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 ofProfessor O.P.CHAWLA

DEPARTMENT OF MECHANICAL ENGINEERINGINSTITUTE 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.

SUMMARYThe 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 . With reference (fig. 2.4) shows

that the condition of oscillation has propagated by the distance in the period T. Thus the

following formula applies to the propagation velocity c:

C=/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=f = 1/T *

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 CL 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=/2 = c/2f0

c=sound velocity of the crystal

f0=resonance frequency of the crystal

d=thickness of the crystal

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

titanate

Barium

titanate

Lead

metaniobate

Lithium

sulphate

Quart

z

Lithium

niobate

sound velocity 4000 5100 3300 5460 5740 7320

acoustic impedance

z 106 kg/m2

30 27 20.5 11.2 15.2 34

electromechanic

coupling factor k

0.6 – 0.7 0.45 0.4 0.38 0.1 0.2

piezoelectric modulus d

(Transmission/

generation)

150-593 125-

190

85 15 2.3 6

piezoelectric

deformation constant H

1.8 – 4.6 1.1–

1.6

1.9 8.2 4.9 6.7

coupling factor for

radial oscillations kp

0.5 – 0.6 0.3 0.07 0 0.1 -

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)

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.

Fig. 2.9 Block diagram of ultrasonic testing technique

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

(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.

Fig. 2.11 Pulse Echo Technique

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 L 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.

(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. 2.12 Immersion Testing Technique

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.

Fig. 2.13 Standing Wave

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.

Basic arrangement for producing Eddy Current in a conducting material is shown in

Figure given below:

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.

Fig. 2.14 Eddy Current

Fig. 2.15 Generation of Eddy 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.

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

Fig. 2.16

Effect of Discontinuity of Eddy Current

Effect of distance of testCoil from the part

(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

Standard depth of penetration =

Where f = frequency

r = relative permeability

= electrical conductivity.

The fig. below illustrates the relationship between depths of penetration against

frequencies for various materials.

1 f r

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.

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

The following figure gives the block diagram.

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

OSCILLATOR

BRIDGE CIRCUIT

SINGLE PROCESSING

CIRCUITS

READ OUT

TEST COIL

TEST PART

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 E2 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.

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

Fig. 2.19 Coil Output Shown on Impedance Diagram

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 (2fL) 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 (eff.). 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 (), relative permeability (rel.). 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 &

Non ferrous sorting

400 Hz to 8 MHz

Coating & Cladding

Thickness

1 MHz & above

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)

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.

Fig. 2.21 Defect Indication in Various Displays

2.2.6.16 Effects of Material and Inspection variable on sensitivity and thickness. (5)

Increase in Variable Effect of Sensitivity Range of

Measurement

Conductivity, Material

permeability and test

Frequency

(1) Increases for thin metallic

parts and platings

(2) Increases for thick metallic

parts and platings

(3) Increases for non conductive

coatings

(1) Decreases for

metallic materials.

(2) Increases for non-

conductive

coatings.

Probe diameter (1) Increases for thick metallic

parts and platings.

(2) Increases for non-conductive

coatings.

Increases for metallic

parts platings and non-

conductive coatings.

Table 2.3 Effects of Material and Inspection variable on sensitivity and thickness. (5)

The above table gives the effects of material and inspection variable on

sensitivity and thickness and also their range of measurements.

Generally alloy sorting, heat treatment checks (non-magnetic materials),

cladding thickness, plating thickness, etc. fall under conductivity measurement.

Magnetic material alloy sorting, plating thickness, case depth, heat-treat control,

fall under magnetic permeability measurement. Measurement of thin foil

thickness fall under geometry measurement. Detection of cracks, seams,

inclusion, etc. fall under flaw detection. Insulation thickness, non-metallic

coating thickness measurements, etc. fall under magnetic coupling (lift-off/fill

factor) variation measurement. Table 2.4 given below shows the effective depth

of penetration for various frequencies for different materials.

