CRACK INITIATION AND GROWTH DURING LOW-CYCLE FATIGUE OF DISCONTINUOUSLY REINFORCED METAL-MATRIX COMPOSITES

The initiation and growth of surface cracks during low-cycle fatigue o f an AA6061 alloy composite containing (a) 15 vol% of SiC particles an d (b) 20 vol% of Saffil short fibres were studied. For both of the com posites, cracks are initiated very early in the fatigue life irrespect ive of the cyclic strain amplitude. A full three-dimensional FEM simul ation demonstrated that the inhomogeneous distributions of stresses an d strains induced by the particles and fibres are enhanced at a free s urface relative to the interior of the material. This promotes surface crack initiation either by cracking of the reinforcement or near inte rfacial cracking in the matrix. The growth rates of the surface microc racks are an order of magnitude faster than the growth rates of long t hrough-cracks at corresponding values of the crack growth parameter DE LTAJ. It was further observed that surface cracks shorter than 100 mum grow at a decelerating rate, yet did not show any threshold behaviour . Despite this early initiation of microcracks in the composites, the particulate composite notably showed a larger tolerance against plasti c strain cycling than the unreinforced matrix in the lower range of cy clic plastic strain amplitudes.