Full field strain measurement of dissimilar laser welded joints

Laser welded tailored-blanks (TWBs) made of dissimilar advanced high strength steel (AHSS) sheets are used in the automotive industry to produce resistant and lightweight components. However, the stamping of AHSS TWBs poses some issues concerning the different formability of the parent materials and of the weld seams. Moreover, the weldment exhibits an inhomogeneous mechanical behavior due to the presence of a fusion zone and of a heat affected zone. For this reason, it is crucial to be able to assess the local mechanical properties of the welded joints in order to optimize the laser welding parameters for the fabrication of AHSS TWBs, and hence to guarantee proper formability of TWBs in the final stamping process. In this paper, digital image correlation is employed to investigate the mechanical properties of laser welded joints. Overall and local behavior are studied by means of two image acquisition systems working at different magnification levels and framing the front and the back side of a loaded specimen in a tensile test. The use of two image grabbing systems has been introduced to measure the strain field as close as possible to the weld seam where high strain gradients are present. To this purpose, it has been necessary to tune the parameters of the image correlation processing algorithm. This is because the method, working on image subsets on which the displacement field is modeled with low order shape functions, leans to smooth displacement gradients and strain singularities. By varying post-processing parameters an elaboration strategy has been suggested to capture the strain gradients correctly. The approach, here applied to laser welded joints, may be also used to measure the surface strain field wherever defects or inhomogeneity create strain discontinuities. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).