IAEA-TECDOC-462

ULTRASONIC TESTING OF MATERIALSAT LEVEL 2TRAINING MANUAL

FOR NON-DESTRUCTIVE TESTING TECHNIQUES

A TECHNICAL DOCUMENT ISSUED BY THEINTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1988

ULTRASONIC TESTING OF MATERIALS AT LEVEL 2IAEA, VIENNA, 1988IAEA-TECDOC-462

Printed by the IAEA in AustriaJune 1988

FOREWORD

The International Atomic Energy Agency is executingregional projects in the Latin American and Caribbean Regionand the Asia and Pacific Region using the syllabi contained inIAEA-TECDOC-407 'Training Guidelines in Non-destructive TestingTechniques . * which has been referenced as being suitable fortraining NOT personnel by the International StandardsOrganisation in the draft standard DP9712, 'The Qualification andCertification of NDT Personnel'.

These ultrasonic notes have therefore been preparedessentially in accordance with the syllabus for Level 2ultrasonic personnel and have been used as the basis for the 80hour model Level 2 ultrasonic regional training coursesconducted by the Asia and and Pacific Project.The notes are theresult of contributions from a number of National NDTcoordinators in the Asia and Pacific Region who have beenorganising National NDT Training Courses.

For guidance it is suggested that the minimum 80hours training recommended by ISO DP9712 be divided as follows;

1. General knowledge 8 hours2. Terminologyphysical principles

and fundamentals of ultrasonics 8 "3. Testing techniques and their

limitations 8 "4. Equipment and accessories 6 "5. Calibration of the testing

system 12 "6. Specific applications 12 "7. Codes, standards,specifications

and procedures 10 "8. Recording and evaluation of

results 109. Special techniques 6 "

EDITORIAL NOTE

In preparing this material for the press, staff of the International Atomic Energy Agencyhave mounted and paginated the original manuscripts and given some attention to presentation.

The views expressed do not necessarily reflect those of the governments of the Member Statesor organizations under whose auspices the manuscripts were produced.

The use in this book of particular designations of countries or territories does not imply anyjudgement by the publisher, the IAEA, as to the legal status of such countries or territories, oftheir authorities and institutions or of the delimitation of their boundaries.

The mention of specific companies or of their products or brand names does not imply anyendorsement or recommendation on the part of the IAEA.

CONTENTS

1. INTRODUCTION ...................................................................................... 11

1.1. Quality and reliability ........................................................................... 111.2. Non-destructive testing (NDT) methods of quality control ............................... 12

1.2.1. Liquid penetrant testing ............................................................... 121.2.2. Magnetic particle testing .............................................................. 131.2.3. Eddy current testing ................................................................... 141.2.4. Radiographie testing method ......................................................... 141.2.5. Ultrasonic testing ....................................................................... 14

1.3. Comparison of different NDT methods ...................................................... 171.4. Destructive versus non-destructive testing ................................................... 19

2. TERMINOLOGY, PHYSICAL PRINCIPLES AND FUNDAMENTALSOF ULTRASONICS ................................................................................... 23

2.1. The nature of ultrasonic waves ................................................................ 232.2. Characteristics of wave propagation .......................................................... 25

2.2.1. Frequency ................................................................................ 252.2.2. Wave length ............................................................................. 252.2.3. Velocity .................................................................................. 252.2.4. Fundamental wave equations ......................................................... 252.2.5. Ultrasonic waves ........................................................................ 272.2.6. Acoustic impedance .................................................................... 272.2.7. Acoustic pressure and intensity ...................................................... 27

2.3. Types of ultrasonic waves ...................................................................... 292.3.1. Longitudinal waves ..................................................................... 292.3.2. Transverse or shear waves ........................................................... 302.3.3. Surface or Rayleigh waves ........................................................... 312.3.4. Lamb or plate waves .................................................................. 322.3.5. Velocity of ultrasonic waves ......................................................... 34

2.4. Behaviour of ultrasonic waves ................................................................. 352.4.1. Reflection and transmission at normal incidence ................................ 35

2.4.1.1. Reflected and transmitted intensities ................................... 352.4.1..2. Reflected and transmitted pressures .................................... 36

2.4.2. Reflection and transmission at oblique incidence ................................ 382.4.2.1. Refraction and mode conversion ........................................ 382.4.2.2. Snell's Law ................................................................. 382.4.2.3. First and second critical angles ......................................... 402.4.2.4. Reflected acoustic pressure at angular incidence .................... 40

2.5. Piezoelectric and ferroelectric transducers .................................................. 412.5.1. Piezoelectric effect ..................................................................... 412.5.2. Types of piezoelectric transducers .................................................. 42

2.5.2.1. Piezoelectric crystal transducers ........................................ 422.5.2.2. Polarized ceramic transducers ........................................... 452.5.2.3. Comparison of piezoelectric transducers ............................... 46

2.6. The characteristics of the ultrasonic beam .................................................. 482.6.1. The ultrasonic beam ................................................................... 48

2.6.1.1. Near field .................................................................... 492.6.1.2. Calculation of near field length ......................................... 502.6.1.3. Far field ..................................................................... 52

2.6.2. Beam spread ............................................................................. 522.7. Attenuation of ultrasonic beams ............................................................... 55

2.7.1. Scattering of ultrasonic waves ....................................................... 552.7.2. Absorption of ultrasonic waves ...................................................... 562.7.3. Loss due to coupling and surface roughness ...................................... 56

2.8. Diffraction ......................................................................................... 61

3. ULTRASONIC TEST METHODS, SENSORS AND TECHNIQUES ...................... 63

3.1. Basic ultrasonic test methods .................................................................. 633.1.1. Through transmission method ........................................................ 633.1.2. Pulse echo method ..................................................................... 643.1.3. Resonance method ...................................................................... 65

3.2. Sensors ............................................................................................. 663.2.1. Ultrasonic probe construction ........................................................ 663.2.2. Types of ultrasonic probes ............................................................ 68

3.2.2.1. Contact type probes ....................................................... 683.2.2.2. Immersion type probe ..................................................... 72

3.3. Pulse echo testing techniques .................................................................. 733.3.1. Contact type techniques ............................................................... 73

3.3.1.1. Normal beam techniques ................................................. 733.3.1.2. Applications of contact type normal beam probes ................... 743.3.1.3. Angle beam techniques ................................................... 793.3.1.4. Calculation of various distances for angle beam probes ........... 803.3.1.5. Surface wave techniques .................................................. 83

3.3.2. Immersion testing techniques ......................................................... 83

4. PULSE ECHO TYPE ULTRASONIC FLAW DETECTOR .................................. 87

4.1. Construction and mode of operation of a pulse echo type flaw detector .............. 874.1.1. Functions of the electronic elements ................................................ 874.1.2. Operation of a pulse echo type flaw detector ..................................... 90

4.2. Scan presentation ................................................................................. 914.2.1. A-scan presentation .................................................................... 924.2.2. B-scan presentation ..................................................................... 924.2.3. C-scan presentation ..................................................................... 944.2.4. Echo amplitude and its control ...................................................... 95

4.2.4.1. Decibel (dB) unit .......................................................... 95

5. CALIBRATION OF THE TEST SYSTEM ....................................................... 995.1. Calibration and reference test blocks ......................................................... 995.2. Commonly used test blocks .................................................................... 99

5.2.1. I.I.W. (VI) calibration block ........................................................ 995.2.3. Institute of Welding (I.O.W.) beam profile block ............................... 109

5.2.3.1. Plotting of beam profile .................................................. 110

5.2.4. ASME reference block ................................................................ Ill5.2.5. Area-amplitude blocks ................................................................. Ill5.2.6. Distance-amplitude blocks ............................................................ 1135.2.7. Blocks ..................................................................................... 113

5.3. Checking equipment characteristics ........................................................... 1145.4. Methods of setting sensitivity .................................................................. 120

5.4.1. Distance amplitude correction curves ............................................... 1215.4.2. DOS (distance-gain-size) diagram method ......................................... 122

5.4.2.1. Setting sensitivity for a normal beam probe .......................... 1245.4.2.2. Setting of sensitivity for angle beam probe ........................... 127

5.4.3. Grain response on time base at maximum testing range ........................ 1295.5. Measurement of attenuation .................................................................... 130

5.5.1. Longitudinal wave attenuation ....................................................... 1305.5.2. Transverse wave attenuation ......................................................... 131

5.6. Determination of transfer loss for angle beam probes .................................... 1325.7. Couplants .......................................................................................... 1335.8. Influence of the test specimen on the ultrasonic beam .................................... 134

5.8.1. Surface roughness ...................................................................... 1345.8.2. Specimen with curved surface ....................................................... 1355.8.3. Coated surfaces ......................................................................... 1375.8.4. Mode conversion within the test specimen ........................................ 1375.8.5. Orientation and depth of flaw ........................................................ 137

5.9. Selection of ultrasonic probe ................................................................... 1385.9.1. Choice of ultrasonic beam direction ................................................ 1385.9.2. Choice of probe frequency ........................................................... 1395.9.3. Choice of probe size ................................................................... 140

