Influence of interfacial stress transfer on fatigue crack growth in SiC-fibre reinforced titanium alloys

An important damage mechanism during fatigue of unidirectional SiC-fibre re inforced titanium alloys is the formation of matrix cracks transverse to th e fibre direction. Due to the relatively low fibre/matrix bond strength the se matrix cracks initially do not break the fibres, so that matrix cracks b ridged by fibres develop. It is shown experimentally, that the strong drop in fatigue strength is caused by the formation of a bridged crack of a crit ical size acid the crack propagation rate (da/dN) for a single load level h as been determined. A prediction of da/dN on the basis of finite element ca lculation of the stress intensity factor range of the bridged matrix crack DeltaK(m) and the DeltaK(m)-da/dN relationship of the used titanium alloy ( Timetal 834) has been performed. Calculation of DeltaK, assuming a negligib le fibre/matrix bond strength and considering shear load transfer at the fi bre/matrix interface due to Coulomb friction (coefficient of friction mu = 0.5 and mu = 0.9) led to a large discrepancy between the measured and predi cted crack growth rate. It can be concluded, that the assumed conditions of stress transfer at the fibre/matrix interface neglecting bonding is the re ason for this discrepancy. (C) 2001 Elsevier Science Ltd. All rights reserv ed.