The plastic behavior of particle-reinforced composites is studied by the continuum theory of stress gradient plasticity. This theory has been quite successful in predicting the size-dependent behavior in micro-torsion and -bending. Analyses are carried out on the axisymmetric unit cell finite element models for two sets of SiC/Al composites. Subsequent stress gradient plasticity predictions of the stress-strain curves in comparison with the corresponding experimental data show excellent agreement of flow strength for composites of varying particle size. Our results reveal that the experimental observations of particle size effect can be explained in terms of a stress gradient plasticity theory with one material length variable, which depends on the matrix microstructure. Therefore, stress gradients in the matrix-originated from the mismatch of elastic moduli between the particles and matrix-together with the matrix microstructure can control the size-dependent behavior in composites. However, the impact of this novel mechanism in competition with other well-known mechanisms can only be revealed by experimental investigations with careful attention on the matrix microstructure of composites.

Abstract. E ects of particle size on metal matrix composites are studied within the Continuum theory of Mechanism-based Strain Gradient (CMSG) plasticity. This theory has been quite successful in predicting the size-dependent plastic behavior in a wide variety of problems. Two-dimensional (plane-strain) analyses carried out on the composite unit cell models with multi-particles of circular shape show that the ow stress of the composites increases by decreasing particle size with high sensitivity to small particle size. The numerical results are in good agreement with experimental data. Subsequently, the e ects of particle shape, orientation, and size distributions on the behavior of composites are investigated. Analyses are carried out on the composites containing squared, rectangular, and elliptical (with aspect ratio of four) particles of various orientations with respect to the loading direction (i.e., vertical, horizontal, and 45 degree inclined directions). The stress inhomogeneity in the matrix, the overall stress-strain curve, and the maximum principle stress in the particles of composites with non-circular particles are investigated and compared with those obtained for the composites containing circular particles. The e ects of particle size distribution on the behavior of composites are also addressed.