6. SPECIFIC APPLICATIONS ......................................................................... 141

6.1. Ultrasonic inspection of welds ................................................................. 1416.1.1. Types of weld joints ................................................................... 1416.1.2. Weld defects ............................................................................. 144

6.1.2.1. Lack of root penetration .................................................. 1446.1.2.2. Lack of fusion .............................................................. 1446.1.2.3. Slag inclusion ............................................................... 1456.1.2.4. Tungsten inclusion ......................................................... 1456.1.2.5. Porosity ...................................................................... 1456.1.2.6. Cracks ........................................................................ 1456.1.2.7. Undercut ..................................................................... 1466.1.2.8. Excessive penetration ..................................................... 1466.1.2.9. Concavity at the root of the weld ...................................... 1466.1.2.10. Lamellar tearing ............................................................ 147

6.1.3. General procedure for ultrasonic testing of welds ............................... 1476.1.4. Examination of root in single Vee Butt welds without backing strip

in plates and pipes ..................................................................... 1526.1.4.1. Scanning procedure ........................................................ 1526.1.4.2. Selection of probe angle .................................................. 155

6.1.5. Examination of weld body in a single Vee Butt weld withoutbacking strip ............................................................................. 1566.1.5.1. Selection of probe angle .................................................. 157

6.1.6. Inspection of single Vee Butt welds with backing strips or inserts ........... 1576.1.6.1. Welds with EB inserts .................................................... 1576.1.6.2. Welds with backing strips ................................................ 159

6.1.7. Inspection of double Vee welds ..................................................... 1596.1.7.1. Critical root scan for double Vee welds .............................. 1596.1.7.2. Weld body examination of double Vee welds ........................ 160

6.1.8. Examination of T-welds ............................................................... 1616.1.9. Examination of nozzle welds ......................................................... 161

6.1.9.1. Fully penetrated set on weld ............................................. 1626.1.9.2. Partially penetrated set in weld ......................................... 1636.1.9.3. Examination of set through nozzle welds ............................. 163

6.2. Ultrasonic inspection of forgings .............................................................. 1646.2.1. Forging defects ......................................................................... 1646.2.2. Testing of semi-finished products: rods and billets .............................. 165

6.2.2.1. Billets ........................................................................ 1656.2.2.2. Rod materials ............................................................... 1696.2.2.3. Use of immersion technique for billet or rod materials ............ 170

6.2.3. Tube testing ............................................................................. 1726.2.4. Testing of forgings ..................................................................... 174

6.3. Ultrasonic inspection of castings .............................................................. 1756.3.1. Defects in castings ..................................................................... 176

6.3.1.1. Shrinkage defects .......................................................... 1766.3.1.2. Defects associated with hindered contraction during cooling ..... 1796.3.1.3. Defects associated with entrapped gas airlocks ...................... 181

7. ULTRASONIC STANDARDS ....................................................................... 183

7.1. Commonly used ultrasonic standards ......................................................... 1837.2. ASME Boiler and Pressure Vessel Code .................................................... 1837.3. BS 3923: Part 1: Manual examination of fusion welds in ferritic steels .............. 186

8. RECORDING AND EVALUATION OF RESULTS ............................................ 195

8.1. Determination of the location, size and nature of a defect ............................... 1958.1.1. Defect location .......................................................................... 1958.1.2. Methods of defect sizing .............................................................. 196

8.1.2.1. 6 dB drop method ......................................................... 1968.1.2.2. 20 dB drop method ........................................................ 1978.1.2.3. Flaw location slide ......................................................... 199

8.1.2.3.1. Plotting the beam spread (vertical plane) on theflaw location slide ........................................... 200

8.1.2.3.2. Using the flaw location slide for flaw locationin welds ....................................................... 201

8.1.2.3.3. Defect sizing using the flaw location slide ............. 2038.1.2.4. Maximum amplitude technique .......................................... 2048.1.2.5. DOS diagram method ..................................................... 205

8.1.3. Determination of the nature of a defect ............................................ 2078.2. Ultrasonic test report ............................................................................ 210

APPENDIX A. WELDING PROCESSES AND DEFECTS ......................................... 217

APPENDIX B. CASTINGS AND FORCINGS AND THEIR RELATEDDISCONTINUITIES ..................................................................... 243

APPENDIX C. ULTRASONIC PRACTICALS ........................................................ 255

I. Use of ultrasonic flaw detector (functions of various controls on theflaw detector) ....................................................................... 255

II. Calibration and use of ultrasonic flaw detector with normal probes ..... 257El. Comparison of various couplants ............................................... 259IV. Calibration with angle beam probe ............................................. 260V. Thickness measurement using the ultrasonic flaw detector ................ 266

VI. Experiment with mode conversion .............................................. 267VII. Understanding decibel (dB) system ............................................. 268

Vin. Velocity measurement in materials by ultrasonics ........................... 269DC. Beam profile in vertical and horizontal plane ................................ 270X. Checking equipment characteristics ............................................. 272

XI. Sensitivity setting using an angle probe and the ASME block ............ 277XII. Determination of the attenuation and surface transfer loss correction

between the reference calibration block and the test plate ................. 278Xu!. Flaw sizing practice ............................................................... 279XIV. Procedure for the examination of a single-Vee Butt weld

according to ASME V ............................................................ 281

Please be aware that all the Missing Pages

in this document were originally blank pages

1. INTRODUCTION

1.1 Quality and Reliability

An industrial product is designed to perform a certainfunction. The user buys it with every expectation that itwill perform the assigned function well and give atrouble-free service for a reasonable period of time. Thelevel of guarantee or certainty with which a trouble-freeservice can be provided by any product may be termed itsdegree of reliability. The reliability of a machine or anassembly having a number of components depends upon thereliability factors of all the individual components.Most of the machines and systems in the modern day world,for example, railways, automobiles, aircrafts, ships,power plants, chemical and other industrial plants etc,are quite complex having thousands of components on whichtheir operation and smooth running depends. To ensure thereliability of such machines it is important that eachindividual component is reliable and performs its functionsatisfactorily.

Reliability comes through improving the quality or qualitylevel of the components or products. A good qualityproduct can therefore be termed one which performs itsassigned function for a reasonable length of time. On theother hand products which fail to meet this criterion andtheir failure or breakdown occurs unpredictably andearlier than a specified time may be termed as bad or poorquality products. Both these types of products differ inreliability factors or quality levels.

The quality of products, components or parts depends uponmany factors important among which are the design,material characteristics and materials manufacturing andfabrication "techniques. Quality may be defined in termsof defects and imperfections present in the materials usedfor making the product or the presence of such defects andimperfections in the finished product itself. Manydefects can also be formed in products during service.The nature of these defects differs according to theprocess of its design and fabrication as well as theservice conditions under which it has to work. Aknowledge of these defects with a view to determining themand then minimizing them in a product is essential toachieve a better or an acceptible level of quality.

An improvement in the product quality to bring it to areasonable quality level is important in many ways. Itincreases, as already mentioned, the reliability of theproducts and the safety of the machines and equipment andbrings economic returns to the manufacturer by increasinghis production, reducing his scrap levels, enhancing his

11

reputation as a producer of quality goods and henceboosting his sales. There is therefore a need to havemethods by which the defects in the products can bedetermined without affecting their serviceability.

1.2. Non-Destructive Testing (NOT) Methods Of Quality Control

The term "Non-destructive testing" is used to describe thematerial testing methods which, without damaging orinfluencing the usefulness of a material or component,give information about its properties. NDT is concernedwith revealing the flaws in an item under inspection. NDTcan not however predict where flaws will develop due tothe design itself.

Non-destructive testing (NDT) plays an important role inthe quality control not only of the finished product, butalso of half finished products as well as the initial rawmaterials. NDT can be used at all stages of theproduction process. It can also be used during theprocess of establishing a new technology by monitoringproduct quality or when developing a new product.Outside the manufacturing field, NDT is also widely usedfor routine or periodic control of various items duringoperation to ascertain that their quality has notdeteriorated with use.

The methods of NDT range from the simple to thecomplicated. Visual inspection is the simplest of all.Surface imperfections invisible to the eye may be revealedby penetrant or magnetic methods. If really serioussurface defects are found, there is often little point inproceeding to the more complicated examinations of theinterior by ultrasonics or radiography. The principle NDTmethods are Visual or Optical Inspection, Dye PenetrantTesting, Magnetic Particle Testing, Eddy Current Testing,Radiographie Testing and ultrasonic Testing.

The basic principles typical applications, advantages andlimitations of these methods with now be brieflydescribed.

A number of other NDT methods exist. These are used onlyfor specialized applications and consequently are limitedin use. Some of these methods are Neutron Radiography,Acoustic Emission, Thermal and Infra Red Testing, StrainSensing, Microwave Techniques, Leak Testing, Holographyetc.

1.2.1. Liquid Penetrant Testing

This is a method which can be employed for the detectionof open-to-surface discontinuities in any industrialproduct which is made from a non porous material. In thismethod a liquid penetrant is applied to the surface of the

12

product for a certain predetermined time, after which theexcess penetrant is removed from the surface. The surfaceis then dried and a developer is applied to it. Thepenetrant which remains in the discontinuity is absorbedby the developer to indicate the presence as well as thelocation, sise and nature of the discontinuity. Theprocess is illustrated in Fig .1.1.

