STUDY OF VARIOUS NON-DESTRUCTIVE
 TESTING TECHNIQUE & APPLICATION OF
THESE TECHNIQUES TO INSPECT AIRCRAFT
WHEEL HUBS & AER...
CERTIFICATE

This is to certify that the project entitled “Study of various non-destructive testing techniques and
applica...
ACKNOWLEDGEMENT

I would like to thank and express gratitude to my project guide Dr. O.P. Chawla for all his
assistance an...
ABSTRACT

The aim of the project is to study the various Non-destructive methods being used in the modern
manufacturing, o...
SUMMARY
The subject matter for the final year major project is the study of various non-destructive testing
techniques bei...
CONTENTS


Certificate
Acknowledgement
Abstract
Summary
1. INTRODUCTION
      1.1 Non-destructive testing techniques and t...
3.2   NDT Inspection-Ultrasonic Inspection of L.P. Compressor fan blade of M/s IAE
           V2500 Engine installed on A3...
1.1 INTRODUCTION


N.D.T is the technology of assessing the soundness & acceptability of an actual component
without affec...
inspections to determine cracks due to fatigue damage/creep, corrosion damage, de-
bonding/delaminating of composite mater...
1.3 MOTIVATION BEHIND THE PROJECT
My major objective to select “Study of various N.D.T. Techniques” as a project is due to...
This chapter also describes about the various remedial measures suggested to minimise
     these failures for smooth engin...
2.      LITERATURE REVIEW
2.1   Review of books and journals
      CIVIL AVIATION AUTHORITY, UK (1) describes about Eddy C...
document also describes about the choice of frequency for carrying out this test on
different material of different size a...
generation of Ultrasonic waves. Various methods of Ultrasonic testing are explained in
details which includes ultrasonic t...
(i)     Examination of engine fan blades internally using Transient Acoustic Propagation (TAP) tester.
        (Reference ...
2.2     Different types of Non-Destructive testing methods:-
2.2.1   Visual Testing Technique (3)
        Visual aids can ...
2.2.2   RADIOGRAPHY TECHNIQUES (3&4)
2.2.2.1 RADIOGRAPHY GAMMA RAY INSPECTION (3&4)
        It is projecting a three dimen...
inspection of boiler tube thinning due to corrosion or erosion in power plants, castings,
weldments, and small thin comple...
2.2.3   MAGNETIC PARTICLE INSPECTION (4)
        This NDT process is normally applied to homogeneous ferromagnetic materia...
cracks, seams, porosity and inclusions. It is extremely sensitive for locating small tight
        cracks of ferromagnetic...
All parts with non-absorbing surfaces (forging, weldments and castings) can be subjected
        to this NDT inspection fo...
Fig. 2.3




                                         Fig. 2.4
                           Figures depicting Propagation of...
f is the frequency of the oscillation, its unit is "number of oscillation/second". The unit is
       named after the phys...
see that the compressions and diminishing move through the test object at an unchanging
distance. The velocity at which th...
Longitudinal and transverse waves can propagate only through the whole volume of a
      workpiece. At the interfaces or s...
c=sound velocity of the crystal
f0=resonance frequency of the crystal
d=thickness of the crystal
λ=wave length




       ...
coupling    factor    for 0.5 – 0.6          0.3       0.07          0           0.1        -
           radial oscillatio...
For the practical application in the material testing, probes are used into which the
          piezoelectric crystal are ...
(i)   Principle of the ultrasonic test instrument
      So far we know that, with the probe, we can transmit ultrasonic pu...
If the whole volume of a workpiece is to be covered it is necessary to scan at least one
          surface completely. E.g...
As soon as an echo appears within the monitor threshold, a voltage is fed to the control
          output which is proport...
Fig. 2.8 Automatic Ultrasonic testing machines (Block diagram)




2.2.5.7   METHODS OF ULTRASONIC TESTING (5)




       ...
In order to test a certain workpiece for certain flaws it is not only important to choose a
        suitable probe but als...
Fig. 2.11 Pulse Echo Technique




(a) Calibration:
         By means of the calibration it is possible to allocate the wh...
Using a straight-beam probe it is impossible to detect reflectors which run inclined or
       perpendicular to the surfac...
with situations were we have to test thin parts or where especially near-surface flaws
are to be detected. In these cases,...
Twin crystal probes are often operated in combination with digital wall thickness gages.
       After coupling the probe a...
probes differ from the straight-beam probes for direct contact only insofar that they are
        watertight moulded inclu...
Fig. 2.13 Standing Wave

The ultrasonic wave is reflected on both interfaces thus traveling through the plate in
two diffe...
2.2.6     EDDY CURRENT INSPECTION TECHNIQUES (1&5)
2.2.6.1   PRINCIPLES OF EDDY CURRENT (5)
          Eddy Current and Its...
The induced eddy current produces its own magnetic field (or flux) in a direction
          opposite to the inducing prima...
(ii)     Effect of in homogeneities and discontinuities:
The inhomogeneities and discontinuities like cracks, inclusions, ...
(iv)   Effect of Test Frequency:

       The magnitude of induced eddy current in an object increases with frequency of
  ...
Fig. 2.17 Depth of Penetration-Frequency for Various Materials.



