Application of synchrotron X-ray diffraction and nanoindentation for the determination of residual stress fields around scratches

Residual stresses and plastic deformation around scratches or scribe marks in ductile materials can affect fatigue life. Scratches of the order of tens of microns may convert into propagating cracks driven by tensile residual stresses at the scratch root. Probing such stresses on a small scale is experimentally challenging in engineering materials. Here we present results of a combined study using synchrotron X-ray diffraction and nanoindentation to determine the residual stresses around scratches in aluminium alloys. The extraction of residual stresses in metallic materials where there is work hardening is challenging using indentation methods, but a method is presented by which this has been achieved, and a good correlation is obtained between the results obtained using diffraction and nanoindentation. The advantage of synchrotron X-ray measurement is that it allows validation of the stresses at the same spatial scale as nanoindentation. It was found that scratches produced by a “ploughing” mechanism where there was significant plastic deformation beneath the scratch showed higher work hardening and tensile residual stresses than those produced by a “cutting” mechanism where there was little plastic deformation of the material. Little effect of fatigue cycling was seen on the peak stresses at the scratch tip.