Material Permeability Conductivity

(σ) %

Depth of Penetration (mm) at

frequency

1 KHz 10KHz 30 KHz 100

KHz

Copper 1 100 2.00 0.63 0.31 0.20

Aluminum 1 65 2.80 0.84 0.48 0.25

Magnesium 1 39 3.30 1.02 0.58 0.33

Brass 1 55 4.30 1.36 0.78 0.43

Mild Steel

(Normal)

500 2.1 0.25 0.10 0.05 0.025

Mild Steel

(Saturated)

1 2.1 6.90 2.27 1.34 0.76

Austenitic

Stainless

Steel

1 2.1 13.40 4.30 2.45 1.34

Table 2.4 Depth of Penetration for various materials and frequencies.

2.2.6.15 STANDARDIZATION AND CALIBRATION (5)

Eddy current method of inspection can only be applied on a ‘GO’/’NO GO’

basis by calibrating the test system against a pre-fabricated standard with

known magnitude of variation, desired to be measured in a specimen which is

identical to the test component except the parameter being measured.

Artificially fabricated standards may contain notches, slots, holes etc. Their

types, advantage, limitations and applicability has been shown in the table 2.5

below.

Type Advantage Limitation Applicability

Drilled Holes (1) Easy to fabricate.

(2) Good dimensional

tolerance.

Maximum response

difficult to detect.

(1) For small crack in Al

& Ti.

(2) Not for small crack in

sheet.

Saw Notches Easy to fabricate in field. Precise dimensional

tolerance possible in

field.

Good for crack standard

in Al, Ti and Steel.

Conductivity

Difference

(1) Easy to use in field.

(2) No tools/ labour

involved.

(1) Response varies

with type of probe.

(2) Primarily for non-

ferrous alloys.

(1) Not for small cracks.

(2) For instrument

operation checking.

(3) Not for use with

steels.

Slots in foils

(Razor cuts)

(1) Easy for field

fabrication.

(2) Formed to curved

surface.

(1) Dimensional

control difficult.

(2) Not for harder

metals.

(3) Response

repeatability poor.

Not for small cracks.

E.D.M. Notches

(Electrical

discharge

machining)

(1) Good dimensional

tolerance.

(2) Response similar in

phase to crack.

(1) Cannot be

fabricated in field

fabrication

(2) Expensive

For small cracks in Al, Ti

and Steel.

E.D.M. slots in

foils

(1) Good dimensional

control.

(2) Formed to curved

surface.

(1) Expensive.

(2) Difficult for field

fabrication.

(3) Response

repeatability poor.

Not for small cracks.

Shrink fit plugs For surface and bolt

holes.

Off scale gross

indication.

(1) Not for small cracks.

(2) For checking

instrument operation.

Milled Notches Less expensive than

E.D.M. notches

(1) Response varies

with type of probe.

(2) Difficult for field

fabrication.

(1) Not for small cracks.

(2) For checking

instrument operation.

Table 2.5 Commonly used Reference Standards

2.3 RELATIVE ADVANTAGE & DISADVANTAGES OF VARIOUS NDTMETHODS :-

Name of the Method Advantages Disadvantages(A)

RADIOGRAPHY (GAMMA RAYS AND X-RAYS) INSPECTION

(i) Low Initial Cost(ii) Permanent

Records(iii) Portable(iv) Small Sources

can be placed with small openings.

(v) Panoramic imaging possible (X-Ray)

(vi) High sensitivity to density changes.

(vii) Nocouplant required.

(viii) Geometry variation doesn't effect direction of X-ray beams.

(i) Trained operator needed.

(ii) Cost related to source size.

(iii) Depth of defect not shown.

(iv) High initial cost (v) Source decays

radiation hazard.(iv) Sensitivity

decreases with increase in scattered radiation.

B. ULTRASONIC INSPECTION

(i) Most sensitive to cracks.

(ii) Test results known immediately.

(iii) High penetration capability.

(iv) Automation and permanent record capability.

(i) Couplant required

(ii) Small Thin & complex parts difficult to test.

(iii) Reference standard required.

(iv) Trained operator required.

