Fatigue crack growth bridging mechanisms in titanium metal-matrix composites

The bridging fatigue crack growth damage mechanisms in a unidirectional SiC/Ti MMC include matrix cracking, fiber/matrix interface debonding and sliding along bridging fibers and fracture of these fibers. The basic components of these mechanisms are examined in this program. The evolution characteristics of residual stresses indicated that extensive stress relaxation occurred in the Ti-alloy matrix phase of the composite following post-fabrication cool down to {dollar}rm 600spcirc C.{dollar} Parametric study on the SiC fiber coating materials showed that the effective residual stress component has an inverse relationship with the thickness of the composite reaction zone. The debonding shear strength of the composite is determined based on localized shear stress distribution along the fiber/matrix interface at the onset of debonding. The resulting shear strength is found to decrease from 221.2 MPa at ambient temperature to 138.6 MPa at {dollar}rm 650spcirc C.{dollar} An interphase debonding model, which combines fracture mechanics equations with finite element results on interphase shear stress and bridging fiber traction range, is proposed to establish a distribution of debonding lengths along a fiber-bridged matrix crack length. The longest debonding lengths in a SiC/Ti MMC was predicted along the first intact fiber at the crack mouth and the lengths decrease for fibers located closer to the crack tip. In addition, the debonding crack length increases with increasing temperature. The driving force for the interface debond crack, however, has an inverse relationship with the test temperature. The concurrent damage events of fiber stress evolution and continuous fiber strength degradation were postulated into a fiber fracture criterion to describe the fracture process of a bridging fiber. Although the strength properties of SiC SCS-6 fibers are found to be unaffected by test temperature of {dollar}rm 650spcirc C{dollar} and below, temperature influenced the fracture process of these fibers through the density of cracks in the outermost carbon-rich fiber coating. This fiber crack density has been correlated with the density of crack initiation sites observed in the interphase region along the reinforcing fibers in a SCS-6/Timetal-21S composite.