Imaging and strain mapping fibre by fibre in the vicinity of a fatigue crack in a Ti/SiC fibre composite

High resolution X-ray micro-tomography has been combined with high spatial resolution diffraction measurements to map the relationship between damage and elastic strain in individual fibres in the bulk of a metal matrix composite containing a matrix fatigue crack in 3D for the first time. Approximately three plies bridge the transverse matrix fatigue crack. The interfacial shear stress distributions in individual bridging fibres have been derived from the respective axial fibre strain profiles. For each fibre these profiles indicate a step jump in interfacial shear stress at the boundary between sliding and perfect bonding. This sudden decrease is taken to be the result of a sharp drop in the sliding stress due to fretting fatigue. Furthermore, the sliding stress appears to decrease towards the matrix crack. The extent of fibre sliding makes a ‘V’ shape in the crack wake, extending around 1·5 mm either side of the crack for ply 1. There is some evidence supporting stick–slip and a lower reverse sliding stress (about 30–50 MPa) than the forward one (about 50–60 MPa).