2.2.6.3 EDDY CURRENT TEST SYSTEM (5)

         Basic Te...
The following figure gives the block diagram.



                                                       OSCILLATOR


     ...
between the test instrument and test object. It serves two main functions. The
           first one is to establish a vary...
optical method of inspection. Main disadvantages are inability to identify the
           exact location or point of defec...
case at point ‘B’ in the sensing element, there is an induction field due to
          current element Idl in the primary ...
selected to suit a specific test situation. This is done depending upon the shape
          of the test object, the sensit...
There are basically three approaches to eddy current testing: (i) Impedance
          testing: (ii) Phase analysis: and (i...
Lower frequencies (1KHz) may provide poor sensitivity to the surface cracks
      but have good sensitivity to conductivit...
hence eddy current of higher intensity is generated. In the subsequent layer the
intensity is decreased due to the nullify...
2.2.6.11 GENERAL GUIDELINE FOR FREQUENCY SELECTION (5)


                     Application                 Frequency Range
...
sample. Since the impedance magnitude test cannot separate, allowed tolerance
         diameter change effects from conduc...
(i)     Uncalibrated Meter:

        This is otherwise known as analog meter and gives continuous reading over a
        w...
the coil in two co-ordinates. The X-Y recording is done by means of an ink
         filled stylus which is activated by th...
2.2.6.15 FLAW DETECTION (5)


       Inhomogeneities in electrically conducting materials appreciably alter the
       nor...
Phase Response from Cracks:


         Phase changes are unique for several eddy current inspection parameters. By
       ...
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
various nondestructive testing techniques and their inspections on aircraft structures.
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to study various nondestructive testing techniques, related inspections - eddy current on aircraft wheel hubs of airbus a320 aircraft and ultrasonic inspections on i.a.e v2500 aero engine fan blades, suggest remedial measures.

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various nondestructive testing techniques and their inspections on aircraft structures.

