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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 three methods, the fiber pressure model (FPM) was the only one able to accurately predict both crack opening displacements (CODs) and the FCG rates for both composites under the various applied loading conditions.
Once the proper interfacial shear strength coefficient, τ, was selected, the shear lag model (SLM) resulted in accurate predictions of the crack opening profiles. While the SLM did account for a substantial decrease in the fatigue crack driving force due to the fiber bridging, it still overestimated the fatigue crack driving force, especially with an increase in the applied stress. It was hypothesized that the overestimation of the crack driving force by the SLM may be due to a higher τ near the crack tip than the rest of the crack wake. The higher τ in the crack tip region is thought to result from a limited amount of interfacial wear that the crack tip region experiences.
The experimentally based approach for predicting FCG behavior accounted for a substantial portion of the fiber bridging effect; however, it also overestimated the resulting fatigue crack driving force.
NASA Lewis Research Center, Cleveland, OH
Sverdrup Technology, NASA Lewis Research Center, Cleveland, OH
Ohio Aerospace Institute, Cleveland, OH
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