Observation of short fatigue crack-growth process in SiC-fiber-reinforced Ti-15-3 alloy composite

Abstract

The microscopic fatigue damage characteristics and short fatigue crack growth of an unnotched SiC(SCS-6) fiber-reinforced Ti-15-3 alloy composite were investigated in tension-tension fatigue tests (R = 0.1) carried out at room temperature for applied maximum stress of 450, 670, and 880 MPa.In situ observation of the damage-evolution process was done using optical and scanning laser microscopies, which were attached in the fatigue machine. The first damage for the composite started from a cracking of the reaction layer followed by fiber fracture. The matrix cracking initiated near the broken fiber when the microhardness of the matrix just to the side of the fracture fiber reached ≈6 GPa, and the number of cycles for the initiation of this cracking decreased with the increase of applied stress. The slope of the relation of surface crack growth lengthvs number of cycles fell into two characteristic stages; in the first stage, the rate was lower than the second stage and accelerated. The surface crack growth rate,d(2c)/dN,vs surface crack length relation also fell into two stages (stages I and II). With the increase in surface crack length, the crack-growth rate,d(2c)/dN, decreased in stage I and increased in stage II. The transition from stage I to stage II occurred due to the fracture of fibers located around the first fractured fiber. It was concluded that the fatigue crack growth resistance of the composite in the short-crack region was controlled by the fiber fracture and matrix work hardening near the fractured fiber. When the fiber fracture occurred, the surface crack growth rate was accelerated and became faster than that of the monolithic matrix.