Understanding the physics of non-linear unloading-reloading behavior of metal for springback prediction

Finite element simulation technique is extensively useful nowadays for die designing by optimizing the springback from the formed state of sheet metal panel. The magnitude of springback is normally calculated in finite element simulation by assuming a completely elastic recovery in non-linear kinematic hardening law. Constant values of elastic modulus and Poisson's ratio are required to estimate the elastic recovery by non-linear kinematic hardening law. Cleveland and Ghosh (Int J Plast 18:769-785, 2002), Li and Wagoner (Int J Plast 1827:1126-1144, 2011), and many other research groups have reported that inelastic strain release during unloading is the main source of extra strain recovery and as a result poor springback prediction by commercial finite element software. In this regard, many theoretical postulates have been proposed to explain such inelastic strain release during unloading. In this work, we show from atomistic simulation that irreversible movement of dislocation, i.e., microplasticity, is the source of inelastic strain release during unloading.