This document discusses ultrasonic testing techniques. It describes different methods for sound generation including hammers, magnetostrictive, and piezoelectric techniques. It then focuses on piezoelectric probes, explaining how they work using polarized crystal materials like lead zirconate titanate. Different probe designs are described for compression and shear waves. Factors that determine probe frequency like crystal thickness are also covered. Finally, automated inspection techniques are briefly outlined.

1. Ultrasonic Testing Part 3 TWI

2. Sound Generation Hammers (Wheel tapers) Magnetostrictive Lasers Piezo-electric magnetostrictive

3. Piezo-Electric Effect When exposed to an alternating current a crystal expands and contracts Converting electrical energy into mechanical - + + - - +

4. Piezo-Electric Materials QUARTZ Resistant to wear Insoluble in water Resists ageing Inefficient converter of energy Needs a relatively high voltage Very rarely used nowadays LITHIUM SULPHATE Efficient receiver Low electrical impedance Operates on low voltage Water soluble Low mechanical strength Useable only up to 30ºC Used mainly in medical

5. Polarized Crystals Powders heated to high temperatures Pressed into shape Cooled in very strong electrical fields Examples Barium titanate (Ba Ti O 3 ) Lead metaniobate (Pb Nb O 6 ) Lead zirconate titanate (Pb Ti O 3 or Pb Zr O 3 ) Most of the probes for conventional usage use PZT : Lead Zirconate Titanate

6. Probes

7. Probes The most important part of the probe is the crystal The crystal are cut to a particular way and thickness to give the intended properties Most of the conventional crystal are X – cut to produce Compression wave Z X X X Y

8. Probes The frequency of the probe depends on the THICKNESS of the crystal Formula for frequency: Ff = V / 2t Where Ff = the Fundamental frequency V = the velocity in the crystal t = the thickness of the crystal Fundamental frequency is the frequency of the material ( crystal ) where at that frequency the material will vibrate.

9. Probes The Thinner the crystal the Higher the frequency Which of the followings has the Thinnest crystal ? 1 MHz Compression probe 5 MHz Compression probe 10 MHz Shear probe 25 MHz Shear probe 25 MHz Shear Probe

10. Probe Design Compression Probe Normal probe 0 ° Damping Transducer Electrical connectors Housing

11. Probe Design Shear Probe Angle probe Damping Transducer Perspex wedge Backing medium Probe Shoe

12. Probe Design Twin Crystal Advantages Can be focused Measure thin plate Near surface resolution Disadvantages Difficult to use on curved surfaces Sizing small defects Signal amplitude / focal spot length Transmitter Receiver Focusing lens Separator / Insulator

13. Sound Intensity Comparing the intensity of 2 signals Electrical power proportional to the square of the voltage produced Hence

14. Sound Intensity Will lead to large ratios Therefore

15. 2 signals at 20% and 40% FSH. What is the difference between them in dB’s?

16. 2 signals at 10% and 100% FSH. What is the difference between them in dB’s?

17. Amplitude ratios in decibels 2 : 1 = 6bB 4 : 1 = 12dB 5 : 1 = 14dB 10 : 1 = 20dB 100 : 1 = 40dB

18. Automated Inspections Pulse Echo Through Transmission Transmission with Reflection Contact scanning Gap scanning Immersion testing

19. Gap Scanning Probe held a fixed distance above the surface (1 or 2mm) Couplant is fed into the gap

20. Immersion Testing Component is placed in a water filled tank Item is scanned with a probe at a fixed distance above the surface

21. Immersion Testing Water path distance Water path distance Front surface Back surface Defect