empirical
component of the magnetic induction data.
field across the boundary surface is
continuous, according to electromagnetic
field theory.

Magnetic Leakage Field Measurements 153

Checking Indicator Reading current coil. Over a small distance along
Accuracy the coil’s axis, a magnetic field can be
considered nearly uniform. As examples,
If inconsistent results occur in matching Table 1 shows a set of magnetic field
magnetic field indicators, it is likely that values for a five-turn coil of 300 mm
some of the devices are malfunctioning. (12 in.) diameter carrying 1500 A direct
Certain indicator malfunctions are easy to current. The table values were calculated
detect, such as an imbalanced or damaged using the following equations.
pointer, or mechanical failures at the
support of the sensing magnet and (26) B = Bo ⎤3/2
pointer assembly. High mechanical impact ⎥
or sudden exposure to a strong magnetic ⎡ ⎛ x ⎞ 2 ⎦⎥
field are among the less obvious causes for ⎢1 ⎜⎝ R ⎠⎟
erratic readings. ⎢⎣ +

A magnetic field indicator can also where Bo is the magnetic field at the
become inaccurate if its magnetization is center of the coil (millitesla), R is coil
changed by exposure to a strong direct radius (meters) and x is distance (meters)
current or a decaying alternating current from the center of coil along the axis.
field. If the fixed reference magnets
become partially demagnetized, the unit (27) Bo = µo ⎛ NI ⎞
can give readings much larger than a ⎝⎜ 2R ⎠⎟
good unit’s results (smaller B* in Eq. 23).
If an indicator’s magnetic components are where I is applied direct current
totally demagnetized, its pointer may not (amperes), N is number of turns in the
return to the zero position, remaining coil and µo is the permeability constant
virtually anywhere on the scale. (4␲ ϫ 10–7).

Two different field indicators may give Sometimes, the Earth’s magnetic field
different results at the same location on can indicate a meter’s accuracy: the
the test object. However, these differences Earth’s field is about 0.05 mT (0.5 G). If
alone do not indicate that one of the field the indicator’s accuracy is within
indicators is malfunctioning. The sensing ±0.03 mT (0.3 G), then with proper
magnets of different devices may have north/south and horizontal orientations,
different sizes and their location inside the device should be able to register an
the units may be different. They may approximate reading of the Earth’s field,
therefore not be measuring the field at provided there are no other magnetic
exactly the same location. In addition, objects nearby.
reference fields inside the units may also
differ. The best magnetic field indicators are
precision calibrated. Their accuracy may
In a highly nonuniform field, readings also be less susceptible to the influence of
may not vary in the same ratio as varying a strong magnetic field. In some
measurement locations. As a result, it may applications, less costly magnetic field
not be possible to verify the accuracy of a indicators may be used to do
magnetic field indicator by comparing its measurements. Precision calibrated units
readings to a known reference unit (other are then used as reference standards,
field measurement devices may be equally verifying the readings of the less costly
inaccurate in the nonuniform field). The devices. Periodically, the reference devices
best way to test the accuracy of a are returned to the manufacturers for
particular device is to perform reference calibration.
comparisons in uniform or nearly
uniform magnetic fields. Table 1. Magnetic flux densities for
five-turn coil carrying direct current,
To set up a uniform magnetic field for compared with linear distance from coil
calibration, a helmholtz coil may be used. center.
This device contains two parallel coils
separated at a distance equal to their Distance from _M__e_a_s_u_r_e_d__V_a_l_u_e_
radius and connected in series adding ___C_o_i_l _C_e_n__te__r __ mT (G)
mode. In about 30 percent of the volume
between the two coils, there is a very m (ft)
uniform magnetic field parallel to their
axes. The field value can either be 0 0 31.0 (309.0)
measured with an appropriate meter or 0.45 (1.5) 1.0 (9.8)
calculated from the coils’ dimensions and 0.9 (3.0) 0.14 (1.4)
the value of applied direct current (in 1.0 (3.3) 0.1 (1.0)
Eq. 26, x·R–1 is 0.5 and Bo is replaced
with 2Bo).

In addition to the helmholtz coil or
commercial calibration fixtures, an
approximately uniform magnetic field
may be established using a large direct

154 Magnetic Testing

Use of Magnetic Field Indicator than the maximum value registered
during rotation and no greater than the
A magnetic field indicator is a convenient average of the readings at the beginning
low cost tool for measuring the residual and the end of rotation. If the rotation
external field intensity of ferromagnetic does not affect the reading, then the field
objects. To measure a uniform field, is perpendicular to the object’s surface
indicators are often calibrated in a way and the reading is the field’s true value.
that requires the operator to align a
special directional marking (line or arrow For a nonuniform field, the reading of
on the device’s casing) with the field’s the magnetic field indicator is an average
known direction. value (at the spot where the indicator’s
field sensing magnet is located).
For an external flux measurement, the
field indicator is positioned against the Good magnetic field indicators have
object with its directional marking near to sound mechanical supports for their
and perpendicular to the object’s surface. reading pointers and these supports
This positioning is based on the fact that cannot be easily damaged. Their magnetic
flux lines are expected to be perpendicular components cannot be easily
to the object’s surface at the location of demagnetized by strong external fields.
significant poles. Also, they do not induce significant
magnetic poles on the objects they test.
In cases where the field direction is
uncertain, the indicator may be rotated For an accurate calibration of a field
about its directional marking, which is in indicator, a uniform magnetic field may
turn positioned normal to the object’s be provided by a helmholtz coil. For a
surface. The rotation moves through quick check of calibration, various
180 degrees to get a maximum reading. approximate, uniform field values along
The component of the magnetic field the axis of a large direct current coil may
normal to the object’s surface is no greater be used.

Magnetic Leakage Field Measurements 155

References

1. Stanley, R.K. and L.[C.] Wong. 8. Zatsepin, N. and V. Shcherbinin.
Chapter 7, “Magnetic Leakage Field “Calculation of the Magnetostatic
Measurements.” Nondestructive Testing Field of Surface Defects: Part 1, Field
Handbook, second edition: Vol. 6, Topography of Defect Models” and
Magnetic Particle Testing. Columbus, “Part 2, Experimental Verification of
OH: American Society for the Principal Theoretical
Nondestructive Testing (1989): Relationships.” Defektoskopiya. No. 5
p 179-198. (1966): p 50-65.

2. Bray, D.E. and R.K. Stanley. 9. Heath, S.E. Residual and Active
Nondestructive Evaluation: A Tool for Magnetostatic Leakage Field Modelling.
Design, Manufacturing and Service. Master of Science thesis. Fort Collins,
Baton Rouge, LA: CRC Press (1994). CO: University of Colorado (1984).

3. Nondestructive Testing Handbook, 10. Swartzendruber, L. “Magnetic Leakage
second edition: Vol. 4, Electromagnetic and Force Fields for Artificial Defects
Testing: Eddy Current, Flux Leakage and in Magnetic Particle Test Rings.”
Microwave Nondestructive Testing. Proceedings of the Twelfth Symposium on
Columbus, OH: American Society for NDE. San Antonio, TX: Southwest
Nondestructive Testing (1986). Research Institute (1970).

4. Hwang, J.H. Defect Characterization by 11. Skeie, K. and D. Hagemaier.
Magnetic Leakage Fields. Dissertation. “Quantifying Magnetic Particle
Fort Collins, CO: Colorado State Inspection.” Materials Evaluation.
University (1975). Vol. 46, No. 6. Columbus, OH:
American Society for Nondestructive
5. Stanley, R. “Basic Principles of Testing (May 1988): p 779.
Magnetic Flux Leakage Inspection
Systems for the Evaluation of Oil 12. Beissner, R., G. Matzkanin and C.
Country Tubular Goods.” Teller. NTIAC-80-1, NDE Applications of
Electromagnetic Methods of Magnetic Leakage Field Methods: A State
Nondestructive Testing. New York, NY: of the Art Survey. San Antonio, TX:
Gordon and Breach (1985): p 97-150. Southwest Research Institute (1980).

6. Foerster, Friedrich. “Nondestructive 13. “Hall Effect Transducers: How to Apply
Inspection by the Method of Magnetic Them as Sensors.” Freeport, IL:
Leakage Fields: Theoretical and MicroSwitch Company (1982).
Experimental Foundations of the
Detection of Surface Cracks of Finite 14. Wong, L.C. “Magnetic Field Indicator:
and Infinite Depth.” Defektoskopiya. Principles and Use.” Materials
Vol. 11 (1982): p 3-25. Evaluation. Vol. 46, No. 6. Columbus,
OH: American Society for
7. Foerster, Friedrich. “On the Way from Nondestructive Testing (May 1988):
’Know How’ to ’Know Why’ in the p 749-754.
Magnetic Leakage Field Method of
Nondestructive Testing” (Part 1).
Materials Evaluation. Vol. 43, No. 10.
Columbus, OH: American Society for
Nondestructive Testing (September
1985): p 1154. Part 2, Vol. 43, No. 11.
(October 1985): p 1398.

156 Magnetic Testing

6

CHAPTER

Equipment for Magnetic
Particle Testing

Joseph E. Monroe, Eastern NDT, Hopewell, Virginia
John J. Flaherty, Bloomingdale, Illinois (Part 7)

PART 1. Basic Magnetic Particle Equipment1

Magnetic particle test equipment can be need to be very sensitive. Extremely small
as small as a handheld yoke or as large as discontinuities must be detected, and
billet testing units in steel mills. Magnetic correspondingly small test indications
particle systems have evolved from the must be produced for evaluation and
small and simple units first produced. interpretation.
Improvements parallel developments in
other technologies and include computer For some applications, high sensitivity
control and improved materials for is actually a problem: excess leakage fields
particles, coatings and vehicles. In its produce false indications and dense
fundamentals, the magnetic particle backgrounds. The magnetic particle test
testing technique has not changed much sensitivity must be set to indicate
since the 1930s. Changes have occurred in discontinuities within a range of severity
three areas: (1) magnetic particle appropriate to the application.
materials, (2) test system configurations
and (3) components automated for Configuration of Test System
manufacturing and production.2,3
Ten considerations help configure a
Magnetic particle test systems must magnetic particle testing system for a
fulfill two basic requirements: (1) accurate specific application: (1) particle type (wet
testing based on amperage requirements, or dry), (2) magnetization requirements of
test object size, magnetic field levels and the test object, (3) degree of automation
suitable testing area and (2) testing with required, (4) demagnetization
or without operator intervention at a rate requirements, (5) current requirements,
required by the particular production (6) test object size and corresponding test
facility. In turn, these two requirements system size, (7) electrical power
determine the size, shape, speed and availability, (8) air supply requirements,
configuration of the magnetic particle (9) accessories needed for the application
testing system. and (10) test specifications requiring
verification.
Effect of Test Parameters
on Equipment Choice Each of these considerations is affected
by other testing or manufacturing
Production Speed parameters. Magnetization, for instance,
may need to be achieved with alternating
Some magnetic particle testing systems current, half-wave rectified alternating
operate at slow production speeds, often current, full-wave rectified alternating
as low as a few large components per day. current or single-phase rectified
In other instances, the testing equipment alternating current, depending on test
is an integral component of a production object and test purpose. As another
line, processing and testing hundreds of example, demagnetization requirements
objects per hour. are determined by the test object’s
subsequent use and the magnetization
If only because of handling means. In addition, demagnetization may
considerations, these two applications need to be performed within the main
require completely different types of testing system or at a separate station.
testing systems, designed and built to
accommodate their specific uses. Before selecting the test equipment,
there must be a thorough understanding
Test Sensitivity of the magnetic particle technique, its
capabilities and limitations. This
In some high production applications, the understanding is applied to the
magnetic particle testing system is used as requirements of the test objects and their
a product screening device: any indication anticipated discontinuities. The test
of a discontinuity becomes cause for object’s magnetic characteristics, geometry
rejection. Once removed from the and intended service affect the choice of
production line, the rejected object may the magnetic particle system. Finally, the
then be reevaluated or reworked as needed. test system must be configured and
installed so that it is an integral part of
In applications such as aerospace or the existing production facility.
plant maintenance, magnetic particle tests
The text that follows is a general view
of magnetic particle testing systems and
can guide specific applications.

158 Magnetic Testing

PART 2. Wet Horizontal Equipment1

The magnetic particle equipment most FIGURE 1. Wet horizontal magnetic particle bench unit.
commonly used for production testing is
the wet horizontal unit (Fig. 1). The
nominal length of such a unit is
determined by the size of the object that
can fit in its clamps. Lengths of 1 to 4 m
(3 to 12 ft) are used for most applications.
Many other system lengths have been
designed, some for objects as small as
aerospace bolts a few millimeters in
length. On the other end of the scale,
very long systems have been built for
testing steel billets, gun tubes, oil field
pipe or railroad engine crankshafts.

