various nondestructive testing techniques and their inspections on aircraft structures.

STUDY OF VARIOUS NON-DESTRUCTIVE
TESTING TECHNIQUE & APPLICATION OF
THESE TECHNIQUES TO INSPECT AIRCRAFT
WHEEL HUBS & AERO ENGINE FAN BLADES
& SUGGEST REMEDIAL MEASURES.
By
KARAN DUGGAL

Under the Guidance of
Professor O.P.CHAWLA

DEPARTMENT OF MECHANICAL ENGINEERING
INSTITUTE OF TECHNOLOGY & MANAGEMENT,
GURGAON, HARYANA
(AFFILIATED TO M.D. UNIVERSITY, ROHTAK,
HARYANA, MARCH 2005)

CERTIFICATE

This is to certify that the project entitled “Study of various non-destructive testing techniques and
application of these techniques to inspect aircraft wheel hubs & aircraft engine fan blades &
suggest remedial measures” is submitted by Karan Duggal as his final year major project the
work is based upon his work under the supervision of Dr. O.P. Chawla and neither his project
report nor any part of it has been submitted for any degree or any other academic award
anywhere before.

Professor O.P. Chawla
Department of Mechanical Engineering, ITM, Gurgaon.

ACKNOWLEDGEMENT

I would like to thank and express gratitude to my project guide Dr. O.P. Chawla for all his
assistance and guidance, which came forward at the advent of our difficulties and problems.
With his guidance & help and his positive approach towards furthering the problems, it would
not have been possible on my part to successfully complete this project.
I would like to express my gratitude to Sh. T.C. Sharma, Chief Manager Accessories Overhaul
Shop, Sh. R.K. Sharma, Chief Manager Jet Engine Overhaul Shop, Sh. S.N. Garg, Sh. R.K.
Arora & Sh. Vivek Inspection Engineers Accessories Overhaul & Jet Overhaul Shop, Indian
Airlines Northern Region, I.G.I. Airport Terminal II, N-Delhi. With their valuable support & on
the job practical guidance, I could succesfully complete this project. I would also like to thank all
the engineering staff of N.D.T. Section of Accessories Overhaul & Jet Overhaul Shops, Indian
Airlines, Northern Region, I.G.I Airport, Terminal II, N-Delhi for all their help and assistance
which helped me to complete this project successfully.
I also thank the college faculty and staff (Mechanical Engineering Department) for all their help
and valuable support for helping me to execute this project successfully.

ABSTRACT

The aim of the project is to study the various Non-destructive methods being used in the modern
manufacturing, overhaul & servicing industry. High Frequency Eddy Current N.D.T inspection
of A320 Airbus main wheel hub assemblies were carried out to identify the surface & subsurface
cracks in their critical areas. The problem areas were studied & remedial measures were
suggested to minimise such failures. Ultrasonic N.D.T inspection of the fan blades of M/s IAE
V2500 Turbo fan engine installed on A320 Airbus Aircraft were studied and problem area of fan
blade root flank area debonding & propagation of fine cracks was identified. This subject report
emphasizes the need to identify such defects in order to obviate in service failure of fan blade
which adversely affects the in-service reliability of the on-wing engines. The desired suggestion
emphasizes on carrying out more frequent on-wing N.D.T Inspections of these fan
blades, detailed inspection of fan blades during shop visit & overhaul of these engines
and their N.D.T inspections whenever any bird hit or F.O.D (Foreign Object Damage) is
reported or observed.

SUMMARY
The subject matter for the final year major project is the study of various non-destructive testing
techniques being used in the present manufacturing, overhaul & service industry. Also, their
important, applications, limitations, relative advantages & disadvantages were also studied.

Further, as a specific application of NDT methods, study of High Frequency Eddy Current
(H.F.E.C) inspections of A320 Airbus Aircraft main wheel Hub assemblies was carried out under
the guidance of N.D.T specialists of aircraft accessories overhaul shop, Indian Airlines, N.R. The
defects so observed were studied in depth & remedial actions suggested to minimize such
failures in future. Continuance of these defects in service without detection could result into
uncontained failure of the component endangering the safety of the aircraft in-service.

Similarly an another application of NDT methods regarding Ultrasonic Inspection of Aero
engine fan blades root flank area & its tap test was done to find out a debond or a crack in this
area to reject such a fan blade. Continuance of this defect in service could lead to fracture of the
blade from the root area causing consequent extensive internal damage to the engine and a
serious flight safety problem. To prevent such in service failure, I suggested more frequent on
wing engine fan blade root flank & tap test N.D.T Inspections during major maintenance checks
of the aircraft. These inspections during scheduled and unscheduled shop visits of the engines
were also suggested to be done scrupulously. I also suggested to carryout these NDT inspections
of fan blade whenever there is any reported or observed engine bird-strike or any time high
engine fan blade vibrations are reported.

These preventive NDT inspections in the critical areas prone to high fatigue stresses in the areas
of high stress concentration can prevent an impending failure of component/system, thereby
improving the component/system in-service reliability.

CONTENTS

Certificate
Acknowledgement
Abstract
Summary
1. INTRODUCTION
1.1 Non-destructive testing techniques and their importance
1.2 Application of N.D.T
1.3 Motivation behind this project
1.4 Consolidation of work
2 LITERATURE REVIEW
2.1 Review of books and journals
2.2 Different non-destructive testing techniques
2.2.1 Visual testing technique
2.2.2 Radiography technique
2.2.3 Magnetic particle inspection
2.2.4 Penetrant (Dye & inspection fluorescent) Inspection
2.2.5 Ultrasonic inspection
2.2.6 Eddy current inspection
2.3 Relative advantages & disadvantages of various NDT methods.

3. EXPERIMENTAL SETUP / SHOP TESTING
3.1 N.D.T Inspection-High Frequency Eddy Current Inspection of main wheel hubs of
A-320 Airbus Aircraft
3.1.1 Instrumentation
3.1.2 Experiments
3.1.2.1 Procedure and Observations

3.2 NDT Inspection-Ultrasonic Inspection of L.P. Compressor fan blade of M/s IAE
V2500 Engine installed on A320 Aircraft
3.2.1 Instrumentation
3.2.2 Experiments
3.2.2.1 Procedure and Observations
4. RESULTS AND DISCUSSIONS
5. CONCLUSION
6. REFERENCES
7. APPENDIX

1.1 INTRODUCTION

N.D.T is the technology of assessing the soundness & acceptability of an actual component
without affecting its functional properties. Non-Destructive Testing is exactly what its name
implies i.e. testing without destroying. Therefore, N.D.T. is an examination of an object or
material in a way which will not impair its future usefulness. N.D.T. is the use of technology for
inspecting the materials to known standard. Since Non-Destructive Testing do not in any way
impair the serviceability, therefore these can be applied, if desired, on all the units produced.
Consequently, there is great reliability in the production.

1.2 APPLICATION
N.D.T. Tests are done for detecting discontinuities which might be inherent, develop during
processing or during in-service. Inherent cast discontinuities relate to inadequate feeding, gating,
excessive pouring temperature or entrapped gasses. Processing discontinuities are usually related
to various manufacturing processes such as machinery, forming, extruding, rolling, welding, heat
treating and plating. While service discontinuities relate to various in-service conditions such as
stress corrosion, fatigue & erosion etc. and are mainly due to design deficiencies, material
imperfection, processing deficiencies, assembly errors and due to in-service deterioration.

Non-destruction testing is an important part of preventive maintenance programme by
identifying a failure or an impending failure, NDT programmes help in the safety of the person,
plant & equipment and adds to the economy by preventing a major break-down. NDT is an
important tool in the present advance manufacturing, assembling, processing, overhauling and
maintenance industry and plays an important role in the development and improvement in the
end product of the company. NDT Techniques are useful in determining critical spots (weak) in
manufacturing operations right from receiving inspection to end item inspection. It helps to
establish and measure quality and acceptance limits at the various stages of manufacturing
thereby useful in development and improvement of material & process for fabrication.
NDT methods help in determining service life of the components. It is useful for failure analysis
and suggests remedial measures to prevent such failures in future. NDT is used for in-service

inspections to determine cracks due to fatigue damage/creep, corrosion damage, de-
bonding/delaminating of composite materials.
As a part of ‘On-Condition’ maintenance management programme, NDT methods are done in-
service on complicated assemblies without their disassembly & health of such critical
components is continuously monitored by carrying out these Non-destructive tests. Acceptable
parts with very high fabrication cost are not lost after testing. Since these tests are rapid, quick
and reliable, these are most suited for high rate production part when tests are to be done on
entire batch. The success of NDT method depends upon the knowledge of various type of
engineering material one comes across, the process by which the component is made of and the
probable stages at which the defect may creep-in. In Non-Destructive Testing language the word
‘defect’ is correctly applied only to a condition which will interfere with the safe or satisfactory
service of particular part in question. A discontinuity will be a defect only when it interferes with
performance of the part or material in its intended service. Non-destruction evaluation is the art
of developing NDT techniques, arriving at acceptance standard for components for which
nothing is available to start with.
The various defect/discontinuous which can be determined by different NDT Techniques include
detection of surface and Sub-Surface Cracks, Blowholes, Weld Penetration Deficiencies,
Detection of Grain Size Variations, Heat Treatment Deviations, Machining Defects, Plating
Defects, Inclusions, Pits & Porosity etc. It also helps to determine chemical composition &
thickness measurement of piping and pressure vessels.
These defects normally arise from initial production of the raw material or during conversion of
base or raw material into manufacturing components. In-service defects could arise due to
components operating under extreme conditions, inadequate preventive maintenance programme
of the components/systems or due to some external cause like F.O.D. (Foreign Object Damage)

1.3 MOTIVATION BEHIND THE PROJECT
My major objective to select “Study of various N.D.T. Techniques” as a project is due to its vast
applications in the Modern Production, Process and Maintenance Industry and to enhance my
knowledge on this subject matter due to my keen interest in aviation maintenance and overhaul
industry where N.D.T. Technique is a very important tool for preventive maintenance
management programme. Moreover this type of industrial project work will strengthen my
knowledge & confidence in the area of my choice & help me in my career growth.

