NDT - By Barry Hull and Vernon John

134 NON-DESTRUCTIVE TESTING
60

50

t 40

i::i::l.

.«~"5~Q..Ec9)~: 30

20

10

Number of cycles -----

FIGURE 7.6 Acoustic emission trace from a low alloy steel during fatigue.

failure occurs. Acoustic emISSIOn inspection can also be employed to monitor
stress corrosion and corrosion fatigue failures. One interesting application is the
recording of welding 'signatures', since the 'noise' of a welding process is a sum of
a11 associated phase transformations, thermal expansion and contraction, and the
formation of cracks and cavities within the welded zone.

7.6 Crack depth gauges

As mentioned in chapters 2 and 3, cracks which appear at the surface of a material
can be readily detected using liquid penetrant or magnetic partic1e inspection
methods, but neither of these methods will give an accurate assessment of the
depth of a crack. Crack depth gauges are frequently used in conjunction with
these other non-destructive tests to give a measure of the depth of flaws which
have been located. One simple but effective device for this consists of two c10sely
spaced electrical contacts. The gauge is placed on the surface of the material and
the electrical resistance between the two contact points measured. When the

OTHER NON-DESTRUCTIVE INSPECTION TECHNIQUES 135

gauge is placed on the testpiece with the contacts on either side of a surface crack,
the measured resistance will be greater as current now has to follow an extended
path around the crack. The meter scale is generally calibrated to give a direct
reading of crack depth.

7.7 Thermography

Thermography me ans the mapping of isotherms, or contours of equal temperature,
over the surface of a component. Reat-sensing materials or devices can be used to
detect irregularities in temperature contours and such irregularities can be related
to defects. Thermography is particularly suited to the inspection of laminates. The
conduction of heat through a laminate will be affected by the presence of flaws in
the structure, resulting in an irregular surface temperature profile. Typical flaws
which can be detected are unbonded areas, crushed cells, separation of the core
from the face plates and the presence of moisture in the cells of honeycomb
structures.

Thermographic methods may either be of the direct contact type, in whlch
heat-sensitive material is in contact with the component surface, or indirect
contact, in which a he at-sensitive device is used to measure the intensity of infra-
red energy emitted from the surface.

Pulses of heat energy, from a source , are directed at the component under test.
It is usual, but not essential, to direct the incident energy on to one surface of a
component and observe the effects at the opposite surface after conduction
through the material. Flaws and irregularities in structure will affect the amount
of conduction in their vicinity. If it is impossible to have access to both surfaces,
the technique can still be used. The heat energy incident on the surface will be
conducted away through the material at differing rates, depending on whether or
not flaws are present.

Direct contact methods include the use of heat-sensitive paints and thermally
quenched phosphors. Indirect contact methods, which offer greater sensitivity,
involve the use of infra-red imaging systems with a TV·video output.

Heat-sensitive paints

Reat-sensitive photo-chromic paints are effective over a temperature range from
about 40°C to 160°C. Some paints show several colour changes within their
reaction temperature range and, with careful application, will have a sensitivity
of the order of ±5°C. When heat reaches the painted surface by conduction
through the material the paint colour changes, usually with a bleaching effect.
Where a flaw impedes conduction the colour will be unchanged. On the other
hand, if heat energy is directed at the painted surface the reverse effect will show
up as heat is conducted away from the surface more rapidly through good regions
than through defective areas.

136 NON-DESTRUCTIVE TESTING
Thermally quenched phosphors

These are organic compounds which emit visible light when excited by ultra-violet
radiation. The brightness of the emission decreases as the temperature of the
compounds increases. Phosphors are available that are useful at temperatures up
to about 400°C and with aresolution of ±1°C.

Thermal pulse video thermography

In this system no physical contact is necessary with the material and very rapid
rates of inspection are possible. A high-intensity heating source is used to send
pulses of infra-red energy into the material. The surface is scanned by an infra-red
thermal imager with a TV-video output. This system, again, can either be used for
sensing heat transmitted through the component or for single-sided inspection
when only one surface IS accessible. Very good sensitivities are possible. Digitised
image processing to provide image enhancement is also possible.

7.8 Surface texture analysis

Arecent development of particular interest is a capacitance-based system for the
rapid measurement of the surface finish or texture of engineering components.
This parameter is of importance to the function and service life of components,
such as engine valves, compressor wheels, turbine blades, bearing cages and extru-
sion dies.

