Effects of the interface on the fatigue crack growth response of titanium matrix composites: modeling and impact on interface design

The influence of the interface on the fatigue crack growth behavior of longitudinally loaded continuous SiC fiber reinforced T1-6A1-4V composites was systematically studied using composites with three different interfaces. The shear stress parameter (f, defined in the text) for the three interfaces, SCS-6/Ti-6Al-4V, SCS-0/T1-6A1-4V and Trimarc 1/TI-6A1-4V, were evaluated to be 3, 10 and 1.7 MPa, respectively. These values showed a similar ranking as the bond strengths measured from transverse tension tests and frictional stresses measured from fiber pushout tests. Fiber strengths that were predicted from the crack bridging model were consistently lower than experimental measurements of extracted fibers, with SCS-6 fibers being most significantly affected. Issues of interface design have been addressed in the perspective of optimizing transverse strength and longitudinal fatigue crack growth properties. Also, the bridging analysis has been presented in a form that can be used by interface designers to predict the effect of interfacial properties and fiber strength on the life of a composite. These results suggest that for a given design stress, increasing the interfacial bond strength and shear sliding resistance will enhance the fatigue life of a com posite provided the fiber strength can be maintained above the maximum fiber stress. Experimental validation of the advantages of using interfaces with a higher value of f is also provided.