BMFB 4283 NDT & FAILURE ANALYSIS

1. BMFB 4283NDT & FAILURE ANALYSIS Lectures for Week 4 Prof. QumrulAhsan, PhD Department of Engineering Materials Faculty of Manufacturing Engineering

2. Issues to address 4.0 Ultrasound 4.1 Introduction 4.2 Theory 4.3 Equipment 4.4 Inspection Principles 4.5 Applications

3. Ultrasonic Testing

4. Introduction • This module presents an introduction to the NDT method of ultrasonic testing. • Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. • Ultrasonic examinations can be conducted on a wide variety of material forms including castings, forgings, welds, and composites. • A considerable amount of information about the part being examined can be collected, such as the presence of discontinuities, part or coating thickness; and acoustical properties can often be correlated to certain properties of the material.

5. Ultrasound – An Introduction • Ultrasonic Testing (UT) uses high frequency sound (ultrasound) energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/ evaluation, dimensional measurements, material characterization, and more.

6. Basic Principles of Sound • Sound is produced by a vibrating body and travels in the form of a wave. • Sound waves travel through materials by vibrating the particles that make up the material. • The pitch of the soundis determined by the frequency of the wave (vibrations or cycles completed in a certain period of time). • Ultrasound is soundwith a pitch too highto be detected by the human ear.

7. Basic Principles of Sound (cont.) • The measurement of sound waves from crest to crest determines its wavelength (λ). • The time it takes a sound wave to travel a distance of one complete wavelength is the same amount of time it takes the source to execute one complete vibration. • The sound wavelengthis inversely proportional to its frequency. (λ = 1/f) • Several wave modes of vibration are used in ultrasonic inspection.The most common arelongitudinal, shear, andRayleigh (surface) waves.

8. Longitudinal wave • a wave formed by individual particles oscillating in the direction of propagation • propagated through solids, liquids and gases • This wave is the most easily generated and detected. • Almost all of the sound energy used in UT originates as longitudinal sound and then may be converted to the other modes for special applications. wave length direction of propagation direction ofosscillation

9. Transverse wave • particle motion is transverse or at an angle to the direction of propagation • only in solids because distance between molecules is so great in liquid and gases • This wave is the most easily generated and detected • speed about half of longitudinal wave length direction of propagation direction ofosscillation

10. Surface wave • Rayleigh wave • elliptical particle motion • penetration approx. one wavelength • velocity of approx. 90 percent of shear wave • Reflection of Rayleigh waves from cracks on the surface or from sub-surface discontinuities may be seen on the CRT

11. Lamb wave (Plate wave) • generated in a relatively thin solid substance whose thickness is about one wavelength • consist of a mixture of zig-zag reflected longitudinal and transverse • detect surface and sub-surface discontinuities

12. Summary of Waves

13. Frequency, Wavelength and Velocity • Frequency is the number of oscillation in one sec • Period is the time is to make on oscillation t = 1/f • Wavelength expressed as  is given as the distance between two successive crests in the waveform, this distance varies with frequency and velocity • The velocity of sound propagation varies from one material to another. • It depends on the elastic property and density of the material.

15. Acoustic Impedance • Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid and the impedance restrict movement of velocity. • The acoustic impedance (Z) of a material is defined as the product of its density () and acoustic velocity (v). • Z = Driving pressure/velocity of particle = P/v or V • Acoustic impedance is important in • the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances. • the design of ultrasonic transducers. • assessing absorption of sound in a medium.

16. Features of Ultrasound • Ultrasonic waves are very similar to light waves in that they can be reflected, refracted, and focused. • Reflection and refraction occurs when sound waves interact with interfaces of differing acoustic properties. • In solid materials, the vibrational energy can be split into different wave modes when the wave encounters an interface at an angle other than 90 degrees. • Ultrasonic reflections from the presence of discontinuities or geometric features enables detection and location. • The velocity of sound in a given material is constant and can only be altered by a change in the mode of energy.