  1. 1. STUDY OF VARIOUS NON-DESTRUCTIVE TESTING TECHNIQUE & APPLICATION OF THESE TECHNIQUES TO INSPECT AIRCRAFT WHEEL HUBS & AERO ENGINE FAN BLADES & SUGGEST REMEDIAL MEASURES. By KARAN DUGGAL Under the Guidance of Professor O.P.CHAWLA DEPARTMENT OF MECHANICAL ENGINEERING INSTITUTE OF TECHNOLOGY & MANAGEMENT, GURGAON, HARYANA (AFFILIATED TO M.D. UNIVERSITY, ROHTAK, HARYANA, MARCH 2005)
  2. 2. 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.
  3. 3. 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.
  4. 4. 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.
  5. 5. SUMMARY The subject matter for the final year major project is the study of various non-destructive testing techniques being used in the present manufacturing, overhaul & service industry. Also, their important, applications, limitations, relative advantages & disadvantages were also studied. Further, as a specific application of NDT methods, study of High Frequency Eddy Current (H.F.E.C) inspections of A320 Airbus Aircraft main wheel Hub assemblies was carried out under the guidance of N.D.T specialists of aircraft accessories overhaul shop, Indian Airlines, N.R. The defects so observed were studied in depth & remedial actions suggested to minimize such failures in future. Continuance of these defects in service without detection could result into uncontained failure of the component endangering the safety of the aircraft in-service. Similarly an another application of NDT methods regarding Ultrasonic Inspection of Aero engine fan blades root flank area & its tap test was done to find out a debond or a crack in this area to reject such a fan blade. Continuance of this defect in service could lead to fracture of the blade from the root area causing consequent extensive internal damage to the engine and a serious flight safety problem. To prevent such in service failure, I suggested more frequent on wing engine fan blade root flank & tap test N.D.T Inspections during major maintenance checks of the aircraft. These inspections during scheduled and unscheduled shop visits of the engines were also suggested to be done scrupulously. I also suggested to carryout these NDT inspections of fan blade whenever there is any reported or observed engine bird-strike or any time high engine fan blade vibrations are reported. These preventive NDT inspections in the critical areas prone to high fatigue stresses in the areas of high stress concentration can prevent an impending failure of component/system, thereby improving the component/system in-service reliability.
  6. 6. 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
  7. 7. 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
  8. 8. 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
  9. 9. 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)
  10. 10. 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.
  11. 11. 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
  12. 12. 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
  13. 13. 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
  14. 14. 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:-
  15. 15. (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.
  16. 16. 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.
  17. 17. 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
  18. 18. 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
  19. 19. 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
  20. 20. 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.
  21. 21. 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
  22. 22. Fig. 2.3 Fig. 2.4 Figures depicting Propagation of wave The time required is the period of oscillation T. On the momentary representation XV we can see that particles zero and twelve are just experiencing their max. deflection to the left, i.e. they are in the same condition of oscillation, The distance between two particles which are in the same condition of oscillation is the wave length λ. With reference (fig. 2.4) shows that the condition of oscillation has propagated by the distance λ in the period T. Thus the following formula applies to the propagation velocity c: C=λ/T From the period of oscillation T the number of oscillations per second can be calculated by the formula F=1/T
  23. 23. f is the frequency of the oscillation, its unit is "number of oscillation/second". The unit is named after the physicist H. Hertz and is abbreviated Hz. Thus the following formula applies to the propagation velocity of the wave: C=fλ = 1/T * λ This equation (wave equation) applies to all wave processes. (ii) Definition of the term ultrasound The frequency of a sound impression (tone) is a direct measure for the pitch of a tone. The higher the frequency the higher the tone. The pitch of a tone which can be received by the human ear has an upper limit. For young people the limit is approximately 20,000 Hz = 20 kHz. Sound having higher frequencies is called ultrasound. Audible sound: f= 20 -20,000 Hz Ultrasound: f> 20,000 Hz = 20 kHz. As we have learned from the spring model an energy transport through a sound wave is possible only when constituent particles are connected to each other by elastic forces. In the case of the sound transmission from the loud-speaker to our ear, the air molecules serve as transmitting medium. Liquids and solid matter are also suitable media for the sound transmission, In the vacuum (space) no matter exists and thus no sound transmission is possible. The satisfactory sound conductivity of liquids and solid matter is nowadays technically utilised in various sectors. 2.2.5.2 PRINCIPLES OF WAVE PROPAGATION (5) (i) Types of Oscillations The sound propagation demonstrated by the spring model is possible in all media, It is characterized by the fact that the direction of oscillation of the particles runs along the direction of propagation of the wave. Thus zones with small particle distance and zones with large particle distance are created, Therefore, this type of wave is called compression wave or longitudinal wave. If we do not look at the momentary representation but at the dynamic process of the propagation of the longitudinal wave, we
