Use of nanoindentation to measure residual stresses in surface layers

Abstract

Equal biaxial residual stresses (of up to about 175 MPa) have been generated in thin copper foils via differential thermal contraction. These foils were subsequently indented, under displacement control, and the load–displacement–time characteristics were measured. The applied load required for penetration to a given depth (in a given time) was found to decrease with increasing (tensile) residual stress, in accordance with predictions from a finite-element model (incorporating both plasticity and creep). The main thrust of this paper concerns sensitivities. Relatively small changes in residual stress (of the order of a few tens of MPa) were observed to generate effects that should be detectable via their influence on the nanoindentation response. This is encouraging in terms of the potential of the technique for characterizing (in-plane) residual stresses in surface layers, particularly for mapping of point-to-point variations (as opposed to obtaining accurate absolute values). In contrast to this, it is shown that changes in the hardness, as a consequence of changes in residual stress level, are smaller and more difficult to analyse.