ASNT NDT Handbook Volume 10 OVERVIEW

RADIOSCOPYAND TOMOGRAPHY I 183

mainly to the opacity of the fibers in long systems or the tor of 1.2 for retrace time, the required bandwidth is on the
acceptance angle of the light in short lengths. order of 525 x 250 x 30 x 1.2 = 4.7 MHz.

Viewing and Recording Systems Although it may appear that resolution could be
improved by increasing the number of scan lines, two prob-
Television Monitors lems result: the charge capacity on the camera's target ele-
ments will be reduced in proportion to the area change,
Television monitors for observing video signals are cath- which may reduce the sensitivity; and an increase in band-
ode ray tubes using a modulated electron beam to write on width will be required, which increases the noise in the
the output phosphor. In the United States the standard for- electronics in proportion to the square root of the band-
mat is 525 lines, interlaced. Interlacing means that the total width. The result is that the standard number of scan lines is
picture frame is composed of two fields: the first field uses often as good as or superior to higher scan line systems.Fv"
every other line on the screen; the second writes between the
lines of the first. The television camera provides the monitor Many cameras and television monitors are designed for
with the appropriate operating format in the video signal. higher bandwidth operation (10 MHz and greater). This
provides greater resolution horizontally (800 lines or more)
Other systems are used routinely in Europe and have than vertically. Because the 525 line vertical is standard, the
various numbers of lines which can be as high as 1,200, resolution of video systems is often quoted by the horizontal
triple interlaced. The tubes vary in size, deflection system resolution value, which is a function of bandwidth.
and component design. Contrast controls can be adjusted to
increase or decrease the gamma over a range of values, typ- Recording Equipment
ically Oto 10.
Video tape recorders may be equipped with pause and
To see an object, there must be a sufficient number of slow motion modes. The pause mode, however, only shows
scan lines in the television image; following is one descrip- one field rather than the full two-field frame. This results in
tion of scan line requirements.17 To visualize n objects there a reduction of information by one half. In place of the sec-
must be 2n lines. Allowing for random orientation, this ond field, some expensive recording equipment repeats the
should be increased by a factor of 2°·5. To see a mesh with first field of information in the pause mode. This improves
five holes per millimeter would require 14 lines per mil- visual display but still represents a reduction in information.
limeter scan rate. Horizontally, the resolution is determined (The reason for showing only one field is to eliminate inter-
by the bandwidth of the signal. One cycle of bandwidth is field jitter caused by movement between field scans of the
required to see the mesh (one half cycle for the holes and camera.)
one half cycle for the spacing between holes). In a conven-
tional system, the viewing matrix uses a 3 x 4 aspect ratio, With digital image processing radioscopic images may be
the horizontal being larger than the vertical. If 525 scan stored as still images in computer memory or storage
lines are used, then the horizontal will require 4/3 (525 + medium. In this case the object is viewed without motion.
2°·5) half cycles or about 250 cycles to maintain the same One or more frames of data are processed for optimum con-
resolution. Using 30 frames per second scanning and a fac- trast and written to computer data storage as an image. Typ-
ically the data are stored as a 512 x 512 eight bit image or
about 250 kilobytes per image.

1 84 I NONDESTRUCTIVETESTING OVERVIEW

PART 2

RADIOSCOPIC IMAGE ENHANCEMENT

Image enhancement is a general term referring to tech- TABLE2. Image enhancementprocedures
niques or processes that modify an original image so that
important information may be more easily detected and dis- Point processing summing
played. Neighborhood processing averaging (temporal)
subtracting (differencing)
Most often the purpose of image enhancement in non- gray level mapping
destructive testing is to display low contrast detail relating to histogram equalizing
cracks, boundaries, part orientation, voids and inclusions. In
some cases the objective is to locate edges more precisely in differentiation
order to improve measurement accuracy. gradient operations
edge detection
Conventional radioscopic systems compare unfavorably spatial filtering
with film based radiography in two major respects: low con-
trast sensitivity and limited resolution. Modern radioscopic examined, system noise and random noise. Examples of the
image enhancement equipment can compensate for both second component, system noise, include fluorescent con-
these limitations. version screen nonuniformities, camera tube ~arget burns,
light leaks, power line interferences etc. The random noise'
Initially, radioscopic images are analog or digital video component is usually a mixture of signal induced noise
signals, generated by television cameras or solid state caused by the statistical nature of the quantum processes
devices. Equipment used to perform image enhancement involved, plus an additive component due to thermal and
must be compatible with the scanning formats of the imag- bias current noise originating in the video amplifiers. Ran-
ing system. Many of these techniques are also useful in non- dom noise may be greatly reduced by summing or averaging
radioscopic applications. For example, by reviewing an successive television frames while some types of fixed pat-
X-ray film on a light box with a television camera, it is possi- tern noise may be reduced using subtraction techniques.
ble to extract and display information that might otherwise
be missed. It is also possible to apply most image enhance- Summation
ment operations to video tape recorded signals.
An essential element of a digital radioscopic system is a
Digital Techniques full frame image memory in which the intermediate or end
result of the processing operation may be stored. In the sum-
Most modem image enhancement systems use digital ming mode, the contents of memory are read out and sent to
processing techniques; although there are exceptions where the arithmetic processor in synchronism with the next
analog signal processing offers some advantages. With digital incoming digitized frame; in this way, matching pixels arrive
processors the first step is to convert the analog video input to at the adder simultaneously. The result of the addition is
digital form using a high speed analog-to-digital converter returned to memory while other picture elements are being
(ADC), and the last step is to reconvert the digital signals processed. The memory output signal is also sent to the digi-
back to analog form with a digital-to-analog converter (DAC). tal-to-analog converter and then to the output for display.

Radioscopic image enhancement processes can be conve- The number of frames that can be summed depends on
niently grouped into two major categories: point processing (1) the capacity of the memory, (2) the alignment of the ana-
operations and neighborhood processing operations. log-to-digital converter output with the memory and (3) the
maximum encoded level of the input video.
Point processors perform the same operation on each
pixel in the image, independent of the values of neighboring Because it is quite possible that the maximum video level
pixels. As the name implies, neighborhood operations are can be encoded at less than the maximum encoder output, it
performed on each pixel, based on the values of neighboring is often possible to sum a larger number of frames. When the
pixels. Table 2 lists typical operations for each category. video is very noisy, using a high resolution encoder is waste-
ful. By shifting the analog-to-digital converter output down-
To improve the capability for displaying low contrast ward relative to the memory, the less signiflcant bits are
information, it is often necessary to improve the signal-to- discarded, leading to coarser resolution but allowing more
noise ratio of the video. The video output of a radioscopic frames to be summed before overflow occurs. As long as the
system is a mixture of signals representing the object being

RADIOSCOPYAND TOMOGRAPHY I 1.85

magnitude of the random noise component is larger than the FIGURE 12. Four 25 mm (1 in.) thick steel
effective encoding interval, the gray scale resolution of the pieces with one percent penetrameters:
summed or averaged image will be governed by the final {aJ unprocessed video display; (b) image
signal-to-noise ratio rather than the effective analog-to-digi- processing using 32 frame (about 1 s]
tal converter resolution. continuousaveraging;(cJ averagingplus real
time qigital image filtering
Continuous Image Averaging (aJ

An alternative to the summing process just described is (b)
to perform a moving average on the input video. The
improvement of the signal-to-noise ratio permits low con- [e]
trast elements of an image to be pulled out of the noise and
made detectable (Fig. 12).

The advantage of this process is the availability of a con-
tinuous dynamic view of an image with improved signal.-to-
noise ratio, rather than an intermittent static image obtained
at the termination of a summation. The amount of image
motion which can be tolerated is a tradeoff with signal-to-
noise ratio improvement. It is possible in some cases to
average moving images without loss of resolution.

Subtraction Operations

An additive fixed pattern noise component can be
removed or greatly reduced with digital image subtraction.
This can be combined with summation so that the dual ben-
efits of random noise reduction and fixed pattern noise
reduction can be achieved. A procedure for doing this is to
sum a series of frames that contain signal, random noise and
fixed pattern noise.

After summation has ended, the data in memory is sub-
tracted from one frame and then optical input is removed so
that output from the sensor is random noise and fixed pat-
tern noise only. An equal number of frames are then added
to the complemented data in memory. Finally, memory con-
tents are inverted (complemented) once more. If the fixed
pattern noise is independent of input level, then it will be
subtracted from the previously summed fixed pattern noise
in equal amounts and thereby canceled. This process does
add some additional random noise to the resulting image.

Edge Enhancemen~,

,~t'

Many radiographic images consist of areas having high
brightness combined with areas having very low brightness,
with both areas containing detail structure of significance.
Neither the display nor the human eye has the dynamic
range needed to cope with these images.

Examination of these images shows that transitions from
very bright to very dark do not occur rapidly, a result to be
expected from a consideration of typical system modulation
transfer function (MTF). Expressed in other terms, one can
say that lower spatial frequency components of the image

186 I NONDESTRUCTIVETESTING OVERVIEW

demand the most dynamic range. On the other hand, the More frequently, the low pass frequency image is sub-
most significant image information is usually contained in tracted from the original image to obtain a linear high pass
higher spatial frequency components. It is these compo- filtered image (Fig. 14). In the original unfiltered image, the
nents that characterize small cracks, voids, inclusions, dynamic range of the display is inadequate for showing
material interfaces, edges etc. From this point of view, edge detail in either the bright or dark parts of the image. After
enhancement operations are essentially image filtering filtering, significant image detail is visible in both areas.
operations, where the low spatial frequencies are attenuated
while high spatial frequency components are emphasized. FIGURE 14. Linear high pass filteringof X-ray
image: (a) original image; (bJ filtered image
Subtraction operations provide a method for accomplish-
ing one-dimensional edge enhancement. One commonly fa)
used scheme is to subtract a slightly delayed video signal
(image shift) from the original, undelayed signal. In regions
of the picture where there is no change taking place (where
there is no detail), subtraction is complete. However,
regions containing detail do not exactly match and subtrac-
tion leaves a residue that serves to emphasize these regions.
This technique can be used in real time when a system has a
memory or a partial memory that will store part of the pic-
ture momentarily and then subtract it from itself. Mathe-
matically, this process is called differentiation. Thus the
signal produced is proportional to the rate of change for
brightness with distance (Fig. 13).

FIGURE 1 3. Edge enhancementobtained
through image shift and subtraction
(differentiaiot n)

fb)

RADIOSCOPY AND TOMOGRAPHY I 1 S7

Figure 15 is an interesting example of filtering out image FIGURE 1 5. Comparison of linear high pass
structure so that underlying details can be more readily filtering in one and two dimensions:
detected. In Fig. 15b, - two-dimensional radioscopic linear (a) original image; (bJ image after application
of two-dimensionarleal time linear filtering;
filtering has been applied to the original image in Fig. 15a; (c) image after removal of horizontally aligned
in Fig. 15c all horizontally aligned components of the image comoonents and low frequency, vertically
have been removed along with the low frequency vertically .::1nnn,=.r1 components
aligned components.

Nonlinear high pass filtering is performed when the orig-
inal image is divided by the low passed image. This type of
filtering is also called homomorphic filtering and is especially
effective in enhancing low contrast detail in dark areas of an
image. In addition to enhancing low contrast detail, high pass
filtering can be effective in compensating for nonuniformi-
ties in the X-ray beam, vignetting effects of the relay optics
and nonuniform response of the television sensors.

Edge enhancement is an important tool for improvin~
the measurement of part dimensions from X-ray images.1
Linear filtering in which the resultant image is proportional
to the second derivative rather than the first derivative is
much more appropriate for dimensional measurement.

Pseudocolor (b)
(cJ
Pseudocolor is a technique used with video systems
wherein a brightness level is displayed as a color. For exam-
ple, dark areas of an image might be displayed as blue, mod-
erate brightness as green and high brightness as red.
Depending on the capabilities of the unit and the applica-
tion, the number of brightness levels translated into distinc-
tive colors may vary from as few as four to more than 100.

With good signal-to-noise ratio, pseudocolor can aid in the
discrimination of small brightness differences since the eye
can distinguish a much larger number of different colors than
it can brightness differences. A second value of this technique
is that parts of an image having the same brightness can be
more easily recognized even when located some distance
apart. The eye is unable to judge absolute brightness levels
and is easily fooled by the brightness of adjacent areas.

.Other Techniques

Among the imagefenhancement techniques not dis-
cussed above are contrast stretching, histogram equalization
and logarithmic processing. The microcomputing revolution
of the 1990s has made a wide range of image processing
capabilities available at low prices. Commercial software
programs offer user friendly menus full of processing tasks
like those mentioned above and permit users to customize
the processing parameters.

All these image enhancement techniques serve to high-
light the information available in radioscopic images and to
thereby expand the overall range of applications.

188 I NONDESTRUCTIVETESTING OVERVIEW

PART 3

X-RAY COMPUTED TOMOGRAPHY

Introduciton FIGURE 16. Comparison of radiography and
computedtomography:[a] conventional
X-ray computed tomography (CT) is a powerful nonde- projectionradiography; (b) computed
structive evaluation technique that was conceived in the tomographyusing slit collimation
early 1960s for medical diagnostics. Both Geoffrey
Hounsfield and Alan Cormack shared the Nobel Prize in faJ
medicine for their independent developments of computed
tomography.20-22 Medical computed tomography developed FILM
rapidly in the 1970s and 1980s with significant advances in
the X-ray sources, detectors, mechanical scanning schemes SOURCE
and reconstruction software.23 Medical computed tomogra-
phy systems are designed for high throughput and low fbJ
dosages specifically for humans and human sized objects.
These systems can be applied to industrial objects that have DETECTOR
low atomic number and are less than 0.5 m (20 in.) in diam- ARRAY
eter. In the late 1970s and 1980s industrial computed
tomography systems were specifically developed and X-RAY
applied to a variety of hardware items. Industrial computed SOURCE
tomography systems do not have dosage and size con-
straints. They are built in a wide range of sizes from the DATA TRANSFER COLLIMATOR
inspection of small jet engine turbine blades using
mid-energy (hundreds of kilovolts) X-ray sources to the 1 COMPUTER RECONSTRUCTED MAP OF
inspection of large intercontinental ballistic missiles requir- SUCE THROUGH OBJECT FROM
ing high (MeV level) X-ray energies.24-28 y
MULTIPLE X-RAY PROJECTIONS
X-ray computed tomography uses measurements of x-
X-ray transmission from many angles completely encircling
a component to compute the relative X-ray linear attenua-
tion coefficient of small volume elements and presents the
data as a cross sectional image map. The clear images of
interior planes of an object are achieved without the confu-
sion of superposition of features often found with conven-
tional film radiography, making computed tomography
results easy to interpret for feature detection and place-
ment. Computed tomography can provide quantitative
information about the density constituents and dimensions
of the features imaged. X-ray computed tomography can be
considered the high end application of radiation measure-
ments because the data obtained are quantitative measures
(directly related to the X-ray linear attenuation coefficient)
for each volume element throughout an object. Accurate
evaluation of dimensions, locations in three dimensional
object space or material density (as related to X-ray linear
attenuation coefficient) can be performed in any orientation
throughout the volume of an object that has been scanned
with the computed tomography system.

