Effect of E-Modulus Variation on Springbackand a Practical Solution 2018-01-0630

Springback affects the dimensional accuracy and final shape of stamped parts. Accurate prediction of springback is necessary to design dies that produce the desired part geometry and tolerances. Springback occurs after stamping and ejection of the part because the state of the stresses and strains in the deformed material has changed. To accurately predict springback through finite element analysis, the material model should be well defined for accurate simulation and prediction of stresses and strains after unloading. Despite the development of several advanced material models that comprehensively describe the Bauschinger effect, transient behavior, permanent softening of the blank material, and unloading elastic modulus degradation, the prediction of springback is still not satisfactory for production parts. Dies are often recut several times, after the first tryouts, to compensate for springback and achieve the required part geometry.

In this study, the effect of Young’s modulus (E-modulus) on springback is investigated. Current challenges in determination of E-modulus through tensile test are discussed and a practical method is proposed which has the potential to improve springback prediction after the first die tryout. In this method, the unloading elastic modulus is adjusted by measuring the springback of the part produced during the first tryout and comparing it with finite element (FE) simulation results. The unloading elastic modulus obtained from this method is called the “apparent E-modulus”. This method is applied to three bending cases: a wipe bending, a U-drawing, and a 3-D crash forming of an actual production part. Results show that the springback can be predicted fairly accurate using the apparent E-modulus and a simple isotropic hardening model.