The determination of interfacial strength in Al/SiC long fibre composites

Abstract

The interfacial properties have been measured for a novel AA6061/SiC-C coated silicon carbide SM3256 monofilamentary fibre (Al/SiC${}_{\text{f}}$) MMC using a variant of the single fibre fragmentation test. In this test, fibre fragmentation is followed in situ using synchrotron X-ray diffraction to probe the axial fibre elastic strains as a function of applied loading, while X-ray radiography is used to follow the fracture sequence. In this way, the variation in axial fibre stress along the fibre is tracked and hence the variation in interfacial shear stress along the fibre inferred at various stages of fibre fragmentation. Prior to loading, the fibre was in a state of axial compression ($\approx$280 MPa) due to thermal residual stresses representative of cooling from 200 °C. During the fragmentation process, the variations in axial strain and interfacial stress show characteristic “stick–slip” behaviour, where the fibre interface must exceed a threshold stress (${\tau }_{\text{deb}}$ = 94 ± 10 MPa), close to the shear strength of the matrix before debonding. Once debonded, the fibre slides at a frictional shear stress, ${\tau }_{\text{fr}}$, initially of around 40 MPa, but falling with increased sliding distance to around ${\tau }_{\text{fr}}$ = 15 ± 5 MPa. Radiography taken during loading, and post-mortem, indicates that interfacial failure occurs at the fibre-coating interface, leaving coating material lining the pull-out within the matrix. The accumulation of coating damage may be responsible for the progressive decrease in sliding stress with increased sliding. These sliding stresses, are much lower than observed for comparable Ti/SiC composites, and would facilitate significant fibre-pull-out and fibre bridging under fatigue conditions.