A FINITE-ELEMENT ANALYSIS OF CRACK BRIDGING IN METAL-MATRIX COMPOSITES

A finite element method is developed to model crack bridging in unidirectional metal matrix composites. The Marshall-Cox-Evans (MCE) model is used for the relationship between the stress of a bridging fiber and the crack opening displacement. Loading, unloading and reloading are modeled in detail for cyclic loads. Fiber failure is assumed to occur when the stress exceeds the tensile strength. The method is applied to Ti-matrix/SCS-6 composites. The critical interface shear stress (tau) predicted from the experimental crack opening displacements is found to be 10 MPa or less. The maximum and minimum stress intensity factors (K-max, K-min) in a fatigue cycle are obtained as functions of the crack length. K-max decreases drastically due to bridging fibers as the crack propagates. K-min at the unloaded state is not negligible and must be accounted for in computing Delta K. The da/dN-Delta K plots show that tau's significantly larger than 10 MPa yield better correlations. However, the quality of correlation is poor even at tau as large as 40 MPa.