Micromechanical analysis of yielding of metal matrix composites

A recently proposed micromechanics model is employed to generate initial yield surfaces of unidirectional and cross-ply metal matrix composites under a variety of loading conditions. The yield surfaces are calculated using two different methodologies: on the basis of local matrix stresses and average stresses in the entire matrix phase. It is shown that the results obtained on the basis of local matrix stresses correlate very well with finite-element predictions for most loading directions considered in the present investigation. A critical direction or cone of directions is found that should be avoided with the outlined micromechanics model. This direction corresponds to a particular combination of longitudinal tension (compression) and equal biaxial transverse tension (compression) whose ratio is a function of the constituent properties. It is also illustrated that the results generated on the basis of average matrix stresses generally underestimate initial yielding (i.e., predict higher yield stresses), the extent of which may be significant depending on the direction of loading. Thus, the use of average matrix stresses in analyzing elastoplastic response of composites should be approached with caution.