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 of an AA6061 alloy composite containing (a) 15 vol% of SiC particles and (b) 20 vol% of Saffil short fibres were studied. For both of the composites, cracks are initiated very early in the fatigue life irrespective of the cyclic strain amplitude. A full three-dimensional FEM simulation demonstrated that the inhomogeneous distributions of stresses and strains induced by the particles and fibres are enhanced at a free surface relative to the interior of the material. This promotes surface crack initiation either by cracking of the reinforcement or near interfacial cracking in the matrix. The growth rates of the surface microcracks are an order of magnitude faster than the growth rates of long through-cracks at corresponding values of the crack growth parameter DELTAJ. 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 plastic strain cycling than the unreinforced matrix in the lower range of cyclic plastic strain amplitudes.