The computed tomography technique is often easiest to
understand by comparing it to film radiography. Figure 16

RADIOSCOPYAND TOMOGRAPHY I 189

shows the radiographic and computed tomography configu- image reconstruction can be found in a number of refer-
ences.23,26·27·29-33 The computer reconstructed computed
rations. In conventional radiography the X-radiation passing
tomography image is a two-dimensional image of a two-
through the object is detected on an image plane ( often dimensional plane in the object. It is as if the object were
cut open and one could view the interior. The data in the
film). For film radiography, three-dimensional object infor- image are composed of information in small voxel units that
are composed of the x­y reconstruction matrix element sizes
mation is compressed onto a two-dimensional image plane. and averaged over the slice thickness of the computed
tomography collimation scheme, shown in Fig. 16 in the
Features in the object are superimposed. In conventional vertical dimension. By taking a series of contiguous com-
puted tomography slices through the object, a volumetric
computed tomography the X-ray beam is collimated to a data set can be created from which cross section images of
any plane through the object may be extracted. The slice
narrow slit and aligned with a solid state X-ray detector thickness used determines the vertical resolution of the vol-
ume data set. The horizontal resolution is determined by
array to define a computed tomography slice plane in the the effective X-ray beam size in the object and the recon-
struction matrix size.
object. Slit collimation reduces scatter and improves the sig-
Computed tomography has a number of variations from
nal to noise in the image. Data are obtained by translating the basic concept shown in Fig. 16. Figure 17 shows the

and rotating the object so that many viewing angles about

the object are acquired. The transmitted X-ray intensity at
each detector element position in the detector array is con-
verted to a digital output level and transmitted to a computer
as a projection for the particular angle through the object. A
computer then back calculates (reconstructs) from the set of
projection views, at all angles about the object, a cross sec-
tional image plane through the object. Mathematics of the

FIGURE 1 7. Computertomographysystemgenerations:(a) first generation;{b) secondgeneration
(rotate-translate); (c) third generation(rotateonly); (d) fourth generation

faJ fbJ
OBJECT

SOURCE . DETECTOR SOURCE

IJ~ 1l
·~
~)

DETECTOR

fcJ (dJ

SOURCE DETECTOR

DETECTOR
RING

190 I NONDESTRUCTIVETESTING OVERVIEW

TABLE3. Advantages and limitationsof computedtomography limitations

Advantages high capital cost
requires 360 degree access
Quantitative volumetric feature detection and configuration control reconstruction artifacts
Digital data; three dimensional information
Sensitivityto small dimensional changes {down to 0.01 percent)
Sensitivityto small density changes {down to 0. I percent)

generations of computed tomography systems developed FIGURE 1 8. Evaluation of cast turbine blade
for medical computed tomography applications. The origi- using 400 kV computed tomographysystem
nal, first generation, computed tomography systems used a showinginternal feature condition and wall
collimated source beam and a single detector with both thickness measurement: (a) radiograph;
translation and rotation of the source and detector relative (b) computedtomographysJice37
to the object. Succeeding generations use detector arrays (aJ
and modification of the translation and rotation schemes to
increase speed. Industrial computed tomography systems (bJ
are primarily based on second or third generation designs,
although the motion of source and detector array about the
human object in medical computed tomography is reversed
to object motion with a fixed source and detector in indus-
trial system design. The second generation, rotate/translate,
scheme is commonly used for industrial objects because
objects larger than the X-ray beam fan angle can be accom-
modated. The third generation, or rotate only, scanning
approach is used on small industrial objects because it is
faster than second generation. Volume computed tomogra­
phy (or cone beam computed tomography) and limited angle
computed tomography are also techniques that are being
increasin~ used for industrial application beyond
research. 6 Cone beam computed tomography, where the
entire cone of uncollimated radiation is measured in an area
detector, is a high throughput technique that acquires the
entire part volume in a single scan. Limited angle computed
tomography does not require that the computed tomogra-
phy data be taken from all angles completely about the part.
Limiting the angles results in less sensitivity to detail than
data with conventional computed tomography but allows
some advantages with certain types oflarge parts.

The advantages and limitations of computed tomography
are listed in Table 3. X-ray computed tomography provides
quantitative volumetric feature detection. The data are digi-
tal with known three-dimensional coordinates relative to a
common origin. Sensitivity to both dimensional and density
characteristics can be quite high. Computed tomography
requires more precise equipment and data processing than
traditional nondestructive test methods and so has a gener-
ally higher capital cost. Conventional computed tomography
technology requires 360 degree access to the parts and
therefore the objects must fit within the field of view of the
computed tomography scanning system. High aspect ratio
(> 15:1) objects are not well suited to conventional com-
puted tomography examination. The data acquisition and

RADIOSCOPYAND TOMOGRAPHY I t 91

processing used in computed tomography image recon- complex structure evaluation overcoming superposition com-
structions do have associated image artifacts that may limit mon in radiography to reveal superior information about the
the detail detection and measurement sensitivity. internal configuration of systems.

Examples of Computed Tomography Computed TomographySystems

Figures 18 to 20 show examples of computed tomography System Characteristics and Component Attributes
images of materials and structures.37-39 Figure 18 compares
computed tomography with film radiography for a turbine As mentioned above, computed tomography systems
blade casting. The part contains a complex internal geometry. require more precise equipment and data processing than
The film radiograph cannot help evaluate· the internal cross traditional nondestructive evaluation hardware. It is therefore
sectional configuration of the part. The computed tomogra- important to consider the various major components that go
phy slice shows the wall thickness of the casting directly and into a computed tomography system and discuss their ramifi-
will show discontinuities in the cast material if they are pre- cations. Figure 21 shows a generic design of computed
sent at the slice plane. The detail evaluation of complex cast- tomography system components.f" The major subsystems
ings is an excellent application of computed tomography that go into a computed tomography system include the
technology. The ability of computed tomography images to mechanical handling subsystem, the data acquisition subsys-
show internal material variations is particularly advantageous tem and the computer interface and software subsystem.
for composite material inspection. Figure 19 shows a com- These major subsystems categories can be further broken
down into components and characteristics that are essential
puted tomography image of a composite J stiffener, where the for a computed tomography system to operate for the desired
output. The selection of certain component attributes or sys-
variations in the consolidation and the ply layups can be eval- tem characteristics will affect the selection of other compo-
uated, particularly at T junctions. Figure 20 shows a film nents or the overall performance and cost of a computed
radiograph and a longitudinal computed tomography slice tomography system. Table 4 lists key attributes of a computed
through a cruise missile engine using a 9 MV X-ray source. tomography system and the ramifications of selections of the
This example shows the power of computed tomography for attributes on system component selection. In the selection of
a computed tomography system to perform nondestructive
FIGURE 19. Computed tomographyimage of inspections it is important to be able to define the desired
graphite epoxywoven J stiffenershowingply inspection characteristics, particularly specimen (size, type
conditionand consolidatio3n8 and weight), inspection parameters (spatial resolution, con-
trast sensitivity, slice thickness and time for inspection) and
the operator interface (system control panel, image display,
processing functions and data archiving).

The most significant point of Table 4 is how the speci-
men to be inspected determines many of the principal
characteristics of the computed tomography system. For
this reason, different computed tomography systems are
designed for different sized objects. The object size and
X-ray penetrability determines the mechanical handling
characteristics. As the object becomes larger, higher energy
X-ray sources and larger mechanical systems are required
- resulting in higher cost for the computed tomography
system. Figure 22 shows the effect of object size and energy
on the cost of a computed tomography system.

Computed Tomography System Sensitivity

The sensitivity to fine detail of computed tomography sys-
tems is a function of resolution and contrast sensitivity. Com-
puted tomography resolution is fundamentally determined by

192 I NONDESTRUCTIVE TESTING OVERVIEW
FIGURE20. A cruise missileengine: [a] radiograph showing complex superposition of information;
(b) computed tomography slice along the axis of engine obtained on a 9 MV computer tomography
systemshowing internal configuration details39

faJ

(bJ

RADIOSCOPY AND TOMOGRAPHY I 193

TABLE4. Computedtomographysystem attributesand their ramifications

Attributes Ramification

Test object size, mass handling mechanical handling equipment
Test object X-raypenetrability loading and unloading features
Spatial resolution
Contrast sensitivity X-ray source
Artifact level X-ray detector type
Speed of test detector/front erd electronics dynamic range

Number of pixels accuracy of mechanical handling equipment
Slice thickness range configuration of source, object and detector
Operator interface source and detector aperture size

Archival requirements strength of X-ray source
integration time

reconstruction algorithm software
accuracy of mechanical handling equipment

size of object
X-ray source strength
number and configuration of detectors
bus structure
speed and architecture of processors
mechanical hardware (motors, brakes etc.)

number and configuration of detectors
amount of data acquired
computer and hardware choice

detector configuration
system dynamic range

instrument control panel
image processing system
control software
interface to remote workstation

computer/hardware choice

the beam width of the X-ray optics design and is driven by the quadratically. Thus, it is impractical to use a very small beam
selection of source and detector aperture sizes and the source width on large parts because of the very long scan time that
object and detector distances. The beam width, size of the will result. Practical resolutions for computed tomography
object and computed tomography image reconstruction systems that handle relatively large components ( > 300 mm
matrix must all be considered in a system design. At the pre- [12.0 in.] diameter) are in the range of 1 to 2 line pair per
sent time the typical reconstruction matrix size for computed millimeter. For components less than 300 mm [12.0 in.]
tomography is 1,024 x 1,024. To a first approximation this diameter, 2 to 4 line pairs per millimeter can be obtained.
would make the resolution limit roughly 1 part in 1,000, and For higher resolution, greater than 4 line pairs per mm (fea-
the system would be designed to match the X-ray optics to ture sensitivity on theorder of 0.125 mm [0.005 in.]), com-
0.001 the part size. For e:&nple, a system designed to handle puted tomography systems are designed to handle objects of
a 0.5 m (20 in.) size part might allow for 0.5 mm (0.02 in.) size only a few centimeters in size.
beam width, and a system designed for a 10 mm (0.4 in.) size
part might have a 0.010 mm (0.0004 in.) beam width. Figure 23 shows how the size and detail sensitivity of
computed tomography systems are related by the design.41
It is of course possible, and routinely performed, to Each type of system (A through D) represents a range of
reconstruct the 1,024 x 1,024 matrix over subregions of a capability that can found in commercially available com-
component so that a higher resolution beam width finer puted tomography systems but no one computed tomogra-
than 1 part in 1,000 of the object can be used effectively. phy system can provide both large object inspection and
However, the scan must still cover the full size of the part. very fine resolution.
As the part size is increased, the source to detector distance
increases and X-ray intensity at the detector falls off Contrast sensitivity is the ability to differentiate
between two regions of an object. Computed tomography

194 I NONDESTRUCTIVE TESTING OVERVIEW

FIGURE 21 . Generic computedtomographysystem components

X-RAY BEAM X-RAY BEAM POSTCOLLIMATOR/DETECTORAPERTURES
PRECOLLIMATOR
X-RAY
X-RAY DETECTORS
SOURCE
DETECTOR
ELECTRONICS DIGITAL DATAOUT

X-RAY ~---MOTION
CONTROL
X-RAY HARDWARE
MONITOR PRIME DATA BUS (REAL TIME) X-RAY BAY

POSITION CONTROL ROOM
ENCODING MAIN BUS

HOUSEKEEPING
DATABUS~~--------~t-r-Tr-t-,

X-RAY DETECTOR
INTERFACE CONTROL
PROCESSOR

OPERATOR ARCHIVES
CONSOLE

FIGURE 22. Computedtomographysystem size FIGURE 23. Computedtomographysystem size
versus cost versus sensitivityto detail

3.0 2.0
2.5
~ 2.0 1.8
1.5
ui 1.0 ~~ 1.6 SYSTEM
0.5 1.4 TYPED
~-t:w Vl QJ 1.2
1.0
t~o].~s_~ ~~ .s_QJ 0.8

w I0.6 I SYSTEM
0.4 SYSTEM TYPE C
a3 SYSTEM
0 I TYPE B I

0 0.2 TYPE A I
0.5
0 I

1.0 1.5 2.0 2.5 3.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

COST RESOLUTION
(millimeters)
(millions of dollars)

RADIOSCOPYAND TOMOGRAPHY I 195

can generate very high contrast sensitivity superior to other Computed Tomography
Applications
radiographic techniques. Each reconstructed volume ele-
In general, the benefit of X-ray computed tomography
ment (voxel) is composed of backprojected rays from many over alternative nondestructive test methodologies is the
orientations about the object. If the X-ray intensity in the ability to map the relative X-raylinear attenuation coefficient
detector array is high so that the statistical noise is low, then of small volume elements throughout a component, permit-
the reconstruction will contain high contrast because the ting the extraction of dimensional and material characteris-
combination of rays from all viewing angles improves signal tics of features and anomalies. With these characteristics,
to noise in each reconstructed image element. Equation 4 derived from the computed tomography data, engineers can
shows an estimate of the signal-to-noise ratio (SNR) in a perform a variety of analyses to arrive at quantitative mea-
voxel element as a function of various computed tomogra- surements of parameters that are of economical value for
phy system characteristics for a reconstruction of a cylindri- improving the overall product. For objects that fit within the
cal object. 29 constraints of size and shape for proper computed tomogra-
phy examination, the computed tomography data offer
SNR 1/2 (Eq.4) unparalleled capability for feature detection and measure-
ment. As complexity of design increases, the value of com-
0.665 µd'1' [ ( :: }-'" ] puted tomography measurement capability increases.

Where: Present computed tomography technology is relatively
expensive. The value of computed tomography is therefore
µ linear attenuation coefficient; realized in applications where the objects are of critical
d X-ray beam width; value or adequate measurements cannot be made by other
v number of views; means. A primary example is rocket motors. Computed
n photon intensity rate at detector; tomography is used extensively on rocket motors because
t integration time of detectors; the objects have very high value and are used in critical
applications, so that the cost of computed tomography
11p ray spacing; and remains a small fraction of the overall mission value. Com-
R radius of object. plex, high value turbine blades are another example where
computed tomography is worth the cost because of the
What the equation shows is that the signal-to-noise ratio accuracy of dimensional measurements that can be obtained
relative to other methods. Beyond these few examples, com-
improves with increases in the value of computed tomogra- puted tomography is not routinely applied to objects as a
final inspection process. Rather, computed tomography is
phy system characteristics of X-ray beam width, number of applied as an engineering tool and enables technology to
support product development activities, speeding products
views, X-ray beam intensity and integration time. The signal to market. Table 5 summarizes the cost effective application
to noise ratio will also be improved by decreasing the ray areas for computed tomography.41

spacing and object diameter. The application of computed tomography as a measure-
These computed tomography system characteristics ment tool for engineering and manufacturing provides a
reflect the tradeoffs involved in optimizing a computed cost benefit to a number of processes. Computed tomogra-
phy is used by engineers on prototypes to fully characterize
tomography system. Fast scan times, fine resolution, high the object. Computed tomography measurements can be
performed on test articles to validate prototypes and models
contrast sensitivity and large object size are mutually exclu- before testing, during certain types of test and post testing,
sive, requiring compromise in system design. · including noninvasive micrographic evaluations. Computed
tomography permits geometry acquisition (often referred to
Because of the high signal-to-noise ratio in any voxel, as reverse engineering), providing a direct cost savings over
traditional approaches to translating existing components
computed tomography can detect features below the resolu- into digital models in computer aided design/engineering
tion limit of the image. For features that are larger than a (CAD/E) workstations. Computed tomography is particu-
larly effective during product failure analysis by noninva-
single voxel the contrast sensitivity improves by the square sively inspecting the interior condition of articles, including

root of the number of pixels making up the feature. For a
feature smaller than a pixel, the apparent density is aver-

aged over the image voxel and therefore the signal for that

image voxel is reduced. This is called a partial volume effect.