Positioning of Test Object repositioning. A suitable reference
standard with known discontinuities
Before magnetic particle testing, the should be used to ensure adequate field
object is clamped between a headstock intensity at the midpoint of the test
and an adjustable tailstock that moves object, or notched shims or quantitative
horizontally along rails in the unit’s bed. quality indicators (QQIs) can be placed in
the areas of interest to verify field
The headstock secures the test object directions and intensities (Fig. 2).4
by means of a compressed air cylinder. An
electrically operated switch, often a foot Following each magnetization
switch, controls the throw of the procedure, the test object is examined
headstock air cylinder. using wet magnetic particle techniques.
Large test objects remain on the unit’s
The tailstock’s position may be bed. Many wet horizontal testing systems
controlled by a gear screw and manual are equipped also for demagnetization.
crank or, in some smaller units, the
tailstock is simply pushed into position Alternating and Direct
along the bed. Some systems use motor Current Systems
driven tailstocks but these are slow and
are normally used only in special Wet horizontal units used for in-service
applications. and in-process testing are usually
single-phase alternating current systems.
Magnetization Procedures Alternating current equipment is less
expensive because costly rectifying
A wet horizontal testing system can circuitry and associated cooling systems
magnetize circularly by direct contact and are not required. In addition, a separate
longitudinally with a yoke or encircling demagnetizing unit is not necessary with
coil. alternating current units and test objects
are demagnetized in position on the unit.
Once the test object is clamped into
position, electrical current is passed Aerospace codes and many
through it or through a conductor to manufacturing standards specify direct
induce a field. For longitudinal current magnetic particle testing
magnetization, most horizontal systems equipment. These are usually three-phase
have a magnetizing coil on a horizontal full-wave rectified alternating current
rail. While the test object is being loaded, systems. In the early days of the magnetic
the coil is clear of the headstock and
tailstock. After circular magnetization, the
coil is moved to encircle the object. On
test objects over 450 mm (18 in.) long,
the encircling coil must be repositioned in
350 to 425 mm (14 to 17 in.) increments.
With yoke procedures, longitudinal fields
are set up along the entire length of the
test object, with no need for

Equipment for Magnetic Particle Testing 159

particle method, direct current was the resulting field follow the path of least
supplied by a bank of acid storage resistance, which is not necessarily the
batteries that were often inefficient and location of discontinuities. Therefore,
unreliable. In 1941, the rectifier circuit alternating current equipment is often
was developed, producing unidirectional chosen because of an improved
current from alternating current and probability of detection. In addition, an
replacing the troublesome battery bank. equivalent relative field intensity in an
alternating current system requires only
Alternating versus Direct Current about half of the current used by a direct
Equipment current system.

There are many differences between the Wet Horizontal System
basic circuitry in direct and alternating Components
current magnetic particle testing systems.
The rated magnetizing output and duty
One difference is that direct current cycle of wet horizontal systems vary with
equipment often has a current timer the model and manufacturer. Alternating
preset for 0.5 s duration so that a low current equipment usually has a
duty cycle can be used. The timer may be maximum output of 1.5 to 3 kA with
bypassed and is adjustable. Test systems some rated as high as 6 kA. Full-wave
using fully rectified alternating current rectified alternating current equipment
require augmented cooling. Fans help usually has a 2, 4 or 6 kA rating. For larger
fulfill the cooling requirements of the test objects, long systems can be
rectifier circuits. Such systems may offer
demagnetization using reversing direct TABLE 1. Components of typical wet horizontal magnetic
current with step down procedures or particle testing unit.
alternating current demagnetizing circuits
with rapid decay. Hood encloses the unit for ultraviolet radiation
Instrument pedestal holds meters and other controls
Because alternating current produces a Ammeter measures amperage of direct current
skin effect (with penetration depth Adjustable timer controls elapsed time of magnetizing current
dependent on line frequency), the Visible light source provides illumination
resulting magnetic field follows the Switches control fan and visible light
contour of the test object. With direct Coil provides longitudinal magnetization
current magnetization, the current and Contact plate provides circuit connection at tailstock
FIGURE 2. Field indicators: (a) pie gage; Tailstock adjustable to test object length
(b) notched shims. Crank adjusts tailstock position
(a) Curtain encloses hood to restrict visible light
Actuator bar triggers actuator to operate magnetizing
(b) current
Transfer switch selects coil or head shot magnetization
Push button starts and stops magnetizing current
Current control selects current levels
Foot switch controls air pressure valve for headstock
contact plate
Actuator connects to actuator bar to operate
magnetizing current
Power indicator indicates when power supply to unit is active
Pump switch activates pump for magnetic particle
suspension
Shelves are headstock and tailstock contact plates to
support test objects
Hose delivers magnetic particle suspension
Nozzle applies magnetic particle suspension to test
object
Contact plate connects circuit at headstock
Headstock supports air cylinder that operates contact
plate
Swing arm allows positioning of ultraviolet lamp
Ultraviolet source provides ultraviolet radiation
Reference standard indicates the direction, penetration and
intensity of field

160 Magnetic Testing

manufactured with 10 kA or more direct TABLE 2. Accessories for magnetic particle tests in quality
current. control.

Table 1 lists the components of a Ultraviolet radiation meter measures ultraviolet radiation intensity
typical wet horizontal magnetic particle with digital or analog readout; may
testing system. Table 2 lists some Test meter also include sensor for visible light
accessories. measures amperage output of
Centrifuge tube magnetic particle unit; must be
Test Systems outside North Tool steel ring standard compatible with unit’s current
America Field indicator tests concentration of magnetic
Calibrated field indicator particle suspension
Magnetic particle testing systems used Magnetic field indicator verifies accuracy of test setup
outside North America differ in several Hall effect meter provides relative measurement of
ways. First, because of demand for Reference standard residual magnetism
alternating current, nearly all systems are provides actual measurement of
designed for use with single-phase input. residual magnetism
Where applicable, rectified circuitry is indicates direction of magnetic field
generally half-wave rectified alternating measures magnetic fields in either
current. Single-phase equipment is dynamic or static modes
appreciably less costly to build than the indicates the direction, penetration
three-phase rectified systems used in the and intensity of field
United States. Single-phase equipment
must have primary current double that of the test object, allowing complete
a three-phase system for the same coverage of potential discontinuities.
secondary or magnetizing current output.
Multidirectional test systems may be
The main advantage of three-phase designed for very specific applications —
rectified alternating current is that the the steel blades in jet engines, for
demanded theoretical power usage is example. For this particular test, the third
lower. However, because most testing phase of three-phase alternating current is
applications have a 10 percent duty cycle used to provide a circular field directed
(0.5 s on and 5.0 s off), the saving in through the dovetail of the blade. The
power is insignificant. Single-phase circular field adds to a longitudinal field
magnetizing current provides far greater supplied by a coil, giving increased
particle mobility for dry and wet particles. magnetic field intensity in the blade’s
critical dovetail area.
The second important difference,
especially in European systems, is the Theoretically, multidirectional
nearly universal use of yokes rather than magnetization can be applied in a
coils for developing a longitudinal field in majority of production applications with
test objects. Many European systems have improvements in resolution and cost.
no magnetizing coil mounted on the unit. Suitable reference standards with known
discontinuities must be used for setup of
A third difference is that most of the multidirectional tests. When properly
electrical or electronic components are applied with assurance of adequate field
separated from the mechanical or and directional balance, improvements in
handling portions of a system. In many discontinuity detection can be as high as
countries, this separation is a code 30 percent over typical unidirectional
requirement with safety to personnel as applications. Reduction in labor costs can
justification. Note that since 1950, no exceed 60 percent when compared to
instance of electrocution has been traced conventional test procedures on wet
to a magnetic particle apparatus, mainly horizontal equipment.
because secondary (magnetizing) circuits
produce relatively low voltage. Time studies have indicated that
forgings weighing 2.25 to 9 kg (5 to 20 lb)
Multidirectional Test Systems require about 35 percent less time for
multidirectional tests because of
Multidirectional testing systems provide reductions in handling and testing times.
magnetizing current in two or more Test objects with multiple apertures are
directions. Because it is desired for the also likely candidates for significant
particles to move freely in the suspension, reductions in labor costs when tested with
wet fluorescent particles are usually used multidirectional methods. Labor savings
in multidirectional systems. up to 90 percent have been reported on
tests of plates in shipbuilding through
Conventional magnetization in one heavy duty multidirectional power packs.
direction is accomplished in a
multidirectional unit by energizing a
specially designed circuit. In addition,
one, two or three circuits can be
individually energized in rapid succession.
These quickly changing magnetizing
currents produce overall magnetization of

Equipment for Magnetic Particle Testing 161

PART 3. Stationary Magnetic Particle Equipment1

Many different stationary bench units are Direct Current
available for specific magnetic particle Magnetizing Equipment
testing applications. The size of the test
system is determined by the size of For magnetic particle testing of large,
anticipated test objects. Larger models are complex castings, welded structures or
commonly used for heavy components plate, overall magnetization with high
such as diesel engine crankshafts, landing magnetizing current is economical. The
gear sections, gun barrels or railroad maximum output for such applications is
wheels (Fig. 3). usually rated around 12 kA.

Stationary magnetic particle units are Multidirectional magnetization is often
generally designed to operate from a used through two or three magnetizing
440 V three-phase alternating current circuits, making it possible to detect
source and to deliver alternating or discontinuities in all directions in a setup.
rectified magnetizing current. Current This multidirectional magnetization can
control is often infinitely variable. In be done mechanically or electronically.
older units, the current was manually Electronic switching provides faster rise
controlled with a step switch. Because of time and programmable cycles for
expanding requirements for close and reliability.
repetitive control over current density,
filtering of the power source may be Equipment listed in Table 2 is used for
necessary. monitoring test parameters for quality
control inspections. The accessories listed
With the increasing use of in Table 3 are used in production
programmable logic controllers, some installations.
stationary bench units can be
programmed with the current test settings TABLE 3. Accessories for production magnetic particle
for various parts. With programmable testing unit.
logic, the operator recalls the part file and
the machine automatically sets itself to Hand spray gun applies wet method suspension
the stored settings. Programmed settings Powder spray bulb applies dry powder materials
ensure that identical parts are tested the Contact block enables cables with bench unit
same way on different occasions and by Cables connect with lug terminals or slip
different operators. (either-end) joints to portable or
FIGURE 3. Horizontal magnetic particle system for fluorescent Contact clamps bench units; have 9 and 12 mm
testing of railroad wheels. Prods (2/0 and 4/0 American Wire Gage)
diameters
Split coil connect electric cables to test object
Central conductor set connect electric cables to test object
Test object supports (commonly for tests of welds or
large objects)
Transport truck provides longitudinal magnetization
Contact pads, copper mesh with mobile units
includes central conductors in
Connectors, slip joint several sizes for bench units
Small part adapters come as headstock, tailstock and rail
mounted supports in various sizes
for tubing, bars and shaft shapes
carries portable testing units and
accessories
ensure contact with test objects
(some models have rubber cushion
backing and V shaped blocks)
connect quickly to prods, clamps,
cables and contact block
lets small objects be tested in
standard wet horizontal unit

162 Magnetic Testing

Automated Magnetic triggered by the line frequency lets the
Particle Systems field direction vary at high speed.

Automatic or semiautomatic magnetic Automated techniques are discussed in
particle testing in most cases requires more detail below.
magnetization in two directions to detect
randomly oriented discontinuities. Quick Break Magnetization
Because two magnetic fields cannot exist
simultaneously in one test object, it is Quick break magnetization is needed in
necessary to switch from one direction to three-phase full-wave rectified systems
the other. using coils for direct current
magnetization. When an object is placed
Electronic switching provides in a coil and magnetized, the field lines
advantages in this configuration, as in leave the test object near the north pole
others. Current can be switched several and reenter near the south pole. The lines
times per second and is often triggered by of force may be normal to the surface at
the line frequency. In this way, the test the ends of the test object. A
object is both circularly and discontinuity near the end of the object
longitudinally magnetized. Switching may therefore be hard to detect with
magnetic particles.
FIGURE 4. Portable magnetic particle testing: (a) particles
applied to weld; (b) excess particles blown from area of To overcome this problem, the direct
interest. current applied to the coil is quickly
(a) turned off. The rapid collapse of the
magnetic field creates low frequency eddy
currents within the object in a direction
favorable for the detection of transverse
discontinuities at the ends of the object.

Prods (Fig. 4), a yoke (Fig. 5) or the
field flow technique makes quick break
magnetization unnecessary because the
test object is part of the magnetic circuit.
Periodic checking of the break is critical:
on electronically triggered equipment, a
malfunctioning firing module could result
in quick break failure.

