  The damage mechanisms and load redistribution of high strength TC17 titanium
alloy/unidirectional SiC fibre composite (fibre diameter = 100 $\mu$m) under
high temperature (350 {\deg}C) fatigue cycling have been investigated in situ
using synchrotron X-ray computed tomography (CT) and X-ray diffraction (XRD)
for high cycle fatigue (HCF) under different stress amplitudes. The
three-dimensional morphology of the crack and fibre fractures has been mapped
by CT. During stable growth, matrix cracking dominates with the crack
deflecting (by 50-100 $\mu$m in height) when bypassing bridging fibres. A small
number of bridging fibres have fractured close to the matrix crack plane
especially under relatively high stress amplitude cycling. Loading to the peak
stress led to rapid crack growth accompanied by a burst of fibre fractures.
Many of the fibre fractures occurred 50-300 $\mu$m from the matrix crack plane
during rapid growth, in contrast to that in the stable growth stage, leading to
extensive fibre pull-out on the fracture surface. The changes in fibre loading,
interfacial stress, and the extent of fibre-matrix debonding in the vicinity of
the crack have been mapped for the fatigue cycle and after the rapid growth by
high spatial resolution XRD. The fibre/matrix interfacial sliding extends up to
600 $\mu$m (in the stable growth zone) or 700 $\mu$m (in the rapid growth zone)
either side of the crack plane. The direction of interfacial shear stress
reverses with the loading cycle, with the maximum frictional sliding stress
reaching ~55 MPa in both the stable growth and rapid growth regimes.