(v) Special probes required

C. EDDY CURRENT INSPECTION

(i) No special operator skill required

(ii) Low cost(iii) Automation

possible for symmetrical parts

(iv) Permanent record possible

(v) No couplant required

(i) Conductive material alone can be tested

(ii) Shallow depth of penetration

(iii) False indication due to variation in part geometry

(iv) Reference standard required

(v) Permeability variation causes difficulties.

D. MAGNETIC PARTICLE INSPECTION

(i) Advantage over penetrant that it indicates sub-surface defects – particularly inclusions.

(ii) Relatively fast inspections.

(iii) Low cost.(iv) Portable &

Fixed both types available

(i) Low sensitivity due thick surface coatings.

(ii) Alignment of magnetic field is critical.

(iii) Demagnetization of parts required after test. Must be cleaned before and after the test.

E. PENETRANT (DYE AND FLUORESCENT INSPECTION)

(i) Portable(ii) Low cost(iii) Indications may

be further examined visually

(iv) Results easily interpreted.

(i) Surface Films such as coatings, scales, smeared metal prevent detection of defect.

(ii) Parts must be cleaned before and after inspection.

(iii) Defects must be open to surface.

3. EXPERIMENTAL SETUP

3.1 HIGH FREQUENCY EDDY CURRENT INSPECTION OF A320 AIRBUS

WHEEL HUB ASSEMBLIES (6)

3.1.1 INSTRUMENTATION

(i) Eddy Current Inspection Defectometer Model H 2.835

(ii) Eddy Current Inspection Non-ferrous surface probe (Shielded)

(iii) Eddy Current Inspection reference standard for calibration

Fig. 3.1 Defectometer Model H 2.835

3.1.1.1 Defectometer Model H 2.835 has the following features (Refer Fig. 3.1):

(i) On/Off switch “O/I”

(ii) “Ready” display

(iii) Pushbutton “L” Lift-Off compensation

(iv) Pushbutton “Z” zero compensation

(v) Pushbutton “M” material-Type (Fe, NFe, Aust.)

(vi) Pushbutton “*” open lock

(vii) Pushbutton “” to increase sensitivity

(viii) Pushbutton “” to decrease sensitivity

(ix) Five LED’s “DEFECT”

(x) Analog instrument “crack severity”

(xi) Threshold switch

(xii) “ZERO” compensation

(xiii) Symbols for “Lift-Off” compensation

(xiv) “Ready” symbol to indicate operability

(xv) Symbol for Material type

3.1.1.2 Eddy Current Tester Calibration Procedure

(i)Lift-Off compensation

Lift-Off may be defined as the change in impedance of a coil when the coil is moved away from

the surface of the specimen. This produces a large indication on the test equipment.

Flashing of the symbol indicates that Lift-Off compensation must be carried out. For this

purpose raise the test probe from the testpiece and depress the push button “L”. This stops

flashing of “L” in the display and symbol remains for upto 3.5 seconds whilst Lift-Off

compensation is carried out. The “L” is extinguished and “Z” beings to flash with the symbol

. (Refer Fig. 3.2)

Fig 3.2 Eddy Current Tester Calibration Procedure

(ii)Selection of Test Probe

The test probes are either Ferrous, Non-ferrous or austenitic, depending upon the type of material

to be tested. For Non-ferrous mode the test frequency is approx. 2 MHz and for ferrous and

austenitic mode it is approx. 4 MHz. Special probes can be obtained for very special inspections

as per the test requirements. There are surface probes which are used for detecting surface or

sub-surface cracks. There are hole probes used to detect radial cracks in round fastener holes

(Cracks inside the fastener holes).

Calibration Standard is supplied along with the apparatus as per the material to be tested and test

probe being used. In order to calibrate the equipment, standard reference pieces, manufactured

from a material similar to that being tested, are necessary. These pieces should contain defects of

known size and shape, so that the change in coil impedance against a known defect could be used

as an acceptance limit.

