Application of the modified critical voids expansion ratio criterion to the prediction of the forming limit of 6016-O aluminum alloy

Abstract

Based on the critical voids expansion ratio criterion, the basic material mechanical parameters such as the strain hardening index n and the hardening coefficient K were introduced in it, and the critical voids expansion ratio criterion was modified. Firstly, the basic hydraulic bulging experiments for the AA6016-T4 sheet metal and AA6016-O sheet metal were performed. According to the comparisons of the experiment results and the simulation results of bulging height, the FEM (finite element model) and the parameters in the numerical simulation were verified accurate. Then, the material constants C1 and C2, which were in the modified criterion were confirmed based on the histories of stress and strain in the forming processes of the two materials, and the specific expression of the modified fracture criterion was obtained. Secondly, the experiments and the numerical simulations of three different hydroforming processes of the box part with a 6016-O aluminum alloy were carried out. By comparing the distribution tendencies of the wall thickness on the two different measurement paths, the numerical simulation results have a good agreement with the experiment results and the FEMs of the hydroforming of the box part were verified accurate. So, according to the numerical simulations, the stress and strain histories of all the nodes in the three different hydroforming processes were introduced into the modified ductile criterion. Then, the positions of the fracture points in the three different processes were predicted by the value of the A, which is the conversion form of the modified fracture criterion. Thirdly, the experiment results and the prediction results of the positions of the fracture points were compared, according to the error analysis between them, it is verified that the prediction of the forming limit of box parts with AA6016-O using the modified critical voids expansion ratio criterion is accurate.