Photographic Study of Stress Wave Propagation and Crack Growth in a Transparent Plastic

Abstract

In order-to develop techniques for studying the transient behavior of plastic materials subjected to mechanical impact, a preliminary photographic investigation was undertaken with an electronic image converter camera system. Two types of events were recorded with CR-39 plastic serving as the specimen material: 1. Stress pulses, produced in the plastic by mechanical impaction, were studied by observation of the resulting fringe patterns. The pulse fronts were characterized in terms of shape and propagation velocity» An empirical equation was derived to express the decrease in slope of segments of the pulse front as it propagated through the specimen. 2. Propagating cracks, generated in the plastic by mechanical impact, were subjected to photographic analysis. Crack prorogation velocities and estimated crack initiation times were obtained. These served to yield;a rough estimate of the magnitude of the incident longituddnal pulse level at which crack initiation occurred. viii pa. ~»*l^-«