3.1.2 EXPERIMENTS (6)

3.1.2.1 PROCEDURE AND OBSERVATIONS

As a part of On-Condition (O.C) preventive maintenance programme of A-320, main

wheel assemblies, High frequency eddy current inspection is carried out during every

shop visit of the wheel assemblies (Shop visit is mainly due to worn out tyres requiring

tyre change or wheel assemblies overhaul).

I associated in carrying out A320 aircraft main wheel hub assemblies high frequency

eddy current inspection in the wheel-bay section of Accessories overhaul shop. The

details of which are given herebelow & copies of their work sheets are enclosed for ready

reference. First I calibrated the defectometer with the help of the testpiece. After

connecting the test probe to the probe socket, the defectometer is switched on. The

display indicates the sensitivity values and the type of material of the last test. For the test

to be carried out one should make sure the displayed material type (Fe, NFe, Aust.)

agrees with that of the test probe. 12.5 seconds after the appearance of the “READY” all

push buttons are locked with the exception of the button “Z” and “L” thus preventing

inadvertent changes to the selected settings. The defectometer is now ready for use. The

crack severity display of the unit can be set with the help of test flaws in the testpiece.

The sensitivity can be modified between 0 and 19.5 db in the steps of 0.5 db. An audible

flaw signal can also be obtained in addition to the visual signal. The audio generator

incorporated in this defectometer can be set with the threshold switch as a threshold value

to 20, 40, 60, 80 and 100 scale divisions of instruments corresponding to 5 flaw

thresholds.

Testing is extremely simple. The test probe is lowered onto the test piece and smoothly

guided over zone to be tested. If the material is flawless, the pointer merely flutters

around the scale reference point due to the inaccuracy of the manual probe guidance. On

the other hand cracks on the surface of the material cause a deflection of the pointer

which is proportional to the depth. The display increases as the crack depth increases. As

the calibrated threshold is reached, an audible signal is given along with large deflection

of the pointer on the scale. If testing is to be carried out in transition from plane surface to

an acutely curved surface undesirable displays appears on the unit. This situation is

eliminated by means of following described calibration method. For testing curved

surfaces, the instrument pointer can be set to the centre of the scale with the zero control

after switching on the unit. To test concave surfaces, Lift-Off compensation is carried out

by placing the probe on the level surface and Zero compensation is carried out in the

curve. To test convex surface, Lift-Off compensation is carried out on the curve and Zero

compensation is carried out on the level surface. The defectometer is now ready to test

the curved surface after “READY” appears on the display.

After testing few of the main wheel hub assemblies, the following five Main wheel hub

assemblies were found to have defects, the details of which are given herebelow:

Fig. 3.3 Eddy Current test Procedure

(i). Main wheel assemblies serial no. AM442 removed from A320 aircraft VT-EPO

durng its ‘4A’ check on 8/1/05 due to uneven wear of the tyre. During shop checks & as

per item A of the WORKSHEET of A320 aircraft Main wheel assembly

servicing/overhaul schedule, high frequency eddy current inspection of inner & outer half

hubs of main wheel assembly serial no. AM-442 was carried out in association with shop

inspection engineers using non-ferrous surface probe & defectometer. Inner half of hub

was found to have a crack of approx ¾” on the web near flange attachment area. Outer

half of the hub was found satisfactory (Refer Fig. 3.4)

Fig 3.4 Crack on the web near flange attachment area of the hub

Refer Appendix I for Shop Inspection Report of Main Wheel Assembly S.No. AM442

Hub Assemblies

(ii). Main wheel assembly serial no. AM 398 removed from A320 aircraft VT-EPM on

8/10/04 during night halt inspection due to its tyre wear. During shop investigation,

H.F.E.C inspection of inner & outer half hubs carried out using non-ferrous surface probe

& defectometer, a circumferential crack of ½” length observed between the tie bolt holes

of outer hub. H.F.E.C inspection of inner hub found satisfactory. (Refer Fig. 3.5)

Fig. 3.5 Circumferential crack between the tiebolt holes.

Refer Appendix II for Shop Inspection Report of Main Wheel Assembly S.No. AM398

Hub Assemblies.

