Tension-tension fatigue damage behavior of an unnotched SiC (SCS-6) fiber-reinforced Ti-15-3 alloy matrix composite at room temperature was examined, applying maximum stresses of 450, 670 and 880 MPa with R = 0.1. The change in stress-strain hysteresis curves was measured. Fiber fracture behavior and matrix cracking behavior were observed in situ and the results were compared with the change of unloading modulus obtained from the hysteresis curves. The fiber fracture behavior inside the specimen was also determined by dissolving the Ti alloy matrix. The results showed abrupt reductions in the unloading modulus of the composite at stresses of 450, 670 and 880 MPa; the normalized unloading modulus decreased by 8%, 12% and 17%, respectively, in the initial stage (N ≤ 10 cycles). This reduction was caused by the multiple fiber fragmentation. Thereafter, the unloading modulus maintained a nearly constant value and non-propagating matrix cracks were initiated adjacent to the end of fractured fiber. The propagation of the matrix crack again led to a rapid reduction of the unloading modulus, and the composite then failed. With higher applied stress, the fatigue life was reduced. The fracture behavior of the composite was discussed with special attention to the fiber fracture behavior and its effect on the modulus of the composite.
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