A method for measuring displacement and deformation of electrodes during resistance spot welding

A new method for measuring the time-dependent displacement and deformation of electrodes during resistance spot welding (RSW) is described. The method allows assessment of the non-stationary thermal expansion of the electrodes. By measuring the electrode indentation close to the weld-piece surface, the method enables a better estimation of indentation than the present methods that rely on measurement of the displacement of the outside part of the electrodes or their holders. The method is based on using a digital video camera to acquire image sequences of electrode caps in the RSW process. A regular pattern of shallow bores is drilled into the caps to enhance the contrast of the acquired images and facilitate image processing. The distances between the bores are analyzed from the image sequences to determine the temporally and spatially resolved displacement and deformation of the caps. The analysis revealed that the cap deformations in some welding regimes can reach up to 20% of the maximum cap displacements, which are approximately 200 mu m. The employed image processing algorithms are presented as well as examples of results that demonstrate the applicability of the method.