Effets du taux de déformation sur la rupture ductile des aciers à haute performance : Expériences et modélisation (2013)

The automotive industry has widely incorporated Advanced High Strength Steels sheets (AHSS) in vehicle structures due to their high strength to weight ratio: they are used to improve the vehicle safety or to reduce the vehicle weight through the use of thinner gages. At the same time, new vehicle design relies heavily on virtual prototyping practices. In the specific example of automotive structures, both the engineering of the production process and of the final product require reliable models of plasticity and fracture. Consequently, great efforts have been undertaken during the last five years to develop models that can predict the fracture of AHSS under static conditions. However, rates of deformation encountered in sheet metal forming operations are typically of the order of 10s-1, while they can be as high as 103 s-1 under accidental crash loading. Therefore, there is a need to investigate the effect of strain rate on deformation behavior and fracture of AHSS, and to assess whether models developed for static loading conditions can satisfactorily be used in industrial applications. The present research work consists of two main parts. The first part aims at developing a reliable methodology for evaluating the influence of strain rate as well as stress state on the ductile fracture properties of initially uncracked Advanced High Strength Steel sheets. An experimental procedure is designed to characterize the deformation behavior and the onset of fracture of sheet materials under tensile loading at high strain rate. Numerical and experimental validations of the proposed setup are performed to evaluate its accuracy. Then an experimental program is carried out at low, intermediate and high strain rates on different type of tensile specimens, thereby covering a range of stress states. Detailed Finite Element analyses of each experiment are used to determine the loading history and the material state at fracture in each experiment. A key component of this hybrid experimental-numerical approach is the constitutive model: a rate-dependent plasticity model is proposed to predict the mechanical response of AHSS over all the range of strains, strain rates and stress states reached in the experiments. The model accuracy is validated by comparing global and local test measurements to the corresponding simulation predictions. In addition, the influence of the geometric discretization used in Finite Element analysis on the accuracy of the hybrid experimental-numerical approach is evaluated. It is shown that fine meshes of brick elements are required for accurate fracture predictions, but cannot be used in industrial applications because of inadequate computational efficiency. A technique of shell-to-solid re-meshing is presented and evaluated, that allows for accurate predictions of the onset of ductile fracture in sheet materials without compromising the numerical efficiency of shell elements. The second part of this work is concerned with the micro-mechanisms responsible for ductile failure. Micrographs of specimens corresponding to different stages of loading prior to failure are analyzed to identify the sequence of damage processes leading to fracture. Observations suggest that the governing failure mechanism is the localization of plastic deformation into shear bands at the grain level. A numerical model based on three dimensional unit cell calculations is developed to assess whether the mechanism of shear localization of the plastic flow at the micro-scale can explain the dependence of the material ductility to both stress state and strain rate that was observed at the macro-scale.