| Experiments have repeatedly shown that metallic materials display strong size effect at the micron and submicron scales when non-uniform plastic deformation exists. These experiments, as well as direct dislocation simulations, therefore, confirmed the existence of a material length scale for plasticity. However, the conventional plasticity theories possess no intrinsic material length and can not explain the above observations, which leading to the considerable recent interest in developing size dependent plasticity theories governing the plastic deformation in small volumes.; It is strongly believed that the size effect is attributed to geometrically necessary dislocations associated with non-uniform plastic deformation. There are, however, many dislocations at the micron scale such that their collective behavior on plastic work hardening of materials should be characterized by a continuum plasticity theory.; We developed a conventional theory of mechanism-based strain gradient plasticity (CMSG) established from Taylor dislocation model but does not involve the higher-order stress. Through several examples of micro-bend, micro-torsion, and void growth, we have shown that CMSG can capture the size effect observed in experiments.; CMSG is used to investigate the stress field around the tip of an interface crack between Nb and sapphire and has shown that the stress at the crack tip is high enough to trigger cleavage cracking in presence of plastic flow in ductile material.; CMSG is also used to study the indentation size effect (ISE) observed in micro and nanoindentation experiments. Different hardness trends with different indenter shapes obtained from CMSG agree very well with the experimental measurement. |
