Residual Stress Distribution in Aluminum/Polypropylene/Aluminum Sandwich Laminates in U-Channel Draw Bending

Residual stresses in sheet metal parts are internal stresses that remain after the release of elastic strains at the end of metal forming operations. When the formed sheet metal part is removed from the forming tools, it not only springs back but also acquires a through-thickness residual stress distribution. Residual stresses for monolithic aluminum sheets and aluminum/polypropylene/aluminum sandwich laminates after springback of a U-channel formed by draw bending are presented in this paper. The forming stresses at the end of punch travel and residual stresses at the end of punch withdrawal are numerically determined using LS-DYNA, a well-established nonlinear finite element software. The through-thickness forming stress distribution is determined using an explicit forming simulation, following which the through-thickness residual stress distribution is determined using an implicit springback simulation. Stress distributions are studied at the die corner, punch corner and along the wall of the U-channel. Both forming and residual stresses in the sandwich laminate are found to be lower than those in monolithic aluminum of equivalent thickness. In sandwich laminates with the same skin thickness, higher residual stresses are observed in the skin layers as the core thickness is increased. The residual stresses at the punch corners of the formed U-channels are more influenced by changes in the die and punch corner radii than at the die corner.