Ut P3

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Ut P3

  1. 1. Ultrasonic Testing Part 3 TWI
  2. 2. Sound Generation <ul><li>Hammers (Wheel tapers) </li></ul><ul><li>Magnetostrictive </li></ul><ul><li>Lasers </li></ul><ul><li>Piezo-electric </li></ul>magnetostrictive
  3. 3. Piezo-Electric Effect <ul><li>When exposed to an alternating current a crystal expands and contracts </li></ul><ul><li>Converting electrical energy into mechanical </li></ul>- + + - - +
  4. 4. Piezo-Electric Materials <ul><li>QUARTZ </li></ul><ul><li>Resistant to wear </li></ul><ul><li>Insoluble in water </li></ul><ul><li>Resists ageing </li></ul><ul><li>Inefficient converter of energy </li></ul><ul><li>Needs a relatively high voltage </li></ul><ul><li>Very rarely used nowadays </li></ul><ul><li>LITHIUM SULPHATE </li></ul><ul><li>Efficient receiver </li></ul><ul><li>Low electrical impedance </li></ul><ul><li>Operates on low voltage </li></ul><ul><li>Water soluble </li></ul><ul><li>Low mechanical strength </li></ul><ul><li>Useable only up to 30ºC </li></ul><ul><li>Used mainly in medical </li></ul>
  5. 5. Polarized Crystals <ul><li>Powders heated to high temperatures </li></ul><ul><li>Pressed into shape </li></ul><ul><li>Cooled in very strong electrical fields </li></ul><ul><li>Examples </li></ul><ul><li>Barium titanate (Ba Ti O 3 ) </li></ul><ul><li>Lead metaniobate (Pb Nb O 6 ) </li></ul><ul><li>Lead zirconate titanate (Pb Ti O 3 or Pb Zr O 3 ) </li></ul>Most of the probes for conventional usage use PZT : Lead Zirconate Titanate
  6. 6. Probes
  7. 7. Probes <ul><li>The most important part of the probe is the crystal </li></ul><ul><li>The crystal are cut to a particular way and thickness to give the intended properties </li></ul><ul><li>Most of the conventional crystal are X – cut to produce Compression wave </li></ul>Z X X X Y
  8. 8. Probes <ul><li>The frequency of the probe depends on the THICKNESS of the crystal </li></ul><ul><li>Formula for frequency: </li></ul><ul><ul><li>Ff = V / 2t </li></ul></ul><ul><ul><li>Where Ff = the Fundamental frequency </li></ul></ul><ul><ul><li> V = the velocity in the crystal </li></ul></ul><ul><ul><li> t = the thickness of the crystal </li></ul></ul><ul><ul><li>Fundamental frequency is the frequency of the material ( crystal ) where at that frequency the material will vibrate. </li></ul></ul>
  9. 9. Probes <ul><li>The Thinner the crystal the Higher the frequency </li></ul><ul><li>Which of the followings has the Thinnest crystal ? </li></ul><ul><li>1 MHz Compression probe </li></ul><ul><li>5 MHz Compression probe </li></ul><ul><li>10 MHz Shear probe </li></ul><ul><li>25 MHz Shear probe </li></ul>25 MHz Shear Probe
  10. 10. Probe Design <ul><li>Compression Probe </li></ul><ul><ul><li>Normal probe </li></ul></ul><ul><ul><li>0 ° </li></ul></ul>Damping Transducer Electrical connectors Housing
  11. 11. Probe Design <ul><li>Shear Probe </li></ul><ul><ul><li>Angle probe </li></ul></ul>Damping Transducer Perspex wedge Backing medium Probe Shoe
  12. 12. Probe Design <ul><li>Twin Crystal </li></ul><ul><li>Advantages </li></ul><ul><li>Can be focused </li></ul><ul><li>Measure thin plate </li></ul><ul><li>Near surface resolution </li></ul><ul><li>Disadvantages </li></ul><ul><li>Difficult to use on curved surfaces </li></ul><ul><li>Sizing small defects </li></ul><ul><li>Signal amplitude / focal spot length </li></ul>Transmitter Receiver Focusing lens Separator / Insulator
  13. 13. Sound Intensity <ul><li>Comparing the intensity of 2 signals </li></ul>Electrical power proportional to the square of the voltage produced Hence
  14. 14. Sound Intensity Will lead to large ratios Therefore
  15. 15. 2 signals at 20% and 40% FSH. What is the difference between them in dB’s?
  16. 16. 2 signals at 10% and 100% FSH. What is the difference between them in dB’s?
  17. 17. Amplitude ratios in decibels <ul><li>2 : 1 = 6bB </li></ul><ul><li>4 : 1 = 12dB </li></ul><ul><li>5 : 1 = 14dB </li></ul><ul><li>10 : 1 = 20dB </li></ul><ul><li>100 : 1 = 40dB </li></ul>
  18. 18. Automated Inspections <ul><li>Pulse Echo </li></ul><ul><li>Through Transmission </li></ul><ul><li>Transmission with Reflection </li></ul><ul><li>Contact scanning </li></ul><ul><li>Gap scanning </li></ul><ul><li>Immersion testing </li></ul>
  19. 19. Gap Scanning <ul><li>Probe held a fixed distance above the surface (1 or 2mm) </li></ul><ul><li>Couplant is fed into the gap </li></ul>
  20. 20. Immersion Testing <ul><li>Component is placed in a water filled tank </li></ul><ul><li>Item is scanned with a probe at a fixed distance above the surface </li></ul>
  21. 21. Immersion Testing Water path distance Water path distance Front surface Back surface Defect

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