FIGURE 5. Yokes for magnetic particle
testing: (a) instrument; (b) flexible legs
adapted to weld.
(a)

(b)

(b)

Equipment for Magnetic Particle Testing 163

PART 4. Mobile Magnetic Particle Equipment1

Mobile magnetic particle testing systems Cables and Connectors
have outputs up to 20 kA and may be
designed to deliver alternating current, Cable length can be varied and many
direct current, half-wave rectified or pulse applications require lengths exceeding
current. Table 4 lists the components of a 30 m (100 ft). Mobile system cables are
typical mobile magnetic particle test unit. usually fitted with slip joint connectors at
In systems before 1970, the current was either end. Cables are connected to the
adjusted with a thirty-point step switch. test system, prods, clamps or to each
Some units have infinitely variable other to lengthen the magnetizing circuit.
current and push button Some high amperage systems use bolted
demagnetization. terminals to permit high currents without
overheating.
In Europe, mobile systems are used as
power packs for stationary testing units. A 12 mm diameter (4/0 American Wire
This precaution is required because Gage) flexible rubber coated cable is most
installing heavy duty electrical equipment commonly used for mobile magnetic
under a kerosene or water suspension tank particle applications. For easier handling,
is prohibited. 9 mm diameter (2/0 American Wire Gage)
cabling is sometimes used for connections
Mobile equipment is widely used for to prods. With short cables 5 to 10 m
many types of magnetic particle testing. (15 to 30 ft) long, mobile equipment
The advantage of these system delivers close to its rated amperage. As the
configurations is that they can be moved cables (magnetizing circuits) are extended,
to the test site, whether that is a flight the amperage available at the test point is
line, a refining tank, large plant considerably decreased. For example, a
machinery or a structural steel weldment. 4 kA unit with two 30 m (100 ft) cables in
The advantage of a mobile system’s high the magnetizing circuit delivers about
amperage is its ability to inspect large 1 kA at the test point, because of internal
castings, forgings, welds or any other test resistance of the secondary circuit.
object requiring strong magnetizing field
intensities. The input power cable to a mobile unit
can be almost any convenient length with
Current and Voltage little or no loss in output. Often the input
Parameters cable is fitted with a heavy duty contact
plug for connection to 230 or 460 V
For certain applications, prods (Figs. 4 and outlets. These outlets are commonly
6) or clamps are used with mobile available for welding equipment and
magnetic particle equipment. A solenoid, other heavy machinery. Mobile magnetic
a cable wrapped into a coil, may be used particle equipment may also be operated
for longitudinal magnetization or from alternating current generators.
demagnetization (Fig. 7). A cable or, with
clamps, a ferromagnetic bar can serve as As the cables and connectors become
an internal conductor for magnetization. worn or overheated, their electrical
resistance increases and current is
Mobile power packs operate on 230 or reduced.
460 V single-phase alternating current.
Most units manufactured through the Input and Output Current
1970s had a maximum output of 2 or 3 kA Requirements
of half-wave alternating current or single-
phase alternating current. Systems are Mobile magnetic particle testing units can
now available with 4 and 6 kA half-wave be operated on 230 or 460 V single-phase
alternating current and single-phase alternating current. The operating voltage
alternating current outputs or a range of is easily changed through an access port
pulse outputs. that exposes the terminals. Jumpers are
alternately positioned to change the
In a typical system, an operator uses a transformer connections and vary the
front panel or terminal to select input requirements. With either voltage
alternating or direct current. input, the output rating is the same.

Mobile test systems with 6 kA output
are not available with 230 V input and,
because of high primary current
requirements, must be operated on 440 to

164 Magnetic Testing

460 V alternating current. Units once used TABLE 4. Components of typical mobile magnetic particle
tap switches. Since the 1980s, infinitely testing unit.
variable current control lets the operator
establish the proper field intensity in a Lifting hooks position the unit with crane
part. The intensity determines the current Current control adjusts alternating or direct current
output for either alternating current or intensity
half-wave alternating current. Line switch provides power to unit
Power indicator indicates power to unit
For demagnetization, mobile magnetic Remote switch allows remote control
particle testing units use a decaying Common connector provides one side of output current
alternating current or a ramp Power outlet provides 120 V for accessories (powder
demagnetizing system. blower, grinder, lamps)
Half-wave connector with common connector, supplies half-
Operation of Mobile wave alternating current to cables
Testing Units Current connector with common connector, supplies
alternating current to cables
Remote Operation Cables supply current to prods (12 mm diameter
[4/0 American Wire Gage] cable)
A cable may be connected to a four-point Connector uses either-end slip joint connector for
outlet to let the operator control the additional cable or accessories
mobile unit remotely. When using prods, Remote control stops or starts magnetizing and
a microswitch for controlling current flow demagnetizing current
may be in the prod handle. With clamps, Casters provide mobility (two swivel and two
coils or an internal conductor, a special fixed)
remote switch station is available or the Current light indicates current is on for output circuit
prod switch may be used. Ammeter indicates amperage in alternating or half-
wave current circuits
Solid state units allow remote control Louvers ventilate electrical equipment
of power, current and demagnetization. A Recess stores cable and accessories
second 120 V outlet is sometimes
provided for remote power to alerts, Maintenance of Mobile
powder blowers, grinders and other Testing Systems
accessories.
Some critical maintenance procedures are
Demagnetization required for mobile magnetic particle
testing systems. For instance, the cooling
Demagnetizing can be accomplished in intake of the unit must be kept clean to
several ways with a mobile system. The permit the free flow of air, especially if the
choice of technique depends on the test unit is moved into a dirty testing
object and magnetization means. Small environment.
objects can be demagnetized by selecting
alternating current and forming a coil Prod tips are another source of
with the cables. The object is drawn concern. They may become corroded or
through the coil while the coil is burned, hindering good contact with the
energized. The test object must be moved test object. Defective prod tips can
sufficiently far from the coil to produce arc burns, sources of cracking.
outdistance the coil’s magnetic field, Similarly, clamps should have good
usually about 1 m (3 ft). copper mesh gripping points.

For larger test objects, demagnetization Frayed cables and continuous
is achieved by touching the surface with overheating decrease the conductivity of
the energized coil and then moving the the cable. This produces heat, increases
coil away from the test object. resistance and reduces the amperage
available for testing. Connectors and cable
An alternate means of demagnetization joints should be tight and solid to avoid
uses demagnetizing current flowing overheating.
through the test object. Cables are
connected to the object with clamps.
With solid state equipment, the decay
current or ramp current reducing system
is used. With older equipment or tap
switch units, demagnetization is done in
successive steps. An internal conductor
can also be used for demagnetization if
the test object geometry lends itself to
such procedures.

Demagnetization is discussed in more
detail elsewhere in this volume.

Equipment for Magnetic Particle Testing 165

PART 5. Portable Magnetic Particle Equipment1

Handheld Equipment Current and Voltage Parameters

Portable magnetic particle equipment is Portable systems vary greatly in electrical
practical for testing objects in the field, so parameters. Lighter weight units are
it is often used for testing structural welds usually designed to operate on 120 V
(Fig. 4). single-phase alternating current. Output
amperage of these units can vary from
The simplest and perhaps most 400 to 900 A, depending on the model
common magnetic particle test system is a and manufacturer. Because the usual limit
handheld magnetic yoke (Fig. 5). For on 120V is 30 A, outputs over 1.0 kA
small test objects (automotive parts, for require a 240 or 480 V unit. Most portable
example) surface discontinuities can be magnetic particle testing systems contain
reliably detected at low production rates half-wave rectified alternating current and
with a portable magnetic yoke. Yokes are alternating current output.
also used for magnetic particle tests of
welds, especially when arc strikes cannot FIGURE 6. Handheld prods for magnetic particle testing:
be tolerated. It is difficult to prevent arc (a) of casting; (b) of bridge girder weld.
strikes when current is applied directly (a)
with prods.

Yokes operate on alternating current at
standard line voltage, either 120 V or
240 V. For areas where a shock hazard
exists, yokes can be made to operate at
42 V. The typical electromagnetic yoke’s
articulated legs assist positioning on
complex shapes.

Yokes with four poles have been
developed. By switching the poles in pairs
at right angles to each other,
discontinuity detection in all directions
can be performed in a single setup.

Portable System (b)
Configurations

Larger equipment is needed for a higher
magnetizing current or a higher duty
cycle. Magnetic particle test systems often
require heavy transformers and, excluding
the magnetizing cables, can easily weigh
34 kg (75 lb). For that reason, portable
units are sometimes mounted on carts
and so become mobile.

Portable systems may operate from a
120, 240, 480 or 380 V single-phase
source. Magnetizing output currents range
from 400 to 2000 A for alternating current
or half-wave rectified alternating current
applications. Dual prods (Fig. 6) or clamps
are used for direct contact magnetization.
Fixed coils are available for mobile
applications; most cables connect at either
end, letting the operator manually form a
magnetizing coil from a standard cable
(Fig. 7).

166 Magnetic Testing

Either two or three connectors are voltage is low in capacitance discharge
provided for dual output systems: a units, providing the additional benefit of
common connector, one connector for increased operator safety.
half-wave rectified and a third connector
for alternating current. Output is Yokes
controlled with a tap switch or a
potentiometer. A single ammeter serves to Magnetic particle yokes have a
measure both the alternating current and multiple-turn coil wrapped around an
the half-wave output. assembly of soft iron laminations. A
power cord and switch are part of the
Accessories and typical yoke design. In service, the yoke is
Components placed on the test object and energized.
Magnetic particles are applied between
Table 5 and Figs. 4 to 7 illustrate the and adjacent to the ends of the yoke. A
components of a portable magnetic magnetic field is set up between the poles
particle test system. of the yoke. To find discontinuities in
both directions, the yoke is rotated
Accessories such as prods, clamps and 90 degrees and the operation is repeated.
cables for portable systems are similar to
their counterparts for mobile testing In the middle of the twentieth century,
units. Cable lengths for portable systems alternating current yokes had a rigid
are usually limited to 10 m (30 ft) or two U shape. A more useful design has the
5 m (15 ft) cables. ends of the U jointed so that wider or
narrower objects can be tested more
With the portable systems, efficiently (Fig. 5b). A rectifier is
demagnetization is accomplished using sometimes used with the yoke, providing
(1) manual alternating current step down a direct current field for penetration
or (2) a coil formed from cables using below the test surface.
alternating current. When step down
demagnetization is used, connection to FIGURE 7. Coils for circumferential magnetization:
the test object is made using clamps or an (a) solenoid; (b) improvised to fit crimper connector of
internal conductor. railroad car.
(a)
A 120 V coil is useful for longitudinal
magnetization. The coil consists of many
turns of fine wire; a switch closes the
circuit when the coil is connected to
120 V alternating current. The coil is
primarily for testing elongated objects
such as spindles or axles. The coil can also
be used to demagnetize many kinds of
test objects.

A capacitance discharge portable
system, although it operates much like a
standard portable unit, weighs
considerably less and can use lighter,
smaller magnetizing cables. The input

TABLE 5. Components of typical portable magnetic particle
testing unit.

Current control provides adjustable or tap switch (b)
Cables need extra flexibility to connect to prods
Prods make in-line electrical contact with test
object
Handle positions and transports unit
Half-wave connector with common connector, supplies half-
wave alternating current to cables
Remote receptacle enables remote cabling hookup
Common connector either-end connector for one side of output
current
Power indicator indicates when power supply to unit is on
Current connector with common connector, supplies
alternating current to cables
Control cable connects to prod remote switch
Ammeter measures alternating and half-wave current
output

Equipment for Magnetic Particle Testing 167

PART 6. Magnetic Particle Testing Power Pack
Systems1

A power pack, a large portable battery, is power pack, circulating pumps, a filtering
used to set up magnetizing fields when an system and application nozzles.
object is too big for the available
stationary testing unit. Many magnetic Automotive Industry
particle systems in production
environments have a power pack as part In the automotive industry, important
of their design. The electrical components components such as spindles, connecting
of the magnetizing and demagnetizing rods and crankshafts are routinely tested
circuits are generally part of the power with magnetic particle techniques. System
pack. Timing circuits adapted to the configuration is based on a power pack for
particular production line are included. the magnetizing current. Timing circuits
are vital in the power pack: magnetizing
Power pack timing circuits are designed current must be applied at a specific time
for specific applications. This might be during the test object’s conveyance
hundreds of objects per hour in the through the system.
automotive industry or an object per
minute in the steel industry. In other Magnetization may be circular or
applications, the power pack could have longitudinal and is routinely followed by
timers that control the length of the demagnetization. Small test objects are
current application to meet product demagnetized with an alternating current
specifications. Power pack design is coil encircling the conveyor. On some
usually dictated by the production systems, the operator drops the test object
requirements of the products being tested. through a coil after testing. For large test
objects, special contact demagnetizing
Power Pack Applications circuitry is designed as integral to the
power pack.
Steel Industry
Large Castings
In the steel industry, magnetic particle
testing is applied to products as diverse as A multidirectional technique described
billets, blooms, bars and tubing. Magnetic below, also known as the swinging field
particle technology is used to test technique, is applied for large, heavy
semifinished products and is sometimes castings. The power pack used in this
applied at an intermediate stage of technique can supply up to three separate
manufacture for process control. magnetizing circuits. Contact is made by
cables clamped to the test object and the
A steel mill testing system typically circuits are arranged so that magnetizing
uses a power pack that supplies current crosses the test object in three
magnetizing current to contacting different directions. The power pack
fixtures. Depending on the test object’s supplies timing circuitry for shifting
size, amperage as high as 20 kA full-wave current flow from circuit to circuit. In
direct current may be used. The test some applications, one of the three
system is like a large bench unit, featuring magnetizing circuits is used to form a coil,
a means of magnetically contacting the providing longitudinal magnetization in
test object after positioning. Steel mill test the test object. Demagnetizing circuitry,
equipment includes handling systems for usually reversing direct current, can be a
positioning steel objects measuring from feature of this power pack unit.
12 to 20 m (40 to 60 ft) long.
The swinging field technique is an
After magnetization, the object is overall testing approach that uses wet
rotated and positioned for testing. fluorescent magnetic particle suspensions
Magnetic particle test results determine to test the entire surface of large cast
where the test object goes next: a grinding objects. In a foundry, for example, this
station, a secondary rolling operation or a technique can save as much as 90 percent
shipping point. of the labor and time required to inspect
an object with prods. In addition, areas
For most magnetic particle tests, steel can be tested that are inaccessible to
mills use wet fluorescent techniques. The prods.
standard steel mill test system includes a

168 Magnetic Testing

Other Power Pack Applications then within milliseconds the field is
changed to longitudinal. This swinging
Full-wave direct current power packs are field sequence is continuous and very fast
used for energizing bench units. These because it can be based on the reversals of
systems are usually designed for testing the 60 Hz power supplied to the unit.
large objects such as landing gear
forgings. A full-wave power pack is also For example, an elapsed time of 0.5 s
useful when large weldments are tested. allows 15 circular field shots and 15
Instead of prods, the test object is longitudinal field shots. The resulting
contacted with clamps and magnetizing magnetic field sweeps across the test
current is passed along the length of the object and crosses the plane of a
weld. After this first test, the clamps are discontinuity regardless of its orientation.
repositioned, contacting the object so that
current is passed perpendicular to the Often, wet fluorescent magnetic
weld line. particles are used with this magnetization
method. The suspension is applied to the
Multidirectional Field test object as the magnetic field swings
Power Packs across the material. The rapidly changing
field requires high mobility from the
In some systems, a power pack is used to particles and that in turn recommends the
supply a circular field to the test object free, small particles used in wet
fluorescent suspensions.

