Evaluation of Non-equal Biaxial Residual Stress Using Modified Berkovich Indenter in Nano Scale

Abstract

Residual stress is generated by manufacturing processes and thermochemical treatments. These residual stresses combines with external loads to influence deformation and fracture properties. At nanoscale, thin films which have already undergone roll-to-roll processing, have non-equal biaxial residual stress, and this will reduce the reliability of materials. Conventional methods of measuring residual stress, like the x-ray diffraction and curvature methods, have stringent requirements on specimen microstructure or can evaluate only the surface mean residual stress. However, instrumented indentation testing is a non-destructive, simple method that can evaluate local residual stress quantitatively. On macroscale, the Vickers indenter is used to measure the magnitude of residual stress and the Knoop indenter to evaluate stress directionality. At nanoscale, the Berkovich indenter is widely used to measure the residual stress magnitude. But no research has been done on evaluating the stress ratio of residual stress using instrumented indentation testing. In this research, in order to measure biaxial residual stress, a Modified Berkovich indenter was designed that is based on the Berkovich indenter but is extended along one direction to yield different load sensitivity. Before manufacture, we used FEA to verify the validity of this new indenter. Since the Modified Berkovich indenter had geometrical self-similarity, conversion factor ratios were measured for the unstressed and stressed states at a given indentation depth. Moreover, by applying various biaxial stresses to cruciform specimens, the non-equal biaxial residual stress was evaluated by analyzing the load-depth curves. The model was verified by comparing the measured stress with applied stress.