COMPARISON OF FAST FRACTURE PROPERTIES OF THERMOPLASTICS

The fast fracture behavior of polyethylene, polycarbonate and poly(met hyl methacrylate) are compared for two different thicknesses of materi al. The aim was to make fracture measurements under precisely defined steady load conditions after the material has reached a stress relaxed state. It was essential that the experimental technique provided good isolation between crack initiation and crack propagation. This was ac hieved in small rectangular shaped specimens by adding a tongue to one side extending the intended crack path. A fast, crack was initiated i n the tongue to propagate into the main section after it had been load ed and allowed to stress relax. When necessary the tongue was embrittl ed by cooling so a crack can be initiated with minimum force. This new method obtains the threshold load to just maintain crack propagation and also the increase of crack velocity with load above this threshold up to the limiting condition.