Non-Destructive Testing of Arc and Plasma Sprayed Coatings by Ultrasonic Imaging Technique

Abstract

In ultrasonic testing of thermally sprayed coatings difficulties often occur, which relate partly to poor lateral and axial resolution, partly to the coarse grained structure of the coatings, and partly to insufficient accuracy in defect description. These problems can be reduced by a test procedure in which the ultrasonic pulses sent into the test body are visualised optically. In practice this is achieved by producing high energy ultrasonic pulses with well defined spectral content and by observing the propagating wave optically by holographic interferometry The object to be tested is illuminated twice by a pulsed laser beam for a very short period, and it has proved possible to incorporate the system into a mobile test unit. The complete sound field and interactions of the sound with existing defects are imaged directly with respect to position, shape, and size. Apart from basic discussion of the test method, results of practical applications to flame, arc, and plasma sprayed coatings are presented.