Dynamic Fracture Under Normal Impact Loading of the Crack Faces

Results of experiments on crack-face impact are presented. The transient stress-intensity factor variation of a crack has been traced by the Stress-Intensity Factor Tracer (SIFT) [1] under time-stepwise uniform pressure loading of the crack faces. To see the effects of various waves generated by the loading, part of the crack faces was left free of traction within the distance l₀ from the crack tip. The crack-face impact loading was produced by an electromagnetic force induced by a square pulse of an electric current flowing through a copper strip inserted in the saw-cut crack of a Homalite 100 plate specimen. The current flowed in opposite directions in the two portions of the copper strip, between the crack faces, causing them to repel each other. The short-time and the long-time behavior of the transient stress-intensity factor variation under the impact loading have been carefully investigated. Brittle dynamic initiation of crack extension and the stress-intensity variation of a running crack have been also examined. The experimental results have been compared with theoretical predictions based on Freund’s crack-face concentrated load solution [2]. The agreement between the theory and the experiment is excellent. In this study, the various waves generated by the loading are shown to play different roles in transmitting the load to the crack tip. In addition, confirmation is given that the SIFT is excellent in tracing the stress-intensity factor regardless of the crack-tip motion.