A Micromechanical Prediction of Initial Yield Surfaces of Unidirectional Composites

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Abstract
For two-phase materials composed of ductile matrices reinforced with plastically nondeformable inclusions, the yield condition that has the same form as Tsai and Wu's phenomenological criterion has been deduced through a micromechanics analysis. Basic assumptions made are that the matrix material obeys the Levy-von Mises flow rule, and that matrix plastic strains are uniform. In general, the shape of the yield surface can be proved to be hyperellipsoidal. A further analysis is devoted to the case where the inclusions can be treated as identical ellipsoids with corresponding axes aligned. Based on the results, numerical computation of yield strength parameters is carried out for unidirectional boron fiber-aluminum composites subjected to uniform temperature changes prior to external loading. It is shown that yielding cannot take place under certain combined triaxial (not hydrostatic) stresses. Some discussion related to the effect of prior temperature changes is also given.

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