Slow-brittle-fracture transition in polymethylmethacrylate

A slow crack growth was achieved in initially edge-cracked specimens made of a high-molecular weight PMMA by regulating the cross-head speed of loading by a computer-driven testing machine. The strain rate \(\dot \varepsilon \) used during the tests varied between \(\dot \varepsilon \) =1× l0^?6 s^?1 and 1×10^?4 s^?1. It was shown that, in this zone of slow quasi-static loading of brittle polymethylmethacrylate specimens under conditions of plane stress, the crack initiated for a critical value of loading, at some characteristic zone of strain-rate variation at the crack tip. It was established that for strain rate between \(\dot \varepsilon \) =0.18×10^?5 s^?1 and \(\dot \varepsilon \) =0.45×10^?4 s^?1 brittle cracks were propagating always slowly with velocities in the range ofc=3 to 5×10^?2 m/s. For values ofv _s outside this transition zone fracture was typically brittle with high crack-propagation velocities. As the strain rate was varying beyond the stable low-velocity region, a two-step crack velocity pattern was operative, where the one step took always low values, and the other step corresponded to crack-propagation velocities significantly higher than these limits, tending to typical brittle-fracture velocities of the material. Oscillations of the velocityc at the transition zones, or, in many cases all over the zone of slow propagation of the crack, indicated the unstable character of crack propagation, influenced by different stress raisers and especially by the opposite longitudinal boundary of the specimen. Stress intensity factor values during crack propagation, evaluated from the front (cuspoid) and the rear (external) caustic, which remained alwaysk _g-dominant, were following similar trends as the variation of the crack propagation velocity.