Mode I Fatigue Crack Growth in a Continuous Alumina Fiber-Reinforced Magnesium Matrix Composite

Mode I fatigue crack growth behavior in a unidirectional alumina fiber reinforced magnesium composite under constant alternating load, and constant alternating stress intensity conditions was studied both experimentally and theoretically. Particular attention was paid to the effect of fiber bridging in the crack faces on the fatigue crack growth rate (FCGR). Fatigue crack growth experiments were carried out using four point bending specimens. Both d.c. electric potential drop and optical techniques were used to monitor fatigue crack growth. Upon completion of tests, the specimens’ fracture surfaces were examined using a scanning electron microscope to identify the micromechanisms of the fatigue crack growth in the composite at different alternating stress intensity levels (ΔK). The crack mouth opening displacement was measured during the constant alternating stress intensity experiments. In order to establish the relation between da/dN, and ΔK under increasing ΔK conditions, some of the fatigue crack growth experiments were conducted with constant alternating load levels. The results showed that fatigue crack growth rates in the alumina fiber-reinforced magnesium composite were much lower than those in the unreinforced magnesium alloys. Through the scanning electron microscopy examination of the fracture surfaces, it appeared that the fatigue crack growth mechanisms were different at low and high alternating stress intensity levels. At the low stress intensity levels, the fracture surface was quite fiat and there was no significant fiber pull-out. However, at the high alternating stress intensity levels, fiber matrix debonding along with the fiber pull-out was the dominant fatigue crack growth mechanism. A transition from a flat fracture surface to a rough fracture surface was observed in the intermediate alternating stress intensity levels. The effect of these various fatigue crack growth mechanisms on the fatigue crack growth rate in composites subjected to sequential loadings was studied by conducting fatigue crack growth experiments at different constant levels of alternating stress intensity. It was found that the effect was much more pronounced in the composite than in the monolithic materials. This behavior was attributed to the fiber bridging on the crack faces. The effects of the fiber bridging on the crack mouth opening displacement and the effective alternating stress intensity level were studied theoretically, and by the finite element analyses. Based on these results, the effective alternating stress intensity (ΔKeff) was determined, and it was shown that the da/dN vs. ΔKeff in the composite was close to that between da/dN vs. ΔK of magnesium alloys.