1.4 CONSOLIDATION OF WORK
1. Chapter 1 deals with Non-destructive testing techniques, their importance, applications of
N.D.T, motivation behind this project and finally it ends with consolidation of project
work.

2. Chapter 2 deals with review of books, journals and manuals used for this project report.
This report also describes about various Non-Destructive testing techniques such as Visual
testing, Radiography, Magnetic particle inspection, Dye Penetrant Inspection, Ultrasonic
inspection and Eddy current inspection technique. Finally, comparison of relative
advantages & disadvantages of various NDT methods have been discussed briefly.

3. In chapter 3 as per the reference manuals, important details of High Frequency Eddy
Current N.D.T Inspection technique of main wheel hubs of A-320 Airbus Aircraft and
Ultrasonic Inspection technique of turbo fan jet engines wide-cord fan blades (Titanium
Alloy Skin with Aluminium Alloy honey comb core) have been discussed in detail. In the
instrumentation, various important features and working of instruments viz Defectometer
and Meccasonic used in these N.D.T techniques have been described. These details also
include calibration procedures of these instruments.

4. In chapter 4 results and discussions include description of various cracks/ flaws on different
areas of main wheel hubs and turbo engine fan blade structure of A320 Airbus aircraft.

This chapter also describes about the various remedial measures suggested to minimise
these failures for smooth engine performance and to improve the inservice reliability of
aircraft wheel hubs.

5. Chapter 5 conclusions mentions about importance of various N.D.T techniques being used
in the present manufacturing, processing and maintenance industry. It also mentions about
importance of High Frequency Eddy-Current N.D.T method on aircraft wheel hubs and
ultrasonic inspection technique on engine fan blades to improve the components and
system reliability.

6. List of references used

7. Appendix

2. LITERATURE REVIEW
2.1 Review of books and journals
CIVIL AVIATION AUTHORITY, UK (1) describes about Eddy Current applications,
its principles of operations, Eddy Current tests, probes types, coil arrangements and types
of circuits used. This leaflet gives guidance on the use of Eddy Current equipment for
detecting cracks, corrosion and heat damage detection, for measurement of coating
thickness or for sorting material. Elementary theory of Eddy Currents is included to show
the variables which are being measured and to indicate interpretation of results which
may be necessary for particular applications. This document also describes about
advantage and limitations of this NDT techniques. It also describes how the size and
shape of test specimen may affect these inspection results which can be overcome by
probe design, equipment calibration, frequency selection or the use of jigs to maintain the
probe in a particular relationship to the material surface. This document also describes in
detail effect of frequency selected and its importance while carrying out this NDT
inspection. It also describes about the effect of LIFT-OFF and its compensation while
accomplishing this procedure. This leaflet also describes the details of the various types
of probes like surface probes, hole probes, special probes and their different usage in
Eddy Current inspection.

CIVIL AVIATION AUTHORITY, UK (2) gives general guidance on the application
and scope of ultrasonic sound waves for detecting surface and internal flaws in material
and parts and for measurement of thickness. This document describes that ultrasonic
inspection is the only satisfactory NDT method when a distant defect lies parallel with
the only available surface of the component. It emphasizes the need for properly trained
and qualified operator for using the ultrasonic equipment. This document describes about
the various applications of ultrasonic methods prior to fabrication, during manufacturing
processes, during periodic preventive maintenance checks to find out fatigue cracks and
other defects arising from operating conditions. It describes about how the ultrasonic
waves are produced and what is piezoelectric effect. This document describes in detail
about various methods of operation of ultrasonic NDT techniques like Transmission
Method, Pulse Echo Method, Immersion Testing And Resonance Technique etc,. This

document also describes about the choice of frequency for carrying out this test on
different material of different size and shape. This document emphasizes the most
important applications of ultrasonic NDT technique for its usage for in-situ examinations
and particularly for detecting corrosion damage which can be found in areas not
accessible for visual examinations. It is a useful NDT method for finding
delamination/debonding of non metallic composite material parts.

ROBERT C Mc MASTER (3) describes NDT techniques like Fluorescent Penetrant
Processes, Magnetic Flaw Detection Methods, Radiological Examination of the parts,
Electromagnetic Methods, Endoscopic Methods and Ultrasonic Methods. It describes
about the principle of operation of the particular NDT method, its application, advantages
and limitations. It also describes how these NDT methods are extensively being used in
present industries and how it will be useful in future advancement in technology.

WILLIAM E SCHALL (4) describes about the various types of flaws/ defects occuring
right from raw material to finish goods stage and how NDT techniques are useful at
various stages of production for economical, reliable and safe operation of the industry. It
defines the various types of surface and sub surface defects and appropriate NDT
technique to detect such defects. It also describes about the accuracy and limitations of
particular NDT method. It also describes about the need for proper training and
experience to become a good NDT engineer. Above all, it also mentions about the high
capital cost involved for procuring the NDT equipments and running cost involved to
keep it going.

In reference (5) EDDY CURRENT AND ULTRASONIC TESTING describes about
the elementary theory of electromagnetic induction, principles of Eddy Current and Eddy
Current Test Systems, the test frequency and distribution of Eddy Currents. This
document also mentions about various methods of Eddy Current testing and their
requirements. It also describes about various applications of electromagnetic testing and
depth of penetration of Eddy Current for various materials. Ultrasonic inspection
describes about basic characteristics of sound, principle of wave propagation and

generation of Ultrasonic waves. Various methods of Ultrasonic testing are explained in
details which includes ultrasonic test equipments-ultrasonic tester, test probes, couplants,
reference standards, scanning devices and recording systems. Effect of surface finish on
correct movement of sound waves is also described.

In reference (6) COMPONENT MAINTENANCE MANUAL, MESSIER HISPANO
BUGATI, ITALY describes in details about the inspection procedure of all the parts of
main wheel assembly which includes inner and outer hub assemblies. It specifies the need
of thorough cleaning of the parts before their detailed visual inspection and mandatory
use of 10X magnifying glass for this method of inspection. This document specifies the
type of damage and area of damage expected during this inspection such as dents, nicks,
scores, notches, corrosions or cracks. Besides it also specifies the need of carrying out
NDT inspections of the critical areas of the inner and outer hub assemblies. This
document also specifies the requirement of additional items of inspection in case of tyre
burst, tyre deflation/ overheating etc.

In reference (7) V2500 ENGINE MAINTENANCE MANUAL describes about the
general inspection of flaws, debonding and wall thickness measurements using Ultrasonic
Inspection. This document describes about the safety precautions, the equipment required
(viz High Frequency Ultrasonic Tester, Meccasonic D125 BJ or D325 BJ, 25 MHz
Focused Immersion Probe, Calibration Standard Specimen, Visual Display Units, etc,.)
for this method of N.D.T. It also decribes in details the calibration and inspection
procedure using the above test equipments. Finally, this document emphasizes the need
for proper training and experience for carrying out the above inspections and
interpretations of test results.

In reference (8) V2500 ENGINE SERVICE BULLETINES AND INSPECTION
PROCEDURES describes about following inspections/ checks of engine low pressure
compressor fan blades:-

(i) Examination of engine fan blades internally using Transient Acoustic Propagation (TAP) tester.
(Reference 8)
(ii) Procedure for Ultrasonic inspection of root debond of engine low pressure compressor
fan blades (Reference 8)..
(iii) Procedure for the C-SCAN inspection of the engine low pressure compressor fan blades
(Reference 8). The subject document gives in detail, the equipment and material required
for carrying out the particular item of inspection as described above. The procedure for
carrying out the above inspection items have been given in details. Finally the document
describes about the analysis of results so obtained including the criterion of the
acceptance or rejection of the part under test.

2.2 Different types of Non-Destructive testing methods:-
2.2.1 Visual Testing Technique (3)
Visual aids can be employed to detect many types of different defects such as surface
cracks & their orientation, weld defects, potential sources of weakness such as notches or
misalignment and oxide film formation etc. The commonly used visual aids to detect
these flaws are mirrors, magnifying lenses, microscopes, telescopes, enlarging projectors,
Comparators, Boroscopes, Photoelectric Systems, Fibre Optics, Image intensifier and
Closed Circuit Television (C.C.T.V.) etc. Some of these special visual aids are explained
herebelow:-

(i) Boroscope (3):-
These are instruments designed to enable an observer to inspect inside of a narrow tube,
bore or a chamber. These are precision built optical systems with arrangements of prisms
and lenses to provide light with maximum efficiency.

