Ultrasonic-image evaluation of microstructural damage accumulation in materials

Ultrasonic imaging techniques for portraying and evaluating cumulative internal microstructural damage in engineering materials are described. A quantitative delineation of the damage is made in terms of acoustic attenuation obtained from computer analyses of digitized ultrasonic images. Acoustic attenuation data are a basic ingredient in previously developed models of damage processes in materials. The ultrasonic imaging methodology has been developed using filled polymer (inert solid rocket-propellant) samples subjected to progressive uniaxial tensile strain. Successive ultrasonic images taken at various levels of applied strain display dewetting and the evolving microvoid formation/growth which occurs. Both initially intact material and that with pre-existing cracks are of interest. Changes in acoustic attenuation with strain, derived from the processing of digital images, have provided results as to the degree of preferential damage accumulation at sites of filler particle agglomerations appearing on the ultrasonic images. Also, the quantitative extent of an asymmetry in the damage-field distribution near the tips of an extending crack was determined in precracked material. Iso-attenuation type contours generated by computer reveal that kidney-shaped damage zones occur in the neighborhood of the propagating crack tips, reminiscent of the plastic-zone shapes near crack tips in ductile metals under strain. Ultrasonic images of precracked samples show that before crack extension begins, the material damage in the neighborhood of the crack already extends over a relatively large volume of the specimen. G.C. Knollman is Senior Staff Scientist and Senior Member, Mechanics and Maternals Engineering Laboratory