포항공과대학교

초록/요약

The present study aims to analyze the influences of material and friction models on springback simulations and to suggest the optimum choice. Dual-phase (DP) steel (1.4 mm thick) and transformation-induced plasticity (TRIP) steel of two different thicknesses (1.2 and 1.4 mm) were chosen for this study. The stress-strain behaviors of the materials were measured using uniaxial tension, balanced biaxial tension and simple shear tests. The frictional behavior was measured using flat-die friction tests under different sliding velocities (0.1-90 mm/s) and contact pressures (12.5-500 MPa). The constitutive and frictional behaviors of the materials were subsequently described using both conventional and advanced models for the springback prediction in U-draw/bending. The combination of advanced constitutive and friction models was able to accurately predict springback and punch load. In particular, the hardening law and elastic unloading modulus were found to have major influences while the friction model had also a significant effect in high blank holding conditions. The analysis was extended to sensitivity studies that considered wider ranges of material and frictional behaviors. These studies suggested that (1) plastic anisotropy has to be modeled using an anisotropic yield function if the yield stresses or r-values affecting the relevant stress state are far from the assumption of isotropy; (2) reverse loading behavior has to be described using an anisotropic hardening model if transient hardening and/or permanent softening are apparent, but the isotropic hardening model is sufficient if the Bauschinger effect alone is present; (3) the elastic modulus should be defined as a function of strain if a visible reduction is observed; (4) the deformation heat and resultant thermal softening can be ignored if the punch speed is less than about 1 m/s; and (5) the friction coefficient should be carefully defined using an advanced friction model if the blank holding force is high.