Size effects at initial yield (prior to Stage II) of micron sized specimens are modeled within a continuum model of plasticity. Two different aspects are considered here: a) specification of a density of dislocation sources that represent the emission of dislocation dipoles, and b) the presence of an initial, spatially inhomogeneous excess dislocation (ED) content. Discreteness of the source distribution appears to lead to a stochastic response in stress-strain curves, with the stochasticity diminishing as the number of sources increases. Variability in stress-strain response due to variations of source distribution is also shown. These size effects at initial yield are inferred to be due to physical length scales in dislocation mobility and the discrete description of sources that induce internal-stress-related effects, and not due to length-scale effects in the mean-field strain-hardening response (as represented through a constitutive equation).

Authors:  Saurabh Puri, Anish Roy, Amit Acharya, Dennis Dimiduk.