Effect of residual stress on the nanoindentation response of aerospace aluminium alloys

Experimental measurements and finite element simulations have been used to study the effect of residual stresses on the nanoindentation response of an aerospace-grade aluminium alloy. Tensile and compressive residual stresses lead to changes in the nanoindentation load-displacement curves. Loading and unloading curves were studied to observe the effect of residual stresses. The maximum load of indentation, curvature of the loading curve, elastically recovered depth, work of indentation, pile-up and contact area were measured and found to have a linear relationship with residual stress. To calculate residual stress from the load-displacement curve, it was concluded that pile-up should be measured carefully. The paper presents a methodology of calculation of area of contact based on the work of indentation which can be extracted from the nanoindentation load-displacement data. This allows extraction of the residual stresses from experimental nanoindentation data for aerospace aluminium alloys which generate pile-up and for which the true calculation of contact area without imaging is very difficult. Methods previously published in the literature have been assessed against the current approach.