Capturing the influence of surface constraints in small and thin samples using polycrystalline plasticity theory

A rate-dependent, single-crystal plasticity model for face-centred cubic crystal structures has been implemented into a large strain elastic-plastic, finite-element code to examine the mechanical influence of the reduced surface constraints of relatively small polycrystalline aggregates. The implemented model simulates deformation of a polycrystal composed of cubic grains where each grain is a single finite element. Mechanical constraint is varied by changing (a) specimen thickness and (b) specimen volume, relative to grain size. Numerical uniaxial tensile tests have been performed to a strain level of 0.01. Direct and statistical examination of the model results revealed the reduced flow stress of grains at specimen surfaces, edges and corners. The results of these simulations are in good agreement with previous experimental studies which suggest that 5-10 grains across the minimum dimension of a structure are necessary to approximate true continuum polycrystalline response.