Crack opening displacements during fatigue crack growth in Ti-SiC fibre metal matrix composites by X-ray tomography

High spatial resolution X-ray microtomography has been used to map the variation of crack opening displacement ( COD) across matrix cracks in unidirectional Ti - 6Al - 4V/ SCS- 6 SiC fibre composites. These show the crack front to bow out between fibres as the crack emerges from a ply and to advance preferentially towards the fibres when located between plies. Quantitative image analysis of 3D volume reconstructions has enabled the COD to be measured at K-min, K-mid and K-max. Hysteresis in crack opening was observed during the fatigue cycle ( loads applied statically). Very little variation in COD has been observed parallel to the crack front irrespective of proximity to the bridging fibres. The changes in COD relative to the initial residual opening at Kmin are not inconsistent with being proportional to the square root of the distance from the crack tip. Considerable crack blunting was observed during an overload leading to a significant residual crack opening upon unloading.