Equipment for Magnetic Particle Testing 169

PART 7. Automation of Magnetic Particle
Testing2

Test Object Handling for Space Utilization. It is often possible to
Magnetic Particle Tests increase output from the same amount of
space by a mechanical handling system.
Manual Handling with Automated Such automation ensures the flow of
Testing objects from process to process and
eliminates the need for temporary storage
The simplest automation of the magnetic spaces before and after each
particle test procedure occurs when test manufacturing process.
objects are manually positioned in the Quality Control. Accurate control of
test system and the magnetizing process handling system speed ensures that each
then proceeds without operator test object receives the same treatment.
intervention. Counters can be installed to monitor
system speeds and this information may
Consistency is achieved with this level be stored on-site or delivered to a remote
of automation (1) by providing accurate controlling center.
levels of magnetizing current or field,
(2) by producing accurate timing of Fully automated magnetic particle
magnetizing field or current flow and testing systems are designed for a specific
(3) by applying consistent volumes of manufacturing site so that the testing
magnetic bath in the same manner and system is compatible with existing
the same direction for each test object. manufacturing facilities. For example, the
This consistency of processing system for moving objects through
considerably improves both the quality manufacturing operations may be an
and efficiency of the testing. To include overhead conveyor or a roller track. The
this kind of automation in a standard testing system must be designed as an
magnetic particle testing system is integral part of the site’s existing handling
relatively simple and can be achieved facility. It is equally important for the
inexpensively with electronic current testing system to accommodate particular
leveling devices and electronic timers. production rates, depending on whether
the plant uses a batch system or
Fully Automated Handling continuous flow line production.

Fully automated magnetic particle testing Another important design
systems can be cost justified only when consideration is the size of the test
large volumes of similarly sized and objects. If large, they may be attached to
similarly shaped test objects are inspected. or loaded onto the conveyor singly and
To move from simple to complete are presented in that way to the testing
automation requires the installation of system. Small objects are often loaded
mechanical handling systems that feed into baskets or fixtures and conveyed
test objects through the testing machine. through the plant in these containers. On
Automatic scanning after magnetization arrival at the magnetic particle testing
and sorting is complex and costly. system (which may have its own
mechanism for presenting test objects
Handling in a manufacturing operation into magnetizing positions), the fixtures
forms a large part of the typical can be unloaded singly by a simple
production cost: in the United States, the robotic arm. Very small test objects
cost of handling is often more than half (fasteners, for instance) may be singly
of the total cost.5 More than any other presented into the testing system by
single factor, materials handling offers an means of a vibrating bowl feeder.
opportunity for cutting production costs
and increasing productivity. The most important considerations for
successful design of a fully automated
In planning a fully automated testing magnetic particle testing system are
system, labor costs may not be the most (1) reducing test object movement to a
important factor. Other considerations are minimum, (2) mechanizing handling in a
safety, space and quality. manner compatible with the rest of the
Safety. Statistics indicate that a very large manufacturing process and (3) controlling
portion of industrial accidents happen test object movement and the testing
during the movement of materials. Many process with a single program installed in
of these accidents can be prevented by the a programmable logic controller.
use of mechanical handling systems.

170 Magnetic Testing

Monitoring of Automated and tailstock simultaneously, with a
Testing Equipment second alternating current 120 degrees
out of phase with the first, applied to the
Magnetic particle testing systems are coil. These currents are two phases of a
designed to meet specific testing three-phase electrical supply.
requirements for detectability of
discontinuity location, direction and size. A multidirectional, contact and
Also included in design considerations for noncontact magnetization technique
automated systems are such parameters as provides a significant increase in the
test object symmetry, size, weight, speed with which magnetization can be
configuration, magnetic properties and established in a given test object. The
throughput requirements. implementation of electronic firing
circuitry enables phase switching rise
All of these factors influence the design times on the order of 4 ms in a 0.5 s shot.
of the load station, conveyor system, test Controlling the current level in each
object positioning fixtures, magnetizing firing circuit is another valuable
method, magnetizing apparatus, bath characteristic. Proper multidirectional
application, test object manipulation, magnetization can reduce magnetization
visual or automatic scanning testing time by half.
stations, demagnetization apparatus,
offloading and tested part segregation. A test system design incorporating
multidirectional magnetization
Automated Magnetizing techniques must account for the
following considerations: (1) the intensity
The proper magnetizing technique for a and direction of the magnetizing fields,
specific test object is fundamental. (2) the sequencing of the magnetizing
Selection of the type and level of shots, (3) the duration of the overall shot
magnetizing current may be dictated by and (4) the application of the magnetic
the controlling process specifications. bath and, in turn, the availability of
Alternating current, half-wave direct magnetic particles on the test object and
current and full-wave direct current, their freedom to move and form
single-phase and three-phase, and indications.
single-shot all have their place in
automated test systems. It takes considerable time and
experimentation to ensure that all these
In most manufacturing applications of parameters are correct. If the magnetic
magnetic particle testing, test objects are field intensities in different directions are
sequentially processed with circular and not correctly adjusted relative to each
longitudinal magnetization using contact other, test indications may not be distinct
and coil techniques. This has long been and significant discontinuities can be
standard practice for manually operated missed. Field intensity adjustments may
equipment and semiautomated systems as not necessarily be accurate if a pie gage or
well as for automated tests. other artificial discontinuity devices are
used (Fig. 2). A multidirectional magnetic
This magnetization technique is
reliable but slow for automated line FIGURE 8. Eddy current response versus magnetization curves
throughput. Test objects magnetized in for two sample test bars 13 × 25 × 150 mm (0.5 × 1 × 6 in.)
both the circular and longitudinal with electron discharge machined notches at 0.6 µm
methods require (1) magnetizing in a (0.015 in.) depth.
circular direction, (2) testing and
interpretation, (3) demagnetizing in some Permeability (H·m–1) µ Eddy current
cases, (4) magnetizing in a longitudinal Eddy current meter response instrument
direction and (5) further testing and reading
interpretation. (relative scale)
800 80
Multidirectional Magnetization
400 40 µ
When automatically processing large
numbers of similar test objects 00
(automotive forgings or castings, for
example), there is a critical need for 1600 3200 4800
performing magnetic particle tests at the (20) (40) (60)
highest reliable speed. This is done by
using a system that indicates Field intensity A·m–1 (Oe)
discontinuities in all directions with one
magnetizing operation. Good indication
Faint indication
Several systems for this kind of testing
have been implemented, including Legend
alternating current swinging field systems. = annealed to hardness of 64 rockwell-B
In a typical alternating current swinging = spheroidized to 91 rockwell-B
field, current is applied to the headstock

Equipment for Magnetic Particle Testing 171

particle system should be set up using a system. A hard alarm immediately shuts
magnetic field indicator (tesla meter) and down the system if a test object is
a reference standard (a typical test object jammed in the clamping mechanism, if
containing known discontinuities). the bath pump fails or if any part of the
hardware or software fails.
Computer Components of Current Leveling. It is necessary to
Automated Testing Systems monitor a test system’s current leveling
device to ensure that it is operating
An automated magnetic particle testing correctly. The magnetizing power unit in
system may be managed by an automated testing system includes
programmable logic with the following current leveling capability to ensure that,
features: (1) central processing program, when the current output control has been
(2) programmable memory, (3) input set, the precise amount of current is
means, (4) output means and (5) a power supplied at each shot. When the output
supply. current is kept consistent, a drop in
current may be attributed to conditions
The central processor uses instructions external to the power unit (bad contact
from the computer’s memory. Among with the test object or loose cable
other functions, the central processor uses connections, for example).
software for the following purposes: (1) to Magnetizing Current. Current passing
interrogate the status of the inputs, (2) to through the test object or the coil must be
set or reset the outputs, (3) to perform monitored. Current levels at 90 to
counting and timing functions and (4) to 100 percent of a preselected value should
monitor the system continuously for be acceptable in most tests. Current values
malfunction. at 80 to 90 percent should cause a soft
alarm. Below 80 percent, a hard alarm
This kind of controller provides should sound and the system should be
flexibility for testing and manufacturing. shut down. These thresholds may be
Magnetizing current levels, shot duration adjusted for different applications.
and magnetic bath application parameters Monitoring current levels for
can be easily monitored and changed. Test multidirectional magnetization is more
object identity and programming can be complex because the current is flowing in
fed from an external computer, and data any one direction only briefly. It is
on inspected parts may be filed as test probably more efficient to integrate the
objects move through the system. current levels in each direction and
average them over the duration of the
Monitoring for System current shot. Monitoring of the duration
Malfunction of the shot itself is also necessary but this
can be a straightforward function of the
With a totally automated magnetic programmable controller.
particle testing system, it is critical to Bath. The concentration of magnetic
verify correct performance of the system. particles in the bath is very important yet
For instance, if the magnetizing current some systems have not monitored it. The
has not reached its full value because of programmable controller should monitor
bad contact, or if the magnetic bath the magnetic bath application time, pump
concentration is low or high, test objects pressure and flow rate. Consistency in the
move through the system but application of the bath helps provide
discontinuities are not reliably indicated. consistent, reproducible test results.
Subsequent interpretation, manual or
automatic, will consider this lack of All of these monitoring steps — current
indications to mean freedom from level of the system, current level in the
discontinuities. test object, bath concentration and dwell
time — are important steps for verifying
It is essential therefore to monitor all the operation of an automated testing
the parameters of the magnetizing process system and, in turn, for ensuring the
and to detect malfunctions of transducers reliability of the system’s test results.
and other devices introduced into the
system. Automating of Magnetic
Malfunction. Malfunction systems are Particle Testing Procedure
normally connected to alarms at two
levels. A soft alarm warns of malfunction In today’s industrial environment, interest
requiring operator intervention. The in automation for production operations,
programmable controller may indicate from basic fabrication to final assembly, is
that the concentration of the magnetic commonplace. Lower cost results from
bath may have moved beyond preset labor reduction, yield improvements and
limits or the magnetizing current may better quality. Better quality comes about
have dropped within ten percent of a because of the uniform manufacture by
preset value. Based on established
procedure, the operator then decides
whether to accept the test objects, to
reprocess the test objects or to stop the

172 Magnetic Testing

automatic machines and from reduction this is a good way to verify that valves are
of human error. working and that the flow is enough. The
cleaning solution should be also
Automation can be used in magnetic monitored to determine if the bath is
particle testing by certain combinations of uncontaminated and at proper
the following processes: (1) automatic concentration.
handling and positioning of the test
object, (2) automatic test object The cleaning of magnetic particle test
magnetization, (3) automatic timing of specimens is usually not critical. Unlike
the bath application relative to the liquid penetrant testing, magnetic particle
magnetization timing, (4) automatic tests are generally very forgiving of
detection and location of magnetic nonmagnetic dirt. For example, it is
particle indications on the test object and possible to have nonferritic foreign
(5) automatic interpretation (deciding if material contained in surface cracks and
indications are caused by discontinuities). still do an excellent job of detecting these
discontinuities with magnetic particle
Process Monitoring techniques. Good judgment and cost
considerations determine how much
Full automation comprises more than cleaning is monitored.
automatic scanning. The term automatic
scanning refers to automated observation. Verification of Magnetization
When the magnetic particle testing
process is fully automated, all steps are Several approaches can verify
performed automatically and there is no magnetization. Current flowing during
inspector. Processing, observation and the correct magnetization time window is
interpretation are performed and a good indication that the test object is
monitored by an automatic apparatus. being magnetized properly.8 Further
assurance occurs when the monitored
Partial automation is achieved when current value falls within predetermined
certain operations are automated. For thresholds. Monitoring of the current
example, it is common to use automatic time and value is straightforward. Flux
cleaning and bath application tied to shunting devices determine proper
automatic magnetization and still use magnetization levels and can serve as
inspectors to observe and interpret the references.
indications.6
It is also possible to measure the
It is important for the inspector to normal and tangential components of the
verify that each stage of the testing surface magnetic flux density with a hall
procedure is performed properly. If full effect device.8 This measurement is a
automation is used, automatic verification relative indicator of internal flux density
is required and maintenance of such for test objects with simple shapes. It is
components becomes a critical procedure. best to measure this value during the
At production speeds, inoperative magnetization time window and use it to
verification equipment can invalidate activate predetermined thresholds.
large numbers of magnetic particle tests.
There are various ways to verify proper Another approach is to use an eddy
performance of processing stages.7 The current device to measure the change in
first decision in this and all verification relative permeability of the material being
processes is determining the need for magnetized (Fig. 8). The optimum value
verification. For example, if cleaning is of permeability is that value at the knee of
critical to the accurate testing of a the initial magnetization curve.9 It is
particular product, then the cleaning possible to monitor this point with eddy
procedure must be verified. The next current techniques. As with other
decision is determining the characteristics methods, predetermined values should be
of interest (in this case, those that affect used, proper threshold levels should be set
magnetic particle test results) and how and permeance should be measured
they can be quantified. during magnetization.

