Materials characterization and nondestructive testing by acoustic imaging techniques

Scanning acoustic microscopy and high-frequency acoustic imaging exploit the interaction of ultrasonic waves with material to image its structure and defects with high spatial resolution. Low frequencies (<100 MHz) are generally used for defect detection, whereas imaging at higher frequencies reveals more details about the microstructure. This paper outlines the principles of acoustic imaging techniques, represents recently developed theoretical fundamentals which allo quantitative evaluation of elastic material properties, and points out a series of practical applications. These include observation of microcrack damage and R-curve behavior in ceramics, interacting effects with grain and phase boundaries and detection of defects down to a size of 30 µm for a frequency of 50 MHz and an attenuation not larger than 1db/cm.