  24. 24. see that the compressions and diminishing move through the test object at an unchanging distance. The velocity at which they move is the sound velocity C L of the longitudinal wave. This sound velocity is a matter constant, i. e. in a test object completely made of same material it can be considered constant. e. g. for steel: CL= 5920 m/s for aluminum: CL = 6300m/s In solid matter the density is very high as compared with that of liquids and gases, i.e. the distance between the atoms or molecules is very small. Moreover, they are arranged in a crystal lattice and the elastic linkage forces between the atoms (molecules) are particularly strong, Due to these two facts the sound can propagate in various ways in solid matter. We already became acquainted with one type of wave, namely the longitudinal wave. Another type of wave is called shear wave or transverse wave, In the case of a transverse wave the particle oscillate vertically to the direction of propagation of the wave. Now we look again at the spring chain. The wave is not excited into the direction of the chain but in cross direction. The springs pull the balls back into their starting position but due to their movement they oscillate over their rest position. At the same time these transverse oscillations are transmitted to the two neighbouring balls which also start oscillating, These oscillations are continued to be transmitted to the neighbouring balls due to the spring connections. Looking at the dynamic process of the wave train, we find out that both wave crests and wave toughs move through the test object at an unchanging distance. The distance between two neighbouring wave crests is the wave length. The energy trasmission of the transverse wave is lower due to the transverse oscillation of the atoms than that of the longitudinal wave. Therefore the propagation velocity of the transverse wave is considerably lower than that of the longitudinal wave, e.g. Steel: CL = 5920 m/s CT = 3250 m/s The velocity of the transverse wave, too, is a matter constant which is characteristic for the corresponding work piece.
  25. 25. Longitudinal and transverse waves can propagate only through the whole volume of a workpiece. At the interfaces or surfaces of the work pieces further types of waves may occur. The surface wave or Rayleigh wave propagates only at the surface of the workpiece. 2.2.5.3. GENERATION OF ULTRASONIC WAVES (5) (i) Piezoelectricity Ultrasonic transmitters and receivers are mainly made from small plates cut from certain crystals (piezoelectric crystals) as shown in (fig. 2.5). If no external forces act upon such a small plate electric charges are arranged in a certain crystal symmetry and thus compensate each other. Due to external pressure the thickness of the small plate is changed and thus the symmetry of the charge. An electric field develops and at the silver-coated faces of the crystal, voltage can be tapped off. This effect is called direct piezoelectric effect. Pressure fluctuations and thus also sound waves are directly converted into electric voltage variations by this effect : the small plate serves as receiver. The direct piezoelectrical effect is reversible (reciprocal piezoelectrical effect). If voltage is applied to the contact face of the crystal the thickness of the small plate changes according to the polarity of the voltage the plate becomes thicker or thinner. Due to an applied high – frequency a.c. voltage the crystal oscillates at the frequency of the a.c. voltage. A short voltage pulse of less than 1/1 000,000 second (1 second) and a voltage of 300-1000 V excites the crystal into oscillations at its natural frequency (resonance) which depends on the thickness and the material of the small plate. The thinner the crystal the higher its resonance frequency. Therefore it is possible to generate an ultrasonic signal with a defined primary frequency. The thickness of the crystal is calculated from the required resonance frequency f0 according to the following formula: d= λ/2 = c/2f0
  26. 26. c=sound velocity of the crystal f0=resonance frequency of the crystal d=thickness of the crystal λ=wave length Fig. 2.5 Piezoelectric Crystals A piezoelectric crystal occuring naturally is the quartz (rock crystal) which was used as crystal material in the beginning of ultrasonic testing. Depending on whether longitudinal waves or transverse waves are to be generated the quartz plates have either been saved vertically to the X-axis of the crystal (X-cut) or vertically to the Y-axis (Y- cut) out of the rock crystal. In modern probes quartz is hardly used, instead sintered ceramics or artificially produced crystals are employed. The most important material for ultrasonic crystal as well as their characteristics are stated in the Table 2.1 below : Lead zirconate Barium Lead Lithium Quart Lithium titanate titanate metaniobate sulphate z niobate sound velocity 4000 5100 3300 5460 5740 7320 acoustic impedance 30 27 20.5 11.2 15.2 34 z 106 kg/m2 electromechanic 0.6 – 0.7 0.45 0.4 0.38 0.1 0.2 coupling factor k piezoelectric modulus d 150-593 125-19 85 15 2.3 6 (Transmission/generatio 0 n) piezoelectric 1.8 – 4.6 1.1– 1.9 8.2 4.9 6.7 deformation constant H 1.6