The 'roughness' of a surface is a function of the 'air space' between the peaks
and valleys of the surface. Air is an electrical insulator and can act as the dielectric
of a capacitor. The electrical capacitance of a capacitor, C, changes as the amount
or average height, t, of dielectric between the plates is changed. If a surface to be
measured is made, temporarily, into a capacitor by placing an electrode plate on
t~e surface, the air space acts as the capacitor dielectric. Hence, the resultant
capacitance depends on the roughness of the surface finish. Figure 7.7 illustrates
the principle of operation. Provided that the probe has been calibrated on reference
specimens of known surface roughness assessed by stylus measurement, the
system permits accurate determination of surface roughness, in either micro-
inches or microns. The result is displayed as a digital read-out on the ancillary
instrumentation.

7.9 Multi-phase flow analysis

A major parameter in the design and use of safe, efficient and economic oil and
gas pipelines is the prediction of multi-phase f10w behaviour of the transferred
medium. A multi-path ultrasonic gas f10wmeter has been designed and developed
by British Gas to meet this requirement. The equipment has a variety of applica-

OTHER NON-DESTRUCTIVE INSPECTION TECHNIQUES 137

A

C C=K ~

t

~:~:,::::~
Sensor pad
Workpiece

FIGURE 7.7 Principle of operation of capacitance probe.

tion areas, such as shore terminals, transfer and compressor stations, and under-
ground storage sites. The principle of operation of the flowmeter is based on
ultrasonic pulse transit time within the flowing medium. A simple twin trans-
ducer set up is shown in figure 7.8.

B
C

A

FIGURE 7.8 Schematic diagram of an ultrasonic flowrneter in operation.

138 NON-DESTRUCTIVE TESTING
The transit time, t, for ultrasonic pulses to travel between a transmitter and

receiver is measured with a high-frequency, electronic c1ock. A pulse travelling
from Tl to T2 is increased in velocity by the resolved component of the gas/oll

flow along its path. Hence, its mean velocity is (V + c(A/B)) where V is the
velocity of sound and c is the mean flow velocity of the medium. A and Bare the
dimensions as shown in figure 7.8. The time taken for a pulse to travel from Tl to

T2 , t 1 , is given by
B

V+ c(A/B)
or

V+ c(A/B)
t1 B
Similarly, a pulse travelling in the opposite direction, that is, from T2 to Tl, is
slowed by the resolved component of the medium flow rate and takes time t2'
Hence

Subtracting

therefore

where k1 = 2BA2 (a constant), or

c=

The volume flow rate Q = cx, where x is the cross-sectional area of the pipe.
Therefore

where k 2 = k 1 x (a constant).
A complete flowmeter system comprises four pairs of transducers, eight trans-

ducers in total, which are aligned to determine the flow rates in the upper, central
and lower parts of a pipeline. In addition, the system accepts temperature, pres-
sure, density and calorific value inputs, and hence volume flowrates, mass flow
rates and heat energy flow rates can be obtained.

OTHER NON-DESTRUCTIVE INSPECTION TECHNIQUES 139

7.10 Conc1usions

The techniques mentioned in sections 7.2 to 7.8 above are aselection of some of
the more recent developments in non-destructive inspection. It will be apparent
that within the realm of inspection there is continual progress, with existing
techniques being improved and refined and their range of applications extended,
and new processes being created to meet increasingly demanding inspection
requirements.

It would probably be true to say that wherever an inspection problem exists a
solution can be found. The solution may be obtained by adapting some existing
technique or, in the longer term, by developing a new inspection process.

Bibliography

BooksandHandbooks

ASM Metals Handbook, Vo1.11, Non-destruetive Inspeetion and Qua/ity Contral,
American Society of Metals.

J. C. Drury, Ultrasonie Flaw Deteetion for Teehnicians, The Unit Inspection Co.
Ltd.

R. Halmshaw (Editor), Mathematies and Formulae in NDT, British Institute of
Non-destructive Testing.

Handbook of Radiographie Apparatus and Teehniques, International Institute of
Welding.

Handbook on the Ultrasonie Examination of Welds, International Institute of
Welding.

Industrial Radiography, Agfa-Gevaert Ltd.
Industrial Radiography, Kodak Ltd.
Krautkramer Booklet, Krautkramer GMBH, Cologne.
M. G. Silk, Ultrasonie Transdueers for Non-destruetive Testing, Adam Hilger Ltd.
J. L. Taylor, Basic Metallurgy for Non-destruetive Testing, British Institute of

Non-destructive Testing.
Ultrasonie Non-destruetive Testing, Baugh and Weedon Ltd.

British Standards

BS 2600, Radiographie examination of fusion welded butt joints in steel.
BS 2704, Specifieation for ealibration blocks for use in ultrasonie flaw deteetion.
BS 2737, Terminology ofinternal defeets in castings as revealed by radiography.
BS 2910, Method for radiographie examination of fusion welded cireumferential

butt joints in steel pipes.
BS 3683 Parts 1-5, Glossary of terms used in non-destruetive testing.
BS 3889, Methods for non-destruetive testing ofpipes and tubes.