Although the signal is reduced by the partial volume effect,
the feature may still be detected. This is a significant point

about the application of computed tomography because

very often relatively large image voxels (compared to very
fine discontinuities) may be used to detect very small fea-

tures although the features are not necessarily resolved. The

contrast sensitivity typically provided by computed tomog-

raphy is in the range of 0.1 to 1 percent.

196 I NONDESTRUCTIVE TESTING OVERVIEW

TABLE 5. Beneficial applications for computed process has been brought into control. For routine produc-
tomography tion quality control the application of computed tomogra-
phy depends on the relation between the object value,
Engineering Manufacturing computed tomographic scanning cost and the cost of alter-
natives. The more complex and costly an assembly, the more
prototype evaluation process development likely that computed tomography can be a cost effective
geometry acquisition feature or anomaly location tool. Ultimately computed tomography can allow the accep-
failure analysis configuration control tance of a product based on quantitative measurements and
performance prediction acceptance by engineering criteria engineering criteria. Such an engineering analysis rather
than qualitative inspection standards has considerable
scans under various operational conditions. Computed potential for reducing scrap and increasing component reli-
tomography evaluation of materials also is useful in perfor- ability. Maintenance, repair and failure analysis activities
mance prediction based on the measurements obtained benefit from computed tomography measurements by pro-
from the computed tomography data. This is where engi- viding information for making decisions on irreversible
neering and nondestructive evaluation need to meet in steps and/or eliminating disassembly or destructive testing
order to create the most cost effective products. to obtain critical data.

Computed tomography can be an important tool in the The long range value of computed tomography technol-
manufacturing and process development stages of product ogy is that it closes the loop between the engineering and
life cycles by providing feature and anomaly location, con- the manufacturing operations by providing quantitative data
figuration control and the direct measurement of dimen- that can be accessed by engineers at their workstations. As
the realization of the value of this methodology grows, the
sions for engineering acceptance. The value of computed use of computed tomography will increase.
tomography evaluation is high for assuring a development

RADIOSCOPY AND TOMOGRAPHY I 197

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33. Bossi, R.H., K.D. Friddell and A.R. Lowrey. "Com-
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Science Publishers (1990).

7. SECTION

ELECTROMAGNETIC TESTING

Ram P Samy, The Timken Company, Canton, Ohio

200 I NONDESTRUCTIVE TESTING OVERVIEW

PART 1 TO ELECTROMAGNETIC

INTRODUCTION
TESTING

Typical Uses of Eddy Current forms of eddy current test instrumentation provide simj
Nondestructive Tests and rapid means for manual quality tests by individual ope
ators and for mechanized test systems to sort mixed lots
Eddy current tests are an important and widely used materials or to track deterioration of materials and equi
method within the broad field of nondestructive materials ment in service.
evaluation and inspection. Modern eddy current and elec-
tromagnetic test techniques offer unique, low cost methods Method of Induction of Eddy
for high speed, large scale inspection of metallic materials Currents in Materials
such as those used in nuclear, aerospace, marine, high pres-
sure, high temperature and high speed engineering systems Eddy current nondestructive tests use time varying ele
where premature failures could represent economic disas- tromagnetic fields as a probing medium to explore (1) ti
ters or the endangering of human life. properties of test materials, (2) discontinuities and (3) vari
tions in geometry and dimensions of test materials. Tl
More recently, the method's special suitability for inspec- varying electromagnetic fields are usually created by a flo
tion of automotive components, engines, machine parts and of periodic, alternating or pulsating electric currents in coi
consumer products has been recognized. The automation of or arrays of conductors called probes. These probes a;
eddy current testing and test data evaluation permits mass placed in close proximity to the surfaces of the test mater
inspection of similar parts at high rates with economies not als. They induce within the test materials a flow of electric
attainable by other commonly used nondestructive tests. currents known as eddy currents. The intensity of eddy cu
Eddy current methods are used both for quantitative mea- rent flow is greatest at the excited surface of the test part
surement of material properties and for sorting of parts by Within the material the eddy current density decrease
dimensions, properties or presence of discontinuities. In exponentially with depth below this surface. The eddy cu
general, eddy current tests provide nearly instantaneous rents induced at the surface of the test material are also tim
measurements. Consequently, they can be used in produc- varying and have magnitude and phase.
tion lines to test swiftly moving bars, tubes, sheets, plates,
welds and other symmetric parts. These parts either pass
through test coils or are scanned by moving test probes.

Metallurgical ProcessApplications of Eddy Current Test Material Properties Influencing
Tests Eddy Current Tests

Electromagnetic induction tests find application in all The magnitude of eddy current flow is directly affectei
stages of forming, shaping and heat treating of metals and by the electrical conductivity and magnetic permeability o
alloys, where the effectiveness of processing steps can be the test material. Inaddition, the flow paths of the eddy cur
quickly evaluated. Materials damaged during processing can rents can be restricted or distorted by the test materia
be detected and removed from production without incur- geometry and the presence of discontinuities within the tes
ring further processing costs. Thermal treatments such as material. Thus, eddy current tests can respond to variation
annealing, normalizing, hardening, case hardening and in electrical conductivity related to composition of alloys
other heat treating processes can be monitored directly in hardness changes or temperature variations. These tests car
many instances. Effects of mechanical processing such as also detect effects of processing and shaping operations dur
machining, drilling, rolling and hot or cold deformation can ing production, as well as corrosion damage or crackinj
be measured; the development of cracks or damage to
mechanical properties can be assessed. Small, portable

ELECTROMAGNETIC TESTING I 201

induced in service, for most nonmagnetic metals and alloys. magnetizing coil. This multiple detector array can be placed
In addition, with ferromagnetic materials such as steels, within or outside the circumference of the magnetizing coil.
effects of thermal or mechanical processing and of heat
treatments (which change the magnetic permeability, elastic Analysisof Eddy Current Test
properties, tensile strength, hardness or ductility of the test Signals (Amplitudes and Phase
materials) can be detected. However, with ferromagnetic Angles)
materials, magnetic anomalies produced by handling, work-
ing or prior magnetization ( as by magnetic lifting cranes)
can interfere with interpretation of material properties.

Methods for Detectionof Eddy The output signals from eddy current test coils and
Current Intensitiesand Flow probes are typically time varying, alternating current (AC)
Patterns voltages or currents. Such signals can be analyzed by circuits
and instrumentation analogous in principle to those used in
The eddy currents induced in a material generate their analyzing alternating current circuit impedance in engineer-
own magnetic field. The magnitudes, time lags, phase ing and industry. In particular, signal amplitudes and phase
angles and flow patterns of the eddy currents within the test angles can be measured with high precision so that quantita-
materials are detected by measuring resultant magnetic tive analyses of eddy current test signals are feasible. Such
fields with another set of sensing coils or solid state mag- signals are appropriate for comparison, manipulation and
netic field detectors (Hall effect devices), usually housed analysis. They can also be readily digitized for display or
within the eddy current test probes. permanent records or for processing by computer.

In some cases, the magnetizing coils also serve as signal Automation of Eddy Current Test Signal Analysis
pickup devices or detectors of the eddy current field reac-
tions. Most commonly, the magnetizing coil system and the The information derived from eddy current tests can be
detectors or pickup devices are combined into a single optimized for automation of test systems, for sorting of test
probe system to provide a one-side, noncontacting test parts, for control of manufactming processes through con-
probe arrangement. For inspecting thin sheets, for example, trol charts and statistical quality control records. The speed
it is also possible to place the magnetizing coil system on of eddy current tests and of modern signal analysis tech-
one side of the test material while the pickup element niques permits such analysis in real time. Materials evalua-
(receiver) is placed on the opposite side of the material to tion test data can be made available as rapidly as test objects
provide a through-transmission eddy current test system. can be fed through the eddy current test system. Such fea-
tures often make eddy current testing preferable to other
Uses of Different or Multiple Eddy Current Detector forms of nondestructivetests that require evaluation of data
Arrays by human operators. Delays in deciding the disposition of
parts or materials can be eliminated by automated eddy cur-
rent test systems.

Often, the magnetizing coil and pickup coil in an eddy Selection of Optimum Eddy
current test probe are of nearly identical size, shape and Current Test Frequencies
location. However, it is also possible to provide two or more
A single eddy current test system can be used for many
magnetizing or pickup coils in different locations. These different measurements through the selection of test fre-
may provide differential arrangements that are sensitive to quencies. The frequencies used in eddy current test systems
small, local variations in test material properties, dimensions vary from below 10 hertz (cycles per second) to many mega-
or discontinuities. Such differential eddy current test sys- hertz (millions of cycles per second). However, most indus-
tems are widely used in locating inhomogeneities, disconti- trial eddy current tests are made in the frequency range
nuities or weld anomalies in tubes, bars and plates during between .5 Hz and 10 MHz. Most types of eddy current test
manufacturing. In systems where the eddy current reaction
fields are measured by Hall devices (which detect the nor-
mal component of the magnetic field directly), a multiplicity
of such pickup elements can be associated with a single

202 I NONDESTRUCTIVETESTING OVERVIEW

equipment provide either variable frequency oscillators or or impossible to penetrate to the center of thick specimens
several fixed frequency steps. Thus, appropriate test fre- because of skin effect, that is, attenuation of the electromag-
quencies can be readily selected by the user to meet special netic field at certain depths below the surface. Eddy cur-
test requirements. These frequencies are those of the exci- rents tend to flow only in paths paralleling the surface to
tation current applied to the magnetizing coils of the eddy which a perpendicular exciting field is applied and usually
current test probes. do not respond to laminar discontinuities that lie parallel to
this surface. They do tend to respond, however, to disconti-
Control of Eddy Current nuities that lie transverse to the flow of eddy currents within
Penetration Depths in Test test materials where these discontinuities interrupt, length-
Materials en or distort the current flow paths.

One of the most distinguishing features of eddy current Correlation of Eddy Current Test
tests with alternating current excitation is the tendency for Indications with Material Properties
the eddy currents to be concentrated near material surfaces and Discontinuities
adjacent to the magnetizing coils or sources of induction.
Low excitation frequencies are used to penetrate deeper Eddy current tests respond specifically only to the elec-
within a conducting test material. High test frequencies can trical conductivity, magnetic permeability and geometric
be used for selective examination of near surface regions, properties of test objects and to the spatial relationship of
for testing of thin materials and for testing of materials hav- the test probes to the surfaces of test objects. Many other
ing low electrical conductivities. With ferromagnetic mate- material properties can be related to these. primary eddy
rials, the high relative magnetic permeability of the test current test measurements but proof of such correlation
material acts to concentrate the eddy currents within very must be obtained for each case. In particular, many differ-
shallow layers of the near surface regions of the test objects. ent metallurgical factors (such as alloy, structure, heat treat-
This effect can be used to advantage in detection of seams, ment, hot or cold working and other processing steps) may
laps or service fatigue cracks in steel and other ferromag- influence the material conductivity or permeability. Resid-
netic alloys. Where deeper penetration of eddy currents ual magnetism within steels and ferromagnetic materials
into ferromagnetic materials is desired, as in evaluation of can affect the eddy current test indications. Sometimes it is
case hardening depth in bearing components, low excitation extremely difficult to separate desired from undesired
(5 to 1,000 Hz) frequencies can be used or the effect of per- effects (spurious indications).
meability can be suppressed by magnetically saturating the
test specimen with a strong permanent magnet or a direct Typically, it is possible that some characteristics not
current magnetizing coil. related to the evaluation of test object serviceability may
result in signals larger than those of critical properties that
Limitations of Eddy Current Tests are vital to the prediction of serviceability. Fortunately,
techniques are available to reduce these undesired effects
In general, eddy current tests are applicable only to test for many applications. The science of eddy current nonde-
materials with significant electrical conductivity, such as structive testing is essentially that of establishing optimum
metals and alloys, or composites with conducting layers or test conditions that respond clearly to the desired effects
reinforcing fibers. They can be used, however, to measure and reduce or eliminate the undesired effects.
thicknesses of nonconducting layers on the surface of con-
ducting metallic materials: in the liftoff effect, the coating Typical Industrial Applicationsof
separates the test probe from the conducting material by Eddy Current Tests
the thickness of the nonconducting coating or sheet mate-
rial. Applications of eddy current tests in industry are numer-
ous and widespread and the total number of test measure-
Eddy current tests provide maximum test sensitivity for ments made annually by this nondestructive test method
the surface and near surface layers of test material adjacent may exceed that of all other types. Although eddy current
to the source of excitation. In some cases, it may be difficult tests respond only to material conditions that influence the

ELECTROMAGNETIC TESTING I 203

material geometry, electrical conductivity and magnetic per- such. In this type of transducer the impedance or the
meability in the region excited by the magnetizing field, the induced voltage in the coil is measured directly (their abso-
test is highly versatile and serves a number of functions: lute value rather than changes in impedance or induced
voltage). In general, absolute eddy current transducers are
1. the ability to measure the thickness of metallic foils, simple to construct and are used in applications such as
sheets, plates, tube walls and machined parts from material sorting and detection of long seams in steel rods.
only one side by noncontacting means;
Differential eddy current transducers consist of a pair of
2. measuring the thickness of coatings over base materi- coils connected in opposition so that the net measured
als where the coating and base material have signifi- impedance or induced voltage is cancelled out when both
cantly different electrical or magnetic properties; coils experience identical conditions. The coils can sense
only changes in the test material and therefore differential
3. identifying or separating materials by composition or eddy current transducers are used to react to changes in test
structure where these influence electrical or mag- materials while cancelling out noise and other unwanted sig-
netic properties of the test material; nals that affect both coils. Their sensitivity to discontinuities
in materials is higher than that of absolute transducers while
4. detecting material discontinuities (which lie in planes sensitivity to liftoff variations and probe wobble is reduced
transverse to the eddy currents) such as cracks, because these factors tend to affect both coils equally.
seams, laps, score marks or plug cuts, drilled and
other holes and laminations at cut edges of sheet or Array transducers are listed separately although they
plate; may be used either in an absolute or differential mode.
Depending on the exact configuration of the coils and their
5. identifying and controlling heat treatment conditions connection, the output will differ greatly. Thus, for example,
and evaluation of fire damage to metallic structures; a number of small coils on the outer surface of a transducer
could be connected in series. In this case, the transducer
6. determining depths of case hardening of steels and qualifies as an absolute transducer. The same transducer
some ferrous alloys; may be used in a differential configuration by connecting
pairs of coils in a differential mode. It is also feasible to use
7. locating hidden metallic objects such as underground the transducer as a multiple absolute (or multiple differen-
pipes, buried bombs or ore bodies, or detecting tial) transducer, where each coil (or pair of coils) is moni-
metallic objects accidentally packaged in foodstuffs; tored separately. The transducer then differs in its ability to
locate discontinuities along the circumference of the test
8. timing or locating the motions of hidden parts of material.
mechanisms, counting metallic objects on conveyor
lines or detecting metallic missiles in flight; and A second kind of transducer classification is based on the
method used for sensing changes in their output character-
9. precise dimensional measurement of symmetric, istics:
machined or ground and polished metallic parts, such
as bearings and bearing races, small mechanism com- 1. the impedance method;
ponents and others. 2. the transmit-receive method.