  27. 27. coupling factor for 0.5 – 0.6 0.3 0.07 0 0.1 - radial oscillations kp Table 2.1 Characteristics of various ultrasonic crystals The efficiency during the conversion from electrical into mechanical energy and vice versa differs according to the crystal material used. The corresponding features are characterised by the piezoelectric constants and the coupling factor. The constant d (piezoelectric modulus) is a measure for the quality of the crystal material as ultrasonic transmitter. The constant H (piezoelectric deformation constant) is a measure for the quality as receiver. The table shows that lead zirconate- titanate has the best transmitter characteristics and lithium sulphate the best receiver characteristics. The constant k (theoretical value) shows the efficiency for the conversion of electric voltage into mechanical displacement and vice versa. This value is important for the pulse echo operation as the crystal acts as transmitter and receiver. Here the values for lead zirconate-titanate, barium-titanate and lead meta niobate lie in a comparable order. As in the case of direct contact as well as immersion testing a liquid couplant with low acoustic impedance z is required the crystal material should have an acoustic impedance of the same order in order to be able to transmit as much sound energy as possible. Thus the best solution would be to use lead meta-niobate and lithium sulphate as they have the lowest acoustic impedance. A satisfactory resolution power requires that the constant kp (coupling factor for radial oscillations) is as low as possible. kp is a measure for the appearance of disturbing radial oscillation which widen the signals. From this point of view lead meta-niobate and lithium sulphate are the best crystal materials. The characteristics of the crystal materials described here show that no ideal crystal material exists. As lithium sulphate presents additional difficulties due to its water solubility the most common materials are lead zirconate- titanate, barium-titanate and lead meta-niobate. 2.2.5.4 Set-up of the probe (5)
  28. 28. 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)
  29. 29. (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
  30. 30. 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.
  31. 31. 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.
  32. 32. Fig. 2.8 Automatic Ultrasonic testing machines (Block diagram) 2.2.5.7 METHODS OF ULTRASONIC TESTING (5) Fig. 2.9 Block diagram of ultrasonic testing technique Today, Non-destructive ultrasonic testing is applied. on a great variety of materials of different processing conditions and geometries. Refer Fig. 2.9 for a typical setup for ultrasonic test set.
  33. 33. 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
  34. 34. Fig. 2.11 Pulse Echo Technique (a) Calibration: By means of the calibration it is possible to allocate the whole width of the CRT-screen to a defined distance range in the material to be tested. This distance range is designated the test range. For the calibration we use a plane-parallel calibration block which has a known thickness and must be made of the same material as the test object. (iii) Locating- reflectors: To test the workpiece we couple the probe onto its surface. If we have chosen a suitable test range we should now obtain the first backwall echo on the screen. If the first backwall echo is preceded by another echo then this echo comes from a reflector in the workpiece. For the evaluation we only use that range before the first backwall echo because in the range behind it secondary echoes may occur due to split transverse waves thus simulating a reflector in the workpiece. The operator's task is now to exactly determine the location of the reflector. By slightly moving the probe on the surface of the workpiece the reflector echo can be optimised (or maximised). At the probe position with the highest echo amplitude the reflector stands exactly on the central axis of the sound beam, i.e. perpendicular under the centre point of the probe. The last step is to determine the exact depth position of the reflector. The sound beam should strike reflectors perpendicularly If this condition is fulfilled the result will be a maximum echo.
  35. 35. Using a straight-beam probe it is impossible to detect reflectors which run inclined or perpendicular to the surface because the sound is not directly reflected to the probe. To be able to detect and evaluate this type of flaw one uses angle-beam probes. To transmit ultrasonic pulses inclined to the surface we glue an X-cut crystal, which generates longitudinal waves, onto a wedge-shaped perspex delay block. That sound portion which is reflected by the sole of the probe strikes a damping element which absorbs this unwanted sound portion. At the interface of the perspex wedge to the workpiece, the sound waves are refracted which may also cause a splitting of transverse waves so that, in this case, there can be two types of waves in the workpiece, namely longitudinal and transverse waves. Both of these two wave types are converted again into a longitudinal wave as they return to the probe. That means, every echo returning to the probe, regardless from which type of wave it originates, is always received by the crystal as a longitudinal wave echo so that it is impossible to decide whether it comes from a transverse or a longitudinal wave. However, due to this it would then also be impossible to locate a reflector since both types of wave propagate in the material in different directions and at different velocities. We therefore use a constructive trick to ensure that only one type of wave may occur in the testpiece. We choose an angle of incidence for the longitudinal wave in the probe so large that a longitudinal wave can no longer occur in steel where by βL is greater than 90° (total reflection). (iv) Testing with twin crystal probes (TR technique) Straight-beam and angle-beam probes are equipped with only one crystal which has both the transmitting and receiving function. Depending on the length of the delay line, this has the effect that the initial pulse is displayed either fully or partially on the CRT- screen, and, consequently, echoes from near-surface flaws are not definitely traceable (dead zone, initial pulse influential zone). In practical testing, however, we often meet
  36. 36. 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.
  37. 37. 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
