Abstract
This work presents description and comparison of internal and state variable models of flow stress in varying processing conditions. Three models were analyzed. The first one is based on dislocation theory and describing the mechanical behavior of f.c.c. polycrystalline structures. The second and third models are standard and modified Sellars’ flow stress models. Models were adapted for two commercial codes based on finite element method: QForm7 and Forge 2005. The compression test of 45 grade steel with instant changes of strain rate was simulated. Calculated compression force and flow stress were compared with the experimental data from plastometric tests. The forging process was simulated by QForm7. Results obtained by both internal and modified Sellars’ models confirm their high accuracy for analysis and prediction of the flow stress under the varying deformation conditions.
Highlights
A proper description of the flow stress under varying conditions is beneficial for computer simulation because the real processing conditions change continuously.For example, the strain rate usually grows at the beginning of the process, reaches a maximum value, maintains it for a certain period at approximately the same level, and decreases to zero at the end of the process
Instant changes of strain rate lead to instant drop of compression force when the table is used for flow stress calculation, while force calculated by internal variables models (IVM) demonstrates smooth changes that are more close to the measured force
The effectiveness of internal variables modeled in varying deformation conditions is discussed in the paper
Summary
A proper description of the flow stress under varying conditions is beneficial for computer simulation because the real processing conditions change continuously. There are well-known models developed by Kocks [1], Roberts [2], Yoshie et al [3], Bergstrom [4], and Estrin and Mecking [5], which use the dislocation density as an internal variable These models are said to have an advantage when a non-stationary process takes place. Kocks and Mecking [6] have shown that in most cases, one internal variable is sufficient to describe the flow stress for materials with the f.c.c. structure in the wide range of the strain rate and temperature. They stated, that one internal variable allows to describe only a process with constant deformation conditions. Models were implemented into two commercial FEM codes, and plastometric tests were simulated
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