Investigation of Non-Linear Springback for High Strength Steel Sheets by ESPI

The springback of the sheet metals after large deformations during deep drawing is not a strongly linear process with a constant Youngs modulus, but the stressstrain behaviour during the unloading phases shows considerably non-linear and inelastic effects. Unloading of two types of steel sheets for cold forming, a cold-rolled high-strength microalloyed steel and a low-carbon steel sheet, has been analysed using the method of electronic speckle pattern interferometry (ESPI). The specimens were investigated by uniaxial tension tests, and the influences of different testing parameters upon springback were analysed. The experimental measurements showed that the stressstrain curve during unloading is non-linear, the influence of the prestrain path upon unloading is minor, and the secant moduli of unloading curves decrease with increasing prestrain. When the prestrain value becomes high enough, a saturated value for the secant modulus is approached. An empirical relation was found to describe the changes in the unloading modulus in accordance with the prestrain value.