Eddy Current Transducers In the impedance method, the driving coil is monitored.
Because changes in coil voltage (for a constant current
The number of different types of eddy current transduc- source) or coil current (for a constant voltage source) are
ers and the even larger number of variations on basic types due to impedance changes in the coil, it is possible to use
make it necessary to classify them according to some conve- this method for sensing any material parameters that result
nient and meaningful factors. in impedance changes. These all relate to changes in the
real part of the impedance (conductivity, losses etc.) or
The most basic distinction between transducers can be changes in the imaginary part of the impedance ( changes in
made on the basis of their mode of operation and includes permeability) or both.
three classes:
The transmit­receive method consists of separate driving
1. absolute eddy current transducers; coil (or coils) and pickup coil (or coils). In this case the
2. differential eddy current transducers; and induced voltage across the pickup coils is measured.
3. absolute and differential eddy current array
There is little fundamental distinction between the two
transducers. methods because in the limit they are identical. For practi-
cal purposes, one may be more convenient or more sensitive
Absolute eddy current transducers consist of a single coil than the other. Both methods may be used with absolute or
or its equivalent. A winding separated into two or more sec- differential eddy current transducers.
tions, as is sometimes done to compensate for coil capaci-
tance, would still be considered absolute if it performs as

204 I NONDESTRUCTIVE TESTING OVERVIEW

A third important method of probe classification is based Factors Affecting Eddy Current
on usage. Although identical or similar transducers may be Transducers
used for different testing situations, the following major
types may be distinguished: Liftoff Curve

1. encircling transducers; An eddy current transducer has an initial impedance that
2. surface transducers; and depends on the design of the probe itself. This is an intrinsic
3. forked transducers. characteristic of any eddy current transducer and is some-
times called infinite liftoff impedance (because the
Encircling transducers (absolute and differential) are impedance is measured in air without the test object). As
used to test outer surfaces of products such as tubes, bars the probe is moved closer to the test object, there is a
and wires and are also widely used to test parts such as bear- change in the real and imaginary parts of the impedance up
ing components for steel mix and heat treatment conditions. to the point where the probe is touching the material sur-
These transducers are also called encircling, annular, cir­ face. This is the zero liftoff impedance. The impedance
cumferential orfeed­through coils. curve described by the probe as it moves between these two
points is the liftoff curve and is a very important factor in
Surface coils (or probe coil transducers) are some of the eddy current testing. Because of the nature of the eddy cur-
most widely used eddy current probes. In most cases, they rent transducers, the curve is not linear (the change in the
consist of flat coils and are used to test flat surfaces or sur- field is larger close to the coils). In many cases, especially
faces with relatively large curvatures. Surface probes may with small diameter probes, for which the field decays
also be curved to fit contours of the test object. rapidly, the range in which measurements may be taken is
very small with a very pronounced liftoff effect. In other
Forked coils are usually used to test flat, thin sheets of cases such as with large diameter probes or with forked
metal. The test material is passed between the coils. This probes, the effect may be considerably smaller.
type of probe is particularly appropriate for testing thin
sheets since they have excellent liftoff characteristics and are Liftoff, because it is troublesome in many cases, is often
very sensitive to material thickness. They also represent the considered an effect that needs to be minimized. Liftoff
through-transmission probes, in which the coils are located effects may be reduced by methods such as surface riding
at opposite sides of the test material, compared to reflection probes,3 liftoff compensation techniques or multifrequency
type probes where both the exciting and the pickup coils (or measurements.4 At the same time, some important eddy
coil) are located on the same side of the test object. current tests depend on the liftoff effect. Measurements of
nonconducting coating thicknesses over conducting sur-
All of these transducers may be used in any of the con- faces and testing for surface evenness are two such tests.
figurations mentioned above. Thus, for example, a feed-
through probe may be absolute or differential and either the Fill-Factor
impedance or the induced voltage may be measured. For
this reason, any classification of eddy current transducers is In encircling coils, the liftoff effect is referred to as fill
at best more of a convenience than a true statement of the factor. This is a measure of how well the tested article fills
differences among various transducers. the coil. The largest signal is obtained with the material
completely filling the coil (fill factor is almost equal to 1.0).
Secondary methods of classification may also be Although it is usually desirable to maximize fill factor, some
employed to further distinguish between various transduc- tests rely on fill factor variations.
ers. For example, it is quite common to refer to particular
transducers by describing their shape. Examples of this Skin Effect
practice are U-core probes, E-core probes, mushroom
probes and pencil probes. A name such as pencil probes Eddy currents are induced in the test object but are not
refers to the fact that the coils are very small on a pencil uniformly distributed throughout the material. The eddy cur-
shaped holder1 but it adds little to our knowledge of the rents are quite dense at the surface and decay exponentially
probe characteristics. Still other probes are commonly with depth in the material. The distance that eddy currents
named according to some feature in their design that may penetrate the material is called the depth of penetration.
have even less importance. For example, winding an abso- Under ideal conditions (i.e., for a plane electromagnetic
lute probe in the cavity of one half of a ferrite pot core pro-
duces a superior probe2 that is sometimes called a pot core
or cup core probe.

Other types of probe classification include: ferrite core
and air core; shielded and unshielded; rotating and nonro-
tating; low and high temperature; and contacting and non-
contacting probes.

ELECTROMAGNETICTESTING I 205

field hitting a semiinfinite, isotropic medium), the standard The standard depth of penetration is a convenient figure
depth of penetration can be calculated as: that states that the eddy current density has decayed to lie
(37 percent) of its surface value. This is also called skin
s (Eq. 1) depth. It is an important figure for practical purposes
because at about five skin depths the eddy current density is
Where: the standard depth of penetration (meters); less than 0. 7 percent of the surface value.
S the conductivity of the material;
o As Eq. 1 shows, the skin depth depends on conductivity,
the frequency (hertz); permeability and frequency but is relatively small for most
f the relative magnetic permeability; and metals (about 0.2 mm or 0.008 in. for copper at 100 kHz).
the permeability of free space.
µ This has two important effects on the design of eddy current
µ0 probes: (1) the transducers are more useful for surface test-
ing and (2) for subsurface testing, lower frequencies may be
necessary in addition to special methods of increasing skin
depth (such as magnetic saturation).

206 I NONDESTRUCTIVETESTINGOVERVIEW

PART 2

REMOTE FIELD LOW FREQUENCY EDDY
CURRENT INSPECTION

The remote field eddy current technique uses internal majority of eddy current instruments operate in this direct
probes to inspect metallic tubes. The technique features the or near field zone.
ability to inspect both ferrous and nonferrous materials with
equal sensitivity to internal or external anomalies. Pits, The remote field shows every evidence of having made a
cracks and general wall thinning can be detected with a sin- double traverse of the pipe walls and of being attenuated
gle probe. and delayed (lagging in phase) as a result. Field plots in the
vicinity of the exciter coil will be presented to demonstrate
The probes employ a simple internal axial exciter coil to this. The penetration shown by signals in this zone makes
induce circumferential eddy currents in the pipe walls at them useful for pipe wall inspection.
low magnetization levels. This technique, unlike more con-
ventional eddy current testing, places the detector coils two Remote Field Zone
to three pipe diameters down the pipe from the exciter coil.
Phase of the detector signal with respect to the exciter is The remote field effect is thought to be the result of two
normally used as the measure of pipe wall characteristics. discrete effects taking place in the tube. The following
explanation is based on observations and measurements of
Commercial instruments employing this inspection tech- remote field devices. Though speculative, it has proven use-
nique have been in use in the petroleum industry since the ful in explaining the characteristics of the technique.
early 1960s and are used to detect casing corrosion and
other problems in oil and gas wells.5 The first effect is the shielding of the direct coupling
exciter flux from the detector along the interior of the pipe.
The internal electromagnetic fields generated in a pipe Magnetic shunting and circumferential eddy currents
by an exciter coil driven with a relatively low frequency sinu- induced on the inner pipe wall rapidly attenuate the direct
soidal signal can be divided into two distinct zones on each coupled energy down the interior of the pipe. Attenuation
side of the exciter coil; these zones are called the direct cou­ in decibels (dB) is rapid and a nearly linear function of dis-
pled and remote field zones as shown in Fig. 1. These two tance.
zones have decidedly different characteristics. Signal levels
in the direct field zone are high while signal levels from a Figure 2 shows actual detector output amplitude and
detector in the remote field zone are relatively faint and phase traces from a steel pipe. For these tests a fixed exciter
require amplification. coil was located at the zero position on the plot and a mov-
able detector coil was moved away from the exciter on both
The direct field zone, while somewhat affected by the the interior and exterior wall surface. The trace labeled
presence of the pipe, is primarily a direct, line-of-sight cou- amplitude ­ inner wall is the horizontal component of field
pling from the exciter in a simple transformer action. The
direct field strength is very large near the exciter coil but
attenuates rapidly with distance from the exciter. The

FIGURE 1 . Remote field representation

•------- - ----- •......
D

__.. DIRECT REMOTE
COUPLING ZONE FIELD ZONE

D

EXCITER COIL DETECTOR ARRAY

ELECTROMAGNETIC TESTING I 207

amplitude along the inner pipe wall as a function of distance which the exterior fields distribute away from the exciter
location. Note also that beyond a short distance from the
from the exciter. The log of amplitude falls rapidly with a exciter the fields at the exterior surface of the pipe have a
substantially higher amplitude than the corresponding inte-
nearly straight line in the direct coupling zone Oto 1.8 inside rior wall fields.
pipe diameters from the exciter. If this path along the inte-
rior of the pipe were the only path the zone beyond 1.8 pipe The circulating sheath of exterior surface currents from
diameters (as estimated by extrapolating the initial slope of these fields acts as a secondary exciter that induces eddy
this curve) would contain almost no electromagnetic field. currents in the pipe wall, some of which diffuse radially
Fortunately for eddy current inspection there is a second back through the pipe wall along the length of pipe. In the
energy path. remote field zone the fields from these currents dominate.
This secondary path is indicated in Fig. 1 by the dotted line.
The second effect contributing to the eddy current
remote field stems from the substantial circumferential Figure 2 also shows a plot of the horizontal component
eddy currents set up in the pipe walls in close proximity to of inner wall flux in the remote field zone. Its slope is the
the exciter coil. The pipe wall in this zone becomes the same as the outer wall field, suggesting the relation between
shorted single tum secondary of the transformer created by them. The outer wall strength is about ten times the inner
the exciter and the pipe. Some of these circumferential wall strength in this zone and shows none of the interaction
eddy currents diffuse radially to the outer wall of the pipe dip seen at the transitional zone between the direct and
and undergo an attenuation and time delay as they do so." remote field zones. This dip on the inner wall field plot is
thought to be caused by the two signals from separate paths,
On reaching the outer surface of the pipe the fields from having about the same amplitude but substantially different
these circumferential eddy currents appear to spread phase angles, which cause a partial nulling of the signals
rapidly in both directions for a considerable distance from when added vectorially.
the exciter. The plot labeled amplitude ­ outer wall on
Fig. 2 shows the axialcomponent of the outer wall field. The Instruments with detectors in the zone where this sec-
slope of this exterior field is much lower than the initial ondary energy path dominates are called remote field
slope of the interior field, indicating the relative rate at devices. Detector signals in this zone contain information
from the double transit of the pipe walls.
FIGURE 2. Electromagnetic field measurements
in vicinity of exciter coil at 40 Hz for 76 mm The detector signal's phase with respect to the exciter as
(3 in.) inside diameter steel pipe with 7 .6 mm a function of distance down the inner wall of the pipe is also
(0.3 in.) wall thickness shown in Fig. 2. The zero point on this curve has been
shifted 180 degrees to facilitate plotting. There is a major
10-1 360 shift in phase as the detector moves through the transition
zone near 1.8 inside pipe diameters from the exciter, where
w 10-2 coupling changes from direct to remote coupling. While the
0 270 shift at first appears to be a slight lead on the plot, the actual
:J shift over the eddy current path through the pipe walls is an
f:-J 10-3 :s!_'.)_ increasing lag. Phase lag or time delay on the outer surface
a'.J of the pipe is about half that indicated for the detector signal
CL in the remote field zone.
fl180 ui ~
~ ~J0--4 CL For the tests shown in Fig. 2, a movable detector was
Oo::'-. 20.. 90 used with a fixed exciter to plot the electromagnetic field
ti I 0-5 characteristics in the vicinity of the exciter coil. In a working
inspection device the detectors are in the remote field zone
w at a fixed distance from the exciter and both are moved as a
~ unit through the pipe.
10--6
0 Eddy Currents in Pipe Waif
Applications
0
Because electromagnetic fields in the remote field zone
8 show a through-transmission effect, eddy current skin depth
equations can be used to approximate remote field behavior.
DISTANCE (INSIDE PIPE DIAMETERS) More exact and complex models for the effect have also
FROM EXCITER COIL

LEGEND
I . AMPLITUDE - INNER WALL
2. AMPLITUDE - OUTER WALL
3. PHASE - INNER WALL
4. DIRECT COUPLING ZONE
5. REMOTE FIELD ZONE