'' '. ,'-. ''''''."'-~- '.'- Ur """"-"--'I

'"-^Zs-(a) (b)

MMULL il!/11 ii i !!,'{'f hjirn . - . . 7-\ |Vf ;:-.-."-: .-

(fini II

(c) (d)

Figure 1.1 Four stages of liquid penetrant process:-

(a) Penetrant application and seepage intothe discontinuity.

(b) Removal of excess penetrant.(c) Application of developer, and(d) Inspection for the presence of

discontinuities.

1.2.2. Magnetic Particle Testing

Magnetic particle testing is used for the testing ofmaterials which can be easily magnetized. This method iscapable of detecting open-to-surface and just-below-the-surface flaws. In this method the test specimen is firstmagnetised either by using a permanent magnet, or anelectromagnet or by passing electric current through oraround the specimen. The magnetic field thus introducedinto the specimen is composed of magnetic lines of force.Whenever there is a flaw which interupts the flow ofmagnetic lines of force, some of these lines must exit and

13

re-enter the specimen. These points of exit and re-entryform opposite magnetic poles. Whenever minute magneticparticles are sprinkled onto the surface of the specimen,these particles are attracted by these magnetic poles tocreate a visual indication approximating the size andshape of the flaw. Fig. 1.2. illustrates the basicprinciples of this method.

1.2.3 Eddy Current Testing

This method is widely used to detect surface flaws, tosort materials, to measure thin walls from one surfaceonly, to measure thin coatings and in some applications tomeasure case depth. This method is applicable toelectrically conductive materials only. In the methodeddy currents are produced in the product by bringing itclose to an alternating current-carrying coil. Thealternating magnetic field of the coil is modified by themagnetic fields of the eddy currents. This modification,which depends on the condition of the part near to thecoil, is then shown as a meter reading or cathoderay tubepresentation. Fig. 1.3. gives the basic principles ofeddy current testing.

1.2.4 Radiographie Testing Method

The radiographie testing method is used for the detectionof internal flaws on many different materials andconfigurations. An appropriate radiographie film isplaced behind the test specimen (Fig. 1.4) and is exposedby passing either X-Rays or gamma rays through it. Theintensity of the X-rays or gamma rays while passingthrough the product is modified according to the internalstructure of the specimen and thus the exposed film afterprocessing, reveals the shadow picture of the internalstructure of the product. This shadow picture, known as aradiograph, is then interpreted to obtain data about theflaws present in the specimen. This method is used on awide variety of products such as forgings, castings andweldments.

1.2.5. Ultrasonic Testing

Ultrasonic inspection is a nondestructive method in whichhigh frequency sound waves are introduced into thematerial being inspected. Most ultrasonic inspection isdone at frequencies between 0.5 and 25 MHz - well abovethe range of human hearing, which is about 20 Ha to 20 KHz.The sound waves travel through the material with someattendant loss of energy (attenuation) due to materialcharacteristics or are measured after reflection at

14

Test object

MagnetCylindrical tesl object

Flux llow-lines

f

l|i

ft * -

K =

r ^

Crack\

-.-ca^^Kvss

'

!,l!S

..'nil. J

;i^

5 N /Mild steel blockto complete path

"\1-

X ' '

j2 - Currenl ^ .

Bar under lest -

p

Flaws.

Flux lines

Threading bar

Melal prods

Figure 1.2

Conductivegauze pads

Current source

MAGNETIC FIELDMAGNETISING COIL

X EDDY , >^ CURRENT '.""^ ^X

a) SURFACE PROBE

EDDYCURRENT

b) ENCIRCLING COIL

ARTICLE

.x Nt

1 -J

INSTRUMENT

COIL -*1

fa)c) INTERNAL COIL

Figure 1.3 Eddy current testing.

15

RADIATION SOURCE

TEST SPECIMEN

RADIATIONDETECTOR

Figure 1.4 Arrangement for radiographie testing method.

Transmitter IJ"driver 11

Transmitter _probe

Amplifier

Boundary

/ Cathode raytube

Receiver probe

_r

0 __ 2 6 B

Figure 1.5 Basic components of an ultrasonic flawdetector.

16

1.3

interfaces(pulse echo) or flaws, or are measured at theopposite surface (pulse transmission). The reflected beamis detected and analyzed to define the presence andlocation of flaws. The degree of reflection dependslargely on the physical state of matter on the oppositeside of the interface, and to a lesser extent on specificphysical properties of that matter, for instance, soundwaves are almost completerly reflected at metal-gasinterfaces. Partial reflection accors at metal-liquid ormetal-solid interfaces. Ultrasonic testing has a superiorpenetrating power to radiography and can detect flaws deepin the test specimen (say upto about 6 to 7 meters ofsteel). It is quite sensitive to small flaws and allowsthe precise determination of the location and size of theflaws. The basic principle of ultrasonic testing isillustrated in Fig. 1.5.

Comparison of different NDT methods

Frequently it may be necessary to use one method of NDT toconfirm the findings of another. Therefore, the variousmethods must be considered complementary and notcompetitive, or as optional alternatives. Each method hasits particular merits and limitations and these must betaken into account when any testing program is planned.

Table 1 gives a summary of the most frequently used NDTmethods.

Table 1. Guide to NDT techniques

Technique

Opticalmethods

Radio-graphy

ultra-sonics

Access requirements

Can be used to viewthe interior ofcomplex equipment.One point of accessmay be enough.

Must be able to reachboth sides.

One or both sides{or ends}

Equip-mentcapitalcost

B/D

A

B

Ins-spec-tioncost

D

B/C

B/C

!

Remarks

Very versatilelittle skillrequired, repaysconsideration atdesign stage.

Despite highcost, large areascan be inspectedat one time;considerableskill requiredin interpre-tation.

Requires point-by-point searchhence expensiveon largestructures,skilledpersonnelrequired.

17

Table 1. Guide to NDT techniques

Technique

Magneticparticle

Penetrantflawdetection

Eddycurrent

Access requirements

Requires a clean andreasonably smoothsurface

Requires flaw to beaccessible to thepenetrant { that is clean and at thesurface)

Surface must (usually)be reasonably smoothand clean.

Equip-mentcapitalcost

D

D

B/C

Ins-spec-tioncost

C/D

C/D

C/D

Remarks

Only useful onmagneticmaterials suchas steel; littleskill required;only detectssurface breakingon near surfacecracks .

For all materialssome skillrequired; onlydetects surface-breaking defects;rather messy.

For surface-breaking or near--surface flaws,variations inthickness ofcoatings, orcomparison ofmaterials; forother than simplecomparisonconsiderable skillis usuallynecessary (theexception isAmlec forsurface-breakingcracks in steels)______________

Where A =

D =

highest cost,

lowest cost.

18

1.4 DESTRUCTIVE VS. NON-DESTRUCTIVE TESTING

The corresponding advantages and disadvantages of destructive andnon-destructive tests are compared in table 2.

Table 2. Comparison of destructive and non-destructive test.

DESTRUCTIVE TESTS

Advantages :

NON-DESTRUCTIVE TESTS

Limitations :

Tests usually simulateone or more serviceconditions. Consequently,they tend to measureserviceability directlyand reliably

Tests usually involveindirect measurements ofproperties of no directsignificance in service.The correlation betweenthese measurements andserviceablity must beproved by other means.

Tests are usually quan-titative measurements ofload for failure, sig-nificant distortion ordamage, or life to fail-ure under given loadingand environmental con-ditions. Consequentlythey may yield numericaldata useful for designpurposes or for estab-lishing standards orspec i f i cat i ons.

Test are usually quali-tative and rarely quan-titative. They do notusually measure load forfailure or life to failureeven indirectly. They may,however, reveal damage orexpose the mechanisms offailure.

The correlation betweenmost destructive testmeasurements and thematerial propertiesbeing measured (particu-larly under simulatedservice loading) isusually direct. Hencemost observers may agreeupon the results of thetest and their signifi-cance with respect to theserviceability of thematerial or part.

Skilled judgment and testor service experienceare usually required tointerpret test indications.Where the essential corre-lation has not been proven,or where experience islimited, observers maydisagree in evaluatingthe significance of testindications.

19

Table 2. Comparison of destructive and non-destructive test.

Limitations Advantages1. Tests are not made on

the objects actuallyused in service. Conse-quently the correlationor similarity betweenthe objects tested andthose used in servicemust be proven by othermeans.

Tests are made directlyupon the objects to beuse in service. Conse-quently there is no doubltthat the tests were madeon representative testobjects.

2. Tests can be made on onlya fraction of the produc-tion lot to be used inservice. They may havelittle value when the

properties vary unpredict-ably from unit to unit.

Tests can be made on everyunit to be used in serviceif economically justifield.Consequently they may beused even when greatdifferences from unit tounit occur in productionlots.

Tests often cannot bemade on complete pro-duction parts. The testsare often limited to testbars cut from productionparts or from specialmaterial specimens pro-cessed to simulate theproperties of the partsto be used in service.

