Prediction of fracture behavior in hole expansion test using microstructure based dual-scale model

A reliable prediction of sheet formability is required for designing automobile parts, especially for the parts made of Advanced High Strength Steels (AHSS) with complex microstructure. Because of the microstructural complexity of AHSS, finite element (FE) simulations based on the representative volume element (RVE) in which microstructural information is incorporated as a submodel have been used to predict macroscopic mechanical properties of materials. In this work, a dual-scale FE approach was proposed to predict the hole expansion ratio (HER) of a multiphase steel sheet. As a large scale simulation, punching analysis followed by the hole expansion simulation was first performed. The strain history of each element was used as a boundary condition for the subsequent small-scale RVE model. Deformation behavior depends on several factors related to microstructural effects such as grain size, and dislocation density. The equilibrium dislocation density in the pile-up configuration was calculated by applying the Peach-Kohler equation and the mean free path was calculated from the derived dislocation density. The dislocation density based flow stress was implemented in the model. For the failure modeling, realistic microstructure-based finite element approach was presented in combination with continuum damage mechanics to consider the microstructure of investigated steel. In the simulation of punching process, a ductile fracture criterion was suggested to predict shear and fracture zones. The experimentally observed hole-expansion formability was reasonably explained by using the presented dual-scale finite element model.