Correlation between residual stresses and the strength differential effect in PM 6061Al-15 vol% SiCw composites:: experiments, models and predictions

An experimental and theoretical study of the strength differential effect (SDE) observed in uniaxial tensile/compressive testing in discontinuously reinforced metal matrix composites has been carried out. The predictions have been compared with experimental data obtained in three 6061Al-15 vol% SiCn, composites. The study has been carried out using two subsequent steps. In the first one, the analysis of the residual stress, RS, state (assumed to be the ultimate responsible of the SDE) has been conducted using an extended Eshelby equivalent inclusion methodology. A good agreement between the experimental values as obtained by neutron diffraction and the values predicted by the Eshelby model is obtained when the real composites microstructure (orientation and distribution of the reinforcement) is taken into consideration. The effective temperature drop responsible of the RS state in each composite has been calculated from this model. In the second step, the SDE values obtained in each of the composites is predicted in two ways. The Eshelby-Withers (EW) model, based on Eshelby's theory, has been used. In parallel, a new method, which combines the mechanisms of load transfer, as described by a Shear-Lag type model, with the RS has been developed. The characteristics of these two models are discussed. It is shown that the results derived from them fit very well with the experimental data presented. All these findings together allow showing that the load transfer mechanism introduces a correction factor, which compares the RS directly with the SDE. It is concluded that RS and the mechanism of load transfer are eventually the origin of the SDE in discontinuously reinforced metal matrix composites. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.