New procedure for determining the strain hardening behavior of sheet metals at large strains using the curve fitting method

The stress-strain relationship of a sheet metal is conventionally evaluated from the experimental data obtained from the uniaxial tensile test before the onset of localized necking. The identification of the hardening behavior beyond the neck or post-necking behavior is essential for studying material fracture. This study presents a new procedure of identifying the flow stresses of sheet metals at large strains. In this procedure, the experimental data obtained from the uniaxial tensile test and the measured forming limit at a plane-strain tension mode FLC0 are adopted in the curve fitting procedure for calibrating the parameters of different hardening laws. The measured FLC0 is introduced in the cost function of the fitting to provide an additional constraint for the hardening parameters in large strain ranges. Using the uniaxial tensile test data ensures the accuracy of the pre-necking description of the hardening. The proposed method decreases the variation of the predicted flow stresses beyond the uniform elongation for different hardening laws. For validation purposes, the developed approach is applied to calibrate the parameters of the Kim-Tuan and the linear combined Swift/Voce hardening models for three sheet metals tested in Numisheet (2014) benchmarks. The hardening laws with newly identified parameters are implemented into the finite element software ABAQUS/Explicit to simulate the standard uniaxial tensile and reverse drawing tests. The comparative results show that the simulated tensile force in the uniaxial tensile tests and the simulated punch force in the reverse drawing tests agree well with the experiments, thereby validating the accuracy and the efficiency of the proposed identification procedure of large strain hardening in sheet metals.

Automated summary

This study presents a new procedure for identifying the flow stresses of sheet metals at large strains by fitting the experimental data from uniaxial tensile tests and measured forming limit values. The proposed method ensures the accuracy of pre-necking hardening description and reduces the variation of predicted flow stresses beyond uniform elongation for different hardening laws. Application and validation of the developed approach on finite element simulation show good agreement with experimental results, thus confirming the accuracy and efficiency of the proposed identification procedure for large strain hardening in sheet metals.