Experiment and modeling based studies of the mesoscaled deformation and forming limit of Cu/Ni clad foils using a newly developed damage model

Lack of systematic researches on mesoscaled deformation and damage criteria would obstruct the development of optimal microforming of clad foils. In this paper, a newly developed damage model, inspired by fracture mechanism, was developed to determine the forming limit of Cu/Ni clad foils via considering size effect and its influences on deformation and fracture behavior. The newly developed model integrates the interface separation model based on GTN-Thomason model and the Marciniak-Kuczynski (M-K) model to represent the two stages of ultimate strain before instability in the fracture process of Cu/Ni clad foils. In addition, the former model describes the first-stage strain of the interface separation dominated by void evolution; while the latter one models the second-stage strain of the localized instability of clad foils at interface separation locations via introducing the effects of surface roughness and interface separation on thickness. The results of the synchrotron radiation experiments indicate that void nucleation, growth and coalescence occurring in the interface layer are mainly located in the Cu layer grain boundary region. The localized high density of the nucleated voids is the direct cause of the localized instability. The results of surface roughness experiments demonstrate that surface roughness increases rapidly with strain and saturates at a low strain condition. The surface roughening becomes severe with the increase of grain size. The experimental results of the forming limit show that forming limit curves (FLCs) shift down and strain concentration occurs prematurely with the increasing grain size. The FLCs of Cu/Ni clad foils influenced by grain size are predicted by the newly developed model successfully. This study reveals the interfacial void evolution and surface roughening behavior and their influences on the forming limit of clad foils, in such a way to provide a theoretical basis for microforming of clad foils.