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Impact tests with precracked bend specimens were performed, and the crack tip loading was measured utilizing the shadow optical method of caustics and high-speed photography. Some results are presented in comparison to a quasi-static solution for the crack loading history, which is based on a simple mass-spring model. The deviations obtained are a quantitative measure of dynamic effects acting at the crack tip and they define dynamic correction functions kdyn. In a normalized form these time-dependent functions are very general curves for impact tests; therefore they can be considered as dynamic key-curves. They allow the prediction of the dynamic crack tip loading history for arbitrary impact test conditons so far as the usual restrictions of the linear elastic fracture mechanics are fulfilled.
The dynamic key-curves show clearly the decrease of dynamic effects with increasing time, allowing us to define transition times tdyn. For times larger than tdyn the crack loading history can be described sufficiently accurately by the quasi-static solution. The established transition time tdyn is compared with transition times defined by Ireland and by Nakamura, Shih, and Freund.
impact test, fracture toughness, impact fracture toughness, dynamic keycurves, transition times, stress intensity factors, caustics, high-speed photography
Fraunhofer-Institut für Werkstoffmechanik, Freiburg,