208 I NONDESTRUCTIVETESTING OVERVIEW

been developcdr? and finite element studies of the effect Example Applications

are underway. For a plane electromagnetic wave hitting a A number of pipe scans are presented to demonstrate
semiinfinite conducting medium, the primary skin depth the capability of the remote field eddy current technique.
equation is: Pipe samples with a length of 1.8 m (6 ft) and a 76 mm
(3 in.) inside diameter were used in the first six tests.
(Eq. 2) Twenty-four scans were used to produce the inner wall elec-
tromagnetic field map. At this 15 degree radial spacing, the
Where: scans were about 10 mm (0.4 in.) apart. The detector was a
small coil that produced localized indications of field
B magnetic flux density at depth d, strength. Thinning is indicated in the upward direction on
B0 magnetic flux density at surface; the records.
d depth;
The effects of simulated pits and cracks on the exterior
f frequency; of the pipe are shown in Fig. 3. An excitation frequency of
40 Hz was used for these results. Discontinuity depths
µ magnetic permeability; ranged from 10 to 67 percent of the 7.6 mm (0.3 in.) full
o electrical conductivity; and wall thickness. Note that a 67 percent saw cut indicates
t time. about two thirds the peak height of a 67 percent hemispher-
ical pit even though the pit has removed about 12 times as
Skin depth may be defined as the depth required to give much metal.
one radian of phase shift to the induced currents in the
semiinfinite medium. The phase lag term in this equation is The 33 percent transverse saw cut gives a higher indica-
given by Eq. 3: tion than a comparable depth longitudinal cut because the
transverse cut interrupts the magnetic field from the eddy
phase lag = d~1tf µcr (Eq. 3) currents and the exciter coil while the longitudinal cut inter-
rupts the currents themselves.
Because of the linear relationship between phase lag and
depth, phase is commonly used as the measure of wall con- Wall thinning over a uniformly machined section of pipe
dition. While wall thickness is usually the variable of inter- is shown in Fig. 4. The pipe has its outside diameter reduced
by five percent, about 0.4 mm (0.015 in.). Because this
est, magnetic permeability and electrical conductivity are reduction is uniformly distributed around the circumference
always involved in the measurement. the responses of both the exciter coil and detector can be
seen. Half the deflection occurs when the exciter passes the
The double transit of the eddy currents through the pipe shoulder of the machined section; the remaining deflection
wall produces characteristic responses to certain discontinu- occurs when the detector passes the same shoulder. Because
ities that can be identified in some of the following exam- the exciter winding is longer in its longitudinal dimension
ples. Uniformly machined sections of pipe will produce a than the detector, its response is more gradual. In this piece
double deflection on chart recordings: half when the exciter of steel pipe at the 40 Hz test frequency the sensitivity of the
coil passes the edge and the remaining half when the detec- instrument was close to one degree of phase shift for each
tor passes the edge. Large pits or other anomalies that are a 0.03 mm (0.001 in.) wall thickness change.
significant part of the circumferential band of pipe near the
exciter coil will produce a double indication on a chart The weld shown in Fig. 5 is a simple single-pass weld with
recording with the same spacing as the exciter-to-detector no known discontinuities. While both the exciter and detec-
tor responses are indicated, the detector response contains a
spacing. better signal-to-noise ratio. Each detector scan, however, is
Velocity effects, particularly from stationary to moderate slightly different, indicating possible differences in thickness,
alloying or heat affected zone. The slight dip on two of the
velocities, are small but at higher velocities, device motion tracks is due to a bead overlap at the ends of the weld.
can be significant during the cycle time of the excitation fre-
quency. Velocities over 0.6 m-s-1 (24 in-s+) have been used. Shown in Fig. 6 is a hard spot in a pipe sample. This was
In addition, spurious signals from device motion can have an created by heating a small area to red heat and quenching it
effect at higher velocities through remnant magnetic fields with water. No dimensional variation was created. In this
in ferromagnetic materials. case, the exciter response gives a more pronounced indica-
tion than the detector response but the detector signals give
Frequencies used with steel depend on the wall thick- more detail on small incremental areas within the hard spot.
ness to be inspected but can vary from 15 Hz at thicknesses Variations in magnetic permeability are probably the cause
over 13 mm (0.5 in.) to several hundred hertz for thick- of the indications.
nesses under 3 mm (O.l in.).

ELECTROMAGNETIC TESTING I 209

FIGURE 3. Remote field eddy current indication of external discontinuities in steel pipe as sensed by
phase plot of internal electromagnetic field at 40 Hz: (a) single trace; (b) complete inner wall scan

tfa)

zz\,'.)
z

:i:

I-

I -- SIMULATED STRESS c_ORRos10N OR FATIGUE CRACKS

SIMULATED CORROSION PITS ------ 25 mm (I in.) RADIUS CIRCULAR SAW CUT --- I
9.5 mm (0.375 in.) DIAMETER BALL MILL TO NOTED DEPTH 0.25 mm (0.01 1n.) THICK TO NOTED DEPTH

00 0

5 mm 4 mm 2.5 mm 1.3 mm 0.8 mm 4 mm 5 mm 2.5 mm 2.5 mm

(0.2 in) (0.15 in.) (0. I in.) (0.05 in.) (0.03 in.) (0.15 in.) (0.2 in.) (0. I in.) (0.1 in.)

67 PERCENT 50 PERCENT 33 PERCENT 17 PERCENT IO PERCENT 50 PERCENT 67 PERCENT 33 PERCENT 33 PERCENT

fbJ

1.8 m (6 ft) -

FIGURE 4. General external thinning in steel pipe as sensed by remote field probe: (a) single trace (wall
thickness values given as measured ultrasonically); (b) cross section drawing showing machined outside
diameter nominal wall reduction of 0.4 mm (0.015 in.); (c) complete inner wall scan

fa) t WALL THICKNESS= 7.25 mm (0.285 in.) 7.0 mm (0.27 in) 6.6 mm (0.26 in.) DETECTOR COIL PASSING
RIGHT EDGE
fbJ zzoz
DETECTOR COIL PASSING
:i:
LEFT EDGE EXCITER COIL PASSING ' 7.25 mm (0.285 in.)
I- RIGHT EDGE

LEFT EDGE

SOME ECCENTRIC/TY AND TAPER

fcJ

1.8 m (6 ft) -

21 0 I NONDESTRUCTIVE TESTING OVERVIEW

FIGURE 5. Weld in steel pipe as sensed by remote field probe: (a) cross section drawing; (b) complete
inner wall scan

fa) WELD

fbJ START OF WELD BEAD 7CAUSED BY EXCITER COIL PASSING WELD ~ CAUSED BY DETECTOR COIL PASSING WELD

FIGURE 6. Hard spot in steel pipe as sensed by remote field probe: (a) single trace; (b) cross section
drawing; (cJ complete inner wall scan

tfa) 1 DETECTOR COIL
PASSING HARD SPOT
l.'J EXCITER COIL PASSING
HARD SPOT HARD SPOT (NO DIMENSIONAL VARIATION)
2z

2I

f-

fbJ

fcJ

ELECTROMAGNETICTESTING I 211

Natural corrosion occurring on a section of 64 mm FIGURE 7. Natural corrosion in oil well tubing
(2.5 in.) oil well tubing is shown in Fig. 7. Pits range in as sensed by remote field probe; numerous pits
depth up to about 50 percent penetration. Equal internal to 50 percent penetration
and external pits are shown in Fig. 8. As could be expected
from a through-transmission system, peak heights are about
equal. An external pit in an aluminum tube is shown in
Fig. 9. A frequency of 1 kHz was used to compensate for the
lower permeability and higher conductivity of aluminum
relative to steel. The low amplitude circumferential bands
are extrusion variations that can be seen when looking down
the tube sample.

Conclusions FIGURE 8. Internal-external pit sensitivity of
remote field probe; two equal simulated pits at
The basic differences and advantages of the remote field 50 percent penetration; 9.5 mm (0.375 in.)
eddy current technique stem from the through-transmission diameter ball mill
effect that is realized even though both the exciter and
detector are located in the interior of the pipe. FIGURE 9. Simulated pit at 50 percent
penetration in aluminum tube as sensed by
Advantages of the technique include the following. remote field probe at 1 kHz

1. Metallic tubes can be inspected with equal sensitivity -==: =
to external and internal anomalies at low magnetiza-
tion levels.

2. Pits, cracks and general thinning in tubes can be
detected with a single, relatively simple probe run
through the pipe.

3. Eddy currents in the through-transmission mode are
predictable, including a linear relationship between
wall thickness and phase lag (time delay).

4. Dirt, scale and probe liftoff have little effect on
results and no couplant is required.

Limitations of the technique include the following.

1. Low frequencies must be used, which limit inspec-
tion speeds.

2. Low level detector signals must be processed.
3. Records indicate wall thickness and do not discrimi-

nate between internal or external metal loss.
4. Encircling tubes around the tube under test can

affect and complicate thickness measurements.

212 I NONDESTRUCTIVETESTING OVERVIEW

PART 3

ELECTROMAGNETIC SORTING
TECHNIQUES

Eddy Current Impedance Plane reactance XL and the resistance R. In Fig. 10, the inductive
Analysis reactance voltage is identified as E1 and the resistance volt-
age is identified as E2. The specific value of the voltage is
Sorting is generally considered to mean the identifica- the product of the current I and either the inductive reac-
tion of mixed materials or the verification of a desired con- tance or the resistance:
dition resulting from a process such as heat treatment.
Absolute identification of chemical composition is fre- (Eq.4)
quently obtained by means of spectroanalysis or chemical
spot testing, while verification of microstructure is obtained and
by microscopic metallography. These methods are destruc-
tive, time consuming, expensive and thereby unsuitable for The voltage E 1 across the inductive reactance is
production applications. Several electromagnetic tech- 90 degrees out of phase with the voltage E2 across the resis-
niques provide a more convenient means of material charac- tance. These two voltages can be represented as shown in
terization. These include eddy current techniques (total
impedance plane analysis, conductivity measurements), hys- Fig. 10.
teresis loop, thermoelectric techniques and direct resistivity Because the current through both the inductive reac-
measurements.
tance and the resistance at any given time is at the same
Eddy current techniques are well suited to the task of value, the voltage values on the voltage plane diagram may
sorting materials. When a particular conductive material is be divided by the current value to give the values of induc-
placed in the magnetic field of an eddy current probe, a spe- tive reactance and resistance in the circuit. The resulting
cific impedance value is established in the test circuit. Any diagram is called a phase vector or phasor diagram, and is
other specimens of the same material will cause the same used to show the amplitude and phase relationship of alter-
impedance value in the test circuit, provided the thicknesses nating current signals having the same frequency (see
of all the specimens exceed the depth of penetration of the
eddy currents. Generally, impedance values are first estab- Fig. 10).
lished on a sample of known material; then the readings When the values of Rand XL are varied, the voltage drop
obtained from test samples are compared with the readings
from the known sample. across the circuit varies, and depending upon these values,
the voltage drop is represented by different impedance pha-
Analytical analysis of eddy current testing is difficult sors Z on the impedance plane. The total voltage drop Er is
under any circumstances but is especially so when consider- the phasor or vector sum of E 1 and E2.
ing materials characterization or sorting. This is because
eddy current instruments are sensitive to only three vari- FIGURE 10. Voltage plane and impedance
ables: conductivity, permeability and geometry. All the met- plane diagrams
allurgical variables of interest in sorting must be inferred
from changes in these three variables. Sorting therefore can L°'< 90 DEGREES ill XL0_~----,
be much more difficult than discontinuity detection due to aI
the sometimes unpredictable appearance of unwanted met- 0~E2 OI
allurgical factors. IR
RESISTANCE INDUCTANCE R
Impedance Plane VOLTAGE PLANE
CIRCUIT IMPEDANCE PLANE
When an alternating current voltage is applied to an
eddy current circuit, current flows through the inductive

ELECTROMAGNETIC TESTING I 21 3

z (Eq. 5) changed. The influence of the test object can be described
by a variation in the test coil characteristics. The apparent
or impedance PO of the coil in air is displaced to P 1 (corre-
sponding to new values of XL and R) under the influence of
Z = ~Xl + R2 the test object (see Fig. 11).

The values of these voltage components depend on the The magnitude and direction of the displacement of the
value of circuit resistance, reactance and frequency. The volt- apparent impedance from P0 to P1 under the influence of
age drop across the resistance is proportional to the current I the test object are functions of the properties of the test
times the resistance R. The voltage drop across the reactance object and the characteristics of the instrumentation. Signif-
is proportional to both the inductance and the frequency. icant properties of the test object include (1) electrical con-
ductivity o, (2) dimensions of the test object, (3) magnetic
Vector addition of the values of inductive reactance and permeability µ and (4) presence of discontinuities such as
resistance, plotted 90 degrees apart, will indicate the cracks. Significant instrument characteristics include
impedance value Z and the lag of phase angle, just as cur- (1) frequency of the alternating current field fin the test
rent vectors will. coil, (2) size and shape of the test coil and (3) liftoff (dis-
tance from the probe to the test object).
In general, the test coil is characterized by two electrical
impedance quantities: (1) the inductive reactance XL (where Liftoffand Edge Effectson
the frequency of the alternating current field is in hertz and Impedance Plane
the self-inductance L of the coil is in henrys) and (2) the
ohmic resistance R. Figure 12a shows the impedance plane response, which
occurs when liftoff is increased. The upper portion of the
It is common practice to plot reactance XL as the ordi- impedance plane is the magnetic domain where responses
nate and resistance R as the abscissa in the impedance occur from ferromagnetic materials. The lower portion is
plane. In this way, the test coil impedance Z is represented the domain where responses are obtained from nonmag-
by a point P formed by two perpendicular components, XL netic materials. Note the nonlinear (logarithmic) changes
and R, on the impedance plane. In the absence of a test among the liftoff locus for equal increments of spacing.
object, the empty test coil has a characteristic impedance
with coordinates Xrn and R0, shown on the impedance plane When the probe is moved near the edge of the specimen
by the coil in air, point P0 of Fig. 11. If the probe is placed or part, an edge effect occurs because a portion of the mag-
on the test object, the original field of the coil in air is mod- netic field is outside the part. For nonmagnetic materials,
ified by the superimposed field of the eddy currents. This the resultant effect produces response similar to the liftoff
field modification has exactly the same effect as would be response (compare Figs. 12a and 12b). For ferromagnetic
obtained if the characteristics of the test coil itself had been materials, the edge effect response line curves to the left of
the liftoff locus line. This evidently occurs because the mag-
FIGURE 11 . Representation of test coil netic field becomes distorted at the edge of the part; the
characteristics on impedance plane magnetic field wants to remain in the part. The angle of cur-
vature A, shown in Fig. 12b, increases or decreases as a
rzLu1J a__ Po COIL LIFTOFF function of operating frequency and coil diameter.
XLO
-- Conductivityand Permeability Loci
~!~,.~u on Impedance Plane
~ Xu Iz,II
Z11 The impedance plane response to the different conduc-
0:::: I tivities of various nonmagnetic alloys is shown in Fig. 13.
I I I 'I The material points trace out a characteristic comma shaped
Lui>=1J I curve with conductivity increasing in a clockwise direction.
l'y The coil liftoff loci are shown for the different metals. Note
:::i vI"' ao that the separation angle between the conductivity locus
~1 \ and the liftoff locus is much smaller for titanium than it is
0~
Ro R1
_J

0u

LEGEND COIL RESISTANCE

=Po COIL IN AIR
=P1 COIL ON PART

214 I NONDESTRUCTIVE TESTING OVERVIEW

FIGURE 12. Liftoff and edge effect loci on for copper. Hence, the unwanted liftoff variable will affect
impedance plane: (aJ liftoff loci; (bJ edge effect test results less when testing copper or aluminum alloys at
loci 100 kHz than it will when testing titanium or graphite.

STEEL 4340 The material points are spaced around the conductivity
locus in a nonlinear fashion. For example, the spacing
(aJ between titanium and 304 stainless steel at the top of the
curve is much greater than it is between bronze and alu-
AIR minum at the bottom of the curve.

x Figure 14 shows the effect of test frequency on the con-
ductivity and liftoff curves for nonmagnetic alloys.As can be
(bJ 7075 ALUMINUM seen, changes in frequency shift the points along the con-
ductivity locus in a nonlinear fashion. This phenomenon,
PROBE-TO-PART SPACING IN also true for other impedance curves, can be used advanta-
INCREMENTS OF 0.025 mm (0.001 in J geously because it allows the material points to be located
for optimum response or suppression. Specifically, a fre-
R quency should be chosen that causes the material points for
the variables to be measured to move in a substantially dif-
STEEL 4340 ferent direction from those points to be suppressed.

At high frequencies (Fig. 14c), the separation angle 8
between the liftoff curve and the conductivity curve for.
bronze is quite small. Thus, it becomes more difficult to
obtain liftoff suppression. Selecting a lower frequency
(Fig. 14a) makes the separation angle for bronze quite large
and allows liftoff suppression and good sensitivity to con-
ductivity variations. For sorting titanium alloys, a frequency
between 500 kHz and 1 MHz would be selected. However,
to sort aluminum alloys, a frequency between 20 kHz and
100 kHz would be chosen.

