Bauschinger effect during unloading of cold-rolled copper alloy sheet and its influence on springback deformation after U-bending

Tension, cyclic tension-compression and partial unloading tests for two types of cold rolled brass sheet (i.e. JIS C2600R-H, C2600R-1/2H) were carried out in order to investigate cyclic plasticity and unloading behavior, and to determine the plastic dependency on Young's modulus. The Young's moduli for the pre-strained materials were determined in different two ways. For the first case, unlading stress-strain data from sigma(0) to 0.0MPa was used, and data from sigma(0) to (sigma(0) -2Y(0)) was used for the second case, where sigma(0) and Y-0 denote 90% of the stress at the beginning of unloading and the initial yield stress used in a constitutive equation, respectively. The saturated Young's moduli were significantly different on C2600R-H sheet, but similar plastic strain dependency on Young's modulus relationship was found on C2600R-1/2H sheet. Subsequently, U-bending experiments and the corresponding finite element simulations using several combinations of plastic strain dependent Young's moduli and isotropic and kinematic hardening rules were conducted. The simulation results indicated that Young's moduli measured using sigma(0) to (sigma(0) - 2Y(0)) data with elasto-plastic unloading is the best choice for springback simulation of the cold rolled sheets. (C) 2014 The Authors. Published by Elsevier Ltd.