Calibration of Elastoplastic Constitutive Parameters for HRB400 Steel Based on Tension-Torsion Tests

To obtain the elastoplastic constitutive parameters of HRB400 steel under tensile and shear stress states， smooth solid round bar and hollow thin-walled circular tube specimens were fabricated， and uniaxial tension and pure torsion tests were conducted respectively. The experimental data were processed using data processing software， where the nominal stress-strain curves in the uniform deformation stage were converted to true stress-equivalent plastic strain curves. Subsequently， the true stress-plastic strain curves after plastic instability were calibrated using the piecewise linear hardening method. A finite element model was established to simulate the experimental process. By repeatedly comparing the load-displacement curves obtained from simulations and experiments， as well as the deformation behaviors of the specimens， the calibrated curves were iteratively corrected to ensure the accuracy of the constitutive parameters. The results showed that when the calibrated hardening curves under tensile and shear states were applied in numerical simulations， the simulation results were in good agreement with the experimental curves and the deformation behaviors of the specimens. Comparative analysis revealed that there were differences in the hardening curves of HRB400 steel under tensile and shear stress states， specifically manifested as a stronger hardening effect under tensile stress than under shear stress. When the elastoplastic parameters obtained from tensile tests were used to simulate torsion tests， the simulated torque values were higher than the measured values， which further confirmed that the hardening of HRB400 steel exhibited directionality. The findings indicate that in practical engineering applications， a more conservative strength value should be adopted for HRB400 steel under shear stress states to ensure structural safety.