Determination of local constitutive properties in similar and dissimilar friction stir welded joints from DIC based surface strain measurement in two mutually perpendicular surfaces

Abstract

In the present work, local constitutive properties of similar and dissimilar friction stir weld (FSW) joints of AA6061-T6 and AA7075-T6 are evaluated using a modified non-uniform stress method (nUSM). The modified nUSM considers the instantaneous cross-sectional area at any section of a macro tensile (MaT) specimen as a two-variable function. These two variables are the strain in width and thickness direction during uniaxial tensile test. The strain in width and thickness direction is measured using two cameras of a stereo digital image correlation (DIC) testing instrument. These two cameras work as two separate 2-dimensional DIC placed perpendicular to width and thickness direction. However, strain data from this two 2D-DIC are arranged in a global euclidian space while maintaining a fixed pixel/mm ratio. The local constitutive properties evaluated from nUSM are then compared with the local properties extracted from longitudinal miniature tensile (MiT) specimens. The local constitutive properties evaluated using modified nUSM for the similar AA6061-AA6061 FSW joint have shown a good agreement with the properties calculated from the MiT specimen. In a similar AA7075-AA7075 FSW joint, the accuracy of the modified nUSM is affected by the relatively less deformation present in some portion of the FSW joints. The nUSM did not evaluate local constitutive properties in the AA7075 side of a dissimilar AA6061-AA7075 FSW joint, as this portion remains elastic during the uniaxial tensile testing of the MaT specimen.