Direct measurement of necking strain using optical contour analysis on isotropic ductile stainless steel

To accurately determine yield stress curves for ductile metals, it is essential to account for the triaxial stress state that develops during necking, which complicates the extraction of the equivalent uniaxial stress state. This study introduces a simple yet effective approach to address this challenge. Using a single-camera setup with backlight illumination, silhouette images of the specimen during tensile testing are captured. From these images, the specimen contours are extracted digitally, enabling strain computation based on changes in contour geometry. Simultaneously, a novel curvature-fitting algorithm is employed to calculate stress triaxiality. The accuracy of this method is validated through comparison with finite element simulations, and its applicability spans from the onset of necking to the point of fracture. This approach is demonstrated on 303 stainless steel, showcasing the accurate recovery of equivalent uniaxial true stress–true strain relationships under varying triaxiality conditions. Furthermore, as these stress and strain measures are energy-conjugate, the mechanical work within the neck can be calculated, enabling a direct determination of the Taylor-Quinney coefficient using infrared thermography. The method offers a robust framework for experimental analysis and provides a straightforward route for mechanical and thermal coupling studies. To facilitate broader adoption, an open-source implementation of the program is made available.