  38. 38. probes differ from the straight-beam probes for direct contact only insofar that they are watertight moulded including the cable connection. The probes are mounted in a holder so that they are oriented perpendicular to the testing surface. Between the probe and the surface of the workpiece is a water delay line which has a defined length. Fig. 2.12 Immersion Testing Technique (vii) Resonance Method All technique described before arc based on the pulse echo method. A continuous ultrasonic wave, however, can also be used in non-destructive testing. A continuous ultrasonic wave which is transmitted into a plane parallel plate can excite natural oscillations of the plate. A pre requirement for this is that the plate can freely oscillate on either side, i.e. on both sides of the plate must be a medium with small acoustical impedance.
  39. 39. Fig. 2.13 Standing Wave The ultrasonic wave is reflected on both interfaces thus traveling through the plate in two different directions whereby the forward wave and the reflected wave superimpose each other. Depending on the wave length in relation to the plate thickness amounts exactly to a multiple of the half wave length. In this case the wave crests of the forward wave meet the wave crests of the reflected wave and a standing wave develops (Refer Fig. 2.13). Such a wave is characterised by the fact that inside the plate there are locations where the particles are always stationary while on other locations the particles always oscillate with the maximum amplitude. Those frequencies which generate standing waves in the plate are designated natural frequencies of the plate. If the plate is excited in one of its natural frequencies then we refer to resonance vibration of the plate. For the detection of flaws the resonance method is difficult to apply. In addition, the flaws must have a surface of ¼ or ½ of the crystal surface in order to be detectable at all. The described disadvantages of this method are the reason why the resonance techniques were replaced by the pulse echo method.
  40. 40. 2.2.6 EDDY CURRENT INSPECTION TECHNIQUES (1&5) 2.2.6.1 PRINCIPLES OF EDDY CURRENT (5) Eddy Current and Its Properties When magnetic flux through a conductor changes, induced currents are set up in closed paths on the surface of the conductor. These currents are in a direction perpendicular to the magnetic flux and are called “Eddy Current”, Figure given below illustrate this. Fig. 2.14 Eddy Current Basic arrangement for producing Eddy Current in a conducting material is shown in Figure given below: Fig. 2.15 Generation of Eddy Current When an alternating current is passed through a coil, an electromagnetic field is set up around it. The direction of magnetic field changes with each cycle of alternating current. If a conductor is brought near this field, eddy currents are induced in it. The direction of eddy current changes with the change in direction of magnetic flux during the cycle of alternating current.
  41. 41. 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’.
  42. 42. (ii) Effect of in homogeneities and discontinuities: The inhomogeneities and discontinuities like cracks, inclusions, voids, etc. in conducting materials also effect the circular pattern of eddy current and its associated magnetic field, fig. below illustrates the effect of inhomogeneities / discontinuities on distribution of eddy current. (iii) Effect of magnetic coupling:- Magnetic coupling refers to the interaction of varying magnetic field of the test coil with the test object. The effect of the primary magnetic field of the coil in inducing eddy current on the surface of a conductor is strongly influenced by the distance of the test coil from the surface of the conductor. This effect is illustrated in fig. below. Effect of Discontinuity of Effect of distance of test Eddy Current Coil from the part Fig. 2.16 Coupling is said to be effective when the distance of separation of the coil from the test object is small, it is said to be poor when the distance of separation between the test coil and test object is large. It is easy to realize that coupling is influenced by configuration and geometry of the test object, Surface condition and Coating on the surface of the test object
  43. 43. (iv) Effect of Test Frequency: The magnitude of induced eddy current in an object increases with frequency of the inducing magnetic field, it has been observed that higher intensity of eddy current results in stronger secondary magnetic field opposing the primary magnetic field. This results in low depth of penetration of eddy current as in case of high conductivity or higher magnetic permeability of the test object as shown in Fig. Eddy current concentration is found to be greater at the surface of conductor and decreases as the depth increases. As the frequency of magnetizing field increases. As the frequency of magnetizing field increases, the concentration of eddy current near the surface also increases and depth of penetration decreases. Increasing magnetic permeability and conductivity of the material further Accentuates this effect. The depth at which the eddy current density is reduced to about 37% of its intensity on the surface, us called standard depth of penetration. This depth is given by 1 Standard depth of penetration = πf µrσ Where f = frequency µr = relative permeability σ = electrical conductivity. The fig. below illustrates the relationship between depths of penetration against frequencies for various materials.
  44. 44. Fig. 2.17 Depth of Penetration-Frequency for Various Materials. 2.2.6.3 EDDY CURRENT TEST SYSTEM (5) Basic Test System An eddy current test system consists of : (i) An oscillator to provide alternating current of required frequency for exciting the test coil. (ii) Test coil-test object combination which brings out desired information in the form of an electrical signal. (iii) Signal processing (iv) Signal display.