Test may be made on theentire production partor in all critical regionsof it. Consequently theevaluation applies to thepart as a whole. Manycritical sections of thepart may be examinedsimultaneously or se-quentially as convenientand expedient.

A single destructive testmay measure only one ora few of the propertiesthat may be criticalunder service conditions.

Many non-destructive tests,each sensitive to differentproperties or regions ofthe material or part, maybe applied simultaneouslyor in sequence. In thisway it is feasible tomeasure as many differentproperties correlated withservice performance asdesired.

20

Table 2. Comparison of destructive and non-destructive test.

Limitations Advantages

Destructive tests are notusually convenient toapply to parts in service.Generally, service must beinterrupted and the partpermanently removed fromservice.

5. Non-destructive tests mayoften be applied to partsin service assemblieswithout interruption ofservice beyond normalmaintenance or idleperiods. They involve noloss of serviceable parts.

6. Cumulative change over aperiod of time cannotreadily be measured on asingle unit. If severalunits from the same lotor service are tested insuccession over a periodof time, it must be proventhat the units wereinitially similar. If theunits are used in serviceand removed after variousprriods of time, it mustbe proven that each wassubject to similarconditions of service,before valid data can beobtained.

6. Non-destructive testspermit repeated checks ofa given unit over a periodof time. In this way, therate of service damage, ifdetectable,and itscorrelation with servicefailure may be establishedclearly.

With parts of very highmaterial or fabricationcost, the cost ofreplacing the partsdestroyed may be pro-hibitive. It may not befeasible to make anadequate number andvariety of destructivetests.

Acceptable parts of veryhigh material orfabrication costs are notlost in non-destructivetesting. Repeated testingduring production orservice is feasible wheneconomically andpractically justified.

21

Table 2. Comparison of destructive and non-destructive test.

Limitations : Advantages :

8. Many destructive testsrequire extensivemachining or otherpreparation of the testspecimens. Often, massiveprecision-testingmachines are required.In consequence the costof destructive testingmay be very high, andthe number of samplesthat can be prepared andtested may be severelylimited. In addition suchpreparation and tests maymake severe demands uponthe time of highly skilledworkers.

8. Little or no specimenpreparation is required formany forms of non-destruc-tive tests. Several formsof non-destructive testingequipment are portable.Many are capable of rapidtesting or sorting and insome cases may be madefully automatic. The costof non-destructive testsin less, in most cases,both per object testedand for overall testing,than the cost of adequatedestructive tests.

The time and man-hourrequirements of manydestructive tests arevery high. Excessiveproduction costs may beincurred if adequate andextensive destructivetests are used as the pri-mary method of productionquality control.

9. Most non-destructive testmethods are rapid andrequire far fewer man--hours or actual hoursthan do typical destructivetests. Consequently thetesting of all theproduction units at a costnormally less than, orcomparable, to, the coastof inspecting destructivelyonly a minor percentage ofthe units in productionlots is feasible.

22

2. TERMINOLOGY, PHYSICAL PRINCIPLES AND FUNDAMENTALSOF ULTRASONICS

2.1 The nature of ultrasonic waves

To understand how ultrasonic wave motion occurs in a medium,it is necessary to understand the mechanism which transferathe energy between two points in a medium. This can beunderstood by studying the vibration of a weight attached toa spring. Fig 2.la

8o"o

----r =sJone cycle

G\l----A

----8

(a)Down

(b)

Figure 2. 1 a) Weight attached to a springb) Plot of displacement of W with time w.r.t

position A.

The two forces acting on w, while it is at rest, are forceof gravity G and tension T in the spring. Now if W is movedfrom its equilibrium position A to position B, tension Tincreases. If it is now released at position B, W wouldaccelerate toward position A under the influence of thisincrease in tension.

At A the gravity G and tension T will again be equal,but asnow W is moving with a certain velocity, it will overshootA. As it moves toward position C, tension T decreases andthe relative increase in gravity G tends to decelerate Wuntil it has used up all its kinetic energy and stops at C.At C, G is greater than T and so W falls toward A again. AtA it possesses kinetic energy and once more overshoots. AsW travels between A and B, T gradually increases and slowsdown W until it comes to rest at B. At B, T is greaterthan G, and the whole thing starts again.

23

The sequence of displacements of W from position A to B, Bto A, A to C and C to A, is termed a cycle. The numberof such cycles per second is defined as the IreausuCY ovib.Ea.tion. The time taken to complete one cycle is known asthe time period T of the vibration, where T = 1

f

The maximum displacement of W from A to B or A to C iscalled the amp_litu,de Q Yib_r.atio.n. All these conceptsare illustrated in Fig.2.1 (b).

All materials are made of atoms (or molecules) which areconnected to each other by interatomic forces. These atomicforces are elastic i.e the atoms can be considered to boconnected to each other as if by means of springs. Asimplified model of such a material is shown in figure 2.2.

ELASTICNTERATOMIC FORCES

ATOMS

Figure 2.2. Model of an elastic body.

Now if an atom of the material is displaced from itsoriginal position by any applied stress, it would start tovibrate like the weight W of figure 2.1 (a). Because ofthe interatomic coupling, vibration of this atom will alsocause the adjacent atoms to vibrate. When the adjacentatoms have started to vibrate, the vibratory movement istransmitted to their neighbouring atoms and so forth. Ifall the atoms were interconnected rigidly, they would allstart their movement simultaneously and remain constantly inthe same state of motion i.e. in the same Eb.a,se. Butsince the atoms of a material are connected to each other byelastic forces instead, the vibration requires a certaintime to be transmitted and the atoms reached later lag inphase behind those first excited.

24

2.2 Characteristics of wave propagation

2.2.1 Frequency :

The frequency of a wave is the same as that of thevibration or oscillation of the atoms of the medium inwhich the wave is travelling. It is usually denoted by theletter f and until recently was expressed as the number ofcycles per second. The International term for a cycle persecond is named after the physicist H . Hertz and isabbreviated as Hz

3 H z =1 cycle per second

1 KHz = 1000 Hz = 1000 cycles per second

1 MHz = 1000000 Hz = 1000000 cycles per second

2.2.2 Wave Length

During the time period of vibration T, a wave travels acertain distance in the medium. This distance is definedas the wavelength of the wave and is denoted by the Greekletter . Atoms in a medium, separated by distance willbe in the same state of motion (i.e. in the same phase)when a wave passes through the medium.

2.2.3 Velocity

The speed with which energy is transported between twopoints in a medium by the motion of waves is known as thevelocity of the waves. It is usually denoted by the letterV.

2.2.4 Fundamental Nave Equations

When a mechanical wave traverses a medium,the displacementof a particle of the medium from its equilibrium positionat any time t is given by :

a = a sin 2TTf t (2.1)o

Where a = Displacement of the particle at time t.

a = Amplitude of vibration of the particle,o

f = Frequency of vibration of the particle.

25

A graphical representation of equation 2.1figure 2.3

is given in

Figure 2.3

Equation 2.2 is the equation of motion of a mechanical wavethrough a medium. It gives the state of the particles (i.e.the phase) at various distances from the particle firstexcited at a certain time t.

= a sin 2 7T f (t - X )o V

(2.2)

Where a =

a =o

V

f

Displacement (at a time t and distance Xfrom the first excited particle ) of a particleof the medium in which mechanical wave istravelling.

Amplitude of the wave which is the same as thatof the amplitude of vibration of theparticles of the medium .

Velocity of propagation of the wave.

Frequency of the wave.

Figure 2.4 gives the graphical representation of equation2.2.

g f *g amplitude

sI

wavelength A

distance

Figure 2.4 Graphical representation of equation 2 . 2 ,

26

Since in the time period T, a mechanical wave of velocity Vtravels a distance in a medium, therefore we have :-

X - V T

or (2.3)

But the time period T is related to the frequency f by

f -(2.4)

Combining equations 2,3 and 2.4 we have the fundamentalequation of all wave motion i.e.

v = (2.5)

2.2.5 ultrasonic Waves

Sound waves are vibrations of particles of gases, solidsor liquids. The audible sound range of frequencies isusually taken from 20 Hz to 20 KHz . Sound waves withfrequencies higher than 20 KHz are known as ultrasonicwaves. In general ultrasonic waves of frequency range0.5 MHz to 20 MHz are used for the testing of materials.The most common range for testing metals is from 2 MHz to5 MHz

2.2.6 Acoustic Impedance

The resistenoe offered to the propagation of an ultrasonicwave by a material is known as the acoustic impedance. Itis denoted by the letter Z and is determined bymultiplying the density (^ of the material by the velocityV of the ultrasonic wave in the material i.e.

Z = (2.6)

Table 2.1 gives acoustic impedances of some commonmaterials.

2.2.7 Acoustic Pressure And Intensity

Acoustic pressure is the term most often used to denotethe amplitude of alternating stresses on a material by apropagating ultrasonic wave. Acoustic pressure P is

27

Table 2.l : Densities, sound velocities and acoustic impedances ofsome common materials.