140

BIBLIOGRAPHY 141

BS 3923, Methods tor ultrasonic examination o[ welds.
BS 3971, Speci[ication tor image quality indicators tor industrial radiography.
BS 4069, Specijication tor magnetic j7.aw detection inks and powders.
BS 4094, Recommendation tor data on shielding [rom ionising radiation.
BS 4124, Part 1,Methods tor non-destructive testing o[ steel [orgings - Ultrasonic

j7.aw detection.
BS 4331, Methods tor assessing the performance characteristics o[ ultrasonic j7.aw

detection equipment.
BS 4408, Part 3, Gamma radiography o[ concrete.
BS 4489, Method tor measurement o[ UV-A radiation (black light) used in non-

destructive testing.
BS 5044, Speci[ication tor contrast aid paints used in magnetic particle j7.aw

detection.
BS 5138. Speci[ication tor magnetic particle j7.aw inspection o[[inished machined

solid [orged and drop stamped cranksha[ts.
BS 5411, Parts 1, 2, 3 & 11, Methods o[ test tor metallic and related coatings.
BS 5650, Specijication tor apparatus tor gamma radiography.
BS 5996, Methods tor ultrasonic testing and speci[ying quality grades o[ [erritic

steel plates.
BS 60n,Method tor magnetic particle j7.aw detection.
BS 6208, Methods tor ultrasonic testing and tor speci[ying quality levels o[[erritic

steel castings.
BS 6443 ,Method tor penetrant j7.aw detection.
PD 6513, Magnetic particle j7.aw detection - A guide to the principles and practice

o[ applying magnetic particle j7.aw detection.
Aero M34, Method o[preparation and use o[radiographic techniques.
Aero M36, Method tor ultrasonic testing o[ special forgings by an immersion

technique.
Aero M39,Method for penetrant inspection of aerospace products.
Aero M40, Methods for measuring coating thickness by non-destructive testing.
Aero M42, Methods for non-destructive testing of fusion and resistance welds in

thin gauge material.

Note: All the above standards publications are available from the British Standards
Institution, Linford Wood, Milton Keynes, MK14 6LE.

Index

'A' scan display 68 coil arrangements 45-6 23
absorption equivalence 129-30 coil impedance 33, 37
acoustic emission inspection 132-4 Compton scattering 104
acoustic impedance 62 conductivi ty 35
angle probe 67-8 continuous method for magnetic testing
coupling agent 62
calibration of 83- 4 crack depth measurement 131-2, 134
re fl ecti on method 72 critical angle 64
transmission method 71 curie 102
application of penetrant 8, 13
applications of dead zone 60-1, 83
eddy current inspection 54-6 defects, origins of 3
magneti c particle inspecti on 27--31 demagnetisation 24-5
neutron radiography 130 development, film 108
penetrant inspection 15-17 development, penetrant 8, 13-14
radiography 119-23 diffraction, ume-of-flight 131-2
ultrasonics 86-9 distance-amplitude blocks 80
area-ampli tude blocks 80-1 dosage rate 123
atomic absorption coefficient 103 dosemeter 124
attenuation of dry magnetic particles 18, 26
neutron beam 129--30 dye penetrant 7, 11
radiation 103-5
sound 65 eddy current testing 3, 32-55 89
attenuator 66 applications of 54-6

'B' scan display 69 edge effect 43
back scatter 113 elastic compression waves 57
barium titanate 59, 66 elastic surface waves 57
barn 103 electrical conductivity 35
beam angle, calibration of 82-3 electrical test methods 3, 32-55
bonding defects 76-8, 135 electro-magnetic acoustic transducers
bore probe 47-8 electro-magnetic spectrum 94
bremsstrahlung 98 electron volt 94
bridge circuit 49 ellipse method 52-3
eq ualisation for radiography 121
'C' scan display 69 exposure chart 112-13
caesium-137 101-2
calibration blocks for ultrasonics 80-5 far zone 60-1 126
coating thickness measurement 56 fibre optic systems
cobalt-60 101-2
142

