Analysis of Quasi-Static Denting Behavior of Automotive Sheet Steels

A two-stage finite element method has been used to evaluate the relative denting characteristics of four commercial grades of sheet steel (DQ, BABH, Rephos. IF, and HSLA-50) in a generic laboratory panel geometry. The first stage forming analysis is performed using LS-DYNA3D, a dynamic, nonlinear, explicit finite element analysis code, while the second stage denting analysis is performed using LS-NIKE3D, a large deformation implicit finite element code, in order to avoid dynamic effects. Material thinning and strain hardening during the first stage (forming) as well as bake hardening effects are explicitly accounted for in the subsequent denting analysis. Simulation results indicate that at a 400 N applied load level and a nominal sheet thickness of 0.76 mm, DQ exhibits the highest dent depth (0.86 mm), BABH and Rephos. IF exhibit similar behavior with a dent depth of approximately 0.5 mm, while no visible dent forms in the HSLA-50 grade for the specific panel geometry evaluated. Simulation predicts that ignoring material hardening effects during forming in the denting analysis results in considerably overestimating the dent depth. For the bake hardenable sample, ignoring hardening (strain and bake) effects resulted in tripling the predicted dent depth at 400 N load from ∼0.5 mm to ∼1.5 mm. The need for better interfaces between implicit and explicit finite element codes to solve complex multi-stage problems is also highlighted in this paper.