Material

aluminiumaluminium oxidebismuthbrasscadmium

cast ironconcretecopperglassglycerine

goldgrey castinghard metalleadmagnesium

motor oilnickelperspexplatinumpolyamide (nylon)polyethylenePolystyrolPolyvinylchloride(PVC hard)

porcel lainequarta

quartz glasssilversteel (low alloy)steel(calibration block)tin

titaniumtungstenuranium owater (20 C )zinc

Density3

kg/m

27003600980081008600

69002000890036001300

19300720011000114001700

87088001180214001100

94010601400

24002650

26001050078507850

7300

4540191001870010007100

ctransm/s

31305500110021201500

2200-

22602560

1200265040007003050_

2960143016701080

92511501060

3500

3515159032503250

1670

31802620-

-

2410

ctransm/s

63209000218044302780

53004600470042601920

32404600680021605770

17405630273039602620

234023802395

56005760

5570360059405920

3320

62305460320014804170

Z 310 Pa s/m

17 06432 40021 36435 88323 908

24 1509 20041 83015 3362 496

62 53233 12074 80024 6249 809

1 51449 5443 22184 7442 882

2 2002 5233 353

13 44015 264

14 48237 80046 62946 472

24 236

28 284104 28659 8401 480

29 607

28

related to the acoustic impedence Z abd the amplitude ofparticle vibration a as :

P = Z a (2.7)Where P = acoustic pressure.

Z = acoustic impedence.

a = amplitude of particle vibration.

The transmission of mechanical energy by ultrasonic wavesthrough a unit cross-section area, which is perpendicular tothe direction of propagation of the waves, is called theintensity of the ultrasonic waves. Intensity of theultrasonic waves is commonly denoted by the letter I.

Intensity I of ultrasonic waves is related to the acousticpressure P, acoustic impedence Z and the amplitude ofvibration of the particle as:

2I = P (2.8)

2 Zand

I = P a (2.9)2

Where

I = intensity.

P = acoustic pressure.

Z = acoustic impedence.

a = amplitude of vibration of the particle.

2.3 Types of ultrasonic Waves

Ultrasonic waves are classified on the basis of the mode ofvibration of the particles of the medium with respect to thedirection of propagation of the waves, namely longitudinal,transverse, surface and Lamb waves.

The major differences of these four types of waves arediscussed below.

2.3.1 Longitudinal Waves

These are also called compression waves. In this type ofultrasonic wave alternate compression and rarefaction

29

zones are produced by the vibration of the particlesparallel to the direction of propagation of the wave.Figure 2.5 represents schematically a longitudinalultrasonic wave.

rCOi

i '{

!* * i

/_.

^PRESSIONi

j.1; .".;'.'';'.

' .'t

'

1

RAREFACTION

'**

1>

;

' ':.

':: '''.

.'/,

. .

*.*

''"'. '.'

. * i* **

."*%*,

DIRECTION 0FPROPAGATION

Figure 2.5 Longitudinal wave consisting of alternaterarefactions and compressions along thedirection of propagation.

For a longitudinal ultrasonic wave, the plot of particledisplacement versus distance of wave travel along with theresultant compression crest and rarefaction trough is shownin figure 2.6.

Direction

Figure 2.6 Plot of particle displacement versus distance ofwave travel.

Because of its easy generation and detection, this type ofultrasonic wave is most widely used in ultrasonic testing.Almost all of the ultrasonic energy used for the testing ofmaterials originates in this mode and then is converted toother modes for special test applications. This type of wavecan propagate in solids, liquids and gases.

2.3.2 Transverse or Shear Waves

This type of ultrasonic wave is called a transverse orshear wave because the direction of particle displacement

30

is at right angles or transverse to the direction ofpropagation. It is schematically represented in figure2.7.

\\

Figure 2.7 Schematic representation of a transverse wave.

For such a wave to travel through a material it is necessarythat each particle of material is strongly bound to itsneighbours so that as one particle moves it pulls itsneighbour with it, thus causing the ultrasound energy topropagate through the material with a velocity which isabout 50 per cent that of the longitudinal velocity.

For all practical purposes, transverse waves can onlypropagate in solids. This is because the distance betweenmolecules or atoms, the mean free path, is so great inliquids and gases that the attraction between them is notsufficient to allow one of them to move the other more thana fraction of its own movement and so the waves are rapidlyattenuated.

The transmission of this wave type through a material ismost easily illustrated by the motion of a rope as it isshaken. Each particle of the rope moves only up and down,yet the wave moves along the rope from the excitation point.

2.3.3 Surface or Rayleigh Waves

Surface waves were first described by Lord Rayleigh andthat is why they are also called Rayleigh waves. Thesetype of waves can only travel along a surface bounded onone side by the strong elastic forces of the solid and onthe other side by the nearly nonexistent elastic forcesbetween gas molecules. Surface waves, therefore, areessentially nonexistent in a solid immersed in a liquid,unless the liquid covers the solid surface only as a verythin layer. The waves have a velocity of approximately 90per cent that of an equivalent shear wave in the samematerial and they can only propagate in a region nothicker than about one wave length beneath the surface ofthe material. At this depth, the wave energy is about 4per cent of the energy at the surface and the amplitude ofvibration decreases sharply to a negligible value atgreater depths.

In surface waves, particle vibrations generally follow anelliptical orbit, as shown schematically in figure 2.8.

31

Direction of wavetravel

AIR

METAL Par fidevibration At rest surface

Small arrows indicate directions of particle displacement.Figure 2.8. Diagram of surface wave propagating at the

surface of a metal along a metal - airinterface.

The major axis of the ellipse is perpendicular to thesurface along which the waves are travelling. The minoraxis is parallel to the direction of propagation.

Surface waves are useful for testing purposes because theattenuation they suffer for a given material is lower thnfor an equivalent shear or longitudinal wave and becausethey can flow around corners and thus be used for testingquite complicated shapes. Only surface or near surfacecracks or defects can be detected, of course.

2.3.4 Lamb or Plate Waves

If a surface wave is introduced into a material that hasa thickness equal to three wavelengths, or less, of thewave then a different kind of wave, known as a plate wave,results. The material begins to vibrate as a plate i.e.the wave encompasses the entire thickness of the material.These waves are also called Lamb waves because the theorydescribing them was developed by Horace Lamb in 1916.Unlike longitudinal, shear or surface waves, thevelocities of these waves through a material are dependentnot only on the type of material but also on the materialthickness, the frequency and the type of wave.

Plate or Lamb waves exist in many complex modes ofparticle movement. The two basic forms of Lamb waves are: (a) symmeterical or dilatational : and (b) asymmetericalor bending. The form of the wave is determined by whetherthe particle motion is symmetrical or asymmeterical withrespect to the neutral axis of the test piece. Insymmetrical Lamb (dilatational) waves, there is alongitudinal particle displacement along neutral axis ofthe plate and an elliptical particle displacement on eachsurface (Figure 2.9 (a) ).

32

PARTICLE VIBRATION

DIRECTION OF WAVE TRAVEL

AT RESTSURFACE

NEUTRAL AXIS

PART/CLE VIBRATION

Diagrams of the basic patterns of (a) symmeterical(dilatational) and (b) asymmeterical (bending)Lamb waves.

This mode consists of the successive "thickenings" and"thining" in the plate itself as would be noted in a softrubber hose if steel balls, larger than its diameter, wereforced through it. In asymmeterical(bending) Lamb waves,there is a shear particle displacement along the neutralaxis of the plate and an elliptical particle displacementon each surface (Figure 2.9 (b) ). The ratio of the majorto minor axes of the ellipse is a function of the materialin which the wave is being propagated. The asymmetericalmode of Lamb waves can be visualized by relating theaction to a rug being whipped up and down so that a rippleprogresses across it.

33

2.3.5 Velocity of Ultrasonic Waves

The velocity of propagation of longitudinal, transverse,and surface waves depends on the density of the material,and in the same material it is independent of thefrequency of the waves and the material dimensions.

Velocities of longitudinal, transverse and surface wavesare given by the following equations.

-{2.10)

(2.11)

0.9XV -(2.12)

Where V Velocity of longitudinal waves.

Velocity of transverse waves.

s

E

G

Velocity of surface waves.

Young's modulus of elasticity.

Modulus of rigidity.

Density of the material.

For steel

0.55 (2.13)

The velocity of propagation of Lamb waves, as mentionedearlier, depends not only on the material density but alsoon the type of wave itself and on the frequency of thewave.

Equation 2.10 also explains why the velocity is lesser inwater than in steel, because although the density for steelis higher than that of water, the elasticity of steel ismuch higher than that of water and this outclasses thedensity factor.

Table 2.1 givens the velocities of longitudinal andtransverse waves in some common materials.

34

2.4 Behaviour of Ultrasonic waves

2.4.1 Reflection and Transmission at Normal Inoidenoe

2.4.1.1 Reflected And Transmitted Intensities

When ultrasonic waves are incidence at right angles tothe boundary {i.e normal incidence) of two media ofdifferent acoustic impedences, then some of the wavesare reflected and some are transmitted across theboundary. The amount of ultrasonic energy that isreflected or transmitted depends on the differencebetween the acoustic impedences of the two media. Ifthis difference is large then most of the energy isreflected and only a small portion is transmitted acrossthe boundary. While for a small difference in theacoustic impedences most of the ultrasonic energy istransmitted and only a small portion is reflected back.