INDEX 143

fill factor 40 magnetic particle inspection 3,18-31
film, radiographie 108 magnetic partieies 18,21, 26-7
fllm badge dosemeter 124 magnetic permeability 36
flow analysis 136-8 magnetic yoke 22, 27, 30
fl uorescent magnetie partieies 27 magnetisation 18-20
fluorescent penetrant 7
fluorescent screens 113 methods of 18, 21-3
fl uoro-metallic screens 114 measurement of radiation 124
fluoroscopy 90, 92, 109-10 multi-phase flow analysis 136-8
freq uencies of ultrasound 57 near zone 60-1
neutron capture 129
gamma h) radiation sources 101-3 neutron radiography 90, 92, 128-30
gamma h) rays 90,93-4 neutron scatter 129
generation of ultrasound 59 normal ultrasonic probes 67

half-li fe period 102 108 reflection method 70
heat-sensitive paints 135 transmission method 70-1
high-defini tion radiography optieal inspection probes 1, 126-8
hole probe 47-8 pair prod uction 104
pancake coil 33, 45
IACS 35 panoramic techniq ue 120
identification markers 114-15 paper radiography 90, 108
identification of defects using ultrasonics penetrameters see image quality indieators
penetrant
73-8 application 8,13
IIW test block 81 development 8, 13-14
image quali ty indicators 114-18, Plates inspection 7-17
removal 8
3 and 8 tests 3
immersion testing 78-9 penetrating ability
impedance 33, 37 of gamma h) radiation 102
of X-radiation 100-1
plane 38 permeability, magnetie 37
plane diagrams 39, 41-2 phase analysis 51-3
triangle 37 photo-electric effect 104
inspection probes, optical 126-8 piezo-electric
intensifying screens 113-14 effect 59
intensity materials 59
of gamma h) radiation 101-3 transducers 59
of X-radiation 97 post-emulsification 11-12
interface effects on sound waves 62-4 protection against radiation 124
interpretation of radiographs 122 quality of inspection 3
ionisation dosemeter 124-5 quartz 59, 66
IQI see image quality indicators
iridium-l92 101-2

laminations in plate 74-5 rad 123
laser-induced ultrasonics 130-1 radiation
. lead intensifying screens 113
lead niobate 59, 66 attenuation 103-5
lead zirconate 59, 66 gauging 90
lift-off factor 40, 50 hazard 123
limit frequency 37-8 protection against 124
linear absorption coefficient 103 sources of 93- 4
liquid penetrant inspection 3, 7-17 radio-active decay 101-2
radiographie
advantages of 14 equivalence 105-6
limitations of 15-17 fllm 108
principles of 8 paper 108
lithium sulphate 66 screens 113-14

144 INDEX

radiographs 23 sources of radiation 93-4, 101-3
interpretation of 122 specific activity 102
viewing of 122-3 standard penetration depth 41-3
surface probe coil 46-7
radiography 3,90-125 surface waves 57, 64, 72
limitations of 91-2 swinging field technique 20
principles of 92-3 thermally quenched phosphors 136
uses of 91 thermography 135-6
thulium-170 101-2
radioisotope carnera 103 time-of-flight diffraction 131-2
Rayleigh scattering 104 transducers 59, 62, 66
Rayleigh waves 57, 58, 64, 68,72 transfer screens 130
reflecti on coefficient 62 transmission coefficient 62
reflection of sound 62- 4 transmission of ultrasound 59, 62--3
refraction of sound 63-4 transmitter probe 59, 65--6
relative magnetic permeability 37 ultrasonic beam characteristics 60
reliability of defect detecti on 4 ultrasonic display 65, 68-9
residual JIlethod for magnetic testing ultrasonic testing 3, 57-89
resistivity 35 ultrasonics, laser-induced 130-1
resonant circui t 49-50 ultrasound, generation of 59
rhm 102 ultra-violet light 7, 8-9, 13-14, 26
roentgen 97, 123 vector analysis 52-3
vector point 52
screens velocity of sound 57-8
for neutron radiography 130 viewing radiographs 122
use in gamma (')') radiography and water-washable penetrants 11
X-radiography 113-14 wavelengths of sound 58-9
weId inspection 71, 76, 119-20, 121, 123
search coil 33 wet magnetic particles 18, 21,23, 26
sensitivities xeroradiography 90, 109
X-ray
in magnetic particle inspection 23- 4
in radiography 118 intensi ty 97
in ultrasonics 79, 85 spectra 98-101
shear waves 57--8, 63-4, 68 tube 94-6
sievert 123 X-rays 90, 93-4
skin effect 41 generation of 94-8
skip distance 71 'Y-radiation SOUIces see gamma ('Y) radiation
Snell' s law 64
soft-face probes 89 sources
solenoid type coil 33, 45 'Y-rays see gamma ('Y) rays
solvent-removable penetrants 12
sound
nature of 57
velocities of 57--8
wavelengths of 58-9
sound waves 57
at interfaces 62- 4
reflection of 62-4
refraction of 63-4
transmission of 59, 62-3

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