Determination of local constitutive properties of aluminium friction stir welds using digital image correlation

In this paper a simple procedure for the characterization of the constitutive behaviour of welds is presented. Digital Image Correlation (DIC) is used for accessing local strain fields in transverse weld tensile samples and the stress distribution is calculated taking into account local strain data and thickness variation across the samples. The constitutive behaviour of the welds is assessed from local tensile stress–strain curves, plotted up to moderate values of plastic deformation, by fitting an appropriate work-hardening model to the experimental results and the ultimate tensile strength of the welds is estimated using the Considère criterion. Based on this information it is possible to assess the constitutive behaviour of different weld sub-zones, which cannot be derived from the hardness measurements, as well as evaluating the mis-match in yield stress and plastic properties across the welds. The proposed methodology is validated by comparing local stress–strain curves obtained by testing transverse weld samples of friction stir welds in very thin plates with those obtained by testing longitudinal samples of the same welds.