(iii). Main wheel assembly serial no. AM208 removed from A-320 aircraft VT-EPC on

20/10/04 during preflight inspection due to uneven wear of the tyre. During shop

investigation, High frequency eddy current inspection of outer & inner wheel hubs

carried out & inner half hub found to have a longitudinal crack of approx. 0.4 inch near

the drive block area (The web area). H.F.E.C of the outer half hub satisfactory (Refer Fig.

3.6)

Fig. 3.6 Longitudinal crack in the drive block area

Refer Appendix III for Shop Inspection Report of Main Wheel Assembly S.No. AM208

Hub Assemblies.

(iv). Main wheel assembly serial no. AM201 removed from A320 aircraft VT-EPR on

3/1/03 during night halt inspection due tyre worn-out. During shop, investigation, outer

half of the hub having a crack of approx. ¼ inch between spoke hole area. Inner half of

the hub satisfactory (Refer Fig. 3.7)

.

Fig. 3.7 Crack between the spoke hole area

Refer Appendix IV for Shop Inspection Report of Main Wheel Assembly S.No. AM201

Hub Assemblies.

(v). Main wheel assembly serial no. AM493 removed from A320 aircraft VT-EPI on

21/9/04 during ‘2A’ check as wheel assembly was due for overhaul. During shop

investigation, outer half of the hub having a circumferential crack of approx. 0.7 inch

between the tie bolt holes & approx. 0.5 inch diagonal crack at the nozzle hole. H.F.E.C

of inner hub – satisfactory (Shop NDT report enclosed) (Refer Fig. 3.8).

Fig. 3.8 Cracks between nozzle hole and tiebolt hole

Refer Appendix V for Shop Inspection Report of Main Wheel Assembly S.No. AM493

Hub Assemblies.

3.2 ULTRASONIC INSPECTION OF L.P COMPRESSOR FAN BLADE OF M/s IAE

V2500 ENGINE INSTALLED ON A320 AIRCRAFT. (7&8)

3.2.1 INSTRUMENTATION

(i) High Frequency Ultrasonic Testing Instrument

Meccasonic D125BJ or Meccasonic D325BJ

(Refer Fig. 3.9)

Fig. 3.9 Meccasonic Ultrasonic Tester

(ii) 25 MHz focussed testing probe

Meccasonic ‘Blueface’ probe 1001225 or equivalent as per Overhaul Manual

(iii) Calibration standard to be made from the same material or a material with same

acoustic velocity as the part to be tested.

(iv) Ultrasonic Couplant Ultragel II

Fig. 3.10Ultrasonic Tester Calibration

Ultrasonic Tester Calibration Procedure Model D325BJ unit (Refer Fig. 3.10)

(i) Switch the set on.

(ii) Set the vertical movement control to mid-setting and push in the vertical control.

(iii) Set the horizontal movement control to mid-setting.

(iv) Set the outer thickness control to position 1 and centre thickness control to its mid

position.

(v) Turn the brightness control to get a trace of necessary brightness and focus the trace.

(vi) If necessary control the display with vertical and horizontal controls.

(vii) Set amplitude control to its mid position.

(viii) Select calibrate position on interface trigger button.

(ix) Turn delay control fully anti-clockwise

(x) Turn horizontal control clockwise to position first pulse at far left of screen.

(xi) Position applicable calibration piece and first multiple echo must be monitored at mid-

screen on the time-base.

(xii) Set thickness control to position 5.

(xiii) Turn delay control clockwise the first pulse will go out of view to the left. Continue to

turn until the first multiple echo is at the far left of the screen but no part of the pattern is

off screen.

(xiv) Push IF button into LOCK position. No multiple echo should be seen at this phase.

Slowly and continuously increase IF LEVEL control from 0 until echo pattern jams and

locks on to the left of the screen.

(xv) Tilt calibration piece to reduce the interface echo amplitude and get the most stable

display possible on the smallest interface echo.

(xvi) Set thickness control to position 3.

(xvii) Using centre THICKNESS control and horizontal control calibrate screen as necessary.

