Characterization and Modeling of the High Strain Rate Response of Advanced High Strength Steels

The push for lightweighting in the automotive industry has motivated metallurgist and steel manufacturers to produce new generations of steel that provide significant improvements over the conventional steels to allow them to compete with the introduction of low-density materials into the industry. To achieve this goal, metallurgist introduced different (stronger and more ductile) phases into the ferrite-dominant microstructure of conventional steels. This has led for generations of AHSSs with significantly improved properties. However, the complex microstructure led to increased complexities and unpredictability in the behavior of these materials, especially in their response to variations in the strain rate. This is particularly important, as the materials in the automotive industry exhibit different strain rates during their lifetime, and the performance should be predictable especially during high strain rates as these are encountered during a crash event where performance dictate safety. This research work aimed at investigating and developing a methodology to allow for the accurate modeling of multi-phased AHSSs at muli-strain rates. First, a set of experimental tests were performed on a selected set of AHSSs having a range of combination of phases in their microstructure at different strain rates. This allowed for the investigation of the effect of the different phases in the microstructure on the response of the materials at multi-strain rates, and it gave an insight into the combination of phases that would result in a material with a favorable response at high strain rates. Then, the focus was shifted into a particular third-generation AHSS (Medium Mn. steel), the selection of which was based on its complexity and importance to the future of AHSSs. Further experiments were performed on this material to characterize its anisotropy at multi-strain rates. The experiments were used to calibrate an anisotropic yield function at different strain rates, and the shape and size of the yield locus obtained were observed to be dependent on the strain rate. Furthermore, the experimental results of Medium Mn. steel was used to develop a constitutive relation predicting the strain sensitivity of the anisotropy of the material at different strain rates. A modification on the Yld2000-2d yield function allowed to develop a unique strain rate dependent anisotropic yield function that captures the yielding and anisotropic behavior of the material at multi-strain rates. The model developed was validated using finite element simulations of the experimental tests. Finally, the developed model was used to perform crash simulation on a railroad tank car. The simulations accounted for the strain rate sensitivity of the hardening and anisotropy of the material. The results highlighted the impact of the current work by extracting the load-displacement and the energy absorbed from three different simulations with variations in accounting for the strain rate sensitivity of the material. The difference in the results emphasizes the impact and importance of utilizing the methodology proposed in this dissertation for the crashworthiness analysis of AHSSs