Quantitatively the amount of ultrasonic energy which isreflected when ultrasonic waves are incident at theboundary of two media of different acoustic impedences{Figure 2.10), is given by :-

R =

Where R

Z

( 2.14)

Reflection Co-efficient

Acoustic impedence of medium 1.

Acoustic impedence of medium 2.

Reflected ultrasonic intensity.

Incident ultrasonic intensity.

Reflected wave -*

Transmitted wave

Incident wave MEDIUM 1

Interface

MEDIUM 2Z,

Figure 2.10 Reflection and Transmission at NormalIncidence.

35

The amount of energy that is transmitted across the boundaryis given by the relation :

I 4 Z ZT = t = 12 (2.15)

Ii

Where

/Z + Z }11 2 J

T = Transmission co-efficient.

Z - Acounstic impedence of medium I1Z = Acounstic impedence of medium 22I = Transmitted ultrasonic intensity,tI = Incident ultrasonic intensity.i

The transmission co-efficient T can also be determined fromthe relation :-

T = 1 - B (2.16)Where

T = Transmission co-efficient

R = Reflection co-efficient

2.4.1.2 Reflected and Transmitted Pressures

The relationships which determine the amount ofreflected and transmitted acoustic pressures at aboundary for normal incidence are :

Z ZP = 2 - 1r (2.17)

Z + Z2 l

2Zand P = 2

-t (2.18)Z + Zl 2

Where

Pr = amount of reflected acoustic pressure.

Pt = amount of transmitted acoustic pressure.

Z = acoustic impedence of material from which1 the waves are incident.

Z = acoustic impedence of material in which2 the waves are transmitted.

36

As is clear from equations 2.17 and 2.18, Pr may be positiveor negative and Pt may be greater than or less than unity,depending on whether Z is greater or less than Z .When Z \ Z

2 1 2 1e.g water-steel boundary then Pr is positive and Pt . 1.

This means that the reflected pressure has the same phase asthat of the incident pressure and the transmitted pressureis greater than that of the incident pressure (Figure 2. 11b). The fact that the transmitted pressure is greater thanthe incident pressure is not a contradiction to the energylaw because it is the intensity and not the pressure that ispartitioned at the interface and as shown by equations 2.17and 2.18 the incident intensity is always equal to the sumof the reflected and transmitted intensities irrespective ofwhether Z \ Z or Z \ Z

1 / 2 2 / 1

The reason for a higher transmitted acoustic pressure insteel is that the acoustic pressure is proportional to theproduct of intensity and acoustic impedance (equation 2.8 )although the transmitted intersity in steel is low, thetransmitted acoustic pressure is high because of the highacoustic impedence of steel.

When Z \ Z e.g. steel-water interface, then P is1 ' 2 r

negative which means that the reflected pressure is reversedas shown in figure 2.11 a.

Sound Pressure

Water

Sound Pressure

Transmittedwave

Water

Reflectedwave

Steel

Transmitted wave

(a) (b)

Fig. 2.11 Acoustic pressure values in the case of reflectionon the interface steel/water, incident wave insteel (a) or in water (b).

37

2.4.2 Reflection and Transmission At Oblique Incidence

2.4.2.1 Refraction And Mode Conversion

If ultrasonic waves strike a boundary at an obliqueangle, than the reflection and transmission of the wavesbecome more complicated than that with normal indidence. Atoblique incidence the phenomena of mode conversion (i.e achange in the nature of the wave motion) and refraction ( achange in the direction of wave propagation) occur. Figure2.12 shows what happens when a longitudinal wave strikes aboundary obliquely between two media. Of course, there isno reflected transverse component or refracted transversecomponent if either medium 1 or medium 2 is not solid.

Figure 2.13 gives all the reflected and transmittedwaves when a transverse ultrasonic wave strikes aboundary between two media. The refracted transversecomponent in medium 2 will disappear if medium 2 isnot a solid.

2.4.2.2 Snell's Law

The general law that, for a certain incident ultrasonicwave on a boundary, determines the directions of thereflected and refracted waves is known as Snell's Law.According to this law the ratio of the sine of theangle of incidence to the sine of the angle ofreflection or refraction equals the ratio of thecorresponding velocities of the incident, and reflectedor refracted waves. Mathematically Snell's Law isexpressed as

Sin. = i (2.19)Sin /-^

' V2

Where

o = the angle of incidence.

* - the angle of reflection or refraction.

V = Velocity of incident wave.1

V = Velocity of reflected or refracted waves.2

Both

Normal to the

'

oft MED/UM

Interface

MEDIUM,

= Angle of incidence of longitudinal wave.

- Angle of reflection of transverse wave.

- Angle of refraction of transverse wave.

- Angle of refraction of longitudinal wave.

Figure 2.12 Refraction And Mode Conversion For AnIncident Longitudinal Wave.

Figure 2.13

-Normal to theinterface

MEDIUM 1

Interface

MEDIUM 2

= angle of incidence of transverse wave.

= angle of reflection of transverse wave.

= angle of refraction of transverse wave.

= angle of refraction of longitudinal wave.

Refraction And Mode Conversion For An IncidentTransverse Wave.

39

2.4.2.3 First And Second Critical Angles

If the angle of incidence o(. (figure 2. 13) is small,1

ultrasonic waves travelling in a medium undergo thephenomena of mode conversion and refraction uponencountering a boundary with another medium. Thisresults in the simultaneous propagation of longitudinaland transverse waves at different angles of refractionin the second medium. As the angle of incidence isincreased, the angle of refraction also increases. Whenthe refraction angle of a longitudinal wave reaches 90the wave emerges from the second medium and travelsparallel to the boundary (figure 2.14 a). The angle ofincidence at which the refracted longitudinal waveemerges, is called the ir.s_ SEitieol aQgie . If theangle of incidence c^. is further increased the angle of

1 orefraction for the transverse wave also approaches 90 .The value ofo

20' 3031' 32' 33'33'2'

on

Of)

70

60

SO

kO

30

20

to

^

\\\

\

\\

\

\

y

\

^

s\

1

\\

/A

/Siir

**

et

/

?/

///

,1;

0 W 20 30 W SO 60 70 SO 90

Figure 2.15 Acoustic pressure of reflected waves Vs angleof incidence.

The angle of incidence of longitudinal waves is shown by thelower horizontal scale and the angle of incidence of shearwaves by the upper horizontal scale. The vertical scaleshows the reflection factor in percentages.

It should be noted from figure 2.15 that :

a) The reflected acoustic pressure of longitudinal wavesis at a minimum of 13 % at a 68 angle of incidence.This means the other portion of the waves is modeconverted to transverse waves.

b) For an angle of incidence of about 30 for incidenttransverse waves, only 13 % of the reflected acousticpressure is in the transverse mode. The remainder ismode-converted into longitudinal waves.

c) For incident shear waves if the angle of incidencelarger than 33.2 , the shear wavesreflected and no mode conversion occurs.

isare totally

2.5 PIEZOELECTRIC AND FERROELECTRIC TRANSDUCERS

2.5.1 Piezoelectric Effect

A transducer is a device which converts one form of energyinto another. Ultrasonic tranducers convert electricalenergy into ultrasonic energy and vice versa by utilizing aphenomenon known as the eiezoel.ectric effect. Thematerials which exhibit this property are known aspiezoelectric materials.

41

In the direct piezoelectric effect, first discovered by theCurie brothers in 1880, a piezoelectric material whensubjected to mechanical pressure,will develop an electricalpotential across it (Figure 2.16). In the inversepiesoelectric effect, first predicted by Lippman in 1881and later confirmed experimentally by the Curie brothers inthe same year, mechanical deformation and thus vibration inpiezoelectric materials is produced whenever an electricalpotential is applied to them (Figure 2.17). The directpiezoelectric effect is used in detecting and the inversepiezoelectric effect in the generation of ultrasonic waves.

f + + + + *

Figure 2.16 Direct piezoelectric effect.

0

(c) Expansion

0)

o

(d) Contraction

Q)

e

Figure 2.17 Inverse piezoelectric effect.

2.5.2 Types of Piezoelectric Transducers

Piezoelectric transducers can be classified into twogroups. The classification is made based on the type ofpiezoelectric materai which is used in the manufacture ofthe transducer. If the transducers are made from singlecrystal materials in which the piezoelectric effect occursnaturally, they are classified as eiSSlectrie crystaltEaQsdU.ee.rg.. On the other hand the transducers which aremade from polycrystalline materials in which thepiezoelectric effect has to be induced by polarization, aretermed polarized ceramic transducers.

