Effect of Pre-strain on Anisotropic Hardening and Springback Behavior of an Ultra Low Carbon Automotive Steel

Abstract

Modeling non-monotonous and/or non-proportional deformation is challenging partly because of lack of constitutive equations and more so because of unavailability of sufficient experimental data. In the present work, two different experiments involving non-monotonous/non-proportional deformation have been proposed. The model based on combined isotropic-kinematic hardening with permanent softening 1) was utilized to simulate the deformation behavior under such deformation conditions and it was found that the above model was capable of correctly predicting the deformations for both cases considered here. It was also noticed that the incorporation of permanent softening often observed during reverse loading, is important for accurately predicting the deformation when bending-unbending (or strain reversals) takes place.KEY WORDS: non-proportional; 2D draw bend; permanent softening; combined isotropic-kinematic hardening.experiments on an ultra low carbon steel, for different combinations of cross tests and tension-compression tests and also proposed modifications to the combined hardening model of Chung et al. 1,28) It was observed for this particular steel that, during the cross test, the hardening curve finally converges to the original monotonic curve suggesting that the isotropic hardening might be good enough (excepting the cross effect observed initially before converging to the original curve) to model the hardening behavior during cross tests. However, it was not clear, whether the hardening curve will saturate to the original monotonic curve even for more complex strain paths. They also observed a significant amount of permanent softening during reverse loading and suggested that the softening behavior is not only incurred by the translation of the yield surface but also by the shrinkage of the yield surface size. They consequently proposed a method to incorporate both of these in the constitutive model.In the present work, two new two-stage experiments under non-monotonous/non-proportional deformation conditions were performed for the same ultra law carbon steel used by Verma et al. 1) : 2D draw bending of a pre-strained sample and uni-axial simple tension with a specimen prepared from the wall portion of a 2D draw-bent component. Simulations were performed using the material model proposed by Verma et al. 1) for these experiments and it was found that the model was capable of successfully representing the deformation even for such complex deformation paths for this steel grade. Experimental ProcedureThe material selected for this study was the same cold rolled, annealed and skin passed interstitial free high strength (IFHS) steel with 1.0 mm thickness, which was used by Verma et al. and the tensile properties of which is given in Table 1. 1) In this study, 2D draw bending experiments were performed for two different cases: the one with samples prepared from the base material without any pre strains and the second with specimens prepared on samples which were already pre-strained by 5% (nominal strain)....

Springback prediction and reduction in deep drawing under influence of unloading modulus degradation

“…The magnitude of springback is found to be significantly influenced by the deformation histories of steel parts. Verma et al [9] stated that for the cases when deformation of blank sheets increases, the dependency of elastic modulus on plastic strain could have a considerable effect on the springback of material. Therefore, in each forming step, the forming history is one of the key factors for the springback effect.…”

Calibration of Distortional Plasticity Framework and Application to U-draw Bending Simulations

“…The different hardening behaviors observed in nonproportional loading have usually been modeled through kinematic hardening approaches, in which the yield surface translates in stress space. [11][12][13][14] These constitutive models have improved the accuracy of numerical simulation results. For instance, the finite element (FE) simulation of springback in sheet metal forming have produced better results compared with the conventional isotropic hardening assumption.…”

An Iterative Compensation Algorithm for Springback Control in Plane Deformation and Its Application

“…Process control method is to control the springback by optimizing the forming process, such as controlling the blank holder force and punch stroke [2,3], changing the loading method [4][5][6], increasing the forming steps [7,8], and improving the forming temperature [9][10][11].…”