Generally, for conductivity measurements (alloy sorting,
heat treat determination etc.) and surface crack detection, a
frequency should be chosen that places the material point
just below the knee in the conductivity curve. At this point,
a large separation angle exists between the liftoff and con-
ductivity curves.

AIR FIGURE 13. Conductivity and liftoff loci on
impedance plane
x
O PERCENT !ACS (AIR)
7075 ALUMINUM
,CONDUCTIVITY LOCUS

/ GRAPHITE (0.2 PERCENT !ACS)

e

x

100 kHz

RR

ELECTROMAGNETIC TESTING I 21 5

FIGURE 1 4. Movement of material points by For magnetic materials, the liftoff and magnetic perme-
frequency changes: (a) low frequency; ability loci curves are virtually superimposed, as illustrated
(b) medium frequency; (c) high frequency in Fig. 15a. However, their respective values increase in
fa) opposite directions. Figure 12 showed that the reactance
component of the test coil impedance is decreased by the
x presence of nonmagnetic materials. This reactance reduc-
tion occurs because induced currents flow in the conductive
R and nonmagnetic object and set up a secondary field that
20 kHz partially cancels the primary field of the coil. The opposite is
true when a magnetic material, such as iron or ferrite, is
fbJ AIR GRAPHITE placed within the field of the coil. This happens because the
presence of the magnetizing force of the primary coil field
t causes magnetic domains of the magnetic material to
become aligned with the field, thus increasing the flux den-
u0t,:. sity. This effect is described by Eq. 6:

x :J B = µH (Eq. 6)

R Where: the magnetic flux density;
100 kHz the relative magnetic permeability; and
B the magnetizing force or magnetic field strength.
fcJ AIR µ
H
x
The NiZn ferrite cores (Fig. 15b) were chosen as exam-
R
1 MHz ples because they have a low conductivity and two different
values for permeability. The effect of the increased flux den-
sity gives a greater induced voltage in the test coil that in
tum raises the impedance. The increase in impedance is in
the reactance direction except for the effect of a small
amount of energy loss resulting from hysteresis. The NiZn
ferrite cores may have an initial permeability of 850, which
would be located high up along the permeability line of
Fig. 15. Usually,practical engineering materials also have an
associated electrical conductivity that affects the
impedance, as shown for 422 and 4340 steel in Fig. 15b. The
relative relationship of the permeability loci lines and the
conductivity curves for three materials is shown in Fig. 15c.

The vector or phasor values of inductive reactance and
resistance, for different material conditions, yield unique
loci or phasor plots on the impedance plane at particular
operating frequencies. The phase angle of the impedance
vectors will change at different frequencies because the
inductive reactance value is a function of inductance and
frequency. Hence, vector points may move relative to one
another along the conditional loci curves when the operat-
ing frequency is changed. This shift in phase was shown in
Fig. 14 for the conductivity values of nonmagnetic materi-
als. Similar phase angle changes for the permeability of
4340 steel are shown in Fig. 16 as the frequency changes
from 75 to 300 kHz. These changes in phase shift at differ-
ent frequencies do not interfere with impedance plane anal-
ysis, provided that the operator is aware of this factor. In
some cases, test results may be improved by the ability to
cause phase shifts by changing the frequency.

2 I 6 I NONDESTRUCTIVE TESTING OVERVIEW

FIGURE 1 5. Permeability, liftoff and FIGURE 1 6. Phase angle changes on
conductivity loci on impedance plane: impedance plane caused by frequency changes
(a) permeability and liftoff locus;
(b) permeability loci for different materials; STEEL 75 KHz
(c) permeability and conductivity loci

IfaJ µ = PERMEABILITY x
LO= LIFTOFF
x AIR

AIR MAGNETIC R
N ON MAGNETIC FROM DOUGLAS AIRCRAFTCOMPANY. REPRINTEDWITH PERMISSION.
1---+------
FIGURE 1 7. Impedance changes in relation to
LO= LIFTOFF one another on impedance plane

COPPER 25 kHz x

R

fbJ µ = I 90 (NiZn)

422 STEEL Iµ LEGEND
4340 STEEL
LO C = CRACK IN STEEL
x µ = PERMEABILITY
I
AIR TITANIUM MAGNETIC r,LO= LIFTOFF
NONMAGNETIC = PLATING (NONMAGNETIC)

ALUMINUM crm = CONDUCTIVITY (MAGNETIC
COPPER
MATERIALS)
25 kHz

R

fcJ µ 1 = FERRITES
µ2 = STEEL
µ3 = NICKEL

x MAGNETIC LEGEND
NONMAGNETIC LO= LIFTOFF
AIR o
C = CRACK IN ALUMINUM
R o., = CONDUCTIVITY

(NONMAGNETIC MATERIALS)
S = SPACING BETWEEN

ALUMINUM LAYERS
T = THINNING IN ALUMINUM
P, = PLATING (COPPER ON

ALUMINUM)
Pa = PLATING (ALUMINUM

ON COPPER)

R

FROM DOUGLAS AIRCRAFTCOMPANY. REPRINTEDWITH PERMISSION.

ELECTROMAGNETIC TESTING I 21 7

With modem phase analysis eddy current instruments, an Linear material values do not produce linear responses
operator can produce his own impedance plane loci plots or on the impedance plane loci. With the eddy current probe
curves automatically on the integral cathode ray tube oscillo- balanced on the metal specimen, the loci values for linear
scope. Such impedance plane plots can be presented for the material conditions are displayed as follows:
following material conditions (see Fig. 17):
1. liftoff (magnetic and nonmagnetic) varies
1. liftoff and edge effects; logarithmically (in X);
2. cracks;
3. material separation and spacing; 2. edge effect (magnetic and nonmagnetic) varies
4. permeability; logarithmically;
5. specimen thinning;
6. conductivity; 3. conductivity (magnetic and nonmagnetic) varies with
7. thickness measurement; and test frequency;
8. heat treatment conditions.
4. magnetic permeability varies with test frequency;
These plots show that ferromagnetic material conditions 5. metal thinning varies exponentially (e2);
produce higher values of inductive reactance than do non- 6. plating thickness (nonmagnetic) varies
magnetic material conditions. Hence, the magnetic domain is
at the upper quadrant of the impedance plane while nonmag- logarithmically; and
netic materials are in the lower quadrant. The separation of 7. material spacing or separation varies exponentially.
the two domains occurs at the inductive reactance values
obtained with the coil removed from the conductor (sample); Electromagnetic induction effects are not easy to under-
this is proportional to the value of the coil's self-inductance L. stand. Neither the magnetic fields nor the eddy currents can
be seen. In a problem solving situation, impedance plane
analysis is useful because it improves the ability to detect
various conditions and provides a better understanding and
interpretation of the eddy current test results.

218 I NONDESTRUCTIVE TESTING OVERVIEW

PART 4

EDDY CURRENT APPLICATIONS IN THE
STEEL INDUSTRY

Eddy Current SystemsThat Rotate The bar rotating and holddown assembly includes two
the Product at Ambient driven skew rolls and two holddown rolls. These rolls clamp
Temperatures the bar for inspection and rotate the product as it moves
through the test station. The entire machine roll assembly is
Numerous automatic bar inspection systems are in use at adjustable as a unit to set up the proper skew angle for
basic steel producing plants and at plants where a large vol- changing bar diameters. Adjustment is made mechanically
ume of bar products are used. These systems use eddy cur- with a calibrated scale.
rent and flux leakage probes for surface inspection.
Surface discontinuities are marked with a carbide cutting
A completely automated installation includes all neces- tool that leaves a clearly visible indication of their location
sary equipment to unscramble a lift of bars, to inspect and
classify each length individually and to sort them into prime, FIGURE 18. Bar inspection system 1 (automatic
salvageable or reject bins. In addition to eliminating human bar classifier)
judgment in the inspection, these systems also eliminate
much of the manual handling. FROM LTV STEELCORPORATION. REPRINTEDWITH PERMISSION.

After a lift of bars is placed on entry skids, pneumatically
powered load agitators spread the lift and the bars roll by
gravity against a stop. Load arms feed one bar at a time into
the inspection section where it is clamped in a roll assembly
skewed at the proper angle for the bar diameter. Inspection
heads are then lowered into contact with the bar surface.
Skewed drive rolls activate to move the bar longitudinally
while rotating it for inspection around the entire periphery
on a helical path.

The first system described will be referred to as bar
inspection system 1 or BIS-1 (shown in Fig. 18).

Inspection Stations

Depending on production requirements, BIS-1 installa-
tions are equipped with 1, 3, 4, 6 or more inspection sta-
tions. The use of multiple stations permits increased
production: each will be required to inspect only a portion
of the total bar length. Each station consists of a main stand
that houses a probe assembly, a bar rotating and holddown
roll assembly, marker and calibration units, bar guides and
discharge arms. The stand itself is mounted on heavy
springs, so that the entire assembly will follow the contour
of a cambered bar without disturbing the probe-to-bar dis-
tance during inspection.

The probe head is pneumatically operated and is sup-
ported on a shock absorbing mount to minimize bar friction.
Carbide wear shoes are contoured to fit the largest bar
diameter that will be tested.

ELECTROMAGNETIC TESTING I 219

and length. The marking tool is timed to actuate one revolu- two 5,000 tum coils of No. 46 wire encased in a nylon hous-
tion of the bar after the scanner head has detected a seam. ing, it is essentially an elongated absolute transducer rather
The mark will be repeated at preset intervals along the than a differential transducer. The series-connected coils
entire length of the discontinuity. An inspection station is (with resistance of 1,775 Q and inductance of 122 mH)
shown in Fig. 19. Modem inspection systems use computer comprise part of an oscillator circuit and the discontinuity
controlled paint marking devices, different colors for differ- signal is derived from the load on this oscillator.
ent discontinuity depths.
This signal is then transferred to the amplifier and
Electronic Instrumentation threshold circuits in the low frequency unit. There are two
threshold circuits with limits that can be preset to specified
The electronic control is equipped with dual threshold values. One limit is for shallow discontinuity depth and the
circuits for each channel. The number of channels depends other limit is for deep discontinuity depth. A seam as deep
on the number of heads (inspection stations) required for as or deeper than the upper limit will trigger both threshold
each installation. circuits.

When six eddy current probes are supplied, only a maxi- If a discontinuity is sensed within the lower seam limit, a
mum of four are used at any one time and these are selec- time delay is activated by this threshold circuit. The time
tively connected to the electronic instrumentation. This delay retains the discontinuity information for one revolu-
separate electronic control cabinet houses four low fre- tion of the bar after a seam has been found. A pulse is then
quency units, four marker delay units, a discontinuity ana- relayed to the mechanical marking equipment to mark the
lyzer, a control unit, two power supplies, a four-channel pen bar exactly on the discontinuity.
recorder and a constant voltage transformer.
The discontinuity analyzer accepts information from the
In general, each of the four detection channels operate two threshold levels of the four channels and automatically
as follows. classifies the bars as good, salvageable or scrap on the basis
of discontinuity depth and length. The discontinuity ana-
Bar seam discontinuities are detected by the probe and lyzer also operates relays to deposit each classified bar in its
sensing circuit. Although the probe transducer consists of appropriate cradle. The four-channel pen recorder continu-
ously monitors the input of the lower threshold circuit of
FIGURE 1 9. A BIS-1 inspectionstationwith bar each channel. One meter is provided per channel to indicate
in inspectionposition the oscillator output level.

Calibration

Each head is provided with a calibration bar in which a
notch of known depth is milled. This is mounted on a motor
driven shaft to rotate the calibration sample under the scan-
ner coils at 0.45 m-s' (90 ft-min"). The shaft is mounted on
a pivot arm and swings into position when equipment cali-
bration is required. An air cylinder is used to move the pivot
arm into the calibration position or into the storage area.

FROM LTV STEELCORPORATION. REPRINTEDWITH PERMISSION. Discontinuity Detection Ability of the BIS- I

At least one user has reported that the inspection sta-
tions of the BIS-1 were able to detect surface discontinuities
of 0.25 mm (0.01 in.) in depth and deeper 100 percent of
the time. Surface discontinuities in the range of 0.13 mm
(0.005 in.) to 0.25 mm (0.01 in.) in depth were correctly
classified according to seam depth 75 percent of the time.

Figure 20 shows the typical signal variation obtained
with such a system when as-rolled bars of five different steel
grades and eight sizes from 30.5 to 71 mm (1.2 to 2.8 in.)
diameter are inspected. Some surface seams less than
0.25 mm (0.01 in.) deep are obscured by the noise level and
are therefore undetected. Because seams vary in width and

220 I NONDESTRUCTIVETESTING OVERVIEW

angle of entry as well as in depth, the signals for a given The BIS-1 is generally applicable for testing off-line at
seam depth also vary. For example, the same signal ampli- speeds less than about 1 m-s-! (200 ft-min-1) but bar produc-
tude occurs for seams from 0.15 to 0.38 mm (0.006 to tion speeds usually exceed 10 m-s! (2,000 ft-rnirr"). With the
0.015 in.) deep. There is no apparent effect attributable to problems of rising production costs and foreign competition,
steel grade, but grit blasting has been effective in reducing major domestic steel companies require in-line testing sys-
the background noise. tems with generally the same capabilities of the BIS-1, but at
higher speeds and usually at higher temperatures.
Some general observations about this system are listed
below. As other systems are described, it will be helpful to refer
to Figure 20 and the detection statistics for the BIS-1.
1. Cluster seams on the bar are additive and show signif-
icantly higher amplitudes than their actual depth. Round Bar Inspection System

