Yielding in Unidirectional Composites Under External Loads and Temperature Changes*

Initial yield surfaces were constructed for the boron-aluminum com posite under arbitrary combinations of applied composite stresses and tem perature changes, by means of a finite element analysis of a regular hexa gonal 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 con structed 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 Ef/Em 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.