Micromechanical investigation of the convexity of yield surfaces of metal matrix composites

This paper examines the convexity of initial yield surfaces of metal matrix composites in the presence of stiffening response of the fiber phase using a recently proposed micromechanics model. The loss of convexity of yield surfaces of unidirectional and multidirectional graphite/aluminum laminates is investigated under selected loading conditions for different fiber stiffening rates and temperature changes. In the case of unidirectional laminae, loss of convexity of initial yield surfaces for the chosen set of material parameters and loading conditions occurs for fibers whose limiting longitudinal response is four times stiffer than the initial response. Application of a negative temperature change of a sufficient magnitude eliminates the concavity. In the case of multidirectional laminates, loss of convexity of initial yield surfaces may occur for fibers which exhibits substantially less stiffering. In particular, initial yield surfaces of angle-ply configurations in the low off-axis range become concave under loading by inplane normal and shear stresses for fibers whose limiting longitudinal response is two times stiffer than the initial response.