Generalized Initial Yield Surfaces for Unidirectional Composites

Abstract

A numerical method is described for determination of generalized initial yield surfaces of unidirectional metal matrix composites under arbitrary external loads and uniform temperature changes. The method leads to the representation of the surface in a three-dimensional system of generalized stress coordinates which, respectively, coincide with the direction of the normal composite stress in the fiber direction, and with the two principal directions of the composite stresses acting in the transverse plane. The initial yield surface of the composite is an irregular ellipsoid with its longest axis inclined toward the hydrostatic stress axis. A thermomechanical analogy is used to show that as a result of a uniform temperature change, the yield surface experiences a rigid-body translation in the direction of the hydrostatic axis in the stress space. The initial yield behavior of a B-Al composite is described in detail. It is shown that microplastic yielding can take place in the composite under relatively small magnitudes of external loads, and hydrostatic stress, or as a result of moderate temperature changes.