The video display must calibrated from at least the first and second backwall echo’s.

Adjust amplitude control to produce a first backwall echo ½ to ¾ screen height.

Selection of Test Probe

The test probe consists of a transducer mounted in a damping material and connected electrically

to the test set. For any particular application it may be necessary to use a probe of particular

design so that a sound beam is injected into the material at an angle normal to the expected

defect. In certain applications a wheel probe, consisting of a transducer mounted inside an oil-

filled plastic tyre, has been found suitable for high speed automatic scanning.

Reference Standards

Reference standards are used for:

(i) Calibration of ultrasonic testing equipment

(ii) Compare test indications of ultrasonic inspection material against defects (real or

artificial) of known size must be specified with the applicable reference number.

3.2.2 EXPERIMENTS(7&8)

3.2.2.1 PROCEDURE AND OBSERVATIONS

Ultrasonic testing is a non-destructive procedure looking for defects or breaks within

materials which can transmit sound at high frequency in these materials, land quality,

voids, cracks, inclusions, porosity can be found throughout the work piece.

As described earlier, in ultrasonic testing an electric pulse is applied to a piezo-electric

crystal, which causes it to vibrate. When this vibration is applied to a sound transmitting

material, a mechanical wave is caused in the material until stopped or reflected by a break

or edge. The reflected energy is found by the same or a different crystal. These detected

signals are applied to an amplifier & visually presented on a cathode ray tube. This

presentation is then analysed to give the size, location & other property of the break or

edge.

The above NDT method was employed to check internal health of V2500 engine fan

blades. These fan blades are Hollow Titanium Hives with Aluminium Honeycomb Core

which are bonded together to form integral structure & are of Wide Cord type for

improved aerodynamic efficiency & for improved F.O.D resistance. I associated in

carrying out ultrasonic inspection on two of such fan blades in the jet overhaul shop &

their detailed inspection reports are as follows.

(I). Engine fan blade part no. 6A7650 serial no. 23475 removed from V2500 engine

serial no. V0079 following shop visit inspections were carried out.

(A)After thoroughly cleaning the fan blade, I carried out ultrasonic inspection of fan

blade dovetail root flank (M/s IAE engine manual task 72-00-00-200-011). The following

procedure was employed after calibrating the ultrasonic testing equipment

COMAT 06-148 ultrasonic couplant was applied to the concave surface of blade root

flank.

(i) Ultrasonic probe was positioned at front of the leading edge of concave blade root flank

& the probe was moved along full cord width of the blade. The signal was monitored

carefully on the display to ensure that signal greater than 60% of screen height between

4.5 & 5.5 division lines on time base is not produced which is criterion for blade

rejection.

(ii) Same procedure was repeated for convex blade root flank.

(iii) Both concave & convex blade root flank inspection were found satisfactory.

Fig. 3.11 Ultrasonic C-scan Inspection

(B) I Carried out C-scan inspection of fan blade aerofoil (M/s IAE engine manual task

72-00-00-200-013). The following procedure was employed after calibrating the C-scan

ultrasonic equipment as follows (Refer Fig. 3.11).:

(i) Switched on the system & 30 minutes system warm up time given

(ii) Entered relevant details of engine & blade serial no. & logged on to the system

(iii) Checked the probe alignment as follows:

a) Checked the front face of the probe holder is parallel to the end of the tank, by placing a

ruler on the front face of the end of the tank. If not parallel, loosen the grub screw & twist

the holder to correct the error.

b) Checked for air bubbles under the probe as follows:

c) Removed the probe from the holder & whilst the probe is in the water gently wipe the probe

face & holder to remove any air bubbles

d) Selected the CALIBRATE icon.

e) Loaded the calibration blade QC6808

f) Scanned the calibration blade QC6808

g) Typed ‘TEST’ in the serial number box & then selected ‘CALIBRATION BLADE’ from

the BLADE TYPE box & scanned the calibration blade again. The system will now reject

the calibration blade.

h) Every four hours, the calibration blade was loaded & tested to ensure the system has not

drifted, then the aerofoil of the L.P Compressor blade was examined as follows:

i) V2500 blade to be inspected was selected.

j) Entered the details of blade to be scanned like serial no., part no., engine serial no.

k) Scanning of the blade done

l) Scanning accepted the blade & as such passed the subject inspection.

m) If the blade is rejected in the C-scan, the system prompts the archive of the blade C-scan

data to an archive directory for future records.

