Variation of mechanical properties in a multi-pass weld measured using digital image correlation

As part of a programme investigating the structural integrity of welds, the Digital Image Correlation (DIC) technique was used to obtain the full-field strain distribution during tensile testing of cross-weld specimens cut from a multi-pass girth welded pipe. The displacement maps were analyzed using Matlab scripts to compute local stress-strain variations from which the local proof stress values were extracted. It has been found that the DIC parameters, such as the size of interrogation windows (subsets) and speckle sizes, have significant effects on the displacement values due to the local variations in the mechanical properties within the weld between the passes. The DIC parameters were therefore optimized using monolithic aluminium alloy specimens with stress concentrations giving similar displacement gradients. The use of a high resolution camera at high magnification allowed the variations between the welding passes to be observed. The variation of the mechanical properties in the weld region is correlated with the full field hardness maps of the same region.