COMPARISON OF THE FATIGUE PROPAGATION BEHAVIOR OF POLYSTYRENE AND 95 5 POLYSTYRENE/POLYETHYLENE BLENDS/

The fatigue crack propagation (FCP) behaviour of polystyrene (PS) and 95/5 PS/high density polyethylene (HDPE) was studied at cycling freque ncies (f) of 0.2, 2 and 20 Hz. At the latter two frequencies, the fati gue crack growth rates (FCGRs) in the blends were lower than in PS, es pecially when a styrene/ethylene-butylene/styrene (SEBS) triblock copo lymer was added as a compatibilizer. The fractographic features observ ed were analysed in detail. Discontinuous growth bands (DGBs), associa ted with the fracture of crazes, formed at low FCGRs in PS and at low and high FCGRs in the blends. Large dimple-like features formed at int ermediate FCGRs and fatigue striations at high FCGRs were observed in PS. The observations indicated that the reduction in FCGR when HDPE wa s added to PS was associated with the presence of stretched HDPE parti cles which fractured behind the crack front, with increased particle-m atrix adhesion favoured by the compatibilizer. An increase in cycling frequency decreased the FCGRs, with the fractographic observations ind icating that this effect was associated with a decrease in the time-de pendent deformation in the fracture process zone slightly ahead of the crack tip. (C) 1998 Chapman & Hall.