2.5.2.1 Piezoelectric Crystal Transducers

Some of the single crystal materials in which thepiezoelectric effect occurs naturally are quartz,tourmaline, lithium sulphate, cadmium sulphide and zincoxide. Among these quartz and lithium sulphate are themost commonly used in the manufacture of ultrasonictransducers,

42

(a) Quartz

Naturally or artificially grown quartz crystals have acertain definite shape which is described bycrystallographic axes, consisting of an X-, Y- and Z-axis.

axes of naturallygrown quartz

_ X-cut quartz forX longitudinal waves

Y-cut quartz fortransverse waves

Figure 2.18 System coordinates in a quartz crystal(simplified) and positions at X and Y-cutcrystals.

The piezoelectric effect in quarts can only be achievedwhen small plates perpendicular either to the X-axis or Y--axis are cut out of the quartz crystal. These are calledX-cut or Y-cut quartz crystals or transducers. X-cutcrystals are used for the production and detection oflongitudinal ultrasonic waves (Figure 2.19) while Y-cutcrystals are used for the generation and reception oftransverse ultrasonic waves (Figure 2.20). Transverse andsurface waves can be produced from an X-cut crystal bytaking advantage of the phenomenon of mode conversion whichoccurs at an interface of two media of different acousticimpedences when a longitudinal ultrasonic wave strikes theinterface at an angle.

Some of the advantages and limitations of quartz whenas an ultrasonic transducer, are given below.

used

Advantages

i) It is highly resistant to wear,ii) It is insoluble in water.

iii) It has high mechanical and electrical stability,iv) It can be operated at high temperatures.

43

directpiezo-electric

effect

inverseDie zo electric

effect

CAUSEcrystalbeing

compressedcrystalbeingexpanded

positivevoltageon faces

negativevoltageon faces

SCHEDULE

tpnnnH &\[mini! V\/

t.

i-

r- o1J-

mun

mKnii

EFFECTpositivevoltageon faces

negativevoftageon faces

expansionof

crystalcontraction

ofcrystal

Figure 2.19.The piezoelectric effect of quartz (X-cut,schematic).

directpiezo- electric

effect

inverseplC

waves. Transverse waves are generated because anX-cut crystal when compressed, elongates in theY-direction also. Production of transverse wavesgives rise to spurious signals after the mainpulse.

iv) It requires a high voltage for its operation,

b) Lithium Sulphate

Lithium sulphate is another piezoelectric crystal whichis commonly used for the manufacture of ultrasonictransducers. Some of the advantages and limitations of alithium sulphate transducer are as follows.

Advantages

i) It is the most efficient receiver of ultrasonicenergy.

ii) It can be easily damped because of its lowacoustic impedence.

iii) It does not age.iv) It is affected very little from mode conversion.

Limitations

i) It is very fragile.ii) It is soluble in water.

oiii) It is limited in use to temperatures below 75 C.

2.5.2.2 Polarized Ceramic Transducers

Polarized ceramic transducers have nearly completelyreplaced quartz and are on their way to replacingartificially grown crystals as transducer elements.Polarized ceramic transducer materials are ferroelectric innature. Ferroelectric materials consist of many "domains"each of which includes large number of molecules, and eachof which has a net electric charge. When no voltagegradient exists in the material these domains are randomlyoriented (Figure 2.21). If a voltage is applied, thedomains tend to line up in the direction of the field.Since a domain's shape is longer in its direction ofpolarization than in its thickness the material as a wholeexpands. If the voltage is reversed in direction thedomains also reverse direction and the material againexpands. This is in contrast to the piezoelectric crystalmaterials which contract for a voltage in one direction andexpand for a voltage in the opposite direction.

45

The ferroelectric mode (i.e. expansion for both positiveand negative voltage) can be easily changed topiezoelectric mode by heating the ferroelectric material toits Curie point (the temperature above which aferroelectric material loses its ferroelectric properties)and then cooling it under the influence of a bias voltageof approximately 1000 V per mm thickness.

In this way the ferroelectric domains are effectivelyfrozen in in their bias field orientations and thepolarized material may then be treated as piezoelectric.

Polarized ceramic transducers, as the name implies, areproduced like ceramic dishes etc. They are made frompowders mixed together and then fired or heated to asolid. The characteristic properties required of atransducer for certain applications are controlled byadding various chemical compounds in different proportions.Some of the advantages and limitations of ceramictransducers are :

Advantages

i) They are efficient generators of ultrasonicenergy.

ii) They operate at low voltages.iii) Some can be used for high temperature

applications eg. lead rnetaniobate Curie pointo

550 C.

Limitations

i) Piezoelectric property may decrease with age.ii) They have low resistance to wear,iii) They suffer from mode conversion.

2.5.2.3 Comparison of Piezoelectric Transducers

Piezoelectric transducers can be compared from data similarto that shown in Table 2.2. The Eiesg.elE2tr.ic. njQdulya ld.1is a measure of the quality of a piezoelectric transduceras an ultrasonic transmitter. Greater values of 'd'reflect a transducer's greater efficiency as atransmitter. Table 2.2 shows that lead-zirconate-titanatehas the best transmitter characteristics. TheeieaQeleetEie sferffiatiQQ estant IB. is a measure of theability of a transducer to act as an ultrasonic receiver.

46

Zf!^i3&&:

No potential applied

Potential applied

Figure 2.21 Domains in Ferroelectric material,

High H values show the greater ability of the transduceras a receiver. From table 2.2 it is evident that lithiumsulphate is the best receiver of ultrasonic energy. Theelectroffigchauisal c.oyp.ling. fStor. 1K1 shows the efficiencyof a transducer for the conversion of electric voltage intomechanical displacement and vice versa. This value isimportant for pulse echo operation as the transducer actsas a transmitter and receiver. Higher values of K meanthat the overall efficiency of the transducer as atransmitter and receiver is better. The values for lead-meta niobate, lead-zirconate-titanate and barium-titanatelie in a comparable order. A satisfactory resolution powerrequires that the soup. ling, factor for radial SQlaQQKP. is as low as possible. Kp is a measure for theappearance of disturbing radial oscillations which widenthe signals. These radial oscillations are because of themode conversion disturbances of the transducers. From thispoint of view lithium sulphate and lead-metaniobate are thebest transducer materials. Since in the case of contact aswell as immersion testing a liquid couplant with a lowacoustic impedance Z is required, the transducer materialshould have an acoustic impedance of the same order to givea better transmission of ultrasonic energy into the testobject. In this respect the best choices are lithiumsulphate or lead-meta-niobate or quartz as all of them havelow acoustic impedances.

47

TABLE 2.2 Boe Characteristics Of Cosson Piezoelectric Transducers.

Soundvelocity

C 1/5

Acousticiipedance

4 2z 10 kg/i B

Electro-echanic

k

Piezoeletricodulus

d

Piezoeletricdefoliationconstant

H

Couplingfactor forradialoscillations

KP

Lead lirconatetitanate

4000

30

0.4 - 0.7

150 - 591

1.8 - 4.4

0.5 - 0.4

,

!1

Bariui titinate ! Lead taniobate!

1

5100 ! 3300!1

1

!27 ! 20.5

11

!

1

1

0.45 ! 0.4!!

1

!125 - 190 ! 85

1t

I

1I

!!

1.1 - 1.4 ! 1.91

!

11

1I

l1

i0.8 ! 0.07

l1

i!!

! Lithitu sulphate ! Buartz11

!5440 ! 5740

11

!t

11.2 ! 15.2

1

i

!!

0.38 ! 0.111

!

I1

!15 ! 2.3

t

1

!II

8.2 ! 4.9i!

1

1t

!11

0 ! 0.11I

1!

I I1

1 Lithiui ni oba te i! t

i ;i 7320 !t :i i.... . {i :

i34 :

ii

i!

0.2

4

4.7

-

i!!

2.6 THE CHARACTERISTICS OF THE ULTRASONIC BEAM

2.6.1 The ultrasonic Beam

The region in which ultrasonic waves are propagated from anultrasonic transducer is known as the ultrasonic beam. Forthe purpose of ultrasonic testing of materials, the greatlysimplified shape of an ultrasonic beam for a circulartransducer is as shown in Figure 2.22 . Two distinctregions of the beam exist and are classified as the nearfield region and far field region.

48

o/ (-6dB)

(-20 dB-

Figure 2.22

2.6.1.l Near Field

A piezoelectric transducer can be considered to be acollection of point sources, each of which is emittingspherical ultrasonic waves to the surrounding medium (Figure2.23). These spherical waves interfere with each other andresult in a system of maxima and minima in intensity in theregion close to the transducer. This region is known as thenear field region. The shape of the wave front in the nearfield, is as shown in figure 2.23 and is planer.

Figure 2.23

The intensity variation along and across the axial distancefor a typical transducer are shown in figures 2.24 & 2.25respectively.

I I0 NI N2 N3 N4

Distance from the transducer in near field lengths-

Figure 2.24 Distribution of intensity along the axialdistance.

49

Figure 2.25

(A) 5MHz Beam Plot in water

Beam plot in water of a 5 MHz, 5/8 inchdiameter round transducer.

Flaws appearing in the near field must be carefullyinterpreted because a flaw occuring in this region canproduce multiple indications and the amplitude of thereflected signal from the flaw can vary considerably if theeffective distance from the probe varies.

