Measurement of Residual Stresses Using Indentation Plastometry

This study concerns the potential use of the indentation plastometry technique to obtain information about residual stresses. This is a logical extension of the technique, which is based on iterative FEM simulation of the indentation procedure so as to infer the true stress-strain curve of the material, with the target outcome being the profile of the residual indent. Once this has been obtained (in the form of parameter values in a constitutive law), a logical extension of the procedure is to create an initial (residual) stress in the model and then converge on the value that gives the best agreement with experiment. In the present work, this has been done for some copper sheet samples, with different levels of residual stress created experimentally via the application of external loads. This has been done for a series of equal biaxial stress levels. The inferring of residual stress levels in this way, and comparison with expected levels, has not been reported previously. The main conclusion is that the sensitivity of the measured indent profile to the residual stress level is low. This is particularly the case for compressive stresses and for relatively deep indents, but it is also true for tensile stresses and shallower indents. The prospects for accurately inferring residual stress levels via this procedure are therefore poor. A positive aspect to this rather negative conclusion is that, when obtaining plasticity characteristics in this way, there need be little concern about the outcome being influenced by the presence of (unknown) residual stresses.