Jörg Hemptenmacher

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

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.

Optimisation of the Fatigue Resistance of Metal Matrix Composites

Fiber reinforced titanium matrix composites (TMCs) are attractive materials for future aerospace applications. Reinforcement of Ti alloys with SiC fibers leads to a significant increase on strength and stiffness, especially under high stress loading or at elevated temperatures. Mechanical properties of TMCs are strongly influenced by thermal residual stresses (TRS). A reduction of TRS leads to an increase in fully reversed high cycle fatigue resistance of TMCs at room temperature. Reduction of TRS can be easily obtained by prestraining during single tension loading of TMCs in the as processed condition.