(ii) Fibre Optic Scanners (3):-
Fibre optic scanning tubes are used to examine inaccessible areas & areas of dangerous
environments. Optical fibres transmit light by the phenomenon of total internal reflection.

(iii) Miniature C.C.T.V. (3):-
A technique of inspection of turbine blades, turbofans, turboshafts and other components
'In-situ' without any disassembling. Direct viewing through Boroscope inserted through
parts in the engine assembly mounted on wing or on test rig requires prolonged
inspection often under difficult and uncomfortable conditions. An improvement has been
introduced of miniature C.C.T.V. camera equipment coupled with boroscope & light
sources appropriate to the type of engine. The results may be transmitted to remote
monitors of video recorders.

2.2.2 RADIOGRAPHY TECHNIQUES (3&4)
2.2.2.1 RADIOGRAPHY GAMMA RAY INSPECTION (3&4)
It is projecting a three dimensional object on a plane with the help of GAMMA RAYS
penetrating radiations resulting from disintegration of radioactive materials. Gamma-
radiations is not in the same form as X-rays and consists of one or more discrete
wavelengths in what is known as a 'line spectrum'. The relative intensities of each
wavelength are always the same for a particular material. The four mostly used isotopes
are Cobalt60, Iridium192 and Thulium170 etc. Radioactive gamma ray sources consist of
a circular disc or cylinder of radioactive material encased in a sealed Aluminium or
Stainless Steel Capsule. The capsule is kept in a container made of lead or deplated
Uranium which will substantially reduce the emission of Gamma rays. This technique is
normally used when there is lack of space or access for X-ray equipment.

2.2.2.2 RADIOGRAPHY X-RAY INSPECTION (3&4)
This particular form of electromagnetic radiation is produced when electron travelling at
high speed collide with matter in any form. The basic requirements for the production of
X-rays are a source of electrons, a means of accelerating the electrons to high speed and a
target to emit the X-rays. The X-ray tube in an evacuated chamber in which the electrons
are derived from a filament, set in a focussing cup & heated to incandescence by a low
voltage current, electrons are released and form 'space charge' around the filament. When
a high potential is applied, electrons accelerate from the filament (the cathode) to the
anode and strike the target which then emits the X-rays (refer fig 2.1 for typical circuit of
an X-ray production set) The films used in radiography are very similar to those used in
photography except that the emulsion covers both sides of the flexible transparent base.
The emulsion is sensitive to X-rays, Gamma rays & light and when exposed to these
radiations a change takes place in its physical structure. When treated with developer, a
chemical reaction results in formation of black metallic silver which comprises of the
image. Both the radiographic techniques are used to detect internal defects and variations,
porosity, inclusion, cracks, lack of fusion geometry variations, corrosion damage,
thickness measurement, determining mis-assembly and mal-alignment. It is used for the

inspection of boiler tube thinning due to corrosion or erosion in power plants, castings,
weldments, and small thin complex wrought products. It is also used to check for dis
bonding/ delamination of non metallic, electrical assemblies, composites, solid propellant
rocket motors and water entrapment in the honeycomb structures. Gamma Ray Inspection
is often used for examination of internal features of turbine engines such as the main
rotor shaft and turbine hot section inspection of Nozzle Guide vanes and rotor blades.

FIG 2.1 Circuit diagram of X-Ray N.D.T Technique

2.2.3 MAGNETIC PARTICLE INSPECTION (4)
This NDT process is normally applied to homogeneous ferromagnetic materials which
can be easily magnetized. This NDT method is suitable for detecting surface and
subsurface cracks.
If a component is subjected to a magnetic flux, any discontinuity in the material will
distort the magnetic field and cause local leakage fields at the surface. Particles of
magnetic material applied to the surface of the magnetised component will be attracted to
the flux leakage areas and reveal the presence of the discontinuity. The sensitivity of
magnetic flaw detection depends largely on the orientation of the defect in relation to the
magnetic flux and is highest when the defect is at 900 to the flux path. Sensitivity is
considerably reduced when the angle between the defect and the flux path is less than 450
so that two tests are normally required with each component, the flux path in first test
being at 900 to the flux path in the second test. Components of complex shape may
require tests in several different directions.

A component may be magnetised either by passing a current through it or by placing it in
the magnetic circuit of a permanent magnet or electromagnet. The magnetic particles
used to reveal defects are either in the form of a dry powder or suspended in a suitable
liquid. They may be applied by spray, pouring or immersion depending on the type of
component. Fluorescent inks are also used where high sensitivity is required. Inspection
of the component, to which fluorescent inks has been applied, should be carried out under
ultraviolet light.

Particles of magnetic ink are attracted to flux leakage fields and these may occur at
defects, brazed joints and heat affected zone in welds. Cracks are revealed as sharply
defined lines on the surface of the specimen, the magnetic particles often building up into
a ridge. So if a discontinuity is present at or near the surface, the magnetic field is
deflected and forms a leakage field. Detection of this field by particle application forms
the basis of this inspection. Finally, the tested component must be demagnetised after this
NDT method is completed. This method is used for determining surface and subsurface

cracks, seams, porosity and inclusions. It is extremely sensitive for locating small tight
cracks of ferromagnetic materials, bars forging, weldments and extrusions.

2.2.4 PENETRANT (DYE & FLUORESCENT) INSPECTION (4)
Penetrant dye processes are used mainly for the detection of flaws in non-ferrous & non-
magnetic ferrous alloys but may also be used for ferrous parts where magnetic flow
detection techniques are not specified or are not possible. The processes can be divided
into two main groups. One group involves the use of penetrants containing an
emulsifying agent (water washable process) whilst in the other group a dye solvent has to
be applied separately after the penetration time has elapsed if the surplus dye is to be
removed by water wash operation. Basically, this process consist of applying a red
penetrant dye to the surface of the part to be tested, removing after the predetermined
time the dye which remains on the surface and then applying a developer, the purpose of
which is to draw to the surface the dye that has entered into defects, the resultant stains
indicating the position of the defects.

Surface preparation is most important for this method of NDT. The surface to be tested
must be free from oil, grease paint, rust, scale, welding flux and carbon deposit etc. The
penetrant dye can be applied to the surface by dipping, spraying or brushing, the method
used depending largely on the size, shape and of quantity of parts to be examined. The
dye penetration time is normally in the range of 5 minutes to 1 hour, the smaller the
defect the longer the time necessary. Any dye remaining on the surfaces of the parts after
expiry of penetration time should be removed as thoroughly as possible but without
disturbing the dye which would have found its way into any defects present. The
developer is usually very fine absorbent white powder suspended in volatile carrier liquid
which rapidly evaporates and the action of absorbent powder is to draw out the dye from
the surface defects, thus indicating their position by the resulting stain. Normally, the
position of defects will be indicated by red marks appearing on the whitened surface.

All parts with non-absorbing surfaces (forging, weldments and castings) can be subjected
to this NDT inspection for detecting defects open to the surface in solids and essentially
non porous materials.URRENT INSPECTION
2.2.5 ULTRASONIC INSPECTION (2&5)
2.2.5.1 BASIC CHARACTERISTICS OF SOUND (5)
(i) Frequency, sound velocity and wave length
How does the sound travel from an oscillating membrane (e.g. loud-speaker) as
transmitter to our ear as receiver? The oscillating membrane excites the neighbouring air
particles into oscillations and pressure fluctuations occur. As the air particles are not
rigidly but elastically connected to each other, we can use balls connected by springs as a
model (fig. 2.2). With reference (fig. 2.3) zero (1st row) balls are at rest. The oscillating
process is started by pushing the left ball to the left, moment I. As the left ball is
connected to the neighbouring ball by a spring the movement is slowed down up to
moment III and finally reversed. Due to the spring connection also the second and then
successively all other balls at the right are being moved. A wave motion develops.
Another examination of the figure shows that each ball oscillates around its rest position
by a certain amount, i.e. merely the condition of oscillation propagates along the direction
of propagation, Only the energy and not the mass is transported, In the period from
moment III to moment XV the particle has carried out one complete oscillation.

Fig. 2.2

Fig. 2.3

Fig. 2.4
Figures depicting Propagation of wave

The time required is the period of oscillation T. On the momentary representation XV we
can see that particles zero and twelve are just experiencing their max. deflection to the
left, i.e. they are in the same condition of oscillation, The distance between two particles
which are in the same condition of oscillation is the wave length λ. With reference (fig. 2.4) shows
that the condition of oscillation has propagated by the distance λ in the period T. Thus the
following formula applies to the propagation velocity c:
C=λ/T
From the period of oscillation T the number of oscillations per second can be calculated
by the formula
F=1/T

f is the frequency of the oscillation, its unit is "number of oscillation/second". The unit is
named after the physicist H. Hertz and is abbreviated Hz.

Thus the following formula applies to the propagation velocity of the wave:
C=fλ = 1/T * λ
This equation (wave equation) applies to all wave processes.

(ii) Definition of the term ultrasound
The frequency of a sound impression (tone) is a direct measure for the pitch of a tone.
The higher the frequency the higher the tone. The pitch of a tone which can be received
by the human ear has an upper limit. For young people the limit is approximately 20,000
Hz = 20 kHz. Sound having higher frequencies is called ultrasound.
Audible sound: f= 20 -20,000 Hz
Ultrasound: f> 20,000 Hz = 20 kHz.
As we have learned from the spring model an energy transport through a sound wave is
possible only when constituent particles are connected to each other by elastic forces. In
the case of the sound transmission from the loud-speaker to our ear, the air molecules
serve as transmitting medium. Liquids and solid matter are also suitable media for the
sound transmission, In the vacuum (space) no matter exists and thus no sound
transmission is possible. The satisfactory sound conductivity of liquids and solid matter is
nowadays technically utilised in various sectors.