Fig. 3.12Ultrasonic Tap Test Technique

(C) I Carried out TAP Test of the subject fan blade (Transient Acoustic Propagation) to

examine the fan blade internally (M/s IAE engine manual task 72-00-00-200-010) using

Transient Accoustic propagation (TAP) tester as follows (Refer Fig. 3.12):

(i) Carried out the system check of the TAP tester by connecting the dedicated probe to the

tester. Carried out the functional check of the TAP test set ensuring the TAP test block

value shown on the tester display is in the range of values engraved on the side of the tap

test block.

(ii) Applied small quantity of COMAT 06-148 ultrasonic couplant to the lower convex

aerofoil adjacent to the annulus filler line & attached the probe to the fan blade. Pressed

the ‘ON’ switch & then pressed the ‘EXE’ switch (Execute). The display showed the

value within approx. 4 seconds. The number displayed was 490 db/sec. Hence blade

‘passed’ the TAP test. If the number displayed is > 700 db/sec, the blade is to be

rejected.

Refer Appendix VI for Inspection Report of Fan Blade S.No. 23475

(II) Similarly, Engine fan blade part number 6A7649 serial no. 43786 removed from

engine serial no V0072 shop inspection were carried out & the ultrasonic inspection of

dovetail root flank of the blade, C-Scan and tap test inspection of the blade were carried

& all the tests were satisfactory. As such this blade also passed all the ultrasonic

inspection checks. (Copies of inspection schedules of both the blades are enclosed for

ready reference).

Refer Appendix VII for Inspection Report of Fan Blade S.No. 43786

4. RESULT AND DISCUSSION

4.1 While carrying out H.F.E.C. inspection of the main wheel assembly hubs, the various

types of sub-surface and surface flaws observed are crack at the hub flange attachment

holes, crack between the wheel nozzle hole and flange attached hole, sub surface/surface

crack at the blending area of the vent holes of the hub, fine crack initiation at the groove

formed by the circlip flange, crack at the corner formed by flange and the web (Area of

sudden change of cross section causing stress concentration) and cracks in the hub bead

area.

(Refer attached figure of inner (Fig. 4.1) and outer (Fig. 4.2) main wheel hub assemblies

with typical crack locations).

Fig. 4.1Cracks in the inner hub area

Fig. 4.2 Cracks in the outer hub Area

As per the available date, there is about 1 to 1.5% wheel hub rejection due to the above

described defects in a year.

4.1.1 The inspection results of the failed main wheel hub assemblies were studied and the

following preventive measures are suggested to minimise these failures

(i) Cracks between the nozzle & tie bolt holes are normally starting from the nozzle. These

tyre charging nozzle assemblies are of hexagonal cross-section with sharp corners. The

sharp corners of hexagonal section cause notch formation and then result in crack

initiation. To obviate these cracks, the sharp edged at corners of the hexagonal section of

the nozzle should be rounded-off thereby eliminating the causes of crack initiation.

(ii) Cracks between the tie bolt holes are traced to their unequal torquing. To obviate these

cracks equal torquing of the tie bolts in criss-cross manner i.e. (proper sequence

diagonal torquing) is suggested.

(iii) Surface/Subsurface cracks of approx. 1 to 2 mm at the cooling holes of the wheel hubs

were observed. These can be eliminated by the grinding of the crack area and restoring

the surface finish by alodine treatment and subsequent primer/ paint of the surface.