2. A lap seam causes a slightlyhigher signal than a radial At least one steel producer has tried to use a portable
seam. commercial eddy current instrument with a rotary straight-
ener to inspect high alloy round bars 13 to 76 mm (0.5 to
3. Overfill will cause the probe coil to lift off the bar, 3 in.) in diameter.t? The in-line rotary straightener provides
causing a signal on every revolution. rotation at approximately 1 m-s? (200 surface ft-min:") and
is a logical choice for such 'an application. However, a fol-
4. Severe overfill can cause damage to the heads. When- lower is necessary to position a probe transducer (1) at a
ever possible, the operator should bypass inspection constant distance from the bar surface and (2) perpendicu-
and reject the bar manually. lar to the bar. This system will be referred to as bar inspec-
tion system 2 or BIS-2. The BIS-2 system operates at a
5. Severe scale or poor surface quality may cause noise frequency of 100 kHz.
levels to exceed the reject threshold.
Figure 21 shows the BIS-2 response to electric discharge
6. Extremely crooked product can cause a false signal machined notches 0.38 to 0.76 mm (0.015 to 0.03 in.) deep
due to coil liftoff. and a natural seam 1.3 to 2 mm (0.05 to 0.08 in.) deep. For
the sensitivity shown, the deep natural seam saturates the
FIGURE 20. Depth of surface discontinuities in system but the 0.38 mm (0.015 in.) deep longitudinal notch
as-rolled bars versus typical bar classifier signals gives a response about four times the noise level. Although
for one station detailed statistical results are not available for this system
the BIS-2 and the following system illustrate how existing
40 in-line rotary equipment can be adapted to perform a useful
inspection.
0UJ 30
:J Rotating Pipe Inspection System
of:-J,-
2 20 Another steel producer has adapted a rotary straightener
to create a multichannel eddy current device referred to
<( here as rotating pipe inspection system 1 (RPIS-1).11The
_J RPIS-1 is a five-channel system for inspecting seamless tube
with 100 to 355 mm (4 to 14 in.) outside diameter (OD) at a
~ normal surface speed of about 1.1 m-s! (225 ft-mirr"). The
usual inspection frequency is 50 kHz. The RPIS-1 was com-
l.9 pared with a magnetic particle inspection for a group of 101
vi 10 seamless pipe ranging in sizes from 140 to 245 mm (5.5 to
9.65 in.) outside diameter. Only significant discontinuities
0.25 0.5 0.75 1.0 1.25 are reported to have been marked by the RPIS-1.
fO.O I J f0.05)
f0.02) f0.03) f0.04) Figure 22 shows that almost five times as many disconti-
nuities are marked by magnetic particle inspection as are
DISCONTINUITY DEPTH marked by the RPIS-1 and five times as much pipe would
be conditioned, perhaps unnecessarily.
millimeters (inches)
Figure 23 shows the RPIS-1 response to 120 natural
LEGEND seams, slivers and pits in 178 mm (7 in.) outside diameter by

e = STEEL GRADE 8620
• = STEEL GRADE 6 I 18
• = STEEL GRADE 4145
o = STEEL GRADE 4 I 18
0 = STEEL GRADE I 030

ELECTROMAGNETIC TESTING I 221

FIGURE21 . Sketchand stripchart showingrespectiveeddy currentsignalsfrom variouselectron
dischargemachinednotches;signalsfrom deepernaturaldiscontinuityare also shown;note that
signalsare full scaleor saturated;test was used to demonstrateability of systemto discriminate
between depthsof discontinuitiesin 0.38 to 7.6 mm (0.015 to 0.3 in.) range

FIGURE22. Bar graph of seamlesspipe FIGURE23. Preliminaryeddy currenttest data
inspectiondata; bar graph shows comparison on pipe taken from lot of N-80 grade seamless
between magneticparticletest method and pipe with 178 mm (7 in.) outsidediameterand
RPIS-1;high ratio indicatesextremesensitivity 8 mm (0.32 in.) wall
of magneticparticletechniqueto minor
discontinuities

3,800 60 12.5 PERCENT
3,600 DEPTH
3,400 5 PERCENT
I~~6oac-:Vzo1 2,000 DISCONTINUITY

o~ l,600 DEPTH

;sDe: :J 1,200 0 10 20 30 40 50 60 70 80 90

llJ 800 LEGEND DEPTH OF EXPLORED DISCONTINUITIES
• = SEAM 0.03 mm (0.001 in.J PER DIVISION
~zCl) f- 400
:zJuO o = SLIVER
0
0V1 c, = PIT

LEGEND

­ = MAGNETIC PARTICLE MARKS
• = EDDY CURRENT INDICATIONS

222 I NONDESTRUCTIVE TESTING OVERVIEW

8 mm (0.32 in.) wall N-80 grade seamless pipe. A total of A complete BILSYS-1 installation incorporates an
eleven discontinuities (two percent) that exceeded the five unscrambler that separates billets and feeds them individu-
percent of wall threshold were not detected with sufficient ally onto inspection line rollers. A grit blast station removes
signal strength to activate the marker. The scatter evident in loose scale and blisters from the billet surfaces prior to
Fig. 23 is typical of hoth eddy current and diverted flux sys- inspection. The billets are then conveyed through the eddy
tems hut a valid comparison can only be made among sys- current inspection station of Fig. 2,5 and into a storage or
tems when signal-to-noise is plotted against seam depth. It classifying section.
will be helpful to compare the scatter of Fig. 23 with the
scatter shown later for other eddy current or diverted flux In addition to detecting and analyzing surface seams, the
systems. This scatter is attributable to the fact that response system also detects scabs, gouges and other types of hot
is a function of the depth of the discontinuity as well as its rolled billet surface discontinuities. Discontinuities are
width, angle of entry and the amount of magnetic scale marked by a paint spray.
entrapment.
The BILSYS-1 test station is essentially a rotating, open
Eddy Current Systems That Rotate ended drum through which the billets pass longitudinally.
the Sensors Eddy current probe and discontinuity marking instruments
are mounted on the drum and rotated with it. Two sensor
Billet Inspection System or BILSYS-1 heads, mounted 180 degrees apart on the rotating drum, are
guided over the flat surfaces and around the corners by
The billet inspection system discussed below is unique in highly flexible, specially designed followers (see Fig. 26).
its ability to rotate eddy current sensors around both square Continuous mechanical contact is maintained between the
and round billets.12 This system is shown in Fig. 24 and is sensor head and the product surface without jerking or
referred to as the BILSYS-1. It belongs to a family of similar bouncing.
systems hy the same manufacturer.
Signal Channels and Search Probes
FIGURE 24. Typical BILSYS-1installation
Each search probe, mounted with its individual oscillator
GRIT BLAST within the rotating drum assembly, constitutes a separate sig-
nal channel. Two types of search probes are used. They are
identical in size but have different electrical characteristics
and operate at slightly different test frequencies to prevent
electrical crosstalk due to mutual coupling between adjacent
channels. All search probes have a winding resistance near

ULTRASONIC INSPECTION FIGURE 25. Sketch of pivoted support for
rotating drum assembly used in billet scanning
inspection system: guide rolls ride on
rectangular billets and position drum
concentrically with respect to cambered billets

A-FRAME

FROM LTVSTEEL CORPORATION. REPRINTED WITH PERMISSION. COUNTER
WEIGHT

BASE

FROM LTVSTEEL CORPORATION. REPRINTED WITH PERMISSION.

ELECTROMAGNETICTESTING I 223

615 Q but those for one signal channel have an inductance disturbing the relationship of the scanner head to the billet
of 20 mH and those for the other signal channel have an surface.
inductance of 24 mH. Two printed circuit oscillator boards
are mounted in each oiltight enclosure on the drum adja- Calibration of the electronic instrumentation is accom-
cent to the search heads. Tuning capacitors are connected in plished by moving the entire test drum assembly to one
parallel with the search unit coils. Each oscillator operates side, out of the line of billet travel. A billet sample, with
at a test frequency determined by the search probe induc- milled slots of specified depths (on the corners, near the
tance and its shunt capacitance. One channel operates near corners and on the flat surfaces), is swung into test position
32 kHz and the second near 28 kHz. by a calibration arm. The test drum is then rotated as in nor-
mal operation· and the signals are recorded. Manual dial
Multiple sensor heads can be mounted on the drum for adjustments provide the desired sensitivity for analysis of
high production rates and to ensure discontinuity detection production discontinuities. After these adjustments are
within the minimum tolerable length. A series of slip rings completed, the drum assembly is returned to its in-line test
around the circumference of the drum connect the electri- position.
cal circuit to the rotating probes.
Electronic Instrumentation
The drum is equipped with a universal pneumatic
mounting arrangement, which permits the entire inspection Electronic instrumentation for the BILSYS-1 is very
assembly to follow twists and bends in the billets without similar to that of the BIS-1 described earlier: it consists
essentially of resonant loss sensing circuits as the primary
FIGURE 26. Search probeassembly for detection circuits. Although no performance data are avail-
scanningrectangularbillets, includingcorners: able for the BILSYS-1, inspection results should be similar
assembly resemblesa rollerchain with eddy to those shown for the BIS-1.
currentprobecoils in place of centerroller

Tests at Elevated Temperatures

SEARCH PROBE Inspection of hot steel products includes continuous butt
FROM LTV STEELCORPORATION. REPRINTEDWITH PERMISSION. welded pipe, rods and bars, electric resistance welded pipe,
seamless pipe and caster billets for in-process quality con-
trol by the eddy current method.13,14Inspection speed
ranges from 0.06 m-s! (12 ft-mirr+) for caster billets to
30 m-s! (6,000 ft.min ") for hot rolled rods. Product tem-
perature is roughly 1,400 K (1,100 °C or 2,000 °F). Various
discontinuities detected on billets, bars, rods and tubular
products are described below. The importance of sounding
alarms on pulses per unit time, as well as on amplitude, is
stressed.

On-line nondestructive inspection of hot steel products
for in-process quality control has been the objective of
research and development efforts for a number of years.
The cost savings in terms of labor and energy conservation,
reduced scrap losses, and increased yields are evident.

Of the available nondestructive testing methods, eddy
current techniques have distinct advantages in that they are
noncontact, can be operated at thousands of feet per minute
and are capable of high signal-to-noise ratios in mill envi-
ronments. In the case of ferromagnetic materials, operation
above the Curie temperature, about 1,050 K (800 °C or
1,500 °F), suppresses troublesome permeability variations.
Although progress has been made in ultrasonic, electromag-
netic, acoustic and radiographic inspection of hot products,
the eddy current method in the 1990s is the most adaptable

224 I NONDESTRUCTIVETESTINGOVERVIEW

to in-line monitoring of steel products, particularly those Eddy Current Detector
made by continuous processes.
The detector circuit for the transducers takes the trans-
The discussion below emphasizes in-line eddy current ducer output and amplifies it, filters it, compares it in ampli-
inspection of hot steel products with high temperature, tude and/or phase with a reference voltage and operates
water cooled eddy current transducers and commercially recording and marking equipment. Discontinuities are
available instrumentation. The purpose is to show the simi- recorded as sharp pulses projecting above many smaller
larities and differences encountered when testing a wide deflections called background noise. The width of these
pulses depends on the discontinuity size, product speed and
range of hot steel products. width of the transducer windings, as shown in Figs. 29 and 30.
The depth of eddy current penetration 8 is related to
FIGURE27. Patentedwater coolededdy
permeability µ, frequencyf and conductivity er, as shown in currentencirclingtransducer

Eq. 7 (see also Eq. 1):

8= 1 (Eq. 7)
~nµfer

For steel above the Curie temperature, both µ and er AMPHENOL CONNECTOR END PLATE
decrease and 8 must therefore increase. The increased eddy
current penetration and the loss of large permeability varia- MACHINE SCREW ~
tions in hot steel make this method of inspection especially CONNECTOR BASE ~
attractive for steel products.
PLUG~
Eddy currents are induced in steel by the action of alter- MACHINE SCREW
nating currents passing through one or more primary wind- COIL LEAD CONDUIT ~
ings. The effects of eddy currents within the product are
measured by reflected changes in electrical impedance, BOBBIN SL£EVE // LEAD WASHER
either in the primary windings or in separate secondary WATER JACKET
windings. For good sensitivity to these changes the trans- STUDS AND NUTS
ducer windings should be relatively close to the product, END PLATE
typically 3 mm (0.12 in.). For inspection of hot rolled prod-
ucts at 1,400 K (1,100 °C or 2,000 °F), this poses some diffi-

cult transducer design problems.

Encircling Transducers FROM MECHANICAL WORKING AND STEEL PROCESSING. REPRINTED
WITH PERMISSION.
A number of considerations are important in transducer
design for the steel industry: conducted and radiated heat; FIGURE28. Water cooled probetransducer
scale on the product; flared ends or end welds on tubular mounted in carriage
products; and hooked or upset ends on rods and bars. The
transducer must be cooled and this is most easily done with
water, remembering that all electrical connections must be
encapsulated against condensation and water leaks. Fig-
ure 27 shows a water cooled eddy current encircling trans-
ducer developed and patented15 for hot product inspection.

Probe Transducers FROM MECHANICAL WORKING AND STEEL PROCESSING. REPRINTED
WITH PERMISSION.
Figure 28 shows a water cooled probe transducer for
scanning flat or nearly flat surfaces. The probe transducer
generally has to contend with less radiant heat energy than
the encircling transducer and it absorbs less heat energy by
conduction. However, because the probe is generally oper-
ated even closer to the product than the encircling coil, the
radiant heat intensity is higher at the probe.

ELECTROMAGNETICTESTING I 225

Figure 29 shows the filter output voltage (x-component, FIGURE29. Pulsewidth WP as functionof
y-component or amplitude only) as a function of the position discontinuitysizeand positionin transducer
of the leading edge of a discontinuity relative to the position
of the transducer. For a short discontinuity such as a pinhole uuuuSECONDARY uuPRllvtARY SECONDARY
the voltage peaks at about the center of each transducer sec-
ondary winding, first in one polarity and then in the other. uu~
The width of one voltage pulse is shown as WP. For a long
uniform discontinuity, such as a notch, the pulse width is TRANSDUCER WINDINGS
twice that for the pinhole. A variety of transducer winding LONG DISCONTINUITY
lengths and inspection speeds results in a family of curves
(Fig. 30) of pulse width versus product speed, with a band of SHORT DISCONTINUITY
possible responses caused by discontinuities ranging in size
from pinholes to long discontinuities. POSITION OF DISCONTINUITY'S LEADING EDGE
IN TRANSDUCER
Noise deflections are caused by a variety of inconse-
quential variables in the product and equipment and in the FIGURE30. Pulsewidth as functionof
mechanical and electrical variations attendant with the test. discontinuitysize, winding length and product
The signal-to-noise ratio (SIN) is a measure of the sensitivity speed
of the test to discontinuities.
100
At a given sensitivity setting any system chosen will have
a certain probability P0 of detecting a real discontinuity but I 90
it will also have a probability P; of detecting a spurious sig- f- 80
nal.16 Both P0 and P, must be optimized for any inspection. 70
30 vi" 60
Eddy Current Testing of Rotating Cast Round Billets ~UJU§ 50
40
Round billets for seamless pipes and tubes are fre- ::) u 30
quently made from continuous cast blooms. The blooms are Cl..-~ 20
rolled to rounds and the hot rounds are inspected by an 10
eddy current testing method to detect surface discontinu- ~U1::S-J-::=:E-:
ities. A round billet rolled from a continuous cast bloom is ~llJ
transferred to the inspection machine where it is rotated by
a set of rollers and scanned by an inspection head moving 0
straight down its length. When discontinuity depths are
more than 2 mm (0.08 in.), the system can detect surface 01 23 456
cracks with a high signal-to-noise ratio. The system uses a fO) f 1) f2) f3) f4) f5) (6) f?) f8) f9) fl O)f 1 I )f 12)
multifrequency method to distinguish a surface discontinu-
ity from a noise signal caused by liftoff ( ovality). PIPE SPEED
meters per second
Figure 31 shows inspection results of the multifrequency
method compared with the single frequency method. By ( 102 feet per minute)
using the multifrequency method, signals of machined
notches can be clearly distinguished from machined ovality LEGEND
signals. 3 mm fO. 12 in.) WINDING LENGTH
IO mm f0.40 in.) WINDING LENGTH
Water and Air Cooled Rotating Probe System 25 mm fl in.) WINDING LENGTH

A second rotating probe system for hot square billets and
slabs is similar to the BILSYS-2. It uses three rotating disks
containing several probes each and one stationary probe per
comer.17 A sketch of the system is shown in Fig. 32. The
paths that the rotating probes take as the flat surface is
scanned are shown in Fig. 33. Both transverse and longitu-
dinal cracks would be crossed by the probes at a 90 degree
angle.

