Fatigue crack growth and load redistribution in Ti/SiC composites observed in situ

Sequential synchrotron X-ray microtomography and diffraction have been applied to follow the growth of fatigue cracks and the associated load redistribution in a Ti/SiC fibre composite. A sequence of micron resolution tomographs reveal for the first time how the cracks progress from ply to ply around the fibres. Complementary high spatial resolution (40 μm) diffraction scans interleaved between the tomographic image acquisitions during the fatigue experiment have enabled the fibre strains and thereby the interfacial shear stress to be mapped as a function of crack growth. The matrix crack front was found to bow out between fibres, eventually reconnecting further downstream. This leads to the prolonged retention of bridging matrix ligaments and increased crack path tortuosity. The rate of crack growth was found to slow somewhat as a fibre is approached. As the crack grew past the fibres under observation the extent of the sliding region and the level of the fibre bridging stress increased. The interfacial shear strength after fatigue was around 60 MPa in the crack-tip region, in common with previous experiments.