Initial yield surfaces were constructed for the boron-aluminum composite under arbitrary combinations of applied composite stresses and temperature changes, by means of a finite element analysis of a regular hexagonal array model.The results suggest that with regard to initial yielding, the macroscopic transverse isotropy of the composite is approximately retained on the microscale. Therefore, the general isothermal yield surface can be constructed as a function of four composite stresses. Yielding in the transverse plane is controlled mainly by matrix properties, whereas yielding under normal composite stresses applied in the fiber direction is controlled by the ratio E f /E m of the Young's moduli of the constituents. In general, high magnitudes of this ratio, as well as higher fiber volume fractions make yielding more difficult.It was found that yielding takes place both under hydrostatic composite stresses, and as a consequence of uniform temperature changes. Therefore, these effects must be accounted for in macroscopic yield criteria and flow rules for composites. It is shown that a uniform temperature increase causes an approximate translation of the yield surfaces in the negative hydrostatic stress direction. Relatively small temperature changes can cause yielding in most composite systems. The typical values were found in the interval of 50° to 100°F, when the tensile yield stress of the matrix was taken as Y = 10,000 psi, and were proportional to Y. *Sponsored by the U.S. Army Research Office-Durham.

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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.

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