A Method for Measuring In-Plane Forming Limit Curves Using 2D Digital Image Correlation

Abstract

With the introduction of advanced lightweight materials with complex microstructures and behaviors, more focus is put on the accurate determination of their forming limits, and that can only be possible through experiments as the conventional theoretical models for the forming limit curve (FLC) prediction fail to perform. Despite that, CAE engineers, designers, and toolmakers still rely heavily on theoretical models due to the steep costs associated with formability testing, including mechanical setup, a large number of tests, and the cost of a stereo digital image correlation (DIC) system. The international standard ISO 12004-2:2021 recommends using a stereo DIC system for formability testing since two-dimensional (2D) DIC systems are considered incapable of producing reliable strains due to errors associated with out-of-plane motion and deformation. This work challenges that notion and proposes a simple strain compensation method for the determination of FLCs using a low-cost single-camera (2D) DIC system. In this study, formability tests are performed on an automotive-grade 6xxx series aluminum alloy using the Marciniak in-plane FLC testing method. The tests are performed on a custom setup that enables simultaneous optical strain measurements using a stereo DIC as well as a 2D DIC system. The results show how 2D DIC FLC points match those obtained by stereo DIC using two popular FLC approaches: ISO 12004-2 section-based spatial method and a time-dependent linear best fit (LBF) method.