2.2.5.2 PRINCIPLES OF WAVE PROPAGATION (5)
(i) Types of Oscillations
The sound propagation demonstrated by the spring model is possible in all media, It is
characterized by the fact that the direction of oscillation of the particles runs along the
direction of propagation of the wave. Thus zones with small particle distance and zones
with large particle distance are created, Therefore, this type of wave is called
compression wave or longitudinal wave. If we do not look at the momentary
representation but at the dynamic process of the propagation of the longitudinal wave, we

see that the compressions and diminishing move through the test object at an unchanging
distance. The velocity at which they move is the sound velocity C L of the longitudinal
wave. This sound velocity is a matter constant, i. e. in a test object completely made of
same material it can be considered constant. e. g.
for steel: CL= 5920 m/s
for aluminum: CL = 6300m/s
In solid matter the density is very high as compared with that of liquids and gases, i.e. the
distance between the atoms or molecules is very small. Moreover, they are arranged in a
crystal lattice and the elastic linkage forces between the atoms (molecules) are
particularly strong, Due to these two facts the sound can propagate in various ways in
solid matter. We already became acquainted with one type of wave, namely the
longitudinal wave. Another type of wave is called shear wave or transverse wave, In the
case of a transverse wave the particle oscillate vertically to the direction of propagation of
the wave. Now we look again at the spring chain. The wave is not excited into the
direction of the chain but in cross direction. The springs pull the balls back into their
starting position but due to their movement they oscillate over their rest position. At the
same time these transverse oscillations are transmitted to the two neighbouring balls
which also start oscillating, These oscillations are continued to be transmitted to the
neighbouring balls due to the spring connections.

Looking at the dynamic process of the wave train, we find out that both wave crests and
wave toughs move through the test object at an unchanging distance. The distance
between two neighbouring wave crests is the wave length. The energy trasmission of the
transverse wave is lower due to the transverse oscillation of the atoms than that of the
longitudinal wave. Therefore the propagation velocity of the transverse wave is
considerably lower than that of the longitudinal wave, e.g.
Steel: CL = 5920 m/s
CT = 3250 m/s
The velocity of the transverse wave, too, is a matter constant which is characteristic for
the corresponding work piece.

Longitudinal and transverse waves can propagate only through the whole volume of a
workpiece. At the interfaces or surfaces of the work pieces further types of waves may
occur. The surface wave or Rayleigh wave propagates only at the surface of the
workpiece.

2.2.5.3. GENERATION OF ULTRASONIC WAVES (5)

(i) Piezoelectricity
Ultrasonic transmitters and receivers are mainly made from small plates cut from
certain crystals (piezoelectric crystals) as shown in (fig. 2.5). If no external forces act
upon such a small plate electric charges are arranged in a certain crystal symmetry and
thus compensate each other. Due to external pressure the thickness of the small plate is
changed and thus the symmetry of the charge. An electric field develops and at the
silver-coated faces of the crystal, voltage can be tapped off. This effect is called direct
piezoelectric effect. Pressure fluctuations and thus also sound waves are directly
converted into electric voltage variations by this effect : the small plate serves as
receiver. The direct piezoelectrical effect is reversible (reciprocal piezoelectrical
effect). If voltage is applied to the contact face of the crystal the thickness of the small
plate changes according to the polarity of the voltage the plate becomes thicker or
thinner. Due to an applied high – frequency a.c. voltage the crystal oscillates at the
frequency of the a.c. voltage. A short voltage pulse of less than 1/1 000,000 second (1
second) and a voltage of 300-1000 V excites the crystal into oscillations at its natural
frequency (resonance) which depends on the thickness and the material of the small
plate. The thinner the crystal the higher its resonance frequency. Therefore it is possible
to generate an ultrasonic signal with a defined primary frequency. The thickness of the
crystal is calculated from the required resonance frequency f0 according to the
following formula:
d= λ/2 = c/2f0

c=sound velocity of the crystal
f0=resonance frequency of the crystal
d=thickness of the crystal
λ=wave length

Fig. 2.5 Piezoelectric Crystals
A piezoelectric crystal occuring naturally is the quartz (rock crystal) which was used as
crystal material in the beginning of ultrasonic testing. Depending on whether
longitudinal waves or transverse waves are to be generated the quartz plates have either
been saved vertically to the X-axis of the crystal (X-cut) or vertically to the Y-axis (Y-
cut) out of the rock crystal.
In modern probes quartz is hardly used, instead sintered ceramics or artificially
produced crystals are employed. The most important material for ultrasonic crystal as
well as their characteristics are stated in the Table 2.1 below :
Lead zirconate Barium Lead Lithium Quart Lithium
titanate titanate metaniobate sulphate z niobate
sound velocity 4000 5100 3300 5460 5740 7320
acoustic impedance 30 27 20.5 11.2 15.2 34
z 106 kg/m2
electromechanic 0.6 – 0.7 0.45 0.4 0.38 0.1 0.2
coupling factor k
piezoelectric modulus d 150-593 125-19 85 15 2.3 6
(Transmission/generatio 0
n)
piezoelectric 1.8 – 4.6 1.1– 1.9 8.2 4.9 6.7
deformation constant H 1.6

coupling factor for 0.5 – 0.6 0.3 0.07 0 0.1 -
radial oscillations kp
Table 2.1 Characteristics of various ultrasonic crystals

The efficiency during the conversion from electrical into mechanical energy and vice
versa differs according to the crystal material used.
The corresponding features are characterised by the piezoelectric constants and the
coupling factor. The constant d (piezoelectric modulus) is a measure for the quality of
the crystal material as ultrasonic transmitter. The constant H (piezoelectric deformation
constant) is a measure for the quality as receiver. The table shows that lead zirconate-
titanate has the best transmitter characteristics and lithium sulphate the best receiver
characteristics. The constant k (theoretical value) shows the efficiency for the
conversion of electric voltage into mechanical displacement and vice versa. This value
is important for the pulse echo operation as the crystal acts as transmitter and receiver.
Here the values for lead zirconate-titanate, barium-titanate and lead meta niobate lie in
a comparable order. As in the case of direct contact as well as immersion testing a
liquid couplant with low acoustic impedance z is required the crystal material should
have an acoustic impedance of the same order in order to be able to transmit as much
sound energy as possible. Thus the best solution would be to use lead meta-niobate and
lithium sulphate as they have the lowest acoustic impedance. A satisfactory resolution
power requires that the constant kp (coupling factor for radial oscillations) is as low as
possible. kp is a measure for the appearance of disturbing radial oscillation which
widen the signals. From this point of view lead meta-niobate and lithium sulphate are
the best crystal materials. The characteristics of the crystal materials described here
show that no ideal crystal material exists. As lithium sulphate presents additional
difficulties due to its water solubility the most common materials are lead zirconate-
titanate, barium-titanate and lead meta-niobate.

2.2.5.4 Set-up of the probe (5)

For the practical application in the material testing, probes are used into which the
piezoelectric crystal are installed. In order to protect the crystals against damage they
are pasted on a plane-parallel or wedge-shaped plastic delay block; the shape of the
delay block depends on whether the sound wave is to be transmitted perpendicularly or
angularly into the workpiece to be tested. The rear of the crystal is closely connected
with the damping element which dampens the natural oscillations of the crystal as
quickly as possible.
In this way the short pulses required for the pulse echo method are generated. The unit
comprising crystal, delay block and damping element are installed into a robust plastic
or metal housing and the crystal contacts are connected with the connector socket.
Probes transmitting and receiving the sound pulses perpendicularly to the surface of the
workpiece are called normal beam probes or straight beam probes. If the crystal is
equipped with a wedge-shaped delay block an angle beam probe is concerned which
transmits/receives the sound pulses at a fixed probe angle into/from the workpiece to be
tested. In both probe types the crystal serves for the both the transmission and reception
of the sound pulses. A third probe type comprises two electrically and acoustically
separated crystal units of which one only transmits and the other receives the sound
pulses. This probe is called Transmitter-Receiver Probe or twin crystal probe (fig 2.6).
Due to its design and functioning, it is used for the testing of this material or detection
of material flaws located near the surface of the workpiece.

