Optical strain measurement through digital image correlation during tensile testing was utilised to study the effect of strain rate and welding defects on localisation of plastic deformation in friction stir welded and electron beam welded phosphorous-alloyed oxygen-free copper. Welds cut from fullsize nuclear waste copper canisters and additionally made defective welds were studied. Optical strain measurement represents a new way of studying deformation of materials and obtaining local material data. The data was used to construct strain maps and estimate local true stress–strain curves. Using Gini oefficients to quantify strain localisation, it appears that the lower strain rate resulted in slightly faster localisation of plastic deformation with lower stress level.