Fatigue crack propagation in short and long glass fiber reinforced injection-molded polypropylene composites

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 stage (range I) and an acceleration stage (range II) could be distinguished. The former was explained by the development and 'stabilization' of the damage zone. The latter range could be adequately described by the Paris-Erdogan power law. Increasing fiber loading resulted in improved resistance against FCP. Incorporation of longer fibers yielded an even higher FCP resistance. The use of LGF reinforcement also resulted in a quasi-isotropic FCP behavior, whereas a clear dependance of the propagation rate on crack direction could be observed for SGF-filled composites. All these differences could be interpreted by differences in microstructural parameters of the LGF in comparison to the SGF systems. Failure processes were studied by light and and scanning electron microscopy, and are 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 effects.