Cover: Measured strain field of a softened heat-affected zone under in-plane tensile loading. Material: boron steel 22MnB5 in the fully hardened, martensitic state. The results, opinions and conclusions expressed in this thesis are not necessarily those of Volkswagen AG. modeling of tailor hardened boron steel for crash simulation PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, prof. dr. H. Brinksma, volgens besluit van het College voor Promoties in het openbaar te verdedigen op vrijdag 10 juni 2016 om 12.45 uur door Summary The automotive industry is continuously working on the weight reduction of their vehicles, while maintaining or even improving the crashworthiness in accordance with increasing safety demands. This has favored the development of innovative materials and production processes. The hot stamping process, in which heated blanks are simultaneously formed and quenched in cooled dies, is one of these recent innovations. Due to the high cooling rates during quenching, an ultra high material strength of up to 1500 MPa can be obtained. By locally reducing the cooling rate during quenching, e.g. by using partially heated dies, regions of lower strength with increased ductility can be introduced. The resulting, so-called tailor hardened components are made out of a single sheet of metal and feature precisely defined zones of varying strength and ductility. To be able to fully exploit the possibilities of tailor hardened components, it is important to attain accurate predictive models of their crash response. In this thesis, a hardness-dependent material model for quench-hardenable, boron-alloyed steel 22MnB5 is presented. The material model is able to predict the material properties both in homogeneous areas of different strengths as well as in hardness transition zones. The model is designed to provide an accurate prediction of the deformation behavior of the material up to the point of fracture. Moreover, it includes a fracture prediction criterion that accounts for the complex loading paths experienced by the material in the event of a crash. For calibration of the material model, five hardness grades of 22MnB5 are considered, covering the full strength-range from 600 MPa in the ferritic/pearlitic range to 1500 MPa in the fully hardened, martensitic state. Separate plasticity and fracture models are calibrated for each of the five grades. Strain hardening is represented by a nonlinear combination of the Swift hardening law and a modified version of the Voce law. For fitting of the initial stress-strain response, force-displacement curves of uni-axial tensile tests are used up to the point of necking initiation. An inverse FEM optimization routine, that takes into account measured force-displacement curves and strain fields, is used to optimize the strain hardening parameters in the post-necking regime. The fracture behavior is represented by a stress triaxiality and Lode angle dependent, strain-based fracture criterion. Five different...
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