Abstract
Abstract An important damage mechanism during fatigue of unidirectional SiC-fibre reinforced titanium alloys is the formation of matrix cracks transverse to the fibre direction. Due to the relatively low fibre/matrix bond strength these matrix cracks initially do not break the fibres, so that matrix cracks bridged 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 critical size and the crack propagation rate (d a /d N ) for a single load level has been determined. A prediction of d a /d N on the basis of finite element calculation of the stress intensity factor range of the bridged matrix crack Δ K m and the Δ K m –d a /d N relationship of the used titanium alloy (Timetal 834) has been performed. Calculation of Δ K m assuming a negligible fibre/matrix bond strength and considering shear load transfer at the fibre/matrix interface due to Coulomb friction (coefficient of friction μ =0.5 and μ =0.9) led to a large discrepancy between the measured and predicted crack growth rate. It can be concluded, that the assumed conditions of stress transfer at the fibre/matrix interface neglecting bonding is the reason for this discrepancy.
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