Verification of Cleaning Monitoring of Particle Bath
Operations
Low Retentivity Objects. Bath application
If a spray cleaning system is used, a flow is crucial for test objects with low
meter may be used to monitor the magnetic retentivity. If such a test object
cleaning bath as it moves through the has a smooth surface and bath continues
pipes leading to the spray head. The flow to flow over developed indications after
meter should be placed as close to the the magnetization cycle, it is probable
spray head as possible and the signal that the developed magnetic particle
generated by the flow meter should be indications will be washed off. In this
compared with the signal to activate the situation, it is crucial that the
flow valve. Timing differences occur but magnetization cycle be carefully timed
when determining the bath procedure.8
Flow conditions should be checked and

Equipment for Magnetic Particle Testing 173

cycles should be carefully adjusted to rough, embedded particles may resist
ensure that all bath flow has stopped in washing even if the test object is
critical areas of the test object before the demagnetized. Remaining particles might
magnetization cycle ends. Then both the indicate discontinuities.
magnetization and bath cycles must be Bath Coverage. It is vital for critical areas
monitored to set thresholds to detect to be covered with magnetic particle bath.
deviations. Flow can be monitored with a Positioning of the test object relative to
flow meter (Fig. 9) as close as possible to the bath applicator is therefore very
the spray heads. Photocells and acoustic important. When an automatic loading
path devices have also been used to system is used, it is possible for the test
measure flow rates. object to shift and critical areas are not
High Retentivity Objects. If the test object then exposed to the bath. For this reason,
has high magnetic retentivity or a rough test object positioning should be
surface, bath application is not as critical. monitored by means of limit switches,
If proper magnetization is used in a high photocells or acoustic path monitors.
retentivity test object, the magnetic Bath Concentration. In manual systems,
leakage field around the discontinuity magnetic particle concentration is
continues to exist and attract particles monitored using a centrifuge (Fig. 11) in a
after the magnetizing current is shut off. settling test.10 In automatic systems, it is
In this case, discontinuity indications possible to monitor particle levels with
develop even if bath flow continues after electromagnetic techniques, optical
the magnetization cycle. Multidirectional techniques and particle counters. A
applied fields are not retained in test number of bath parameters may be
objects8 (only one of the directions checked at the same time by using a
remains after the current is stopped). The standard test object containing a known
usual compromise is to end magnetization discontinuity. The reference standard is
with a field orientation covering the most passed through the entire test system in
critical discontinuity orientation. If front of each part being tested or in some
alternating current magnetization is used,
the current should be cut off at the same FIGURE 10. Flux density versus magnetizing
point in the cycle for every test object for current. To ensure maximum retained field,
consistency. For maximum retained field, current should be shut off during the two
the current should be cut off at zero or parts of the waveform illustrated as solid
when it is decreasing from a maximum lines. Shutoff must be at the same point for
value (Fig. 10). If a coil or yoke is used for all test objects to ensure consistency.
alternating current magnetization, it is
easy to demagnetize the test object as it B
moves through the tapering field at the +Bmax
exit end of the coil or yoke.8 As with low
retentivity materials, little or no field is
then retained around the discontinuity
and indications might be washed off.
However, if the test object’s surface is

FIGURE 9. Typical magnetic particle bath flow versus –Imax Io +Imax H
magnetization time.

Maximum Bath flow

Bath flow Magnetization
(unspecified volume per unit of time)
–Bmax
Magnetization
+Imax

Io 17 ms
at 60 Hz

–Imax

0123 Legend
Time (s) I = current (A)
B = magnetic flux density (T)
H = magnetic field intensity (A·m–1)

174 Magnetic Testing

frequent, repetitive manner. The standard crests, valleys, hole edges and other
is cleaned, magnetized, bathed and tested anomalies.
automatically. When the system is
working correctly, the standard’s A trained operator ignores nonrelevant
indications confirm that test objects are indications and so should an automated
being inspected properly. system. This discrimination capability
dramatically increases system complexity,
Automated Scanning of Magnetic yet most automated testing applications
Particle Tests require pattern recognition capabilities
including optical techniques, dedicated
Fluorescent magnetic particle indications digital processing, microprocessor
are scanned to automate the viewing of algorithms and neighborhood processing
test results. Automatic scanners usually algorithms.
comprise the following components: (1) a
source of ultraviolet radiation, (2) a video Optical Pattern Recognition
camera, (3) a computer for imaging,
(4) amplification and discrimination In laser inspection, a linear beam is aimed
equipment to evaluate indications and at a discontinuity and along the
(5) material handling accessories. orientation of that discontinuity.
Figure 12 illustrates the formation of this
Material handling accessories are line of light using an anamorphic lens. An
needed to move test objects to and from anamorphic lens is curved so that it
the scanner, to index or move the test focuses the collimated laser light into a
objects under the scanner so that all line parallel to the lens axis. The beam’s
desired areas are inspected uniformly and cross sectional width in the orthogonal
to mark or separate rejected test objects. direction is maintained because there is
no focusing in that axis.
Automated Interpretation
of Magnetic Particle For optical pattern recognition, a
Testing phototube detects and integrates the
fluorescent flash that occurs when the
Recognition of significant test indications exciting beam is coincident with the
and distinguishing them from background indicated discontinuity. The need for
indications are generally the two most parallel alignment of the beam with the
complex problems for an automated discontinuity limits system flexibility.
testing system. Background often appears
as a foggy glow containing a number of A similar result can be obtained using a
isolated bright spots. Frequently, there are scanning laser system and integrating the
also some false indications that share the output of the photodetector across the
characteristics of discontinuities. These scan. This has the advantage of a larger
include tool marks, scratches, thread depth of field (no optics are used for
focusing) but requires more complex
FIGURE 11. Centrifuge tube for monitoring scanning and electronics.
of magnetic particle concentration.
Sensitivity is reduced if the beam and
the discontinuity are not parallel. A
parallel line of exciting radiation permits
optical integration of the fluorescent light
from the entire length of the indication.
Slits of excitation illumination can permit
optical pattern recognition for straight
line indications in other orientations.

FIGURE 12. Optical pattern recognition of straight, linear
fluorescent test indications.

Collimated light

Phototube

Anamorphic lens

Line of
laser light
Discontinuity

Test object

Motion of
scanned object

Equipment for Magnetic Particle Testing 175

References

1. Haller, L., S. Ness and K.[A.] Skeie. 5. Chudvk, G. TP75PUB370, Cost
Section 15, “Equipment for Magnetic Reduction and Improved Productivity
Particle Tests.” Nondestructive Testing through Material Handling. Dearborn,
Handbook, second edition: Vol. 6, MI: Society of Manufacturing
Magnetic Particle Testing. American Engineers (January 1975).
Society for Nondestructive Testing
(1989): p 349-379. 6. Flaherty, J.[J.] “Automation and NDT.”
Manufacturing Systems. Vol. 4, No. 8.
2. Flaherty, J.[J.] and C. Exton. Section Chicago, IL: Hitchcock Publishing
10, “Process Automation of Magnetic (August 1985).
Particle Testing.” Nondestructive Testing
Handbook, second edition: Vol. 6, 7. Foley, Eugene. “NDT: A Real Profit
Magnetic Particle Testing. American Center.” Machine and Tool Blue Book.
Society for Nondestructive Testing Vol. 30. Chicago, IL: Hitchcock
(1989): p 245-269. Publishing Company (December 1984):
p 58.
3. Magnetic Particle Inspection Symposium
Reference Book [Nashville, TN, March 8. Betz, C.E. Principles of Magnetic Particle
1993]. Columbus, OH: American Testing. Chicago, IL: Magnaflux
Society for Nondestructive Testing Corporation (1967).
(1993).
9. Lorenzi, D.E. United States Patent
4. ASTM E 1444, Standard Practice for 4290016, Method and Apparatus for
Magnetic Particle Testing. West Establishing Magnetization Levels for
Conshohocken, PA: ASTM Magnetic Particle Testing or the Like
International (2005). (1981).

10. ASTM E 709, Standard Guide for
Magnetic Particle Inspection. West
Conshohocken, PA: ASTM
International (2008).

176 Magnetic Testing

7

CHAPTER

Magnetic Particles1

Bruce C. Graham, Arlington Heights, Illinois
David G. Moore, Sandia National Laboratories,
Albuquerque, New Mexico

PART 1. Introduction

Magnetic particle testing is magnetic flux the presence of certain linear
leakage testing in which the indications discontinuities slightly below an object’s
are produced by ferromagnetic particles surface. The precise degree of the
applied to the test surface. magnetic particle test sensitivity has not
been firmly established, although this text
Magnetic particle testing provides a test discusses several applications that
indication very near actual material demonstrate how fine a crack may be
discontinuities, describing their size and reliably detected with careful testing
shape on the test object surface. In this procedures. In practical situations,
important way, the magnetic particle absolute sensitivity is not as important as
method differs from most other the probability of detection. This
nondestructive tests. In other techniques, probability is a product of the magnetic
test indications are typically produced in particle characteristics, the magnetization
a medium separate from the test object, as technique, the level of magnetization, test
an oscilloscope trace, through an acoustic environment lighting intensity and
transducer or on radiographic film, for inspector training.
example. By outlining and precisely
locating the discontinuity, magnetic There are two types of magnetic
particle indications are comparatively particles in general commercial use: dry
simple to interpret. and wet. Dry particles are applied to the
test surface as a solid suspension or as a
The magnetic particle test method is cloud in air. Wet particles are applied as a
best suited for locating small surface suspension of particles in a liquid vehicle,
discontinuities on ferromagnetic test either oil or water.
objects. The technique may also indicate

178 Magnetic Testing

PART 2. Dry Technique Testing Materials

Dry Technique Particle in air, not to be mixed with a liquid
Characteristics vehicle. Reclaiming dry powders is not
recommended.
Particles used in the dry technique of
magnetic particle testing are Fine dry particles that can pass through
manufactured to emphasize three sets of mesh sieve apertures of 50 µm (0.002 in.)
physical properties: (1) magnetic diameter show much greater sensitivity to
characteristics, (2) size and shape and small discontinuities and their small
(3) visibility. leakage fields when compared to particles
of 150 µm (0.006 in.). The coarser
Magnetic Properties particles, nearly three times the diameter
of the 50 µm (0.002 in.) mesh particles,
Nearly all dry magnetic powders are finely are more than 20 times heavier and are
divided iron particles coated with too large to be held by weak leakage
pigments. These ferrous materials are fields.
chosen to provide the characteristics
critical to magnetic particle test However, for two reasons, a dry testing
procedures. Primary among these are low powder cannot be made exclusively of
magnetic retentivity, high magnetic small magnetic particles: (1) small
permeability and low coercive force. particles adhere to all surface anomalies
(oil traces, dampness, fingerprints or
Low retentivity increases the dry roughness), producing dense particle
powder’s ability to clearly indicate backgrounds and (2) the testing
discontinuities. If the particles retain environment becomes extremely dusty
magnetism (high magnetic retentivity), and unsafe for the inspectors. As a result,
they adhere to each other and cannot be a practical dry magnetic powder contains
properly applied. When such particles a range of particle sizes. Small particles are
reach the test object, they adhere to its needed for sensitivity to fine
surface and cause an intense background, discontinuities. Coarser particles are
masking discontinuity indications. needed to bridge large discontinuities and
to diminish the powder’s dusty nature. In
High magnetic permeability also addition, large particles can reduce
increases the powder’s ability to indicate masking by dislodging the background
discontinuities. Particles with high often formed by fine particles.
permeability are easily attracted to the
small magnetic leakage fields from Particle shape also has important
discontinuities, where they are trapped effects on background, particle application
and retained for interpretation. and test results. Elongated particles (large
length-to-diameter ratios) are easily
The concentration of the magnetic attracted to leakage fields. When
material in dry powders is a fourth critical compared to particles of equal
consideration. Increasing the amount of permeability but compact shape, long
magnetically inert pigment in a dry particles more readily form linear
powder composition naturally lowers its discontinuity indications, possibly
magnetic sensitivity, or coercivity. Dry because of their ability to easily achieve
magnetic powders are therefore magnetic polarity. However, elongated
manufactured as a compromise between particles cannot be used alone because of
the primary need for sensitivity and the their tendency to mat and to form
secondary need for high visibility. clusters hard to apply.

Size and Shape Particles with a compact shape flow
easily, have high mobility and can be
Particle size and shape are to some extent simply dispersed into clouds for proper
more critical than magnetic permeability application. As a result, the most sensitive
for achieving sensitivity and ease of use in dry powders contain both shapes, in a
a dry powder. Magnetic powders are not ratio determined by the test application
simply an aggregate of metallic filings. and empirical data.
The particles are made from carefully
selected magnetic materials of specific In SAE AMS 3040, Magnetic Particles,
size, shape, magnetic permeability and Nonfluorescent, Dry Method,2 the upper size
retentivity. They are designed to be used limit for sensitive dry particles is about
180 µm (0.007 in.) diameter. Larger
particles can plug powder applicators and
do not add sensitivity.