(iv) Bead area cracks are mainly due to pitting and stress corrosion of the area due to high

level of stress accompanied with some moisture in the tyre inflating nitrogen. To obviate

these cracks, it is suggested that the tyre filling nitrogen should be moisture-free. The

pitting of the bead area should be critically monitored and same should be removed by

smooth grinding. The surface protection be restored by alodining and subsequent

primer/ paint. By removing the pitting damage, we shall be eliminating the cause of the

crack initiation. Moreover, it is suggested that a water base paint be applied in this area

(Resin + Hardener + Water) which will give great resistance to this area against pitting

corrosion.

(v) Cracks in the web area are observed at very few occasions and the matter has been taken

up with the hub manufacturers to improve the surface treatment of the area or change in

the alloying elements of Aluminum to improve its fatigue strength. The matter is under

study by the hub manufacturers.

4.2 The various ultrasonic inspections were carried out on two of the different V2500 engine

fan blades & no defect was found in any of these inspections.

4.2.1 However, after studying the shop reports of the fan blades in the last one year, I observed

that there were few engine fan blade rejections due to their failure in dovetail root flank

inspections. Similarly, few fan blades were rejected due to their failure in ‘C-scan’ &

‘Tap-test’ inspections.

The following preventive measures are suggested on the on wing engines and during

scheduled and unscheduled shop visits of the engines:-

(i) It is suggested that the subject engine fan blade NDT inspections must be done more

frequently to detect these hidden defects. As these initial defects if not detected timely

will propagate in service & could result in fracture of fan blade and subsequent

extensive internal damage to the engine. These NDT inspections must be done on on-

wing engines during all major maintenance checks of the aircraft and during scheduled

& unscheduled shop visits of the engines in the overhaul shop.

(ii) The special bird-strike/FOD schedules must include an item of NDT inspection relating

to fan blades – specially the fan blade root area NDT inspections.

(iii) Whenever any engine fan module (N1) high vibration is reported, the subject NDT

inspections of fan blades (specially the blade root area) must be called out.

5. CONCLUSION

(i) After studying the various NDT techniques, their applications, advantages & limitations,

I have concluded that NDT is a very important tool for the modern production,

maintenance & process industry. It helps in a great way to operate these industries

economically, safely with high reliability & produce quality end-products. This is the

ultimate goal of any industry.

(ii) Regarding high frequency eddy current inspection of the aircraft wheel hubs, the various

defects observed were studied & their remedial measures have been suggested after

studying these problem areas.

(iii) Similarly, various ultrasonic inspections of engine fan blades can pinpoint a smallest

possible hidden defect, the continuance of which can cause a catastrophic failure or an in

flight shut down of the engine. As such more frequent on wing and shop NDT

inspections of the fan blades have been suggested. These inspections have also been

suggested during reported engine F.O.D / Bird ingestion or in case of reported high fan

module (N1 rotor) vibrations.

(iv) Above remedial measures if carried out scrupulously shall go a long way in improving

the in-service reliability of aircraft wheel hub assemblies & engine fan blades.

6. REFERENCES

1. Civil Aviation Authority, U.K., Civil Aircraft Inspection Procedure Non–Destructive

Examination – Eddy Current Methods – Chapter bl/8-8 Issue 1, page 1-14, 1973.

2. Civil Aviation Authority, U.K., Civil Aircraft Inspection Procedure Non–Destructive

Examination – Ultrasonic Flaw Detection – Chapter bl/8-3 Issue2, page 1-12, 1971.

3. Non-Destructive Testing, Robert c. Mc Master, The Ronald Press Company, London, page

16-34, 1963.

4. Non-Destructive Testing, William e Schall, Machinery Publication Ltd., London, page 28-

56, 1968.

5. Eddy Current & Ultrasonic Testing (snt-tc-1a), Satyakiran School of NDT, New Delhi,

page 3-96, 1996.

6. Component Maintenance Manual of Main Wheel Assembly – A320 Airbus Aircraft., Messier

Hispano Bugatti, Spain, page 501-511, 2003.

7. V2500 Engine Maintenance Manual, International Aero Engines, U.S.A, page 1-10, 1998.

8. V2500 Engine Service Bulletins & Inspection Procedures, International Aero Engines,

U.S.A, page 1-10, 1999.

7. APPENDIX