Behaviour and Modelling of Dual-Phase Steels

Dual-phase steels are being increasingly considered for application in vehicle structural crash components because of their combined attributes of high-strength and good formability characteristics. Accurate prediction of such components, which are often made of formed components, is necessary to reduce the cost of physical tests. The non-linear finite element method is an efficient and reliable tool for the design of new components. The reliability of the finite element analyses depends on the accuracy of the constitutive and fracture models. To support the engineering applications of dual- phase steels for crash and forming events, both strain hardening and strain rate hardening must be thoroughly modelled for general loading paths. In this paper, an elasto-viscoplastic phenomenological model is adopted to represent the material behaviour of dual-phase steels. The constitutive model is formulated in the framework of phenomenological continuum mechanics. The main ingredients of the model include a non-quadratic yield criterion, the associated flow rule and non-linear isotropic and kinematic hardening. The model is within the class of plasticity models proposed by Chaboche [1] and Lemaitre and Chaboche [2] for application to monotonic, non-proportional and cyclic loading conditions. The constitutive model is also able to depict viscous characteristics of the material. The material was experimentally characterized under different loading conditions suited for crash and forming events. Figure 1 depicts the approach embraced in this research for identification of material parameters for the utilized model from material tests. The conventional material tests are providing data for the calibration of the model. Simple tension tests were used to characterize the elastic parameters, the yield surface and the isotropic hardening parameters. Non-proportional tension tests were used to identify the kinematic hardening parameters. Additionally, viscous parameters were identified from the corresponding tension tests over a wide range of strain-rates. The predictive capability of the adopted numerical model is assessed against the material behaviour at elevated rates of strain and the material behaviour under strain-path changes. Particularly, phenomena like dynamic localisation, large plastic deformations have given due importance for validity of the numerical model. Moreover, the same model is validated for crashworthiness performance of dual-phase steel generic components. The model provides results which are in good agreement with the experimental observations. In this work, the non-linear explicit FE code LS-DYNA was used for numerical computations.