FATIGUE PROPAGATION BEHAVIOR OF POLYSTYRENE POLYETHYLENE BLENDS/

Fatigue crack propagation (FCP) of injection-moulded polystyrene (PS) and 95/5, 85/15 and 70/30 PS/high-density polyethylene (HDPE) blends a t loading frequencies of 2 and 20 Hz was studied. The FCP results show ed that increasing the HDPE content caused a progressive reduction of the fatigue crack growth rates, especially when a styrene/ethylene-but ylene/styrene (SEBS) terpolymer was added as a compatibilizer. Increas ing the loading frequency also led to a fatigue crack growth rate redu ction. Moreover, the fatigue crack growth rates were lower at a given cyclic stress intensity factor range, Delta K, when the crack propagat ed normal, instead of parallel, to the melt-flow direction during inje ction moulding. Fractographic observations indicated that discontinuou s growth bands (DGBs), associated with the fracture of crazes in the p lastic zone, were present through most or all of the fracture surfaces of the PS/HDPE specimens. In the presence of sufficient HDPE, these D GBs were formed by the initiation, growth and coalescence of large dim ples initiated at HDPE particles ahead of the microscopic crack front, similar to a multiple crazing effect. The loading frequency effect on the FCP behaviour of these blends is attributed to a time-dependent d eformation process. It is concluded that the FCP behaviour of these bl ends is strongly affected by the loading direction with respect to the matrix and minor phase orientation, by the presence of a compatibiliz er, by the composition of the blend and by the testing conditions. (C) 1998 Chapman & Hall.