Simple shear deformation of sheet metals: finite strain perturbation analysis and high-resolution quasi-in-situ strain measurement

Abstract

Accurately evaluating the simple shear in sheet metals has proven to be challenging owing to three reasons: a) the detailed shear strain evaluation is different from one publication to another; b) the measured strain is strongly dependent on the employed virtual strain gauge size (LVSG) when digital image correlation (DIC) is utilized; and c) the negligence of the strain perturbation phenomenon (Swift effects) will only result in a rough estimation of the strain–stress behavior. Thus, in this study, a finite strain perturbation analysis according to the configuration of experimental tests was constructed to understand the shear strain–stress evolution, and a high-resolution quasi-in-situ strain measurement technology was developed to capture the localized shear strain with reliable results. Material anisotropy and simple shear specimen geometry were identified as the two major contributors to the Swift effects. Hence, a finite strain perturbed shear kinematics was first proposed, revealing that some specific shear strain estimations led to a certain degree of error (∼12–25%) at large deformation levels. This work demonstrated that the finite element simulation of simple shear tests, performed using the carefully calibrated YLD2004 yield model, can be utilized to reasonably predict perturbation. A Vickers-indentation-based quasi-in-situ strain measurement was further developed to obtain reliable local strain of the shear zone with LVSG =28 μm. The technique was found to provide high-resolution strain detection to satisfy the necessary conditions of accurate strain measurement when a narrow shear band is present. The results yielded a 30% higher von Mises strain under simple shear deformation than that reported in previous publications for a similar DP980 sheet. The said result was also found to be larger than the fracture strain determined through the so-called “hybrid method”. The proposed perturbed shear kinematics was further implemented in the anisotropic elastoplasticity framework to show that the magnitude of normal stresses did not always remain in the second order with respect to the shear stress due to strain perturbation. Thus, the concept of “perturbed” shear stress was designed to consider the effects of perturbation, and the findings signified a 6–13% change in shear stress compared with the commonly used shear stress for tested scenarios.