Mechanisms of fatigue fracture in injection-molded polypropylene sheets

Mechanisms of fatigue fracture in injection-molded sheets of polypropylene under cyclic tension have been investigated. Under low stress amplitude E' and tan delta were maintained almost constant during the fatigue process. Just before fracture, E' and tan delta drastically decreased and increased, respectively. In this specimen crazing was observed. Crazing was suggested to increase the fatigue lifetime and to cause brittle fracture. At a higher temperature even under low stress amplitude E' and tan delta monotonously decreased and increased, respectively, until fracture. The specimen showed thermal failure promoted by high ambient temperature, and as a result the lifetime decreased. In both the intermediate and outer core layers of this specimen the crystallinity homogeneously increased. Under high stress amplitude crazing was rather suppressed, and shear flow preferentially took place to lead the specimen to ductile fracture. In this case the lifetime decreased. In the core layer of this specimen the molecular orientation was enhanced, which was suggested to induce the crystallization.