ELASTIC STRESSES, TWIN BOUNDARIES AND FATIGUE CRACKING

Prior finite element method calculations have previously shown that high cycle fatigue cracks start at twin boundaries, because elastic interactions produce the highest stresses there. This investigation was undertaken to test this idea. Three diffusion-bonded bicrystals of 70-30 alpha-brass, having a boundary plane at 45-degrees to the loading axis, were fatigued at low strain amplitudes. The component crystals were so oriented that bicrystal 1 produced low elastic interaction stresses and bicrystals 2 and 3 were of matrix-twin orientations, with the highest total stresses on slip systems nearly parallel to the twin boundary. Only for the initial few cycles did bicrystal 1 reveal no slip bands near the boundary, whereas bicrystals 2 and 3 produced a significant slip band nearly parallel to the twin boundary at the outset. With repeated cycling, the matrix of bicrystal 2 also showed a secondary slip band at the boundary where it produced large incompatibility strains and subsequent cracking. Fatigue cracks were initiated along slip band extrusions away from the bicrystal boundary in bicrystal 1 and along the twin boundary in bicrystal 3. For all three bicrystals, cracking commenced where the stresses and therefore the strains were the highest.