CRACK DEFLECTION DURING FATIGUE AND MONOTONIC FRACTURE OF A SIC WHISKER-REINFORCED 2009 ALUMINUM-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 f atigue tests a range of orientations of the starter notch, with respec t to the extrusion direction, L, was examined. Slanted crack propagati on was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T remained consta nt whilst the ratio of Mode I to Mode II loading was varied. Again cra ck deflection was observed in all cases apart from the L-T specimen un der pure Mode I loading. Whisker debonding was found to be the dominan t factor controlling crack deflection, independent of the mixity of th e loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to t he asymmetrically distributed maximum principal stress. In fatigue loa ding, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted i n terms of the different sampling volumes at the crack tip in monotoni c fracture and in fatigue crack growth.