Dynamic crack propagation and unloading behavior of brittle polymers

The method of caustics in combination with a Cranz-Schardin high-speed camera was utilized to study dynamic crack propagation and unloading behavior of epoxy, PMMA and Homalite-100 specimens. Dynamic stress intensity factor K_ID and crack velocity ȧ were evaluated in the course of crack propagation. Caustic patterns at the loading points were also recorded to estimate load P applied to the specimen. Unloading rate Ṗ, the time derivative of P, was determined as a function of time t, and its time correlation with K_ID or ȧ was examined. The findings showed that the change in Ṗ was qualitatively in accord with the change in K_ID or ȧ. However, there existed slight differences among the values of t giving the maximum Ṗ, ȧ and K_ID, so that their order was ȧ, Ṗ and K_ID.