  45. 45. The following figure gives the block diagram. OSCILLATOR TEST COIL TEST PART BRIDGE CIRCUIT SINGLE PROCESSING CIRCUITS READ OUT Fig. 2.18 Block Diagram of Eddy Current Test System Here an Oscillator provides alternating current of required frequency to the test coil, which generates eddy current in the test object. The test object variables, like conductivity, permeability, discontinuities, etc. modulate the test coil impedance. The modulated impedance signal is processed and displayed over a readout mechanism. The commercially available eddy current equipment falls in the following two categories: (1) amplitude detector and amplitude – phase detector; and (2) special purpose equipment like conductivity meters, thickness measuring equipment for conductive materials and for non-conductive coating on conductive materials, flaw detector/metallurgical condition monitors, etc. 2.2.6.4 SENSING ELEMENTS TYPES AND ARRANGEMENTS (5) In most of the non-destructive inspection equipment applications of eddy current, the test coil (also called sensing element) serves as the main link
  46. 46. 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
  47. 47. optical method of inspection. Main disadvantages are inability to identify the exact location or point of defect over the circumferencential inspection and more Wear and tear of the test coil. (iii) Surface Coil: Surface coil is a type of coil arrangement in the form of a spring mounted flat probe or a pointed pencil type probe which scans the surface or selected location and this arrangement is very useful in exactly locating the defect. In surface probes, the distance between the probe and the specimen is very critical and this must be taken care of in probe design or in equipment design, to compensate for this ‘lift-off’ effect. In some surface probes, the coil is spring mounted, such that independent of the pressure applied, a constant spring pressure is applied to the coil and holds it firmly against the specimen. Main advantage of this coil is that it pin points the defect and disadvantages are speed of testing is slow being manual and lift-off and edge effect create problems. 2.2.6.6 TEST COIL FUNCTION AND SIGNAL FORMATION (5) Although the same coil can be used for excitation and for supplying the response signal, this is not necessary and often not desirable. One coil can be used for excitation purposes with a second coil or multiple coils used for monitoring the electromagnetic field conditions. The use of separate coils for excitation and sensing gives greater flexibility in meeting the test system requirement. For example, the primary electromagnetic field may be established by the use of a few turns of relatively large wire driven from a low impedance generator and the number of turns not the sensing coil can be adjusted to meet the input impedance requirements of sensing circuits. If desired, sensing circuits having very high input impedance can be used and the sensing coil may be wound with many turns of small wire gives a simple illustration of a sensing coil inside an excitation coil. As a simplified case, let ‘I’ be the current flowing through the excitation loop. Let us consider an element current Idl at ‘A’ in the excitation loop. Because of this elemental current at point ‘A’ there will be an induced voltage E1 at Point ‘B’ in the sensing loop and E 2 at a point ‘P’ in the test material. The induced electric field E2 at P causes a current Im to flow in the test object at point ‘P’. This current in turn causes induction resulting in a electric field Em at point ‘B’ in the sensing coil. Thus in the above simplified
  48. 48. case at point ‘B’ in the sensing element, there is an induction field due to current element Idl in the primary exciting loop and another filed Em due to the current intensity at point ‘P’ in the conductive object. Hence the total current intensity at ‘B’ in the sensing coil due to a current element Idl flowing at point ‘A’ is given by ET = El + Em. Similarly, each elemental point around ‘P’ in the part contributes its own filed and hence the information carried by the sensing coil is a cumulative field information which is a complex factor when contribution of ‘I’ though the excitation loop is considered as a whole. This resultant field intensity is fed to analyzing circuit. The test coil’s output signal is shown in the phasor diagram. The curved locus ABCD represents the test coil output signal locus for variation of conductivity of test object. A standard test object might have the conductivity represented by signal point D. The signal phasor OD represents the test coil output for this standard condition. Now let us assume that the original test object is replaced by a second test object which has a lower conductivity than the first one. This might give a test coil signal represented by a phasor OC. These two cases give distinct difference in readings as shown in fig illustrated below. Fig. 2.19 Coil Output Shown on Impedance Diagram 2.2.6.7 TEST COIL SELECTION CONSIDERATIONS (5) Selection of test coil depends on the nature and shape of the specimens to be tested, the type of information sought, the location of information sought, the distribution of information during the course of testing and the magnitude of testing required. Normally to achieve satisfactory test results, test coils are
  49. 49. selected to suit a specific test situation. This is done depending upon the shape of the test object, the sensitivity and resolution required in a test situation. The depth of eddy current penetration depends on test frequency, conductivity and permeability. Hence selection of frequency for a test situation on part of known conductivity and permeability is one of the main considerations. Each test requirements requires high duty cycle system. High production rate testing like tube, wires, etc. requires a specific test system. High production rate testing like tubes wires, etc. requires high duty cycle system with a good mechanical, electrical and thermal stability. The system should have least vibration between coil and job, whereas in a scanning type surface coil testing, good resolution and sensitivity are very essential. The test object conductivity varies due to temperature. Power dissipation in coil-object combination would result in temperature variation leading to change in conductivity. This thermal drift effect is troublesome, only when the coil assembly scans the same location of the test part for more duration. 