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Transcript

  • 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