DIC-based approach to predict post necking behavior for AA6061, AA7075 and their friction stir welded joints

Abstract

In a tensile test, the precise measurement of the area at the necking section is crucial for predicting the post-necking true stress-strain behavior. In the conventional theoretical method, this area is linked to the engineering strain using the incompressibility criteria, which is not applicable in the post necking domain. The present work proposes a digital image correlation (DIC)-based technique with two cameras for estimating real-time area reduction at a necking section of a rectangular tensile specimen during a uniaxial tensile test. In the current study, the area at a given section is computed by measuring strain synchronously in the width and thickness planes of a tensile specimen and translating the change in lengths in both planes into a global coordinate system. The area computation of the proposed approach is successfully validated by measuring the necking area of a terminated tensile specimen (stopped before fracture) by optical microscope to an error in the range of 0.12–1.10%. Following verification, the post necking behaviors of aluminum alloys such as AA6061-T6 and AA7075-T6 are assessed. The current approach is further implemented to evaluate the area reduction and post necking behavior of similar and dissimilar friction stir welding (FSW) joints of AA6061-T6 and AA7075-T6 that possess process induced microstructural inhomogeneity. The current experimental approach effectively assessed the area reduction of a similar and dissimilar FSW joint influenced by weld inhomogeneity, which is otherwise complicated to contemplate using an analytical approach.