Fatigue crack growth behavior of off-axis Ti–6Al–4V/SCS-6 metal matrix composites

The fatigue crack growth (FCG) behavior of five different laminate orientations ([0/90]2s, [90/0]2s, [±45]2s, [150/60]2s and [60/150]2s) of cross-ply Ti–6Al–4V/SCS-6 has been investigated at room temperature in air. All these tests were performed on testpieces with a similar notch depth of 0.5 mm under three-point bending (3PD). The FCG behavior was investigated as a function of initial stress intensity factor range (ΔKini), fiber orientation, surface condition and composite architecture to determine the crack arrest/catastrophic failure (CA/CF) transition of these laminates. For a given ΔKini, the laminates containing 0° (or on-axis) plies to the loading direction (e.g. [0/90]2s and [90/0]2s) show higher FCG resistance than the laminates containing off-axis plies (e.g. [±45]2s, [150/60]2s and [60/150]2s). The damaging of fibers on the surface ply due to polishing also reduces the CA/CF transition. Regarding crack propagation behavior, laminates with 0° fibers show mode-I cracking, whereas, the laminates with off-axis fibers show fatigue crack propagation at an angle to the mode-I direction. It is also found that the dominant direction of crack propagation in these laminates is controlled by the angle of the central double plies with respect to the mode-I direction.