Fig. 2.6 Twin Crystal Probe

2.2.5.5 ULTRASONIC TEST EQUIPMENT (5)

(i) Principle of the ultrasonic test instrument
So far we know that, with the probe, we can transmit ultrasonic pulses into the
workpiece. If this has two plane-parallel surfaces, the sound pulse will be reflected on
the surface opposite to the probe and return to it.
Our interest concerns the measurement of the pulse transit time.
This time is too short to be measured mechanically. We therefore use a cathode ray
tube or Braun tube as measuring instrument. The Braun tube contains a heater coil
which brings the cathode K to glow whereby the so “eveporating” electrons are
accelerated by a voltage between between cathode K and anode A. The result is an
electron beam. The voltage at the wehnelt cylinder W focuses the electron beam and so
makes it appear on the fluorescent screen F as a light spot. As the electrons travel to the
CRT-screen, they pass two pairs of deflecting plates which are arranged
perpendicularly to each other. If one applies a voltage to the horizontally orientated pair
of plates then the electron beam will be reflected vertically. Analogue to this, the
vertically oriented pair of plates serves for the horizontal deflection.
(a) A-scan representation
The standard application of the pulse echo method normally uses ultrasonic flaw
detectors having a CRT-screen with A-scan representation. The A-screen shows the
amplitudes of the echo signals in the vertical Y direction and the distance of the
corresponding reflectors are represented in the horizontal X-direction.
This allows a direct allocation between the echoes on the screen and the depths of the
associated reflectors.
(b) B-scan representation
Especially for semi or fully automatic tests, the ultrasonic testing technique uses special
instruments which allow a special type of displaying the test results. Instruments with
B-scan representation display a cross-section of the test object on the screen after the
probe has been moved on a scanning track running across the test object. The probe
movement is mostly displayed in the X-direction while the distance of occurring
reflectors is displayed in the Y-direction.

(c) C-scan representation

If the whole volume of a workpiece is to be covered it is necessary to scan at least one
surface completely. E.g. an automatic plate testing machine scans the whole plate using
a great number of probes and a measuring scanning track. The test results can then be
displayed by the C-scan representation for which one normally uses an X-Y recorder or
a printer. The workpiece is represented in a top view in which the flaw locations can
then be marked true-to-scale. The use of printer allows the indication of further
information (e.g. the depths of reflectors, echo amplitudes) by means of various
symbols.

Fig. 2.7 C-Scan
2.2.5.6 Monitor function
In a case of a manual test using an Ultrasonic Flaw Detector, the operator scans the
workpiece with the probe and simultaneously observes the CRT-screen thereby
concentrating on echo indications which originate from the interior of workpiece or, in
other words, from the flaw expectancy range. The start and end of the flaw expectancy
range can be there by be marked by means of a step on the base line of the screen or an
additionally displayed bar on the screen. If now an echo appears within this range then
this releases a visible and/or audible alarm signal. The response threshold of the
monitor is also variable so that an echo indication only releases the alarm when it has
reached a certain height.
In addition to the monitor function, most of these instruments have a control output
which can be used to further process the information.

As soon as an echo appears within the monitor threshold, a voltage is fed to the control
output which is proportional to the echo height and which can be immediately used for
automatic recording. By means of this monitor function together with a path pick-up
which is fixed onto the probe, C-scans of workpieces can be easily printed on an X-Y
recorder. They can be regarded as test reports and filed.

2.2.5.7 Test mechanism (5) (Fig. 2.8)
All cases where continuously large amounts of equal parts are tested are suitable for
automatic testing machines. These consist essentially of one or several probes which
are coupled to the test specimen by a control unit and are moved across the test object
according to a predetermined scanning pattern. The ultrasonic signals are processed by
the evaluation unit (e.g. an ultrasonic flaw detector) and displayed on a CRT-screen if
available. All measured data are fed to a computer where they are further processed and
evaluated. At the same time information about the probe position is also fed to the
computer. The test report is produced by means of a printer. The computer controls
additionally the marking and sorting device which marks the flaw locations on test
objects. Test objects which have unacceptable flaws are rejected. A further task of the
computer is to control the transport of the workpiece and to signal defined test
conditions.

Fig. 2.8 Automatic Ultrasonic testing machines (Block diagram)

2.2.5.7 METHODS OF ULTRASONIC TESTING (5)

Fig. 2.9 Block diagram of ultrasonic testing technique

Today, Non-destructive ultrasonic testing is applied. on a great variety of materials of
different processing conditions and geometries. Refer Fig. 2.9 for a typical setup for
ultrasonic test set.

In order to test a certain workpiece for certain flaws it is not only important to choose a
suitable probe but also the right testing method.
(i) Direct contact testing
Almost all non-automatic tests employ direct coupling, i.e. The operator moves the
probe manually in direct contact over the surface of the test object. The acoustic
coupling agent are oil, water, glycerine, wall paper glue, etc,.
(ii) Straight-beam testing
If a probe, which contains an X-cut crystal, is coupled to a workpiece a longitudinal
wave pulse will be generated and transmitted into the workpiece. Straight-beam probes
transmit the sound pulse perpendicular to the surface of the workpiece into the material
(Refer Fig. 2.10). If the pulse that passes through the workpiece has plane-parallel
surfaces then the reflected pulse returns to the probe and generates a signal (backwall
echo) on the CRT-screen of the instrument. Only a small portion of the reflected pulse
returns to the probe itself while the greater portion is reflected on the surface and passes
through the workpiece a second time. This generates further backwall echoes on the
CRT-screen.

The speed at which the electron beam travels across the CRT-screen from the left to the
right is set in accordance with a defined proportion to the sound velocity of the
workpiece. If this proportion is known then the thickness of the workpiece can be
directly read off by the distance between two sequential backwall echoes on the CRT-
screen (Refer Fig. 2.11). To be able to set the above speed ratio the instrument must be
calibrated.

Fig. 2.10 Normal & Alternate Transmission Technique

Fig. 2.11 Pulse Echo Technique

(a) Calibration:
By means of the calibration it is possible to allocate the whole width of the CRT-screen
to a defined distance range in the material to be tested. This distance range is
designated the test range. For the calibration we use a plane-parallel calibration block
which has a known thickness and must be made of the same material as the test object.

(iii) Locating- reflectors:
To test the workpiece we couple the probe onto its surface. If we have chosen a suitable
test range we should now obtain the first backwall echo on the screen. If the first
backwall echo is preceded by another echo then this echo comes from a reflector in the
workpiece. For the evaluation we only use that range before the first backwall echo
because in the range behind it secondary echoes may occur due to split transverse
waves thus simulating a reflector in the workpiece.
The operator's task is now to exactly determine the location of the reflector. By slightly
moving the probe on the surface of the workpiece the reflector echo can be optimised
(or maximised). At the probe position with the highest echo amplitude the reflector
stands exactly on the central axis of the sound beam, i.e. perpendicular under the centre
point of the probe. The last step is to determine the exact depth position of the reflector.
The sound beam should strike reflectors perpendicularly

If this condition is fulfilled the result will be a maximum echo.

Using a straight-beam probe it is impossible to detect reflectors which run inclined or
perpendicular to the surface because the sound is not directly reflected to the probe.

To be able to detect and evaluate this type of flaw one uses angle-beam probes.

To transmit ultrasonic pulses inclined to the surface we glue an X-cut crystal, which
generates longitudinal waves, onto a wedge-shaped perspex delay block. That sound
portion which is reflected by the sole of the probe strikes a damping element which
absorbs this unwanted sound portion. At the interface of the perspex wedge to the
workpiece, the sound waves are refracted which may also cause a splitting of transverse
waves so that, in this case, there can be two types of waves in the workpiece, namely
longitudinal and transverse waves.
Both of these two wave types are converted again into a longitudinal wave as they
return to the probe. That means, every echo returning to the probe, regardless from
which type of wave it originates, is always received by the crystal as a longitudinal
wave echo so that it is impossible to decide whether it comes from a transverse or a
longitudinal wave. However, due to this it would then also be impossible to locate a
reflector since both types of wave propagate in the material in different directions and
at different velocities.

We therefore use a constructive trick to ensure that only one type of wave may occur in
the testpiece. We choose an angle of incidence for the longitudinal wave in the probe so
large that a longitudinal wave can no longer occur in steel where by βL is greater than
90° (total reflection).

(iv) Testing with twin crystal probes (TR technique)
Straight-beam and angle-beam probes are equipped with only one crystal which has
both the transmitting and receiving function. Depending on the length of the delay line,
this has the effect that the initial pulse is displayed either fully or partially on the CRT-
screen, and, consequently, echoes from near-surface flaws are not definitely traceable
(dead zone, initial pulse influential zone). In practical testing, however, we often meet

with situations were we have to test thin parts or where especially near-surface flaws
are to be detected. In these cases, we use a probe with 2 crystal units which are
electrically and acoustically separated, i.e. one only transmits sound pulses and the
other one has only a receiver function.

Each TR Probe crystal unit consists of a perspex delay line having the shape of a semi-
cylinder. The crystal, which is semi-circular, is bonded to the delay line. Both crystal
units, separated from each other by an acoustic separation layer, are built into a probe
housing and are connected with 2 electrically separated sockets. An additional increase
in sensitivity within the near-surface zone is attained by a slight inclination of the
crystals towards each other. This angle of inclination, which we also call roof angle,
varies in size from 00 to approx. 120 depending on the purpose of application and probe.
If we wish to operate a TR probe in connection with an ULTRASONIC FLAW
DETECTOR then the instrument must be switched to TR-operation (twin crystal
operation). At one connection socket stands the initial pulse and at the other connection
socket is the input of the amplifier (receiver). The ultrasonic pulses are generated in
transmitter part of the probe and transmitted into the delay line. Echoes from the delay
line, however, are not displayed on the CRT-screen due to the fact that the transmitter
crystal has no receiving function. If we now couple the probe onto a plane parallel plate
we then receive an echo because the sound pulse, being reflected on the backwall, is
directed into the receiver part of the probe.