226 I NONDESTRUCTIVE TESTING OVERVIEW

FIGURE 31 . Cathode ray tube displays of In laboratory tests, the probes have been operated at a
multifrequency tests of hot round billets: probe-to-metal distance of 2 to 10 mm (0.08 to 0.4 in.).
(a) single frequency method ( 128 kHz); Sh_orter . distances yield higher sensitivity. Each probe is
(bJ multifrequency method (128 and 256 kHz); dnven simultaneously at two frequencies and the response
(cJ test object for each frequency provides four pieces of information from

faJ FIGURE 32. Rotating probe eddy current
inspection system for hot slabs

fbJ NOTCH DEPTH FROM AB SANDVIK BERGSTRAND. REPRINTED WITH PERMISSION.
2.1 mm (0.08 in.)
FIGURE 33. Scanning pattern from three
2.7 mm (0.1 in.) NOTCH DEPTH spinning disks when moved over surface

fcJ FROM AB SANDVIK BERGSTRAND. REPRINTED WITH PERMISSION.

MACHINED OVAL SHAPE

LEGEND
I 1.77 mm (0.07 in.) NOTCH DEPTH
2 1.77 mm (0.07 in.) NOTCH DEPTH
3. 2. I mm (0.08 in.) NOTCH DEPTH

2.7 mm (0.1 in.) NOTCH DEPTH
2.7 mm (0.1 in.) NOTCH DEPTH

ELECTROMAGNETICTESTING I 227

t It(' impedance plane: amplitude R; phase angle p; width of FIGURE34. Impedanceplane information
tl1c signal lobe D; and time T required for the lobe to go processedby rotating probeeddy current
[rom its maximum to half its maximum point. These quanti- instrument
t ics are shown in Fig. 34. The resulting eight pieces of infor-
mation are analyzed by a microcomputer and the surface FROM AB SANDVIK BERGSTRAND. REPRINTEDWITH PERMISSION.
discontinuity is classified into one of three categories: seri-
<111s, harmless or unknown. Position is calculated for the

xoriousand unknown discontinuities and these are displayed
graphically in color on a cathode ray tube (CRT). Signals
Irorn the harmless discontinuities are not processed. The
position and depth information can be fed to a scarfing or
grinding machine or to an X, Y recorder. The signals from
unknown discontinuities are stored in memory for later
evaluation.

Although it is generally possible to inspect hot steel
products with an accuracy equal to or better than that
obtained at room temperature, care must be taken that the
range of product temperatures does not pass through the
Curie temperature where permeability variations generate
noise. Direct current saturation could be used to avoid this

problem - but at some added expense and complexity.

228 I NONDESTRUCTIVE TESTING OVERVIEW

PART 5

EDDY CURRENT INSPECTION OF BOLT
HOLES

Eddy Current Bolt Hole Inspection areas, a crack of this magnitude may continue to grow to
failure at a rapid rate.
Eddy current bolt hole inspection is a reliable way to
detect cracks in material adjacent to fastener holes. Recent Finding small cracks at fastener holes sometimes
developments in automated bolt hole scanners provide a requires removing the fastener and performing an eddy cur-
method for obtaining reliable and repeatable test results. rent check. Autoscan techniques-" may also be used to
The use of eddy current testing techniques for detecting detect cracks as small as 0.1 mm (0.004 in.). Figure 3,5 illus-
small fatigue cracks, particularly in aluminum fastener trates an eddy current bolt hole probe. The probe coil axis is
holes, is well known.lf:U\l Small cracks in material adjacent to perpendicular to the material adjacent to the hole. An
fastener holes may go undetected until they grow to a size adjustable collar is used to position and locate the coil a
that allows detection at a surface not covered by the head of desired distance inside the hole. Usually, inspection is
the fastener or nut. Unfortunately, in many highly stressed started with the coil just below the test material surface and
the probe is manually rotated 360 degrees. After each rota-
tion, the probe is advanced about 1..5 mm (0.06 in.) so that

FIGURE 35. Eddy current hole probe

COIL ASSEMBLY

COIL WIRES SEALED IN BODY PROBE WITH EPOXY CEMENT

ADJUSTABLE COLLAR INSULATING SLEEVE
SETSCREW SOLDER COIL WIRE TO
MICRODOT TERMINAL
SEAL CONNECTOR AND SHIELD IN
PROBE BODY WITH EPOXY CEMENT MICRODOT CONNECTOR
(FEMALE TYPE 3102 OR EQUIVALENT)
SOLDER COIL WIRE TO SHIELD
SHIELD
FROM DOUGLA.S AIRCRAFT COMPANY. REPRINTED WITH PERMISSION.

ELECTROMAGNETICTESTING I 229

the depth of the fastener hole is scanned at 1.5 mm single-loop crack response for an absolute bolt hole probe
inserted in an aluminum sample is shown in Fig. 36a. Fig-
(0.06 in.) increments. The spherical end of the probe is ure 36b shows the typical double-loop response obtained
often split in the middle so that a small rubber or plastic V when using a differential probe.21
wedge may be inserted to make the probe snug in oversized
holes. These wedges are not generally used for probes less Reference Standards for Bolt Hofe
than 6 mm (0.25 in.) diameter. Inspection

During bolt hole inspection the absolute method is gen- Calibration standards aid in adjusting instrument controls
erally used. The absolute method uses one coil on a ferrite and are used to ensure that cracks will be detected with a
core; measurements are made while the material is in direct predetermined sensitivity based on depth, length and loca-
contact with the probe coil. The differential method uses tion (see Fig. 37). Standards have been fabricated by simu-
two coils. This permits a comparison of measurements lating cracks with electric discharge machined (EDM) radial
obtained from the (uncracked) material under one coil with slots of various depths and lengths (see Fig. 38). This type of
the (cracked) material under the other coil. A bridge circuit standard works well if the notches are less than 0.1 mm
is unbalanced if one coil is located over sound material and (0.004 in.) wide. However, these simulated flaws do not
the second coil is located over a discontinuity. The typical respond exactly the same as real fatigue cracks.19 Some
investigators have produced cracks by introducing an elec-
FIGURE 36. Normalizedimpedance graph tron discharge machined slot in the hole and then subjecting
showingcrack responsesfor absoluteand the specimen to fatigue loading. A crack initiates and grows
differentialprobes: (a) absoluteprobecrack outward from the electric discharge machined slot. The
response; (b) differentialprobecrack response21 depth and length of the crack are qualitatively controlled by

*(aJ CRACK FIGURE 37. Typical locationof cracks adjacent
SCAN to fastenerholes

COIL TOP VIEW

x

N~ --0
ALUMINUM

100 kHz

R

co,,t(bl CRACK
SCAN

x

500 kHz

R FROM DOUGlAS AIRCRAFT COMPANY. REPRINTED WITH PERMISSION.

230 I NONDESTRUCTIVE TESTING OVERVIEW

FIGURE 38. Calibration block for bolt hole and the stress and the number of cycles. After the crack has
grown to the approximate size, the electric discharge
surface probe mspections/? machined slot is removed by drilling the hole to a sufficient
predetermined oversize. The depth of the resulting crack
CALIBRATION SLOTS FOR can only be estimated. The true depth can be determined
HOLE PROBE only by fracturing the standard.

CALIBRATION SLOTS FOR Procedure for Bolt Hole Inspection
SURFACE PROBE
FROM DOUGLAS AIRCRAFT COMPANY. REPRINTED WITH PERMISSION. A typical procedure for performing eddy current bolt
hole inspection is listed below.

1. Determine diameter and depth of holes to be
inspected from engineering drawing or other applica-
ble document.

2. Select proper size probe and reference standard.

FIGURE 39. Cathode ray tube displays and detection probe: (aJ amplitude/time (Y-TJ cathode ray tube
presentation; (bJ lissajous phase analysis presentation of X,Y components on impedance plane;
(cJ hand held automatic probe scanner

(cJ

i11.•~I·.1

Lifl

(bJ

FROM ROTOTEST, INCORPORATED. REPRINTED WITH PERMISSION.

ELECTROMAGNETIC TESTING I 231

3. Calibrate instrument response to reference standard. structural fastener holes and it reduces the errors of conven-
4. Visually check holes for excess corrosion or galling. tional hand scanning methods. It has been reported that the
system can detect cracks as small as 0.25 mm (0.01 in.) long
Remove material from hole by manually passing a by 0.1 mm (0.004 in.) deep in aluminum.P" Scan depths are
reamer through the hole or drilling out to next over- adjustable from 6 to 40 mm (0.25 to 1.5 in.), using a thumb-
size diameter. wheel on the top housing of the hand held scanner. The
5. Scan for discontinuities by rotating the probe at con- variable speed (Oto 150 rpm) scanner provides simultane-
stant depths, at intervals not exceeding 1.5 mm ous linear and rotational (spiral) motion. Either filtered or
(0.06 in.) in depth. Conduct scanning in approxi- unfiltered signals from the scanner can be displayed on a
mately one-revolution intervals. If suspect areas are chart recording or oscilloscope. The analog recorder pro-
noted near the ends of scans, retest these areas so vides a detailed profile of discontinuities for analysis. The
that the suspect areas are located in the middle por- system is fully portable and all equipment is contained in a
tions of the second scans. protective case.

Fatigue crack depths may be estimated from the ampli- Other rotating eddy current inspection systems are avail-
tudes of eddy current readings. When observing meter able for rapidly detecting small cracks in aircraft bolt holes.
deflections, make sure the deflection is not due to scratches, They use small, light, hand held scanners (Fig. 39c) and var-
surface irregularities, hole out-of-roundness or probe tilting. ious sizes of bolt hole probes. These instruments are simpli-
fied, easy to use and portable. They generally operate at
Bolt Hole Probes 500 kHz and use differential probes rotating at 50 s-1
(3,000 rotations per minute [rpm]).
Bolt hole probes are generally available from eddy cur-
rent equipment manufacturers. It must be remembered The resolution of the differential scanning coil system
that different types of probes are used for inspecting follows the physical laws for general eddy current testing;
straight or tapered holes and that different collars are used when the probe moves on an orthogonal path over a crack,
for flat, countersunk or curved outer surfaces. the crack depth is the principal factor affecting the ampli-
tude, provided its length is equal to or greater than the coil
AutomatedBolt Hole Inspection diameter.

When numerous bolt holes need to be inspected, it is The main application is rapid inspection of fastener holes
often desirable to eliminate manual scanning and use a for cracks. The high revolution speed of 50 s-1 (3,000 rpm)
device that automatically rotates the probe in the hole. of the rotating probe and the clear display on the screen
Eddy current equipment manufacturers have been develop- permit a high inspection sensitivity. The saving of time com-
ing such systems. The rotating probe is attached to the end pared to the hand held method is about 80 percent. In prin-
of a relatively short spindle, which is rotated in a hand held, ciple, all metals can be tested, including titanium, aluminum
rotating head. and steel. The detection sensitivity depends on the rough-
ness of the surface and begins at approximately 0.2 mm
The United States Air Force has also been actively (0.008 in.) depth. Two different kinds of display may be
engaged in bolt hole inspections. The initial development used:
from these investigations was the automatic eddy current
discontinuity detection system. The system detects and 1. Y­t with one sweep per revolution (Fig. 39a);
records discontinuities on conductive metal surfaces and in 2. X,Y vector for differentiation of the signal, i.e.,

complex impedance plane display (Fig. 39b).

The lightness and small size of the detection probe (Fig. 39c)
allowsthe instrument to be used in confined areas.

232 I NONDESTRUCTIVETESTING OVERVIEW

PART 6

AUTOMOTIVE APPLICATIONS OF EDDY
CURRENT TESTING

Eddy current tests are the most frequently used nonde- be in the range from 0.2 to 9 mm (0.008 to 0.35 in.) so that
structive tests in the automotive manufacturing industry. optimum test parameter values may differ significantly
Primary among their valuable characteristics is the speed between applications.
with which tests can be performed; this makes them suitable
for automatic testing of high production quantities. Material composition monitoring is performed on a lim-
ited number of components. Broad application to standard
Eddy current tests are applied to automotive compo- ferrous materials used in automotive components is gener-
nents in the full range of the method's capabilities: crack ally not possible. The reason is that the chemical composi-
detection, material composition monitoring, material struc- tion limits permit such a range of values that magnetic
ture monitoring (such as after heat treatment), and film characteristics overlap in the same manner as the composi-
thickness measurement. Although ferrous metals dominate tion values. Thus, an eddy current test for composition is
the applications, nonferrous metals, mostly aluminum, are usually applied to components in a plant where other steels
also included. of the same size are present for other components, and a
mix of these materials must be avoided. Specific test param-
Table 1 lists representative automotive components eters can often be established to discriminate between a
tested in production by eddy current techniques. Note that limited number of steels.
many components are subjected to more than one eddy cur-
rent test because more than one characteristic is monitored. Film thickness measurements are a conventional applica-
tion of commercial eddy current instruments for the mea-
A nondestructive test may be the key to implementation surement of prime and color paint films on metals, primarily
of a new production process and may then become unneces- steel automobile and truck bodies.
sary when the process has been fully developed. Other tests
can remain in service even though years go by without a part Hardness and Case Depth
being rejected. Inspection of Axle Shafts

Crack detection techniques listed in Table 1 include fer- A system containing a computer controlled multifre-
rous as well as nonferrous components. Some applications quency eddy current instrument is used to test cold
are simple, such as when a crack is gross, as is sometimes the extruded axle shafts for case depths ranging from zero up to
case with castings. Other applications, such as tests of spring as much as 9 mm (0.35 in.) in four zones (spline, midshaft,
wire, push the technology to its sensitivity limit. bearing and fillet, as shown in Fig. 40) and surface hardness
in two zones (button and bearing). The system in Fig. 41
Hardness determination is the longest list in Table 1. uses two absolute dual winding coils. One coil rests on the
Hardness of metals is important in many components button while the other traverses the shaft. At each measure-
because it is related not only to wear characteristics but also ment zone, the appropriate coil is excited at sequentially
to fatigue strength. Conventional hardness measurement applied frequencies ranging from 5 to 10 kHz, depending
methods, such as Rockwell or Brinell, are not only slow rel- on location. The maximum empty coil field strength is about
ative to eddy current methods but also are often destructive 7 mT (7 x 10-3 Wb,m-2 or 70 gauss root mean square). A
to the test object. Because permissible variations in compo- desktop computer is used to control the scanner and eddy
sition affect magnetic properties, hardness measurement by current instrument, to acquire and analyze data and store
eddy current methods is not always as precise as necessary, the results.
especially with cast iron components. In such cases, eddy
current techniques are useful for ensuring that a certain The equipment is calibrated by making measurements
proportion of the components are of acceptable hardness, on a master shaft, then storing the results on magnetic tape.
thus significantly reducing the number of components that All test results are normalized to the responses for the mas-
must be tested by potentially destructive methods. ter. The computer uses the responses at certain frequencies
to estimate the case depth. Then, on the basis of the esti-
Case depth (depth of hardening) of a ferrous component mate, it selects algorithms to make a final calculation of case
is often paired with hardness in eddy current tests because
the combination gives the best assurance of both durability
and strength of the component. Gears and shafts of various
kinds are the general category of components that are tested
by eddy current techniques. Depths that are monitored may