Abstract
It has been widely observed that below the yield stress the loading/unloading stress-strain curves of plastically deformed metals are in fact not linear but slightly curved, showing a hysteresis behaviour during unloading/reloading cycles. In addition to the purely elastic strain, extra dislocation based micro-mechanisms are contributing to the reversible strain of the material which results in the nonlinear unloading/reloading behaviour. This extra reversible strain is the so called anelastic strain. As a result, the springback will be larger than that predicted by FEM considering only the recovery of the elastic strain. In this work the physics behind the anelastic behaviour is discussed and experimental results for a dual phase steel are demonstrated. Based on the physics of the phenomenon a model for anelastic behaviour is presented that can fit the experimental results with a good accuracy.
Highlights
There has been an increasing interest by the automotive industry towards employing Advanced High Strength Steels (AHSS) in the past years
In the past years most researches were focused on the development of novel plasticity models to give an accurate stress prediction
Little has been done in modeling the material behaviour during unloading upon release of the constraining force
Summary
There has been an increasing interest by the automotive industry towards employing Advanced High Strength Steels (AHSS) in the past years. It has been observed experimentally that the material shows a nonlinear unloading behaviour as well as reloading behaviour after being plastically deformed [2,3,4] This is caused by an extra reversible strain recovered during unloading along with the pure elastic strain [5]. Various researchers have adopted an approach attributed to E-modulus degradation [9,10,11,12,13,14] In this approach the E-modulus of the material is made a function of the equivalent plastic strain in the simulations. The draw back with this approach is that it is assumed that all the points in the material are unloaded to the zero stress This is not a realistic assumption in industrial forming processes. In this work by quantifying the anelastic strain, a model for describing the nonlinear unloading behaviour is proposed and the model prediction is compared with experimental data
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