The remaining wall thickness measurement on tubes and containers which are exposed
to corrosion or erosion is one of the principal fields of application: The main advantage
is the fact that the installations or plants to be tested do not need to be put out of
operation and therefore no standstill losses will occur. The measuring accuracy is 1/10
mm with wall thickness from 0.5 mm onwards and measurements are possible on
systems which have temperatures of up to approx. 5000 C.

Twin crystal probes are often operated in combination with digital wall thickness gages.
After coupling the probe a digital display indicates directly the wall thickness in mm, or
inch.

Due to the high sensitivity of twin crystal probes regarding the near-surface zone they
are also suitable to trace very small flaw locations from a depth of approx. 0.6 mm.
Twin crystal probes arc therefore employed for flaw detection on thin parts.

(v) Through transmission
The through transmission technique is the oldest method applied in ultrasonic testing.
One probe is used to transmit sound into the test object and the other side receives it.
Using this method we compare the sound intensity from a flaw-free zone with that from
a flawed zone. A flaw in the sound field shades a portion of the sound energy off, so
that the intensity measured at the receiver is lower as compared with a flaw-free zone.

A disadvantage of this method is that no statement can be made regarding the depth and
the extension of the flaw. Despite this disadvantage, the through transmission method is
still in use, mainly for testing thin plates or saucer type test objects which are accessible
on both sides and have flat flaws extending parallel to the surface. This concerns
mainly plates of whole thickness range which have laminar defects, and short tubular
bodies such as bearing bushes, laminated plastics and platting. To avoid high coupling
variations these objects are normally tested in the immersion technique.
If the test objects are not accessible on both sides but have plane-parallel surfaces, one
can also employ the V-through transmission or make use of the lamb wave
transmission.
(vi) Immersion technique
With the immersion technique (Fig. 2.12) both the testpiece and the probe are totally
immersed in water. This guarantees a continuously good coupling effect. As a rule we
use water to which we admix an anticorrosion agent, fungicides as well as an additive
to reduce the surface tension of the water. The water should stand for a longer time to
de-aerate, i.e. to reduce disturbing air bubbles to a minimum. The immersion type

probes differ from the straight-beam probes for direct contact only insofar that they are
watertight moulded including the cable connection.
The probes are mounted in a holder so that they are oriented perpendicular to the testing
surface. Between the probe and the surface of the workpiece is a water delay line which
has a defined length.

Fig. 2.12 Immersion Testing Technique

(vii) Resonance Method
All technique described before arc based on the pulse echo method. A continuous
ultrasonic wave, however, can also be used in non-destructive testing.
A continuous ultrasonic wave which is transmitted into a plane parallel plate can excite
natural oscillations of the plate. A pre requirement for this is that the plate can freely
oscillate on either side, i.e. on both sides of the plate must be a medium with small
acoustical impedance.

Fig. 2.13 Standing Wave

The ultrasonic wave is reflected on both interfaces thus traveling through the plate in
two different directions whereby the forward wave and the reflected wave superimpose
each other. Depending on the wave length in relation to the plate thickness amounts
exactly to a multiple of the half wave length. In this case the wave crests of the forward
wave meet the wave crests of the reflected wave and a standing wave develops (Refer
Fig. 2.13). Such a wave is characterised by the fact that inside the plate there are
locations where the particles are always stationary while on other locations the particles
always oscillate with the maximum amplitude. Those frequencies which generate
standing waves in the plate are designated natural frequencies of the plate. If the plate is
excited in one of its natural frequencies then we refer to resonance vibration of the
plate. For the detection of flaws the resonance method is difficult to apply. In addition,
the flaws must have a surface of ¼ or ½ of the crystal surface in order to be detectable
at all. The described disadvantages of this method are the reason why the resonance
techniques were replaced by the pulse echo method.

2.2.6 EDDY CURRENT INSPECTION TECHNIQUES (1&5)
2.2.6.1 PRINCIPLES OF EDDY CURRENT (5)
Eddy Current and Its Properties
When magnetic flux through a conductor changes, induced currents are set up in closed paths on
the surface of the conductor. These currents are in a direction perpendicular to the
magnetic flux and are called “Eddy Current”, Figure given below illustrate this.

Fig. 2.14 Eddy Current

Basic arrangement for producing Eddy Current in a conducting material is shown in
Figure given below:

Fig. 2.15 Generation of Eddy Current
When an alternating current is passed through a coil, an electromagnetic field is set up
around it. The direction of magnetic field changes with each cycle of alternating
current.

If a conductor is brought near this field, eddy currents are induced in it. The direction of eddy
current changes with the change in direction of magnetic flux during the cycle of
alternating current.

The induced eddy current produces its own magnetic field (or flux) in a direction
opposite to the inducing primary magnetic field. The secondary magnetic field due to
eddy currents interacts with the primary magnetic field and changes the overall
magnetic field and magnitude of the current flowing through the coil. In other words,
the impedance of the coil is altered due to influence of eddy current. During non-
destructive testing, using eddy current, change in impedance is displayed either on a
meter or on a cathode ray tube screen.

2.2.6.2 FACTORS AFFECTING EDDY CURRENT (5)
The magnitude and distribution of eddy current in a given conductor is influenced by
conductivity of the material, the magnitude of primary magnetic field of the coil,
Permeability of the conductor, Geometrical variations of the part, In homogeneities and
discontinuities, Test frequency and Skin effect. Some of these important factors are
discussed herebelow:-
(i) Effect of Geometrical variation of the part:
The shape, thickness and presence of conducting materials in close proximity of the part affect
distribution of eddy current and associated magnetic field.

Edges, corners and radii, obstruct the circular pattern of eddy current. This limits the
volume of eddy current and changes the magnitude and distribution of eddy current and
consequently the associated magnetic field is also effected. This is called ‘edge effect’.

(ii) Effect of in homogeneities and discontinuities:
The inhomogeneities and discontinuities like cracks, inclusions, voids, etc. in conducting
materials also effect the circular pattern of eddy current and its associated magnetic
field, fig. below illustrates the effect of inhomogeneities / discontinuities on
distribution of eddy current.

(iii) Effect of magnetic coupling:-
Magnetic coupling refers to the interaction of varying magnetic field of the test coil with the
test object. The effect of the primary magnetic field of the coil in inducing eddy
current on the surface of a conductor is strongly influenced by the distance of the
test coil from the surface of the conductor. This effect is illustrated in fig. below.

Effect of Discontinuity of Effect of distance of test
Eddy Current Coil from the part
Fig. 2.16

Coupling is said to be effective when the distance of separation of the coil from the
test object is small, it is said to be poor when the distance of separation between the
test coil and test object is large. It is easy to realize that coupling is influenced by
configuration and geometry of the test object, Surface condition and Coating on the
surface of the test object

(iv) Effect of Test Frequency:

The magnitude of induced eddy current in an object increases with frequency of
the inducing magnetic field, it has been observed that higher intensity of eddy
current results in stronger secondary magnetic field opposing the primary
magnetic field. This results in low depth of penetration of eddy current as in
case of high conductivity or higher magnetic permeability of the test object as
shown in Fig. Eddy current concentration is found to be greater at the surface of
conductor and decreases as the depth increases. As the frequency of
magnetizing field increases. As the frequency of magnetizing field increases,
the concentration of eddy current near the surface also increases and depth of
penetration decreases. Increasing magnetic permeability and conductivity of the
material further Accentuates this effect. The depth at which the eddy current
density is reduced to about 37% of its intensity on the surface, us called
standard depth of penetration. This depth is given by

1
Standard depth of penetration =
πf µrσ

Where f = frequency
µr = relative permeability
σ = electrical conductivity.
The fig. below illustrates the relationship between depths of penetration against
frequencies for various materials.

Fig. 2.17 Depth of Penetration-Frequency for Various Materials.

2.2.6.3 EDDY CURRENT TEST SYSTEM (5)

Basic Test System

An eddy current test system consists of :
(i) An oscillator to provide alternating current of required frequency for exciting the
test coil.
(ii) Test coil-test object combination which brings out desired information in the form
of an electrical signal.
(iii) Signal processing
(iv) Signal display.

The following figure gives the block diagram.

OSCILLATOR

TEST COIL
TEST PART
BRIDGE
CIRCUIT

SINGLE
PROCESSING
CIRCUITS

READ OUT

Fig. 2.18 Block Diagram of Eddy Current Test System

Here an Oscillator provides alternating current of required frequency to the test
coil, which generates eddy current in the test object. The test object variables,
like conductivity, permeability, discontinuities, etc. modulate the test coil
impedance. The modulated impedance signal is processed and displayed over a
readout mechanism. The commercially available eddy current equipment falls in
the following two categories: (1) amplitude detector and amplitude – phase
detector; and (2) special purpose equipment like conductivity meters, thickness
measuring equipment for conductive materials and for non-conductive coating
on conductive materials, flaw detector/metallurgical condition monitors, etc.

2.2.6.4 SENSING ELEMENTS TYPES AND ARRANGEMENTS (5)

In most of the non-destructive inspection equipment applications of eddy
current, the test coil (also called sensing element) serves as the main link

between the test instrument and test object. It serves two main functions. The
first one is to establish a varying electromagnetic field which induces eddy
current in the test object and induces increased magnetic effect in magnetic
materials. The second purpose is to sense the current flow and magnetic effect
within the test object and feed the information to signal analysis system.

