Charaterization of dynamic hardening behavior at intermediate strain rates using the Virtual Field Method

Abstract

Intermediate/High strain rate experiments are seriously affected by inertia, stress wave, noise and etc. and those give big influence on the results. Several methods were proposed to solve these problems but those are complex and take much times in data processing. To overcome the problems, the Virtual Fields Method with a strain rate dependent hardening equation was used to derive dynamic hardening parameters. Before the experiment, the Virtual Fields Method (VFM) with a rate-dependent model was validated by using a Finite Element (FE) simulation. For automotive steels, popular dynamic hardening models such as the Johnson–Cook or Cowper–Symonds model do not properly describe dynamic behavior at intermediate strain rates. Therefore, the LIM–HUH model was used and validated by comparing input and output material behavior in the FE simulation. Experiments were performed after the validation was completed. Dual-phase steel (DP780, DP980) was selected for the initial stage of the study and TRIP780 was applied for intensive study. A newly developed impact frame high speed (IFHS) tensile tester was used mainly to generate intermediate strain rates (100 /s – 300 /s) in the experiment, and a high-speed camera was used to take images during deformation for digital image correlation (DIC) analysis. After the dynamic tensile test was conducted, the VFM was performed and dynamic parameters of the LIM–HUH model were derived. In addition, stress–strain curves at various strain rates could be derived using a single dynamic experiment.