INITIATION OF FATIGUE CRACKS ALONG GRAIN- BOUNDARIES IN ALUMINUM BICRYSTALS

Fatigue crack initiation along boundaries was studied by using aluminu m bicrystals. The bicrystals were grown by the Bridgman method to have three different inclinations of 0, pi/36 and pi/15 rad between the gr ain boundary and the primary slip plane. They were tested by a bending fatigue machine under strain amplitudes of 0.13, 0.08 and 0.05%, and the effects of the strain amplitudes on crack initiation and propagati on were examined. Fatigue cracks are nucleated along the grain boundar ies at all the strain amplitudes down to 0.05% in the 0 rad aluminum b icrystal. In the aluminum bicrystals with the inclination of pi/36 and pi/15 rad, the fatigue cracks are nucleated along grain boundaries at the high strain amplitude of 0.13% only, but are not nucleated at the low strain amplitude of 0.05%. The number of cycles to failure of the 0 rad bicrystal is lower than those of the pi/36 and pi/15 rad bicrys tals, suggesting that the active slip on the planes parallel to the gr ain boundary enhances the crack initiation along grain boundaries.