Magnetic Particles 179

Not all dry powder test procedures Dry Particle Uses
require high sensitivity. In some
applications, high sensitivity is actually Dry magnetic particles are often used to
detrimental to the testing procedure. In test welds or castings for surface
these circumstances, size and shape are discontinuities. Dry powder is not
less critical and the most important recommended for the detection of fine
particle characteristic is low residual discontinuities such as fatigue cracks or
magnetism. grinding checks. Wet technique testing is
much more sensitive to small surface
Visibility and Contrast discontinuities.

Dry technique magnetic testing powders Tests of Welds
are commercially available with three
types of colors: (1) visible colors, for In weld testing, the typical magnetic
viewing under visible light, (2) fluorescent particle technique uses prods or yokes,
colors, for viewing under ultraviolet with the inspector magnetizing and
radiation and (3) daylight fluorescent testing short overlapping lengths of the
colors. The daylight fluorescent powders weld. The continuous magnetization
fluoresce brightly in visible light, greatly technique is used (the magnetic field is
amplifying their visible color. continuously activated while the
inspector applies the powder and removes
The visible light powders are normally the excess).
available in gray, red, black, yellow, blue
and metallic pigments. This range of Automatic processing has been used for
colors allows the user to choose the one testing linear welds on large diameter
that contrasts most strongly with the test pipe. However, most welded structures are
object surface. shaped in ways that make them difficult
to handle automatically. In addition,
Fluorescent colors are seldom used in much weld testing is done on site and
dry powder applications, partly because only portable testing systems can be used.
dry powder tests are typically performed
on site or on large structures, making it Direct current magnetization is usually
difficult or impossible to enclose and preferred for weld testing because it
darken the testing area. In addition, penetrates more deeply and allows the
fluorescent dry powders often produce indication of slightly subsurface linear
background that is bright enough to be discontinuities. Half-wave direct current
objectionable. also has the advantage of providing
increased particle mobility and increased
Daylight fluorescent colors are potential for forming accurate
occasionally used when a test indication’s discontinuity indications while reducing
high visibility or contrast is more background.
important than absolute sensitivity
(fluorescent color augments the visible Tests of Castings
color and indications are extremely bright
and visible). With these powders, there is Dry particles are particularly useful for
no need for an enclosed, darkened test magnetic particle testing of large castings.
environment. In fact, the brighter the Cast objects are normally tested using
ambient light, the brighter the prods or yokes, with the test covering
indications. Daylight, even in deep small overlapping areas. Large portable
shadow, excites their fluorescence, as does power supplies may be used. In these
blue mercury vapor lighting or the light applications, the magnetization
from white fluorescent tubes. Ordinary equipment is set up in advance, with
incandescent lights work less effectively connectors for through-current shots
than white or blue light sources. However, firmly clamped in place and coils or
the yellow light from sodium vapor lights looped cables wound where needed. The
does not excite fluorescence in daylight current is applied throughout powder
powders and cannot be used for this application making this a continuous
application. technique. Because the test procedure
may take several minutes on a large
In some applications, particle contrast casting, all test system cabling must be
may be enhanced by coating the test properly rated to avoid excessive heating.
object surface with a thin white lacquer.
The characteristics of the lacquer then The choice of magnetizing current
become important considerations for the depends on the type of discontinuities
magnetic particle inspector. The white being sought.
surface coating must dry immediately, lest
it slow the testing procedure, and must be
easily removed after the test to avoid
delays and additional costs.

180 Magnetic Testing

Application of Dry Reuse of magnetic powders also
Magnetic Particles encourages fractionation, the separation
of different sized particles from the
Particle Applicators composite. Particles sprayed over a test
object are designed to have different sizes
Dry magnetic powders must be applied in and to fall in different trajectories.
a gentle stream or cloud. If applied too Generally, finer particles float further
rapidly, the stream may dislodge away from the point of introduction.
indications already formed and will Unless great care is taken to collect all the
certainly build up too much background particles, reused powders gradually
for easy removal or interpretation. If become coarser and so less sensitive.
applied with too much velocity, typical
particles gain too much momentum to be Particle Storage
reliably trapped by discontinuity leakage
fields. Manual and mechanized powder The storage condition for dry technique
applicators can help provide proper powders is critical to their subsequent use.
density and speed of particle application. The primary environmental consideration
is moisture. If magnetic particles are
The simplest and most common exposed to high levels of moisture, they
particle applicators are rubber bulbs or immediately begin to form oxides.
shaker bottles. For most magnetic particle Rusting alters the color, but the major
applications, these simple devices provide problem is that the particles adhere to
the kind of application required for each other, forming lumps or large masses
accurate testing. Mechanical powder useless for magnetic particle testing.
blowers are another application option.
They are designed to float a cloud of Though not severe, there are also
particles onto the test object surface and limitations on the temperatures at which
then to provide a gentle stream of air to dry powders can be stored and used.
remove the lightly held background Visible light powders work on surfaces as
particles. For best results, magnetizing hot as 370 °C (700 °F). Near this
current should be present throughout the temperature some particle materials
application of particles and the removal of become sticky. Others lose much of their
background. color, although their magnetic properties
remain intact and they can still indicate
Monitoring of Particle Application discontinuities. Beyond 370 °C (700 °F),
magnetic powders can ignite and burn.
Regardless of the applicator used, the
inspector must carefully monitor the test Fluorescent and daylight fluorescent
object while particles are applied. It is powders lose their visible contrast at
critically important that powder 150 °C (300 °F) and sometimes at lower
application be done properly to ensure temperatures. This occurs because the
the reliability of the test. This is especially pigments are organic compounds that
true if subsurface discontinuities are being decompose or lose their ability to
sought. Their particle indications are fluoresce at particular temperatures.
weakly held and not well delineated, so
that they are very susceptible to damage Other Considerations
from particles applied later.
Dry technique powders must not be used
Particle Reuse in wet technique applications. Dry
particles are designed and manufactured
It is recommended that each batch of dry at densities that work well in air but cause
magnetic particles be used only once. them to settle quickly out of liquid
There are occasions when reuse is suspensions. In water, typical dry powders
permitted, but inspectors should settle at rates around 152 mm·s–1
understand the effects of frequent reuse. (6 in.·s–1) and accordingly cannot be kept
suspended. In addition, dry powders are
Ferrous magnetic powders are dense susceptible to oxidation when exposed to
(specific gravity of 7.68). When agitated water.
in bulk, as in a powder blower or a bulb, a
lot of shearing and abrasion results and Note also that ferrous powders in
wears off some pigment. Each reuse, with general and particularly many of their
its additional rehandling, wears off more pigments have been classified as nuisance
pigment. As a result, color and contrast dusts by the Occupational Safety and
diminish to the point that indications are Health Administration (OSHA) and must
not visible. be handled accordingly.3

Magnetic Particles 181

Viewing and Interpreting visibility. Incandescent light is somewhat
Dry Particle Test less effective and yellow sodium vapor
Indications light is totally ineffective. Daylight
fluorescent powders also fluoresce brightly
Producing a discontinuity indication is under ultraviolet radiation but cannot be
the first stage of a magnetic particle test. recommended for this technique of
Viewing the indication and interpreting viewing. They exhibit a high level of
its meaning is the second important step, fluorescent background when viewed in
and both procedures depend critically on the dark, high enough to nearly obliterate
the characteristics of the magnetic fine discontinuity indications. Testing
particles and radiation intensities. under full visible light hides this
background and keeps contrast high.
Visible Particle Light Intensities
Dry Magnetic Particle
ASTM E 1444, Standard Practice for Specifications
Magnetic Particle Testing,4 calls for
minimum light intensities of 1000 lx Although the manufacturers of dry
(100 ftc) for magnetic particle testing with magnetic powders have their own quality
nonfluorescent powders. Other light level control tests, additional testing is often
recommendations range from 800 to needed to prove conformance to user
2000 lx (80 to 200 ftc). specifications. One important
specification is SAE AMS 3040, Magnetic
The optimal light level is often a Particles, Nonfluorescent, Dry Method.2
compromise between operator fatigue and SAE AMS 3040 places an upper limit on
visibility. On bright or reflective surfaces, particle size: 98 percent must be finer
high light intensities can cause glare that than a 180 µm (0.007 in.) diameter mesh.
interferes with interpretation. On darker The specification also defines the limiting
surfaces or those covered with thin scale, amount of loose pigment (in a test called
rust or other staining, the 1000 lx (100 ftc) magnetic properties) and specifies a
level may be barely adequate. minimum sensitivity: six indications
visible on the tool steel ring standard.
Fluorescent Particle Radiation
Intensities Other techniques have been developed
for dry magnetic particle testing of pipe,
Because of their limited industrial including recommendations for the dry
application, there is little empirical data particles.5 Powder is required to have a
for radiation levels in dry fluorescent range of particle sizes: at least 75 percent
testing. In addition, there is presently no in mass should be finer than 125 µm
standard or specification for dry (0.005 in.) mesh and at least 15 percent in
fluorescent intensity ranges. The few dry mass should be finer than 44 µm
fluorescent powders that have had (0.0017 in.) mesh.
marginal applications were much brighter
than typical wet technique powders. Their This technique also outlines a magnetic
viewing conditions were correspondingly permeability test that uses a cylindrical
less demanding than the conditions for powder sample as the core of a simple
wet fluorescent particle viewing (see transformer. For an 11 kA·m–1 (140 Oe)
below). input in the primary circuit, at least 2.5 V
shall show up across the secondary circuit,
Daylight fluorescent dry powders also with the prescribed circuitry. Such tests
have no specifications for required are sensitive to the amount of powder
radiation intensities. Sunlight, shaded or packed into the cylinder and effectively
direct, as well as artificial blue or white put a ceiling on the amount of
light all excite daylight fluorescent magnetically inert pigment that the
powders to a high level of brightness and powder can contain.

182 Magnetic Testing

PART 3. Wet Technique Testing Materials

Wet Technique Particle of forming low retentive powders
Characteristics (grinding or precipitating from a highly
agitated solution) preferentially break
In wet technique magnetic particle down long, narrow particles. Exceptions
testing, particles over 25 µm (0.001 in.) in to this are the high retentivity, high
diameter are considered coarse. These coercive force oxides and ferrites used in
large particles exhibit diminished magnetic tapes.
sensitivity to fine surface cracks and
produce no more than four indications on Fluorescent wet particles have a
the tool steel ring standard. These coarse definite and measurable size, as do
particles settle out of suspension in 5 to particles based on finely divided metallic
15 min and are difficult to keep in iron. Synthetic iron oxides are more
suspension. difficult to measure, with diameters
around 0.1 µm (4 × 10–6 in.). They are
Sensitive wet technique particles range almost too fine to settle out of
from 5 to 15 µm (0.0002 to 0.0006 in.) in suspension.
diameter; unpigmented ferromagnetic
oxide particles are an order of magnitude Because of their slight residual
finer. magnetism, oxide particles collect to form
loose clusters that settle out of suspension
The small size and generally compact much faster than individual particles. The
shape of wet technique particles dominate degree of clustering depends on the
their behavior. Their size makes intensity of agitation. At high agitation
permeability measurements inexact and rates, the clusters are small. At low
not useful. In addition, the size influences agitation rates, they become larger.
the brightness of fluorescent powders Clusters can often be seen on smooth
made from such particles. shiny test surfaces after bath application.
In this quiescent state, the clusters grow
Wet Particle Composition large enough to be seen with the unaided
eye, often tens of micrometers in
Commercial wet technique particles are diameter.
made from finely divided iron, from black
iron oxide, from brown iron oxide and Magnetic Properties
experimentally from ferrites, nickel and
nickel alloys. Because metals and ferrites can be shaped
into ideal toroidal samples of 100 percent
Black iron oxide or magnetite (Fe3O4) is density, their magnetic properties can be
available as ground ore or preferably as a measured precisely. Powders cannot be
fine synthetic powder. Brown iron oxide accurately measured because samples
(gamma Fe2O3) is chemically identical to cannot be made with densities higher
nonmagnetic red iron oxide (alpha Fe2O3) than about 50 percent. As an example,
but has the same ferromagnetic cubic pure iron has an initial permeability of
crystalline structure as magnetite. 1000 at 8 kA·m–1 (100 Oe) field intensity.
Pure iron has a maximum permeability
Ferrites are hard ceramic materials around 5000. In laboratory tests, 10 µm
difficult to make into fine powders. Some (0.0004 in.) iron powder showed a
nickel alloy powders show good magnetic permeability of about 10 (5 for iron
test sensitivity, if fine enough, but are oxides) at 8 kA·m–1 (100 Oe). With the
slightly denser than iron and even harder lower field intensities associated with
to keep in suspension. small leakage fields at discontinuities,
permeabilities may well be lower still (the
Fluorescent powders also contain data above cannot be used to predict a
fluorescent pigments as well as a binding ferromagnetic powder’s behavior).
resin to attach the fluorescent pigment to
the ferromagnetic core. Because of their compact shape, the
true material permeability of fine wet
Size and Shape technique particles may not be of
practical importance. However high the
At sizes of 10 µm (4 × 10–4 in.) and under, core material’s permeability might be, the
practically all low retentivity apparent permeability of the individual
ferromagnetic powders have a compact particles does not exceed a value of about
shape, with length-to-diameter (L·D–1) 2.5. This is because of the large
ratios around one, because the techniques

