Fatigue and Failure Behavior of Short and Long Glass Fiber Reinforced Injection-Molded Polypropylene

Abstract

Fatigue crack propagation (FCP) in unfilled and short (SGF) and long glass fiber (LGF) reinforced injection-molded polypropylene (PP) composites was studied on notched compact tension (CT) specimens in tension-tension mode. In the FCP response a fatigue crack deceleration (range I) and an acceleration (range II) stage could be distinguished. The appearance of the former was attributed to development and “stabilization” of the damage zone. The latter range could be adequately characterized by the Paris-Erdogan relationship. Increasing fiber volume fraction resulted in improved resistance against FCP. Incorporation of longer fibers yielded an even higher FCP resistance. The use of LGF reinforcement resulted in a quasi-isotropic FCP behavior, whereasa clear dependence of the propagation rate on crack direction could be observed for SGF-filled composites. All these differences could be explained by differences in microstructural parameters of the LGF in comparison to the SGF systems. Fracture surfaces and damage zones were studied by light and scanning electron microscopy and discussed. Increased matrix ductility at higher FCP rates and corresponding changes in the fiber-related events, especially in fiber pull-out length, were attributed to crack tip heating. © 1991, De Gruyter. All rights reserved.