2.2.6.8 TEST FREQUENCY AND DISTRIBUTION OF EDDY CURRENT (5) (i) Effect of Frequency on Eddy Current Testing: Eddy current testing is based on the principle of electromagnetic induction, wherein the test object is placed under the influence of varying magnetic field of a test coil driven by an alternating current of required frequency. In general, the test coil is characterized by a change in impedance which consists of two electrical impedance parameters. Impedance change due to magnetic variable causes a change in inductive reactance component ‘XL’. Alternatively, a change in electrical variable causes a change in resistive component ‘R’ of the impedance. In the case of inductive reactance XL (2πfL) the test coil’s inductive reactance is frequency dependent since ‘f’ is the frequency of the applied A.C. field. Since the inductive reactance is directly proportional to the frequency of the test coil, it plays an important role in eddy current testing.
  50. 50. There are basically three approaches to eddy current testing: (i) Impedance testing: (ii) Phase analysis: and (iii) Modulation analysis. These methods can identify changes in conductivity, permeability or dimensional variations of the test specimen cumulatively or separately. A change in any one or more of the above characteristics, or a test part will be identified accordingly, depending upon the test requirement and choice of the method. Similarly important instrument characteristics which influence the eddy current testing are: (i) frequency of the A.C. applied to the test coil; (ii) size and shape of the test coil; and (iii) distance of the test coil from the object or electromagnetic coupling (lift-off/fill factor). Basic factors which influence the eddy current testing are (i) the effective permeability (µeff.). Which is determined by the frequency ration f/fg ; (ii) the limit frequency fg which is a function of physical characteristics of the test object such as conductivity (σ), relative permeability (µrel.). Diameter (d) for round specimens; (iii) test frequency f, and (iv) electromagnetic coupling (fill factor in case of cylindrical test object–encircling coil combination). The optimum test frequency for a specific test problem is determined by theory or experiment to provide the highest sensitivity to detect variation in conductivity, dimensions or permeability. 2.2.6.9 SELECTION OF TEST FREQUENCY (5) Generally, test frequencies used in eddy current inspection range from 200 Hz to 6 MHz. Frequency has a direct relationship with the ability of any eddy current test system to accurately and reliably measure the desired property of the test object. Frequencies can be selected to provide a maximum response signal caused by the variable. Usually lower frequencies of the order of 1 KHz are used for magnetic materials and relatively higher frequencies for non- magnetic materials. Actual frequency used for any specific case/instrument depends upon the thickness of test material, desired depth of penetration, degree of sensitivity/resolution required and the purpose of the inspection. For example, mid range frequency (say 100 KHz) might be used to detect surface cracks in stainless steel plate. Higher frequencies (1MHz) provide less sensitivity to cracks and greater sensitivity to lift off/ dimensional variations.
  51. 51. 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
  52. 52. 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.
  53. 53. 2.2.6.11 GENERAL GUIDELINE FOR FREQUENCY SELECTION (5) Application Frequency Range Ferrous Sorting 1 to 400 Hz Crack detection & 400 Hz to 8 MHz Non ferrous sorting Coating & Cladding 1 MHz & above Thickness Table 2.2 Frequency selection for different materials 2.2.6.12 EDDY CURRENT TEST SYSTEM REQUIREMENTS (5) Basically any electromagnetic test system consists of :- (i) a generator to provide a.c. to required frequency which will excite the test coil. (ii) A modulating device consisting of test coil – test object combination. Varying property of the component modulates the impedance magnitude of the coil. (iii) A signal preparation unit consisting of bridge / null balancer, filters, amplifiers, etc. (iv) Demodulation and signal analysis unit consisting of phase discriminators, compensators, etc. (v) Read out mechanism like meters, CRT, relays, recorders, etc. Equipment design consideration depends upon the nature and requirement of the test conducted. Mostly impedance magnitude type equipments are used when the change in the coils impedance is displayed over a meter or CRT screen. The deflection of meter reading is proportional to the magnitude of variation in the
  54. 54. 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.
  55. 55. (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
  56. 56. 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.).
  57. 57. 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.
  58. 58. Phase Response from Cracks: Phase changes are unique for several eddy current inspection parameters. By determining the phase change of an eddy current response, it is possible to isolate the response of a specific variable such as conductivity, lift-off, thickness, permeability and cracks (Refer Fig. 2.21) Fig. 2.21 Defect Indication in Various Displays In most applications of eddy current inspection, difference in phase between lift-off response and crack response is essential for the detection of cracks. When conductivity, magnetic permeability, frequency, etc. are low and crack size is small, the phase angle response between lift-off and crack indication will be small. As the magnitude of one or more of the above variables increases, the phase angle increases. When the crack length increases, the phase angle approaches more closely the phase angle for conductivity changes.

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