Experimental Studies of Slow Stable Brittle Crack-growth in Polymethyl-methacrylate

A controlled crack growth was achieved in single edge—cracked specimens made of a high—molecular weight PNNA by regulating the cross— head speed of loading of the specimens by a computer—driven testing device. The crosshead speeds used during the tests were varied between v =1x107m/s and 1X1O5m/s. 5 It was shown that in this area of very slow quasi—static loading of brittle plexiglas specimens under conditions of plane stress the crack initiated at some critical value of loading, characteristic of the crosshead speed and almost independent of the initial crack length. It was established that in the interval of crosshead speeds between v5=O.2x106m/s and v5=O.6x105m/s brittle cracks propagated always slowly with velocities of the order c=3 to 5X1O2m/s. For lower or higher values of v5 than the lowr or higher limits of this transition zone, fracture was typically brittle with high crack—propagation velocities. In the vicinity of the lower bound the velocity of the crack at the initiation zone strongly oscillated and then stabilized to some, almost constant velocity of the order of 5 to lOXlO2m/s, which was stable along the whole width of the specimen and only at the vicinity of the opposite boundary was reduced considerably oscillating again. As the crosshead speed v5 was increasing beyond the stable low—velocity region, a two—step crack velocity region was appeared. The first step was manifested after the initiation of propagation of the crack, which took always low values, and a second step, where the crack—propagation velocity was increased considerably, tending to typical brittle—fracture velocities of the material. Oscillations of the velocity c 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 were remained always K1—dominant and rather insenstitive to variation of c, following similar trends as the variation of the crack propagation velocity.