METHODOLOGY FOR PREDICTION OF FIBER BRIDGING EFFECTS IN COMPOSITES

A review of three different methods used at NASA Lewis to account for the effect of fiber bridging on fatigue crack growth (FCG) in metallic and intermetallic titanium-based composites is presented. Of the thre e methods, the fiber pressure model (FPM) was the only one able to acc urately predict both crack opening displacements (CODs) and the FCG ra tes for both composites under the various applied loading conditions. Once the proper interfacial shear strength coefficient, tau, was selec ted, the shear lag model (SLM) resulted in accurate predictions of the crack opening profiles. While the SLM did account for a substantial d ecrease in the fatigue crack driving force due to the fiber bridging, it still overestimated the fatigue crack driving force, especially wit h an increase in the applied stress. It was hypothesized that the over estimation of the crack driving force by the SLM may be due to a highe r tau near the crack tip than the rest of the crack wake. The higher t au in the crack tip region is thought to result from a limited amount of interfacial wear that the crack tip region experiences. The experim entally based approach for predicting FCG behavior accounted for a sub stantial portion of the fiber bridging effect; however, it also overes timated the resulting fatigue crack driving force.