Magnetic Particles 183

demagnetization factor6 associated with The size of the particle strongly influences
an L·D–1 ratio of one. its fluorescent brightness, as the following
data illustrate.
(1) 1 = 1 − Nd
µ µ′ In 1 kg (2.2 lb) of 125 µm (0.005 in.)
iron particles, the pigmented surface area
= 1 − N is about 6 m2 (65 ft2) and can be made
µ′ 4π brightly fluorescent with very little
pigment. A finer iron powder, about
where N is the demagnetizing factor, Nd is 40 µm (325 mesh or 0.0017 in.) has a
the shape demagnetization factor, µ is the surface area around 18 m2 (190 ft2). In
true permeability of the parent 1 kg (2.2 lb) of a 6 µm (2.4 × 10–4 in.)
ferromagnetic material and µ′ is the oxide based powder, the pigmented
apparent permeability of the sample surface area is about 420 m2 (4500 ft2).
particle. Because the inspector’s eyes register the
fluorescent pigment on the particle’s
At lower material permeabilities, the surface, ultrafine particles require 30 to 60
apparent permeability decreases, times as much pigment as the coarser
becoming 2.0 at a material permeability of particles to achieve the same relative
10. The apparent permeability decreases brightness. Such ratios make
rapidly at still lower permeabilities. As a manufacturing of bright ultrafine particles
result, magnetic particles made from very impossible (the particle would contain
high permeability material are slightly virtually all pigment with a trace of
more effective than those of moderate ferromagnetic core and no
permeability values (about 20 to 100). The electromagnetic sensitivity).
most important consideration is avoiding
the use of particles with very low An important consideration in
permeabilities. fluorescent particle contrast is its
durability. In an agitation system where
True material permeability becomes the bath constantly passes through a
important when two or more particles centrifugal pump, the particles are
touch and align in a leakage field. The subjected to constant high speed impact
L·D–1 ratio of the joined particles begins and shearing action from the pump’s
to effectively exceed a value of one, and impeller. This slowly breaks the particle
the demagnetization factor of the string down to fragments. In the extreme case,
of particles shrinks, allowing the string to two kinds of particles are formed:
become more effectively magnetized and (1) nonfluorescent magnetic fragments,
more firmly attached to the sites of which form indications that cannot be
discontinuity leakage fields. seen, and (2) nonmagnetic fluorescent
fragments, which do not indicate
Visibility and Contrast discontinuities but which do cause
background. In practice, baths never reach
Wet technique particles are commercially this stage of total deterioration, but as
available as fluorescent and breakdown progresses, indications become
nonfluorescent materials. Most dimmer while background fluorescence
nonfluorescent particles are simply increases (indication-to-background
ferromagnetic iron oxides, either black or contrast diminishes).
brown. These are used in their natural
color with no added pigment. As a result, The onset of particle breakdown can be
these particles are very slightly more detected in an already operating bath by
sensitive than the pigmented fluorescent an extension of the settling test.
particles. For this reason, the Fluorescent fragments are both less dense
unpigmented iron oxides are preferred for and finer than the intact particles and
some applications where sensitivity is settle out of suspension much more
more important than easy visibility. slowly, requiring 10 to 15 h. After a
Bearing testing is an important settling test is performed, allow the
application of these oxides: sensitivity is settling tube and sample to sit unagitated
critical and the smooth reflective surface overnight. Loose fluorescent pigment will
of the test objects gives the best possible produce a thin, brightly fluorescent layer
contrast to the dark oxide particles. on top of the sediment. This is the settling
test, described below.
On darker surfaces, indications from
brown and black particles are very No firm correlation has been made
difficult to see and locate, though a thin between the extent of particle breakdown,
white lacquer painted on the test surface the relative amount of free pigment and
can improve contrast, much as it does the reliability of the bath. This settling
with dry powders. test simply detects the occurrence of
breakdown. The individual user can best
Fluorescent magnetic particles are relate the evident breakdown to the
composites, containing a ferromagnetic quality of the fluorescent magnetic
core, a fluorescent pigment and preferably particle bath.
a binder to hold the composite together.

184 Magnetic Testing

Oil Vehicles for Wet points. The flash point minimum given in
Technique Particles SAE AMS 3161 is only 65 °C (150 °F).15

There are two kinds of vehicles used for Flash point is important for two
wet technique testing: water and oil. Oil reasons. Fire safety is the primary
vehicles are preferred in certain consideration: the Occupational Safety
applications: (1) where lack of corrosivity and Health Administration has placed
to ferrous alloys is vital (finished bearings costly restrictions on the use of lower
and bearing races, for instance); (2) where flash point solvents in open tanks).16-18
water could pose an electrical hazard; These restrictions may be avoided if the
(3) on some high strength alloys, where vehicle’s flash point is over 93 °C (200 °F).
exposure to water may cause hydrogen Health considerations are less obvious but
embrittlement (hydrogen atoms from also important. Low flash point vehicles
water can diffuse into the crystal structure are more volatile than high flash solvents.
of certain alloys and so cause A typical petroleum solvent having a flash
embrittlement. point of 66 °C (150 °F) is much more
volatile than one having a flash point of
Water vehicles are preferred for the 93 °C (200 °F) or more, and burdens the
following reasons: (1) lower cost, (2) little inspector’s breathing air with many times
fire hazard, (3) no petrochemical fumes, more solvent vapor.
(4) quicker indication formation and
(5) little clean up required on site. The MIL-STD-1949A has been replaced
by ASTM E 1444, Standard Practice for
History of Oil Vehicle Magnetic Particle Testing.4 The
Specifications requirements for magnetic particle oil
vehicles can be found in SAE AMS 3045
In the past, a variety of petroleum and SAE AMS 3046 documents.19,20
solvents and oils were written into DLA A-A-59230, Fluid, Magnetic Particle
magnetic particle testing specifications Inspection, Suspension, supersedes DOD F
but most of these early oil vehicles were 87935, Fluid, Magnetic Particle Inspection,
designed for other purposes. For example, Suspension Medium. The requirements of
federal specification P-D-680, Dry Cleaning DOD F 87935 were as follows.14,21
and Degreasing Solvent,7 and federal
specification VV-K-220 (kerosene, 1. The maximum viscosity is 5.0 mm2·s–1
deodorized)8 were referenced in (5.0 cSt) at bath temperature, in
MIL-I-6868E (1976), Magnetic Particle accordance with ASTM D 445.22
Inspection Process.9 These standards were
all superseded.10-12 2. The minimum flash point is 93 °C
(200 °F), in accordance with
P-D-680 originally applied to ASTM D 93;23
nondestructive testing before high flash
point fluids were required. This 3. The vehicle fluoresces no more than
specification called for: (1) a minimum does the reference standard.
flash point of 59 °C (138 °F), (2) a low
distillation range (no contemporary oil 4. There is no odor.
vehicle would meet this specification) and 5. Particulate matter is no greater than
(3) two inappropriate, purely chemical
tests (the doctor test and sulfuric acid 0.5 mg·L–1, in accordance with
absorption).7 ASTM D 2276.24
6. Total acid number is 0.015 mg·g–1
The VV-K-220 document allowed a maximum, in accordance with
similarly low flash point and low ASTM D 3242. Acidity is expressed as
distillation range and required a different the amount (milligram) of base
sulfuric acid reactivity test.8 (potassium hydroxide, KOH) needed
to neutralize 1 g of the measured
ASTM E 1444, like MIL-STD-1949A material.
before it, calls for magnetic particle oil 7. The ASTM color number is 1.0
vehicles that met the requirements of maximum, in accordance with
AMS 3161 and DLA A-A-59230.4,12-14 ASTM D 1500.25
The requirements of AMS 2641 in 2007
SAE AMS 3161, Odorless Heavy Solvent were as follows.26
Inspection Oil,15 addresses viscosity, a 1. The minimum flash point is 93 °C
specific concern of magnetic particle (200 °F) for a Type I vehicle and is 60
testing. Viscosity is measured as square to 93 °C (140 to 200 °F) for Type II
meter per second (m2·s–1) and is often vehicle (ASTM D 93).23
expressed in square millimeter per second 2. Viscosity must be no more than
(mm2·s–1) or its equivalent in the 3.0 mm2·s–1 (3.0 cSt) at 38 °C (100 °F);
centimeter gram second system, no more than 5.0 mm2·s–1 (5.0 cSt) at
centistokes (cSt). The specified viscosity of bath temperature, according to
2.0 to 2.3 mm2·s–1 is low, typical of light, ASTM D 445.22
volatile petroleum solvents with low flash 3. Fluorescence must be the same as
specified in DOD F 87935.21
The AMS 2641 specification prescribes
the same limits as DOD F 87935 on the

Magnetic Particles 185

amount of particulate matter, acidity, odor particles, with their water repellent
(inoffensive) and visible color.21,26 organic pigments and binders,
(3) minimize foaming caused by wetting
Viscosity Check agents necessary in the bath and
(4) retard rusting of test object surfaces.
Viscosity, a measure of a fluid's resistance
to flow, is an important property of oil Where the bath does not at first cover
vehicles for magnetic particle testing. the test object surface, there can be no
Contaminants from the surface of test particles and no indication formation.
objects build up in the particle suspension Beyond this, the bath film must cover the
and increase its viscosity. Precleaning test test object surface (without breaking)
objects to remove oil and grease helps throughout the magnetic particle test
resolve but does not eliminate this procedure. If the water film does break or
problem. peel from the surface to form separate
drops, it also peels off particles in
Depending on the specifications in indications. The result is a poor and
force, viscosity measurements are usually unreliable test.
performed monthly, often in specialized
commercial laboratories. A 90 mL (3 oz) Wetting Abilities
specimen from the tank is enough for this
check. Different surfaces require different degrees
of wetting. Steel billets, with porous,
If the viscosity exceeds specified values, oxidized surfaces are easily wetted by
the suspension is discarded and the tank untreated water. At the other extreme,
is drained, cleaned and refilled with fresh very smooth surfaces covered with a trace
suspension. The results of viscosity tests of oil require very strong wetting ability.
are reported in a log book, along with any In such cases, surface tensions as low as
third party reports. 25 mN·m–1 (25 dyn·cm–1) may be
required.
Suspensions, solvents and similar
materials must be discarded in compliance Magnetic iron oxides used in
with federal, state and local laws. nonfluorescent wet technique baths are
easily wetted by untreated water.
Vehicles for Wet Technique However, fluorescent particles typically
Particles Water contain organic pigments and binding
Conditioning resins that tend to be water repellent.
Fluorescent baths must therefore include
Water cannot by itself be used as a treated water to achieve the wetting
magnetic particle testing vehicle. It rusts ability needed to cover common oily test
ferrous alloys (including the testing surfaces and to adequately wet and
equipment), it wets and covers test disperse the fluorescent particles. When
surfaces poorly and does not reliably fine fluorescent magnetic particles are not
disperse fluorescent magnetic particles. wetted by the vehicle, they float on the
Water conditioners or wetting agents must bath surface like dust and no amount of
be added to remedy these shortcomings. agitation will disperse them.

Water conditioners are well covered in Foaming Solutions
the ASTM E 1444 specification.4 SAE AS
4792, Water Conditioning Agents for Because they contain powerful wetting
Aqueous Magnetic Particle Inspection,27 agents, magnetic particle water baths
requires only that a conditioned water easily generate stable foams when agitated
vehicle (1) wet a test surface without the at the surface. Masses of foam that reach
film of water breaking and (2) have an the test surface during bath application
alkalinity not exceeding a pH of 10.0. slide to the lowest edge of the surface,
(The term pH is used as a logarithmic erasing indications in their path. It is
measure of hydrogen cation therefore important that water baths
concentration.28 Water has a pH of 7; acid (1) do not foam excessively and
solutions have more hydrogen ions and a (2) generate unstable foam that disappears
pH less than 7; alkaline solutions have quickly. Although careful formulation of a
fewer hydrogen ions and a pH greater water conditioner can minimize foaming,
than 7.) full foam control occasionally requires
antifoaming agents. Such agents cover the
Common sense indicates that a bath surface with a microscopically thin
conditioned bath should not rust test layer of an oily substance. Antifoaming
objects and should not foam so as to agent concentrations are critical: if too
interfere with the formation of much is present, it acts like an oily
indications. In magnetic particle tests, a contaminant, coagulating the magnetic
water conditioner needs to perform the
four following functions: (1) reliably wet
and cover all test surfaces, (2) encourage
wetting and dispersion of fluorescent

186 Magnetic Testing

particles and destroying the bath’s wetting the bath. Emulsifiers must be added
ability. carefully: too much will markedly increase
the bath’s viscosity and fluorescence.
Corrosion Inhibition
Brilliance and Contamination
Corrosion was at one time controlled by Checks
including small amounts of sodium nitrite
or traces of sodium chromate in the Each week that magnetic particle
magnetic particle bath. These chemicals equipment is used, the brilliance of
have high levels of toxicity and, when fluorescent suspension should be checked
present in the United States, must be by comparing it to a fresh, unused
listed by suppliers on “Materials Safety sample. When a suspension is mixed,
Data Sheets,” as ordered by the 200 mL can be retained in a dark glass
Occupational Safety and Health bottle for reference.
Administration.29 Sodium nitrite and
sodium chromate are also among the Used suspension is compared with the
waste water contaminants regulated by unused sample under ultraviolet
the Environmental Protection Agency. radiation. If the brilliance is noticeably
These safety restrictions limit the use of different, the test system's tank should be
the chemicals, and alternative corrosion drained, cleaned and refilled with fresh
inhibitors may have found acceptance suspension. The results of these checks are
after 2008. recorded in the log book.

