1) Ultrasonic testing techniques include pulse echo, through transmission, and transmission with reflection. Pulse echo uses a single probe to send and receive sound to detect defect depth and orientation. Through transmission uses probes on opposite sides to detect defects but not location. Transmission with reflection can locate defects. 2) The sound beam has a near zone where intensity varies and a far zone with exponential decay. The near zone length depends on probe frequency and diameter, with higher frequency and larger diameter increasing length. 3) Beam spread is smaller with higher frequency and larger diameter probes. Compression waves have a smaller beam spread than shear waves. Snell's law and critical angles determine how sound refracts between materials

1. Ultrasonic Testing Part 2 TWI

2. Ultrasonic Testing techniques Pulse Echo Through Transmission Transmission with Reflection

3. Pulse Echo Technique Single probe sends and receives sound Gives an indication of defect depth and dimensions Not fail safe

4. Defect Position No indication from defect A (wrong orientation) A B B

5. Through Transmission Technique Transmitting and receiving probes on opposite sides of the specimen Presence of defect indicated by reduction in transmission signal No indication of defect location Fail safe method Tx Rx

7. Through Transmission Technique Advantages Less attenuation No probe ringing No dead zone Orientation does not matter Disadvantages Defect not located Defect can’t be identified Vertical defects don’t show Must be automated Need access to both surfaces

8. Transmission with Reflection Also known as: Tandem Technique or Pitch and Catch Technique R T

9. Ultrasonic Pulse A short pulse of electricity is applied to a piezo-electric crystal The crystal begins to vibration increases to maximum amplitude and then decays Pulse length Maximum 10% of Maximum

10. Pulse Length The longer the pulse, the more penetrating the sound The shorter the pulse the better the sensitivity and resolution Short pulse, 1 or 2 cycles Long pulse 12 cycles

11. Ideal Pulse Length 5 cycles for weld testing

12. The Sound Beam Dead Zone Near Zone or Fresnel Zone Far Zone or Fraunhofer Zone

13. The Sound Beam NZ FZ Main Beam Distance Intensity varies Exponential Decay

14. Main Lobe Side Lobes Near Zone Main Beam The main beam or the centre beam has the highest intensity of sound energy Any reflector hit by the main beam will reflect the high amount of energy The side lobes has multi minute main beams Two identical defects may give different amplitudes of signals

15. Sound Beam Near Zone Thickness measurement Detection of defects Sizing of large defects only Far Zone Thickness measurement Defect detection Sizing of all defects Near zone length as small as possible

16. Near Zone

17. Near Zone What is the near zone length of a 5MHz compression probe with a crystal diameter of 10mm in steel?

18. Near Zone The bigger the diameter the bigger the near zone The higher the frequency the bigger the near zone The lower the velocity the bigger the near zone Should large diameter crystal probes have a high or low frequency?

19. Which of the above probes has the longest Near Zone ? 1 M Hz 5 M Hz 1 M Hz 5 M Hz

20. Near Zone The bigger the diameter the bigger the near zone The higher the frequency the bigger the near zone The lower the velocity the bigger the near zone Should large diameter crystal probes have a high or low frequency?

21. Beam Spread In the far zone sound pulses spread out as they move away from the crystal   / 2

22. Beam Spread Edge,K=1.22 20dB,K=1.08 6dB,K=0.56 Beam axis or Main Beam

23. Beam Spread The bigger the diameter the smaller the beam spread The higher the frequency the smaller the beam spread Which has the larger beam spread, a compression or a shear wave probe?

24. Beam Spread What is the beam spread of a 10mm,5MHz compression wave probe in steel?

25. Which of the above probes has the Largest Beam Spread ? 1 M Hz 5 M Hz 1 M Hz 5 M Hz

26. Beam Spread The bigger the diameter the smaller the beam spread The higher the frequency the smaller the beam spread Which has the larger beam spread, a compression or a shear wave probe?

27. Testing close to side walls

28. Sound at an Interface Sound will be either transmitted across or reflected back Reflected Transmitted How much is reflected and transmitted depends upon the relative acoustic impedance of the 2 materials Interface

29. The Phenomenon of Sound REFLECTION REFRACTION DIFFRACTION

30. The Phenomenon of Sound REFLECTION REFRACTION DIFFRACTION

31. Law of Reflection Angle of Incidence = Angle of Reflection 60 o 60 o

32. Inclined incidence(not at 90 o ) Incident Transmitted The sound is refracted due to differences in sound velocity in the 2 DIFFERENT materials

33. REFRACTION Only occurs when: The incident angle is other than 0 ° 30 ° Refracted Water Steel Steel Steel Water Steel

34. REFRACTION Only occurs when: The incident angle is other than 0 ° 30 ° Refracted The Two Materials has different VELOCITIES No Refraction 30 ° 30 ° 65 ° Steel Steel Water Steel

35. Snell’s Law I R Material 1 Material 2 Incident Refracted Normal

36. Snell’s Law C Perspex Steel C 20 48.3

37. Snell’s Law C Perspex Steel C 15 34.4

38. Snell’s Law C Perspex Steel C 20 S 48.3 24

39. Snell’s Law Perspex Steel S C C When an incident beam of sound approaches an interface of two different materials: REFRACTION occurs There may be more than one waveform transmitted into the second material, example: Compression and Shear When a waveform changes into another waveform: MODE CHANGE C C S

40. Snell’s Law Perspex Steel C If the angle of Incident is increased the angle of refraction also increases Up to a point where the Compression Wave is at 90 ° from the Normal 90 ° This happens at the FIRST CRITICAL ANGLE C S C S C S

41. 1st Critical Angle C 27.4 S 33 C Compression wave refracted at 90 degrees

42. 2nd Critical Angle C S (Surface Wave) 90 C Shear wave refracted at 90 degrees 57 Shear wave becomes a surface wave

43. 1st Critical Angle Calculation C Perspex Steel C S 27.2

44. 2nd Critical Angle Calculation C Perspex Steel C S 57.4

45. 1 st . 2 nd . 33 ° 90 ° Before the 1 st . Critical Angle : There are both Compression and Shear wave in the second material At the FIRST CRITICAL ANGLE Compression wave refracted at 90 ° Shear wave at 33 degrees in the material Between the 1 st . And 2 nd . Critical Angle: Only SHEAR wave in the material. Compression is reflected out of the material. At the 2 nd . Critical Angle : Shear is refracted to 90 ° and become SURFACE wave Beyond the 2 nd . Critical Angle: All waves are reflected out of the material. NO wave in the material. S C C

47. Summary Standard angle probes between 1st and 2nd critical angles (45,60,70) Stated angle is refracted angle in steel No angle probe under 35, and more than 80: to avoid being 2 waves in the same material. C S One Defect Two Echoes C S

48. Snell’s Law Calculate the 1st critical angle for a perspex/copper interface V Comp perspex : 2730m/sec V Comp copper : 4700m/sec