Effects of Surface Stress on the Load-Depth Curves of Depth-Sensing Indentation
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Effects of Surface Stress on the Load-Depth Curves of Depth-Sensing Indentation Q. Wang and K. Ozaki Digital Manufacturing Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8564, Japan Email: [email protected] ABSTRACT Based on the effects of residual surface stress on the unloading curves of indentation loaddepth responses, an experimental scheme for determination of the residual stress by depthsensing indentation is proposed. From the point that the elastic unloading portion of the loaddepth curves can be expected to be unaffected by the residual stresses, the formula for evaluating surface stress by indentation is derived based on energy method. The proposed formula is verified by using FEM simulated indentation load-depth responses for different surface stress levels. The levels of surface stress evaluated by the proposed formula show a good agreement with the ones used as input parameters in FEM simulation. INTRODUCTION Depth-sensing indentation, widely applied in characterizing mechanical properties [1,2], is a useful tool for measuring the residual surface stress at the micrometer and sub-micrometer scales. In the past decade, many studies on how to measure the residual stress by depth-sensing indentation have been undertaken and have found that the effect of residual stress on the indentation load might be useful in the measurement of the residual stress by depth-sensing indentation [3-10]. Suresh and Giannakopoulos [5] proposed a method for the determination of the residual stress based on the difference in contact areas of residual-stressed and unstressed samples indented to the same depth. However, the effects of the residual stress on the contact areas are relatively small for a sharp indenter. For this reason, Swadener, Taljat and Pharr [7] proposed to use a spherical indenter while using contact pressure instead of contact area as a stress indicator. But after carefully examining these methods, Lepienski et al. [11] found that all of these methods were not easily applied to thin film/layer structures. Similar to Suresh and Giannakopoulos [5], Lee and Kwon [9,10] also proposed a stress-relaxation model for calculating the residual stresses from indentation load shift induced by the residual stress at the same indentation depth. However, in considering indentation process as an elasto-plastic
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problem, the indentation load shift at the same indentation depth implies a different plastic deformation although indenting to the same depth. Therefore, the stress-relaxation model based on indentation load shift at the same indentation depth is vague and need more critical evaluation. In this paper, an easy-to-use experimental scheme for determination of the residual stress by depth-sensing indentation is suggested, based on the effects of residual surface stress on the unloading curves of indentation load-depth responses. From the point that the elastic unloading portion of the load-depth curves can be expected to be unaffected by the residua
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