Water baths have never been expected Contamination of the suspension by
to provide long term corrosion protection foreign matter should also be checked at
for the test object after testing is least once a week. If the suspension is
complete. Accomplishing this protection contaminated, the tank should be
requires separate treatment after testing. drained, cleaned and refilled with fresh
suspension. The following procedure is
Bath Contamination used to verify contamination.
1. Run the circulating pump for 1800 s
Both water and oil vehicles can be
contaminated by solid materials, as in the (30 min).
discussion of settling tests, below. 2. Fill a graduated 100 mL centrifuge tube
Introduced oil may also be a contaminant
of water and oil baths. In addition, oil with suspension and let it stand for
vehicles can be contaminated by water. 1800 s (30 min).
3. Examine the liquid above the
The most common form of precipitate under ultraviolet radiation.
contamination is oil in a water bath. This If the oil or water fluoresces green or
occurs in recirculating systems where bath yellow green, the tank should be
runoff returns to the bath reservoir and drained, cleaned and refilled with
oil on the test surface is washed into the fresh suspension.
water. When enough oil is present, it 4. Examine the precipitate. If two distinct
causes magnetic particles to congeal and layers are visible and if the top layer of
also destroys the bath’s wetting ability. Oil contaminate exceeds 50 percent of the
contamination can be avoided by bottom layer's magnetic particles, then
effectively precleaning test objects. If this the tank is drained, cleaned and
is not possible, stringent control of the refilled with fresh suspension.
bath is required and should include 5. Record the results in the log book.
regular visual inspections and monitoring Specifications may vary in the
of production levels. Contaminated bath allowable ratio between particles and
is discarded and replaced. contaminant in the precipitate. Results
of these tests are always logged in
Highly fluorescent oil or grease on test writing.
object surfaces can easily dissolve into oil Because these tests together take at
based magnetic particle baths. The least 3600 s (60 min), it is normal to
accumulation of introduced oil produces a begin magnetic particle testing before the
blue fluorescent background that can brilliance and contamination procedures
hinder discontinuity detection. Frequent are completed. If the suspension does not
inspection of the bath and monitoring of meet specification, objects tested during
production levels are again the solutions, the 1800 s (30 min) must be retested with
if precleaning is not possible. fresh suspension.

Particle coagulation also occurs when Bath Preparation
water contaminates an oil bath. The
condition is accompanied by a sticky Wet magnetic particle baths may be
buildup on the bath tank walls. Adding mixed by the supplier or may be sold dry
small (0.1 percent or less) amounts of a for mixing by the user. When the
suitable oil soluble emulsifier will restore magnetic particle system contains a

Magnetic Particles 187

recirculating bath, mixing is relatively result from these concentrations. Most
easy. industry standards specify only the
acceptable settling volume.
Recirculating System Bath
Preparation Visible Particle Concentrations

After the liquid vehicle is added to the The Society of Automotive Engineers
bath tank, the manufacturer’s required specifications AMS 3042 and AMS 3043
amount of powder is added directly above for visible wet technique particles call for
the sump. The bath is allowed to settling volumes from 10 to 24 mL·L–1 or
recirculate for five minutes while the (using a 100 mL centrifuge tube) 1 to
pump disperses the powder. 2.4 mL (1 to 2.4 cm3).30,31 ASTM E 1444
calls for a range of 1.2 to 2.4 mL after
When a conditioning agent is needed 1800 s (30 min) settling in a vibration free
for a water vehicle, the agent is added to location.12
the bath and dissolved before the
magnetic powder is added. Because the All three standards specify the pear
conditioning agent must be dissolved shaped tube of ASTM D 96, Water and
before it becomes active, it is faster to Sediments in Crude Oils (Fig. 1 shows a
begin the process with warm water. similar settling tube).32 The ASTM D 96
tube has a 1.5 mL (1.5 cm3) stem
Nonrecirculating System Bath graduated in 0.1 mL (0.1 cm3) increments.
Preparation The next graduation is at 2.0 mL (2.0 cm3)
in the conical portion of the tube. If the
If the magnetic particle system has an air expected settling volume is in an
agitated bath, or if the bath is applied inadequately graduated portion of the
from spray guns, then bath preparation is settling tube, a permanent mark should be
more difficult. Magnetic particles often made at the correct concentration so the
stick together during manufacture, storage bath can be kept constant without
or shipment and simple stirring does not guesswork.
separate them. Manually mixed baths are
therefore susceptible to uneven Fluorescent Particle
distribution of particles and are unreliable Concentrations
for testing.
SAE AMS 3044 and SAE AMS 3045
The best way to manually prepare a standards for wet particles specify
finely dispersed bath is to add concentrations for fluorescent particle
premeasured powder to a small food baths but again the requirements differ
blender along with enough oil or slightly. The SAE AMS specifications call
conditioned water to nearly fill the
container. At low speed this takes about FIGURE 1. Typical centrifuge tube used for
30 s. magnetic particle settling tests.

An alternative technique is to add
premeasured powder to a small container
along with enough liquid to form an
easily worked paste. Mixing by hand
precedes adding the paste to the bath.

Bath Maintenance 100

The effectiveness and reproducibility of a 50
magnetic particle bath depend on its
concentration. If the concentration is too 25
low, indications will be weak and difficult 20
to locate. If the concentration is too high, 15
the background will be intense enough to 1860
camouflage indications. Correct particle 5
concentrations fall between these 43 2
extremes.
1.5
Keeping the concentration at a
constant level eliminates one variable in 1.0
the test: the indication-to-background 0.5
contrast. It is important to monitor bath
concentrations regularly throughout the
testing cycle, not only after bath
preparation. Suppliers of magnetic
powders specify bath concentration both
in mass of powder per volume of bath
and also in settling volume ranges that

188 Magnetic Testing

for a range of 0.2 to 0.5 mL (0.2 to particle depletion is likely. Most magnetic
0.5 cm3) whereas the older specification particles adhere to the test object whereas
MIL-STD-1949A had called for 0.1 to most of the vehicle returns to the
0.4 mL (0.1 to 0.4 cm3) settling volumes. reservoir. The particle concentration
The ASTM D 96 tube is required for either decreases steadily with usage over time
test. MIL-STD-1949A has been superseded and may not be noticed by inspectors. For
by ASTM E 1444.4,12,19,32,33 reliable, reproducible magnetic particle
tests with low bath concentrations, the
Fluorescent particle baths are much frequency of concentration tests must be
more dilute than visible baths, so that all increased.
likely settling volumes fall within the
graduated stem of the tube. More Settling Test
reproducibility for small settling volumes
can be achieved if the tube has a 1 mL Since the 1940s, a settling test has been
stem graduated in 50 µL intervals. used to measure magnetic particle bath
concentrations.35 It is a convenient
Particle Settling technique that requires little equipment, a
simple procedure and only 30 to 60 min
Settling Ranges to perform. Its accuracy is sometimes less
than 80 percent but the levels of precision
It is not productive to assume that all are appropriate for most applications.
concentrations within broad specified
ranges are equally effective or desirable. ASTM E 1444 specifies that the filled
For example, it has been shown that settling tube be demagnetized before the
fluorescent particle concentrations around settling test begins.4 Because the magnetic
2 mL·L–1 give the best results on a tool condition of a bath can affect the speed of
steel ring standard.34 Higher settling and the final settling volume, the
concentrations produce backgrounds that demagnetization procedure is an effort to
obscure faint discontinuity indications. standardize the magnetic level of the bath
Lower concentrations produce indications regardless of its use.
too faint to be easily detected. Settling Parameters. It is essential that the
settling test take place in a location free
In another special case, it was found from vibration. The settling tube must be
that increasing particle concentration positioned in an area that is proven to be
from 1 to 4 mL·L–1 gives increasingly free of strong magnetic fields.
brighter indications with very little
increase in background. In this Freshly magnetized bath settles very
application, clusters of very fine cracks rapidly, often in 900 s (15 min) or less.
were located under a chromium plate. The Magnetization causes the particles to
cracks were very close together (less than clump together quickly and form large
0.5 mm apart) so that expanding clusters that sink fast (agglomerated
indications totally depleted the narrow settling). However, these clusters form a
strips of background between the cracks, much larger settling volume than if the
producing dense bright indications. individual particles were unmagnetized:
the structure of the clusters cannot be
Some users prefer a substantial compacted by gravity.
fluorescent background where it makes
bath concentration measurements less Speed of settling and settling volume
frequent. Such modifications of depend on the particles’ magnetization
recommended procedure are allowed only level, so the settling tube sample must be
after firmly establishing and verifying the demagnetized. Vibration during settling
results of such tests for the individual does not affect the speed of settling but
application. can compact the sediment to give falsely
low settling volumes.
The broad concentration ranges Settling Tube. Settling test equipment is
outlined in most specifications cover the simple: (1) a 100 mL pear shaped
limits that are allowed and that may be graduated glass centrifuge tube (Fig. 1),
required. Testing beyond either specified (2) a stand for supporting the tube
extreme may not produce the best results vertically and (3) a timer to signal the end
and higher or lower concentrations of the specified settling period.
should be considered.
The tube referenced in most
There is a strong tendency in some specifications has a 1.5 mL (1.5 cm3) stem
industries to test at the lowest possible graduated in 0.1 mL intervals. According
concentration. Low concentrations are to ASTM D 96, the maximum reading
valid for economic reasons and also error of this tube is 30 µL.32 Another
ensure that excessive background does not common tube has a 1.0 mL stem
become a problem. One precaution is graduated in 50 µL intervals. This
necessary for this approach: if particles are configuration is easier to read for baths
applied from a recirculating system where with small settling volumes, including
the excess bath returns to the reservoir, most fluorescent particle baths.

Magnetic Particles 189

For very dilute baths, even 0.05 mL particles adhering to the sloping walls of
intervals are too large. A tube with a the settling tube.
0.2 mL stem in 0.01 mL graduations gives
the most reproducible results. This stem is Stages of Settling
very nearly the size of a capillary tube and
is extremely difficult to clean after a The sedimentation of magnetic particles
settling test. during the settling test consists of four
separate but overlapping stages.36
The typically specified tube is probably
a compromise. It can be used to measure The first stage is simple unhindered
dilute fluorescent particle baths (under settling of the individual particles. This
0.2 mL with diminished accuracy) as well changes to agglomerated settling where
as more concentrated visible particle baths their slight residual magnetism causes the
(also with diminished accuracy if the particles to collect into larger and faster
settling volume is over 1.5 mL). The three settling clumps. When most of the
tubes do not show the same settling particles have settled into the narrowing
volumes for the same bath. Tubes with portion of the centrifuge tube, hindered
narrower stems show higher settling settling begins. In this stage, the local
volumes. Hindered settling behavior particle concentration is high enough that
mentioned below acts to retard settling in the falling particles get in each others way
the more constricted stems. and restrict further settling. In addition,
the upward flow of the displaced liquid
Effect of Contaminants now becomes rapid enough to further
retard settling. The final stage is compact
Recirculating magnetic particle baths can settling, where all the particles are in
pick up solid contaminants from three contact and apparently settled but the
main sources. The first source includes sediment slowly shrinks in volume as
solids washed into the bath from test more liquid is displaced.
object surfaces. Vapor phase degreasing, a
common procedure for cleaning test Condition of Vehicle after
objects before magnetic particle testing, Settling
does not remove nonoily contaminants
such as sand, dust, lint or grit. In its early versions, MIL-STD-194912
required that the supernatant liquid (the
A second source is particulate matter in vehicle after settling) be essentially
the testing atmosphere, matter that settles nonfluorescent. Naturally, fluorescence in
into the bath. The quantity of these solids the liquid vehicle can detract from the
is determined by the geographic location contrast of fluorescent indications. It can
and the nature of the manufacturing also signal the breakdown of fluorescent
facility, but some airborne particulates can magnetic particles into their components.
be in quantities sufficient for increasing A fluorescent magnetic particle bath
the settling test volume. composed of unattached fluorescent
pigment along with nonfluorescent
A third source, unique to water baths, particles, whose indications cannot be
is a highly dilute but bulky and gelatinous seen under ultraviolet radiation, is not
precipitate. The contaminant is caused by usable and should be discarded.
increasing water hardness in the bath as
water is added to replace evaporation loss. Three different conditions can lead to
Water hardness is determined by the fluorescence in the supernatant liquid
concentration of certain salts, including after settling is complete, but not all of
calcium and magnesium. The resulting them indicate a substandard bath.
precipitate does not affect settling tests of Fluorescent oils or grease can be swept
baths with high particle concentrations into the bath from test surfaces, making
because too little of it is present and the the whole liquid fluorescent. Particles can
weight of the settled ferrous particles break apart, leaving tiny slow settling
compacts it. In dilute baths (settling fragments of fluorescent but nonmagnetic
volumes of 0.1 mL or less in the 100 mL pigment behind. Finally, some very small
centrifuge tube), the precipitate adds bulk but complete particles can escape
to the sediment and can give falsely high agglomeration and, settling at
settling volume readings. individually slow rates, may require
another hour or more to finally settle out.
Contaminating particles do not
necessarily settle out first or last to form The fluorescing of a supernatant liquid
obvious layers in the sediment. The effects is a warning to further monitor the
of contamination contribute strongly to condition of the bath. Is the blue
the low absolute accuracy of the settling fluorescence due to oils bright enough to
test. Contamination of oil baths by water interfere with indication contrast? If not,
and contamination of water baths by oil then it is not excessive. Does the
tend to produce bulk sediments and
substantial amounts of the magnetic

190 Magnetic Testing