17. BEHAVIOR OF SOUND WAVES • Behavior of sound waves: • Reflection • Refraction • Diffraction • Mode conversion • Attenuation

18. Reflection at Normal Incidence • When an ultrasonic wave incidents normal to an interface between two materials and the materials have different acoustic impedance , both reflected and transmitted waves are produced. • The difference in Z is commonly referred to as the impedance mismatch.  • The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface

19. Reflection at Normal Incidence • The fraction of the incident wave intensity that is reflected can be calculated with the equation below and known as the reflection coefficient  • Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one.  T = 1 - R = 4Z1Z2 / [Z2+ Z1]2

20. Reflection at Oblique Incidence When an ultrasonic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced. • The reflected energy follows the same laws as light, i.e. the angle of incidence is equal to the angle of reflection

21. Refraction at Oblique Incidence • Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials. • The refraction can be calculated by Snell’s Law as shown in the following equation. Where: VL1 is the longitudinal wave velocity in material 1. VL2 is the longitudinal wave velocity in material 2.

22. MODE CONVERSION • Ultrasonic energy when reflected, may change from one waveform to another (compressional to shear, shear to surface, etc) • This mode change is accompanied by the appropriate change in velocity.

23. L reflectedwave incidentwave surfacewave perspex steel T O  = 2= 57° ( second critical angle) T = 90° Critical Angles L L reflectedwave reflectedwave L L reflectedwave incidentwave incidentwave incidentwave L perspex L perspex medium 1 L steel steel medium 2 refractedtransverse wave refractedwave refractedwaves T T  = 1 = 27.5°° (first critical angle) T = 33.3 L = 90°  = 36.4° T = 45° L L T T

24. Ranges for incident waves 27,5° L 57° perspex 90° steel T 33,3°

25. DIFFRACTION • Diffraction is the apparent bending of sound waves around the tips of a narrow reflector. • Diffuse reflection or scatter occurs at these positions resulting in a small amount of energy being bent around the defect

26. ATTENUATION • Attenuation is the loss intensity of the ultrasonic beam as it passes through a material and is dependent on the physical properties of the material. • Two factors affect the sound attenuation: • Absorption • Cause by the interaction of the particles as they vibrate during the passage of sound waves • The movement of particles cause friction, and dissipated as heat • As the frequency increases, the absorption greater due to raid movement • Scatter • Cause by grain boundaries, porosity, non metallic inclusions, etc • Larger grain size greater scattering • Coarse grain will be more attenuative than fine grain material • Rough surface also cause attenuation

27. Attenuation • Attenuation can be represented as a decaying exponential. The amplitude change of a decaying plane wave can be expressed as: • A0 is the unattenuated amplitude of the propagating wave at some location. • The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location. • The quantity  is the attenuation coefficient of the wave traveling in the z-direction. The dimensions of are nepers/length, where a neper is a dimensionless quantity. Attenuation can also measured in terms of decibels (dB) dB = 20 log (I0/I)

28. The transducer is capable of both transmitting and receiving sound energy. Ultrasound is generated with a transducer. Ultrasound Generation A piezoelectric element in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa.

29. Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference . These are sometimes also referred to as diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area. Sound beam Where: N = Near Field Length or Transition from Near Field to Far Field D = Diameter of the Transducer F = Frequency of the Transducer l = Wavelength, V= Velocity of Sound in the Material

30. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. The area just beyond the near field is where the sound wave is well behaved and at its maximum strength. Therefore, optimal detection results will be obtained when flaws occur in this area. Sound beam The transition between the near field and the far field occurs at a distance, N, and is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer. Where: N = Near Field Length or Transition from Near Field to Far Field D = Diameter of the Transducer F = Frequency of the Transducer l = Wavelength, V= Velocity of Sound in the Material

31. Sound beam • Far zone is beyond near zone. The beam diverges resulting in decay in sound intensity as the distance from the crystal is increased. In the far zone, large and small reflectors follow different laws: • Large reflectors: follow inverse law- the amplitude is inversely proportional to the distance • Small reflectors: follow inverse square law- the amplitude is inversely proportional to square of the distance. near field N = near field length far field acoustical axis (central beam)  = angle of divergence

32. Beam Divergence/Beam Spread • the energy in the beam does not remain in a cylinder, but instead spreads out as it propagates through the material. • The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction. • beam spread is twice the beam divergence.

