Damage accumulation during high temperature fatigue of Ti/SiCf metal matrix composites under different stress amplitudes

The damage mechanisms and load redistribution taking place under high temperature (350 degrees C), high cycle fatigue (HCF) of TC17 titanium alloy/unidirectional SiC fibre composites have been investigated in situ using synchrotron X-ray computed tomography (CT) and X-ray diffraction (XRD) under two stress amplitudes. The three-dimensional morphology of the fatigue crack and fibre fractures has been mapped by CT. At low stress amplitude, stable growth occurs with the matrix crack deflecting by 50-100 mu m in height as it bypasses the bridging fibres. At higher stress amplitude, loading to the peak stress led to a burst of fibre fractures giving rise to rapid crack growth. Many of the fibre fractures occurred 50-300 mu m above/below the matrix crack plane during rapid growth, contrary 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 over the fatigue cycle and after the rapid crack growth by 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 over the loading cycle, with the maximum frictional sliding stress reaching -55 MPa in both regimes. In accordance with previous studies, it is possible that a degradation in fibre strength at elevated temperature is responsible for bursts of fibre fracture and rapid crack growth under higher stress amplitude. (c) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

The study investigated the damage mechanisms and load redistribution in TC17 titanium alloy/unidirectional SiC fibre composites under high temperature and high cycle fatigue conditions using synchrotron X-ray computed tomography and X-ray diffraction. The results showed that the behavior of fatigue crack and fibre fractures varied at different stress amplitudes. The changes in fiber loading, interfacial stress, and fiber-matrix debonding in the vicinity of the crack were mapped over the fatigue cycle and after rapid crack growth.