NONCONTACT ACOUSTO-ULTRASONICS USING LASER GENERATION AND LASER INTERFEROMETRIC DETECTION

Laser generation and interferometric detection of ultrasound offer various advantages over contact piezoelectric transducers (PZTs). A compact, portable fiber-optic heterodyne interferometer has been designed and built for detection of out-of-plane motion on surfaces. The interferometer gives a linear output for displacements over a broad frequency range, making it useful for ultrasonic, acoustic emission, and acousto-ultrasonic (AU) testing. Using this interferometer with a compact pulsed neodymium:yttrium-aluminum-garnet (Nd:YAG) laser yields a powerful, portable noncontact testing system. Various tests were run on this noncontact testing system to determine its usefulness for the AU technique. Ultrasonic and acoustic emission tests were run to test the interferometer, and then ultrasound was generated in a composite specimen by using the Nd:YAG laser; the resulting displacements were detected by the interferometer. This waveform was compared with a waveform obtained from this same specimen by using a PZT to detect the ultrasound.