Effects of temperature, cyclic frequency and environment on fibre degradation and fatigue crack growth resistance in a fibre-reinforced titanium metal-matrix composite under displacement-range loading

The fatigue crack growth resistance of a [0/90°]_2s cross-ply SCS6 fibre-reinforced Ti-6Al-4V alloy metal-matrix composite has been assessed under displacement range control (i.e. under load shedding conditions with crack extension) to investigate potential fibre degradation and the process of crack extension at room temperature, and at 450 °C, in air and in vacuum. Attention is focused on initial conditions that will promote crack arrest at room temperature. Under the test conditions employed here, regions of crack growth can occur where the applied nominal stress intensity factor range (ΔK) is relatively constant. This `constant' ΔK range is the result of a fortuitous balance between the particular test-piece geometry, loading conditions utilized, matrix crack growth and the rate of fibre fracture. It allows the influence of environment, cyclic frequency and temperature on fatigue crack growth resistance to be analyzed more easily than for tests carried out under load control. The crack growth rate remained almost constant but with some steep local retardations in growth rate in the constant ΔK region at a temperature of 450 °C, while crack arrest occurred at room temperature for the same initial ΔK. The average crack propagation rate in this `constant ΔK region' at a temperature of 450 °C in air was much greater than that at a temperature of 450 °C in vacuum. This indicates that environment plays an important role in the process of fibre degradation. The effect of cyclic frequency is saturated at a frequency of less than 1 Hz. The process of crack growth at various frequencies is also discussed.