Fibre bridging during high temperature fatigue crack growth in Ti/SiC composites

Synchrotron X-ray diffraction has been used to map the crack tip stress field, load redistribution and the variation in interfacial shear stress along bridging fibres local to a matrix crack during fatigue crack growth in Ti-6Al-4V/unidirectional SCS-6 SiC monofilament composite at elevated temperature. Quasi-static fatigue cycles were applied in a diffractometer at the same stress amplitudes and temperatures (120 and 300 °C) as those used in prior off-line fatigue testing. The elastic fibre strains were measured ply by ply along the fibres and in the matrix in the region of the crack. In this manner the crack affected zone was mapped, and subsequently the interfacial shear stress levels deduced as a function of distance from the crack at these temperatures. The results are compared with previous studies of load redistribution at room temperature and the fibre sliding stresses compared with those needed to slide pristine fibres to evaluate degradation of the interfacial shear strength caused by interfacial wear during fatigue. The implications for the use of such composites at elevated temperatures, for example in aero engines, are discussed.