Size Effects at Dwell Stage of Micro-Indentation for Pure Aluminum

Pure aluminum L2 was used to study the size effect during the dwell stage of micro-indentation. The experiments reveal that an indentation depth increases further and then a load-displacement platform appears when the load is held in a constant after reaching the maximum load. The results show that the emergency of instantly formed load-displacement platform has a little relevance to the dwell time. Subsequently the micro-plastic constitutive model with intrinsic material length and the reduced intrinsic material length was imported into the FEM of whole indentation process. The abnormal phenomenon at the dwell stage could be treated as a rule through comparative analysis from simulated and experimental results. The intrinsic length of pure aluminum L2 was obtained as 5.09 μm, while the reduced intrinsic length at the end of the dwell stage decreased to 4.90 μm. The simulated results indicate that depth variances at the dwell stage are mainly affected by the reduction of the strain gradient. This is equivalent to that the density of geometrically necessary dislocations (GNDs) reduces to about 96.27% compared with that of the beginning of the dwell stage. It is summarized that the forming mechanism of the load-displacement platform may be attributed to the geometric size effect and the mechanical size effect.