33. Dead zone dead zone The dead zone is the ringing time of the crystal and is minimized by the damping medium behind the crystal. It is not possible to detect defect in this zone. 0 2 4 6 8 10

34. Principle of Ultrasonic Inspection • Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface. • At surface interfaces some of the wave energy is reflected and some is transmitted. • The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector. • The travel time of the sound can be measured and this provides information on the distance that the sound has traveled.

35. Principle of transit time measurement finish signal (echo) transmitter Stop-watch start signal (pulse) probe transit timemeasurement sound transitpath work piece

36. Equipment Equipment for ultrasonic testing is very diversified. Proper selection is important to insure accurate inspection data as desired for specific applications. In general, there are three basic components that comprise an ultrasonic test system: - Instrumentation - Transducers - Calibration Standards

37. Transducers • Transducers are manufactured in a variety of forms, shapes and sizes for varying applications. • Transducers are categorized in a number of ways which include: • - Contact or immersion • - Single or dual element • - Normal or angle beam • In selecting a transducer for a given application, it is important to choose thedesired frequency, bandwidth, size, and in some cases focusing which optimizes the inspection capabilities.

38. Chac: of Transducers • Some transducers are specially fabricated to be more efficient transmitters and others to be more efficient receivers. • A transducer that performs well in one application will not always produce the desired results in a different application. • For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. • Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer. • Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

39. Chac: of Transducers The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available. Transducers are constructed to withstand some abuse, but they should be handled carefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the backing material. Damage to a transducer is often noted on the A-scan presentation as an enlargement of the initial pulse.

40. Contact Transducers Contact transducers are designed to withstand rigorous use, and usually have a wear plate on the bottom surface to protect the piezoelectric element from contact with the surface of the test article. Many incorporate ergonomic designs for ease of grip while scanning along the surface.

41. Probe construction - Contact Transducers • The cork separator and corrugations in the Perspex reduce the cross-talk/chatter between crystals • The probe are used mainly for testing thin sections (thickness measurement) and to detect near surface defect

42. Contact Transducers (cont.) • A way to improve near surface resolution with a single element transducer is through the use of a delay line. • Delay line transducers have a plastic piece that is a sound path that provides a time delay between the sound generation and reception of reflected energy. • Interchangeable pieces make it possible to configure the transducer with insulating wear caps or flexible membranes that conform to rough surfaces. • Common applications include thickness gauging and high temperature measurements.

43. Straight beam probe housing socket matching-element damping-block protecting face(probe delay) crystal workpiece Sound pulse

44. Probe construction - Contact Transducers • Contact transducers are available with two piezoelectric crystals in one housing. These transducers are called dual element transducers. • One crystal acts as a transmitter, the other as a receiver. • This arrangement improves near surface resolution because the second transducer does not need to complete a transmit function before listening for echoes. • Dual elements are commonly employed in thickness gauging of thin materials.

45. TR-probe / dual crystal probe transmittersocket receiversocket acousticalbarrier damping blocks crystal delay

46. BE IP 0 2 4 6 8 10 TR-probe work piece Probe delay with TR-probes

47. 0 2 4 6 8 10 Cross talk at high gain BE IP flawecho cross talkecho TR-probe flaw

48. Transducers (cont.) • Angle beam transducers incorporate wedges to introduce a refracted shear wave into a material. • The incident wedge angle is used with the material velocity to determine the desired refracted shear wave according to Snell’s Law) • Transducers can use fixed or variable wedge angles. • Common application is in weld examination.

49. Angle beam probe damping blocks housing socket perspex wedge(probe delay) crystal workpiece Sound pulse

50. Transducers (cont.) • Immersion transducers are designed to transmit sound whereby the transducer and test specimen are immersed in a liquid coupling medium (usually water). • Immersion transducersare manufactured withplanar, cylindrical or spherical acoustic lenses (focusing lens).