Rate-dependent size effects and material length scales in nanoindentation near the grain boundary for a bicrystal FCC metal

This paper addresses the size effects encountered in nanoindentation experiments of a copper bicrystal specimen in close proximity to the grain boundary. The experimental results show that as the distance r between the indentations and the grain boundary decreases, the hardness of the tested specimen increases. The single grain boundary in the bicrystal specimen is believed to play an important role to the increase in the hardness. The impact of the distance r on the hardness can be considered as a new type of size effect. The strain rate dependency of the length scale is investigated by incorporating a strain rate variable in the expression of the length scale. A temperature and rate-dependent indentation size effect model is used in order to determine the material intrinsic length scale using the corresponding data from the experiments. Nanoindentation experiments with different strain rates are performed in order to investigate the effect of the strain rate. In order to obtain more robust simulation results, the required numerical analysis is performed using ABAQUS Explicit software with a VUMAT user-subroutine which gives the physically based viscoplastic constitutive relations for the tested material.