A MODEL TO PREDICT THE FATIGUE LIFE OF FIBER-REINFORCED TITANIUM MATRIX COMPOSITES UNDER CONSTANT AMPLITUDE LOADING

A model based on micro-mechanical concepts has been developed for pred icting fatigue crack growth in titanium alloy matrix composites. In te rms of the model, the crack system is composed of three zones: the cra ck, the plastic zone and the fibre. Crack tip plasticity is constraine d by the fibres and remains so until certain conditions are met. The c ondition for crack propagation is that fibre constraint is overcome wh en the stress at the location of the fibre ahead of the crack tip atta ins a critical level required for debonding. Crack tip plasticity then increases and the crack is able to propagate round the fibre. The deb onding stress is calculated using the shear lag model from values of i nterfacial shear strength and embedded fibre length published in the l iterature.If the fibres in the crack wake remain unbroken, friction st resses on the crack Banks are generated, as a result of the matrix sli ding along the fibres. The friction stresses (known as the bridging ef fect) shield the crack tip from the remote stress, reducing the crack growth relative to that of the matrix alone. The bridging stress is ca lculated by adding together the friction stresses, at each fibre row b ridging the crack, which are assumed to be a function of crack opening displacement and sliding distance at each row. The friction stresses at each fibre row will increase as the crack propagates further until a critical level for fibre failure is reached. Fibre failure is modell ed through Weibull statistics and published experimental results. Fibr e failure will reduce the bridging effect and increase the crack propa gation rate. Calculated fatigue lives and crack propagation rates are compared with experimental results for three different materials (32% SCS6/Ti-15-3, 32% and 38% SCS6/Ti-6-4) subjected to mode I fatigue loa ding. The good agreement shown by these comparisons demonstrates the a pplicability of the model to predict the fatigue damage in Ti-based MM Cs.