Factors that influence selection of a test coil are:-

(i) Nature of the test specimen, e.g. Flat (sheet & plates), cylindrical (rods, wires,
tubes and pipes), spherical (ball).

(ii) type of information required and likely distribution of variables, e.g. Crack
detection, conductivity variation, permeability variation, etc.

(iii) accessibility, e.g. complexity of shape can make a test location on a component
very difficult and the test may require a special configuration coil for testing.

(iv) quantum of inspection: Depending on the production rate of a component and
percentage inspection or service monitor needs, the selection of coil is made.

2.2.6.5 TYPES OF COILS (5):
(i) Encircling Coil:

An encircling coil is a coil arrangement in which the coil is in the form of a
solenoid into which the test part is placed. With this arrangement the entire
outside circumferential surface of the test part covered by the coil is scanned at
a time. Its main advantages are Evaluating entire circumference at one time,
High speed of testing and No coil wear problem. Main disadvantages are that it
does not identify the exact location or point of defect in the circumference.

(ii) Inside Coil:

An inside coil is a coil arrangement in which the coil is in the form of a winding
over as bobbin, which passes though the parts like tube, bolt holes, etc., to be
tested and this arrangement scans the entire inside circumferential surface of the
tube or bolt hole at a time. The main advantages are evaluation of the entire
internal circumference at a time, which is otherwise not accessible to any other

optical method of inspection. Main disadvantages are inability to identify the
exact location or point of defect over the circumferencential inspection and
more Wear and tear of the test coil.

(iii) Surface Coil:

Surface coil is a type of coil arrangement in the form of a spring mounted flat
probe or a pointed pencil type probe which scans the surface or selected location
and this arrangement is very useful in exactly locating the defect. In surface
probes, the distance between the probe and the specimen is very critical and this
must be taken care of in probe design or in equipment design, to compensate for
this ‘lift-off’ effect. In some surface probes, the coil is spring mounted, such
that independent of the pressure applied, a constant spring pressure is applied to
the coil and holds it firmly against the specimen. Main advantage of this coil is
that it pin points the defect and disadvantages are speed of testing is slow being
manual and lift-off and edge effect create problems.

2.2.6.6 TEST COIL FUNCTION AND SIGNAL FORMATION (5)

Although the same coil can be used for excitation and for supplying the
response signal, this is not necessary and often not desirable. One coil can be
used for excitation purposes with a second coil or multiple coils used for
monitoring the electromagnetic field conditions. The use of separate coils for
excitation and sensing gives greater flexibility in meeting the test system
requirement. For example, the primary electromagnetic field may be established
by the use of a few turns of relatively large wire driven from a low impedance
generator and the number of turns not the sensing coil can be adjusted to meet
the input impedance requirements of sensing circuits. If desired, sensing circuits
having very high input impedance can be used and the sensing coil may be
wound with many turns of small wire gives a simple illustration of a sensing
coil inside an excitation coil. As a simplified case, let ‘I’ be the current flowing
through the excitation loop. Let us consider an element current Idl at ‘A’ in the
excitation loop. Because of this elemental current at point ‘A’ there will be an
induced voltage E1 at Point ‘B’ in the sensing loop and E 2 at a point ‘P’ in the
test material. The induced electric field E2 at P causes a current Im to flow in the
test object at point ‘P’. This current in turn causes induction resulting in a
electric field Em at point ‘B’ in the sensing coil. Thus in the above simplified

case at point ‘B’ in the sensing element, there is an induction field due to
current element Idl in the primary exciting loop and another filed Em due to the
current intensity at point ‘P’ in the conductive object. Hence the total current
intensity at ‘B’ in the sensing coil due to a current element Idl flowing at point
‘A’ is given by ET = El + Em. Similarly, each elemental point around ‘P’ in the
part contributes its own filed and hence the information carried by the sensing
coil is a cumulative field information which is a complex factor when
contribution of ‘I’ though the excitation loop is considered as a whole.

This resultant field intensity is fed to analyzing circuit. The test coil’s output
signal is shown in the phasor diagram. The curved locus ABCD represents the
test coil output signal locus for variation of conductivity of test object.

A standard test object might have the conductivity represented by signal point
D. The signal phasor OD represents the test coil output for this standard
condition. Now let us assume that the original test object is replaced by a second
test object which has a lower conductivity than the first one. This might give a
test coil signal represented by a phasor OC. These two cases give distinct
difference in readings as shown in fig illustrated below.

Fig. 2.19 Coil Output Shown on Impedance
Diagram

2.2.6.7 TEST COIL SELECTION CONSIDERATIONS (5)

Selection of test coil depends on the nature and shape of the specimens to be
tested, the type of information sought, the location of information sought, the
distribution of information during the course of testing and the magnitude of
testing required. Normally to achieve satisfactory test results, test coils are

selected to suit a specific test situation. This is done depending upon the shape
of the test object, the sensitivity and resolution required in a test situation. The
depth of eddy current penetration depends on test frequency, conductivity and
permeability. Hence selection of frequency for a test situation on part of known
conductivity and permeability is one of the main considerations. Each test
requirements requires high duty cycle system. High production rate testing like
tube, wires, etc. requires a specific test system. High production rate testing like
tubes wires, etc. requires high duty cycle system with a good mechanical,
electrical and thermal stability. The system should have least vibration between
coil and job, whereas in a scanning type surface coil testing, good resolution and
sensitivity are very essential. The test object conductivity varies due to
temperature. Power dissipation in coil-object combination would result in
temperature variation leading to change in conductivity. This thermal drift effect
is troublesome, only when the coil assembly scans the same location of the test
part for more duration.

2.2.6.8 TEST FREQUENCY AND DISTRIBUTION OF EDDY CURRENT (5)

(i) Effect of Frequency on Eddy Current Testing:

Eddy current testing is based on the principle of electromagnetic induction,
wherein the test object is placed under the influence of varying magnetic field of
a test coil driven by an alternating current of required frequency. In general, the
test coil is characterized by a change in impedance which consists of two
electrical impedance parameters. Impedance change due to magnetic variable
causes a change in inductive reactance component ‘XL’. Alternatively, a change
in electrical variable causes a change in resistive component ‘R’ of the
impedance. In the case of inductive reactance XL (2πfL) the test coil’s inductive
reactance is frequency dependent since ‘f’ is the frequency of the applied A.C.
field. Since the inductive reactance is directly proportional to the frequency of
the test coil, it plays an important role in eddy current testing.

There are basically three approaches to eddy current testing: (i) Impedance
testing: (ii) Phase analysis: and (iii) Modulation analysis. These methods can
identify changes in conductivity, permeability or dimensional variations of the
test specimen cumulatively or separately. A change in any one or more of the
above characteristics, or a test part will be identified accordingly, depending
upon the test requirement and choice of the method. Similarly important
instrument characteristics which influence the eddy current testing are: (i)
frequency of the A.C. applied to the test coil; (ii) size and shape of the test coil;
and (iii) distance of the test coil from the object or electromagnetic coupling
(lift-off/fill factor).

Basic factors which influence the eddy current testing are (i) the effective
permeability (µeff.). Which is determined by the frequency ration f/fg ; (ii) the
limit frequency fg which is a function of physical characteristics of the test
object such as conductivity (σ), relative permeability (µrel.). Diameter (d) for
round specimens; (iii) test frequency f, and (iv) electromagnetic coupling (fill
factor in case of cylindrical test object–encircling coil combination).

The optimum test frequency for a specific test problem is determined by theory
or experiment to provide the highest sensitivity to detect variation in
conductivity, dimensions or permeability.

2.2.6.9 SELECTION OF TEST FREQUENCY (5)

Generally, test frequencies used in eddy current inspection range from 200 Hz
to 6 MHz. Frequency has a direct relationship with the ability of any eddy
current test system to accurately and reliably measure the desired property of
the test object. Frequencies can be selected to provide a maximum response
signal caused by the variable. Usually lower frequencies of the order of 1 KHz
are used for magnetic materials and relatively higher frequencies for non-
magnetic materials. Actual frequency used for any specific case/instrument
depends upon the thickness of test material, desired depth of penetration, degree
of sensitivity/resolution required and the purpose of the inspection. For
example, mid range frequency (say 100 KHz) might be used to detect surface
cracks in stainless steel plate. Higher frequencies (1MHz) provide less
sensitivity to cracks and greater sensitivity to lift off/ dimensional variations.

Lower frequencies (1KHz) may provide poor sensitivity to the surface cracks
but have good sensitivity to conductivity variations in the base material.

Selection of frequency is a compromise so that penetration is sufficient to reach
any sub-surface flaw. At lower frequencies, penetration is greater, but at the
same time sensitivity to flaw decreases. Therefore, inspection frequency as high
as possible that is still compatible with the required depth of penetration is
selected. Generally, small flaws remain undetected as the depth increases.
Optimum frequencies are often determined experimentally. Frequency selection
can be within a range since often there will be wide band frequencies, which
produces nearly the same results. The following Fig. gives a general guideline
for frequency selection for different purposes.

Ferrous Crack Detection & Coating

Sorting Non-Ferrous Sorting Cladding

Low Freq. Med. Freq. High Freq.