2.6.1.2 Calculation Of Near Field Length

The length N of the near field depends upon the diameter ofthe transducer and the wave-length of the ultrasonic wavesin the particular medium. The near field length for a probeincreases with increase in its diameter and frequencyand can be calculated approximately from

Where

N = D2

= D f

4 V(2.20)

N = Near field length.

D = Diameter of transducer.

V = Velocity of sound in material,

f = Frequency.

(i) Longitudinal Wave Probe With Circular Transducer

Since because of glueing the effective transmitting area ofa transducer is 97 % V, he near field length for acircular transducer of a longitudinal wave probe is moreaccurately given by :

D 2Near field length = N = eff = 0.94 D = 0.94. D.f

4 V(2.21)

50

Where D = 0.97 Deff

D = diameter of the transducer.

^ = wave-length of ultrasound in thematerial.

f = frequency of ultrasound in thematerial.

V = velocity of ultrasound in thematerial.

(ii) Angle Probe With Circular Transducer

2 velocity in perspex (longitudinal)N = D - Perpex depth x

___ velocity in steel (transverse)4?) (2.22)

(iii) Angle Probe With Square Or Rectangular Transducer

length of the width of the frequency of the1.3 x x x 0.97 x

N = transducer transducer transducer

Transverse wave4 x velocity in the (2.23)

material

velocity in perspex (longitudinal)- Perspex depth x

velocity in steel (transverse)

where the factor 1.3 in equation (2.23) is the factor ofconversion from circular shape to rectangular shape and thefactor 0.97 is because of the fact that only about 97 % area ofthe transducer is active due to glueing. A simplified formulafor probes with almost square transducers, where the differencein lengths of the edges is a maximum of 12 % , is

2 2a a

Near field length = N = 1.3 eff = 1.3 eff . f (2.24)^ V

where a is half the effective length of the larger sideeff

which is given in the manufacturer's data sheet of the probe.

51

2.6.1.3 Far Field

The region beyond the near field is known as the far field.The wave front of ultrasonic waves in the far field beyond adistance of three near field lengths from the transducer isspherical as compared to the wave front in the near fieldwhich is planer. The region in the far field between onenear field length and three near field lengths is known asthe transition region because transition in shape of thewave front from planer to spherical occurs in this region.

The intensity in the far field along the axial distance fromthe transducer beyond three near field lengths, falls offwith distance in accordance with the inverse square law i.e.the intensity decreases inversely with the square of thedistance (Figure 2.24). The intensity in the transitionregion of the far field, varies exponentially with distancewith an exponent of distance between 1 and 2.

The reflected intensity of ultrasonic waves from flawsoccuring in the far field, depends upon the sise of the flawwith respect to the beam dimensions. If the flaw is largerthan the beam then the reflected intensity follows theinverse proportional law i.e.

Intensity of reflection OC __________distance

On the other hand if the siae of the flaw is smaller thanthe beam dimensions then the reflected intensity variesinversely as the square of the distance i.e.

, 1Intensity OC 2

(distance)

2.6.2 Beam Spread

There is always some spreading of the ultrasonic beam in thefar field as the waves travel from the transducer. Theintensity of the beam is a maximum on the central axis anddecreases in proportion to the distance from the transducer.The angle of beam spread or divergence angle % (Figure 2.22)can be calculated from the following equation :

-l K )\X n = Sin nu (2.25)

D

Where A is the wave length of the ultrasonic waves, D isthe diameter in case of a circular transducer and K is aconstant which depends :

52

i) on the edge of the beam which is considered.Usually the value of K is determined with respectto the reduction of the beam intensity to 50 % (6dB), 10 % (20dB) and 0 % (extreme edge) of themaximum amplitude. The subscript "n" in Y and K

denotes the respective edge e.g.

divergence angle for 6 dB edge and

divergence angle for 20 dB edge.

Y0

n Vvthe

the

ii) the method which is used to determine beam spread.In one method the through transmission technique isused. In this case a very small diameter probe ismoved over the back wall surface of several plane -parallel specimens of different thicknesses and arecord is made of the amplitudes of the CRT screenindications. The beam spread is then plotted byjoining together those points which have the sameindication amplitude. The sound beam thus obtainedis also referred to as the " free field " .

In the second method the beam spread is measured bymaking use of the pulse echo technique. In thismethod small reflectors of constant size atdifferent depths are used to plot the beam. Theplot of the beam made by this method is known as the"echo field".

iii) the shape of the transduceror rectangular.

i.e. whether circular

Values of K for a circular transducer determinedby the first method are given in Table 2.3 whileTable 2.4 gives different values of K determined bythe second method for both circular and rectangulartransducers.

Table 2.3. Values of K for circular and rectangulartransducers as determined by throughtransmission technique.

Edge

X (dB)

0 % (0 - 00)

10 X {20 - dB)

50 % (6 - dB)

Kcircular

1.22

1.08

0.54

K

1.00

0.60

0.91

53

Table.2.4 Values for K for circular and rectangulartransducers as determined by pulse echotechnique.

Edge% (dB)

0 % (0 - 00)

10 % (20 - dB)

50 % (6 dB)

Kcircular

1.22

0.87

0.51

_ _ _ _ _ _ _ _ _ ___

Krectangular

1.00

0.74

0.44

In the case of angle beam probes with rectangulartransducers, the width of the beam in the horizontal planeis not equal to the width of the beam in the vertical plane.The half beam widths at one near field distance in thehorizontal and vertical planes, in this case, are called thefocal length and focal width and are denoted by FL and FBrespectively (Figure 2.26).

FOCUS

ACOUSTICALAXIS

(MAIN BEAM)Figure 2.26. Definition Of Sound Field Data.

In Figure 2.26 the other notation used are :

angle of refraction of the beam in steel.

angle of beam divergence in the verticalplane.

Because of refraction this angle might bedifferent for the upper and lower parts ofthe beam, this is, therefore, usuallygiven by .Cp = + or a> =in the data sheet of a probe, if there isappreciable difference in the two angles.Accordingly the focal width (FB }for the two parts are given as FB^ = +and FB., = - ........ in the data sheets.tvangle of beam divergence in the horizontalplane.

54

In the data sheets of the pulse echo type probes, data isusually given for the 6 dB or 50 % intensity edge. Todetermine the data for the 20 dB edge from the data sheetsthe following equations are used :

FB = 1.74 FB ............ (2.26)20 6

FL = 1.74 FL ............ (2.27)20 6

1.7420 6

(2.28)

where FB and FB are half the widths of the beam at one20 6

near field distance from the transducer (They are known asfocal widths) and v an

n ndepending on which plane the beamwidth is required to be determined.

2.7 ATTENUATION OF ULTRASONIC BEAMS

The intensity of an ultrasonic beam that is sensed by areceiving transducer is considerably less than the intensityof the initial transmission. The factors that are primarilyresponsible for the loss in beam intensity are discussedbelow :-

2.7.1 Scattering of Ultrasonic Waves :

The scattering of ultrasonic waves is due to the fact thatthe material in which the ultrasonic wave is travelling isnot absolutely homogeneous. The inhomogeneities can beanything that will present a boundary between two materials

55

of different acoustic impedance such as an inclusion orpores and possibly grain boundaries containingcontaminants. Certain materials are inherentlyinhomogeneous, such as cast iron which is composed of amatrix of grains and graphite particles which differ greatlyin density and elasticity. Each grain in the agglomerationhas radically different acoustic impedance and consequentlyproduces severe scattering. It is possible to encounterscattering in a material of just one crystal type if thecrystals exhibit velocities of different values whenmeasured along axes in different directions. A material ofthis type is said to be Anisotropie. If individual grainsare randomly oriented throughout a material, scattering willoccur as if the material is composed of different types ofcrystals or phases. Materials exhibiting these qualitiesnot only decrease the returned ultrasonic signal because ofscattering, but also often produce numerous small echoeswhich may mask or "camouflage" real indications.

2.7.2 Absorption Of ultrasonic Waves

Absorption of ultrasonic waves is the result of theconversion of a portion of the sound energy into heat. Inany material not at absolute zero temperature the particlesare in random motion as a result of the heat content of thematerial. As the temperature increases, there will be anincrease in particle activity. As an ultrasound wavepropagates through the material it excites the particles.As these particles collide with unexcited particles, energyis transmitted causing them to oscillate faster and throughlarger distances. This motion persists after the sound wavehas passed on, so energy of the passing wave has beenconverted to heat in the material.

2.7.3. Loss due to coupling and surface roughness

A third cause of attenuation is transmission loss due tothe coupling medium and the surface roughness. When atransducer is placed on a very smooth surface of a specimenusing a couplant, the amplitude of signal from the backsurface varies with the thickness of the couplant. Atypical example is shown in Fig. 2.27 .

Figure 2.27

2.25Q 29N PL-3USF-513

^A .1-0.1 0-2 0.3 0.4 0.5 0.6 0.7

Thickness of Couplant (Oil)Variations of signal amplitude andthickness.

couplant

56

The variation of signal amplitude with the type ofcouplant for various surface roughnesses is shown in Figure2.28, which indicates that the surface roughness shouldpreferably be l