https://www.nde-ed.org/NDETechniques/Ultrasonics/index.xhtml
Introduction
- Advantages
- Senses both surface and subsurface discontinuities
- Minimal part preparation
- Limitations
- Coupling medium required
- Difficulty with rough, irregular, small, thin parts
- Difficulty with coarse-grain materials
- Straight Beam Testing: transducer (and waves) are normal to test surface
- Transducer: a device that converts electrical energy to acoustical energy, or vice versa
- Thickness Measuring: If material speed of sound is know, calculate time of signal going to backwall and bouncing back
- A defect will reflect waves sooner and absorb some energy, meaning backwall peak is lower
- Angle Beam Testing
- Add an angled plastic block between transducer and part
- non-normal angle of incidence means shear waves are produced
- Shear/S Wave: particle motion is orthogonal to direction of propagation
- Compression/Longitudinal/P Wave: particle motion is parallel to direction of propagation
- Waves are also refracted, Knell’s law
- Speed of shear waves is slower than longitudinal waves, so refraction angle is different
- Immersion Ultrasonic Testing
- Both transducer and part are immersed in water
- Can also refer to when sound travels through water jets
- Additional medium introduces a front wall peak, which is after the initial pulse peak
- Both transducer and part are immersed in water
Transducers and Other Equipment
- Piezoelectric Transducers
- Piezoelectric effect: crystal produces electric field when undergoing strain, and vice versa
- Active element thickness is proportionate to output frequency
- thickness is 1/2 the desired wavelength
- Impedance matching element sits between active element and external face
- matching layer thickness should be 1/4 of desired wavelength
- Contact transducers’ matching layer has acoustical impedance between active element and steel
- Contact transducers also have a wear plate
- Immersion transducers’ matching layer has acoustical impedance between active element and water
- Backing material behind active element influences damping characteristics
- Backing material impedance should be similar to active element for effective damping
- Effective damping means wider bandwidth and higher sensitivity
- Higher mismatch means increasing material penetration but reduced sensitivity
- Transducer efficiency, bandwidth, and frequency
- Sensitivity to small defects is proportional to transmission efficiency * receiving efficiency
- Resolution requires a highly damped transducer
- Resolution: ability to locate defects near surface
- Bandwidth: range of frequencies
- Highly damped transducers will respond to frequencies above and below central frequency
- Broad frequency range means high resolving power but less penetration
- Lower frequencies provide more penetration but less sensitivity
- sensitivity also includes thickness measurement capabilities