1Hz 10Hz 100Hz 1KHz 10KHz 100KHz 1MHz 10MHz
Frequency

Fig. 2.20 Frequency Distribution

2.2.6.10 EDDY CURRENT DISTRIBUTION (5)

In eddy current testing, the coil’s field intensity decreases as the distance from
the coil surface increases. The amount of eddy current generated in a specimen
increases as the field intensity increases. If we consider an empty test coil, the
filed would have a constant intensity across the coil’s inside diameter. When the
test coil carrying A.C. is placed near a conductor, the electrons in the conductor
move back and forth generating eddy current which follow circular path. This
path will always be parallel to the surface of the specimen.

The eddy current generated near the surface will be more. As the depth from the
surface increases eddy current intensity decreases. The reason is that the flow of
eddy current generates a secondary magnetic field decreases the intensity of the
coils’ magnetic field. Near the surface, the test coils’ full intensity is applied and

hence eddy current of higher intensity is generated. In the subsequent layer the
intensity is decreased due to the nullifying effect of the secondary magnetic
field due to eddy current. Thereby the eddy current produced in subsequent
layers below will be less than the previous upper layer. This phenomenon
continues until the eddy current intensity becomes negligible. Thus at the center
of a circular conductor of reasonable diameter, eddy current intensity will be
almost negligible.

Distribution of field strength or eddy current density within the test object
determines the sensitivity of the test method. At higher f/fg ratio, eddy currents
concentrate near the surface which results in sensitivity restricted to surface
cracks with reduction in sensitivity to conductivity variations. Too low test
frequencies, no doubt, would have greater penetration but are less sensitive. At
very high f/fg ratio (say 100), the eddy current density fails off very rapidly as
indicated by a steep falling trend of the curve. At f/fg of 4, the intensity falls off
gradually indicating the presence of eddy current even at lower depths. The rate
of reduction of the field strength decreases and the percentage of fields strength
decreases. The eddy current density at the surface decreases as the frequency is
decreased. As the frequency is increased, the eddy current concentrates near the
surface and decreases as the depth increases with virtually no field strength at
the center. Stronger the eddy current, more sensitive is the system for detection
of discontinuities and sensitivity is always greater near the surface.

Inhomogeneities and discontinuities act to obstruct the passage of eddy current,
thereby distorting the circular path. This changes the effective conductivity of a
test piece by concentration of eddy current into a relatively small volume. This
results in lowering the effective conductivity of the specimen. Further, the
response to surface cracks and discontinuities is greatly reduced at low f/fg
ratios. If the magnetizing field strength is increased, the eddy current density
also increases proportionately. But if very high eddy current is developed, it
heats up the test sample which is turn changes the electrical conductivity of the
test specimen resulting in erroneous test results.

2.2.6.11 GENERAL GUIDELINE FOR FREQUENCY SELECTION (5)

Application Frequency Range
Ferrous Sorting 1 to 400 Hz

Crack detection & 400 Hz to 8 MHz
Non ferrous sorting

Coating & Cladding 1 MHz & above
Thickness

Table 2.2 Frequency selection for different materials

2.2.6.12 EDDY CURRENT TEST SYSTEM REQUIREMENTS (5)

Basically any electromagnetic test system consists of :-

(i) a generator to provide a.c. to required frequency which will excite the test coil.

(ii) A modulating device consisting of test coil – test object combination. Varying
property of the component modulates the impedance magnitude of the coil.

(iii) A signal preparation unit consisting of bridge / null balancer, filters, amplifiers,
etc.

(iv) Demodulation and signal analysis unit consisting of phase discriminators,
compensators, etc.

(v) Read out mechanism like meters, CRT, relays, recorders, etc.

Equipment design consideration depends upon the nature and requirement of the
test conducted. Mostly impedance magnitude type equipments are used when
the change in the coils impedance is displayed over a meter or CRT screen. The
deflection of meter reading is proportional to the magnitude of variation in the

sample. Since the impedance magnitude test cannot separate, allowed tolerance
diameter change effects from conductivity changes, it is having limited scope in
practice.

Four basic types of instruments are :-
(i) measuring the change of magnitude of the total impedance of the test coil
regardless of phase.

(ii) measurement of the resistive component of the test coil impedance (core loss).

(iii) measurement of the reactive component of the coil impedance.

(iv) Phase sensitive measurements which separate the resistive and reactive
components of the coil impedance, as required.

Eddy current instrumentation system is designated to sense and indicate
variations in the output of the coil assembly resulting from changes in
electromagnetic field caused by discontinuities in the part under test. The
detection system may include an adjustable phase selective system as well as
filter circuit for the purpose of enhancing the response to specific kind of
variations present in the output of test coil assembly and reducing unimportant
variations. When such selective methods are present, means must be provided to
ensure that their correct adjustments are achieved. This is done by the use of
calibrated controls. The stability of eddy current system should be such that
repeatable results are obtained when a calibration standard is passed through the
test system at various times.

2.2.6.13 READ OUT (DISPLAY) MECHANISMS (5):

An important part of eddy current inspection system is the readout system used
to display the demodulated signals for interpretation. The display device may be
an integral part of the system or a replaceable plug-in module type. The readout
mechanism should be of required speed and accuracy to meet the test
requirement depending upon production speed and variable of interest. A single
test requirement may have more than one device. There are various types of
display devices available.

(i) Uncalibrated Meter:

This is otherwise known as analog meter and gives continuous reading over a
wide range. They are rapid in operation and the scale can be calibrated for any
specific parameter by having standard specimen with known parameter. The
accuracy of such meters is ±1% of full scale.

(ii) Calibrated Meters:

These types of equipment have a meter with a needle, over which calibrated
scales for specific variables to be measured (usually coating thickness, etc.) is
attached. For each measurement different scales are attached and the needle
reading against the calibrated scale given the test parameter directly.

(iii) Digital Meters:

These provide greater accuracy than analog meters. Here the chance of operator
error is negligible, since the reading is direct and no interpretation is involved.
This system is faster than direct meter type.

(iv) Audio and Video Alarm:

Both these two system alert the operator that the test parameter level has
exceeded the pre-set level. These two systems can be set to any threshold level
by adjusting as potentiometer type control. If any input signal exceeds the set
level an automatic warning alerts the operator by way of a blinking lamp or a
buzzer.

(v) X-Y Recorder:

Here the output signal activates a pen which records continuous line as a chart.
When scanning long specimens like rod, tube, etc. the recording reveals position
of a given flaw in one co-ordinate (the longitudinal direction of the specimen) in
the form of a ‘go’ and ‘no-go’ trace or proportional trace. X-Y recorders makes
it possible to simulate on a stationary sheet of chart, the scanning movement of

the coil in two co-ordinates. The X-Y recording is done by means of an ink
filled stylus which is activated by the output signal voltage.

(vi) Storage Oscilloscope:

This is similar in principle to X-Y recorder but is comparatively fast. The
storage pattern is available over a long persistence fluorescent screen of CRT as
along as desired by the operator, but can be cleared when the interpretation is
completed.

(vii) Magnetic Tape Recorder:

The output signal can be recorded in a magnetic tape at a good speed and can be
played back and analyzed later at any time.

(viii) Computers:

These types of mechanisms are very useful for complex test variables and the
output signals from various channels are fed to a high speed computer. These
mechanisms are highly useful in high production rate process lines. The
computer separates parameters and isolates the variable of interest and
significance, catalog data, print the summaries of the result and store them on
tape for reference in future study.

2.2.6.14 APPLICATION OF ELECTROMAGNETIC TESTING (5)

Eddy current method of non-destructive testing is put to variety of applications.

Broadly, eddy current application can be grouped into (i) Conductivity
measurement (sorting, hardness, heart-treatment, & alloy segregation,
carburization, etc.); (ii) Discontinuity testing (cracks, dimensional charges,
surface conditions, etc.); (iii) Thickness measurement (coating, platting, sheet
metal gauging, etc.).

2.2.6.15 FLAW DETECTION (5)

Inhomogeneities in electrically conducting materials appreciably alter the
normal circular eddy current flow pattern and can be detected by eddy current
test coils. The inhomogeneities include crack, inclusions, voids, seams, laps etc.
and are termed as discontinuities.

Single surface probe coil with defectometer is normally employed for detection
of crack depth in forgings, castings, extrusions which are electrically
conductive. The system is balanced with the probe in air and further balanced to
a zero value on sound material of the same composition, heat- treatment and
surface conditions as the component under test. Alternatively, a known defect
can also be used for balance calibration. Flaw suspected areas of the surface are
scanned with the probe which searches for imbalance due to flaw. The
indications for defective component, in meter type, ellipse type and vector point
type presentations are shown in fig. below. For wires, solid and hollow
cylindrical parts, rods, etc. encircling coil is used.

Phase Response from Cracks:

Phase changes are unique for several eddy current inspection parameters. By
determining the phase change of an eddy current response, it is possible to
isolate the response of a specific variable such as conductivity, lift-off,
thickness, permeability and cracks (Refer Fig. 2.21)

Fig. 2.21 Defect Indication in Various Displays

In most applications of eddy current inspection, difference in phase between lift-off
response and crack response is essential for the detection of cracks. When conductivity,
magnetic permeability, frequency, etc. are low and crack size is small, the phase angle
response between lift-off and crack indication will be small. As the magnitude of one or
more of the above variables increases, the phase angle increases. When the crack length
increases, the phase angle approaches more closely the phase angle for conductivity
changes.

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