Crack deflection during fatigue and monotonic fracture of a SiC whisker reinforced 2009 aluminium alloy

Fatigue crack growth under Mode I loading, and static fracture in both symmetrical and asymmetrical notched four point bend specimens, have been examined in SiC whisker reinforced 2009 aluminium alloy. In the fatigue tests a range of orientations of the starter notch, with respect to the extrusion direction, L, was examined. Slanted crack propagation was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T) remained constant whilst the ratio of Mode I to Mode II loading was varied. Again crack deflection was observed in all cases apart from the L-T specimen under pure Mode I loading. Whisker debonding was found to be the dominant factor controlling crack deflection, independent of the mixity of the loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to the asymmetrically distributed maximum principal stress. In fatigue loading, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted in terms of the different sampling volumes at the crack tip in monotonic fracture and in fatigue crack growth.