Durability and Damage Tolerance of High Temperature Advanced Titanium Metal Matrix Composites

摘要

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 features of residual stresses indicated that stress relaxation occurred in the Ti matrix phase of the composite following post-fabrication cool down to 600 00. Parametric study of the SiC fiber coating materials showed that the effective residual stress component has an inverse relationship with the thickness of the fiber reaction zone. The debonding shear strength of the composite is determined by the localized shear stress distribution along the fiber/matrix interface at the onset of debonding. An interphase debonding model, which combine fracture mechanics with FE 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 as a function of 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. Furthermore, the 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/ Ti-21S composite.