Modelling size effects in micro/nano-systems by including interfacial effects in a gradient plasticity framework

The effect of the material microstructural interfaces increases as the surface-to-volume ratio increases. It is shown in this work that interfacial effects have a profound impact on the scale-dependent plasticity encountered in micro/nano-systems. This is achieved by developing a physically-based higher-order gradient-dependent plasticity theory that enforces microscopic boundary conditions at interfaces and free surfaces. These non-standard boundary conditions relate the micro traction stress at the interface to the interfacial energy. Application of the proposed framework to size effects in biaxial tension of a thin-film on an elastic substrate is presented. Three film-interface conditions are modelled: soft, intermediate, and hard interfaces.