Effects of Microstructure on the Strain Rate Sensitivity of Advanced Steels

The dependence of the strain rate sensitivity of advanced ~1 GPa tensile strength steels on the phases present in their microstructures was studied by testing different steels at 0.005 and 500 s−1. The high strain rate tests were performed using a Kolsky bar setup, while the quasi-static tests were performed using a universal testing machine. The two main steels of interest were the Ferrite-Martensite DP980 and the Ferrite-Martensite-Austenite QP980; the latter being a transformation induced plasticity (TRIP) assisted steel. For comparison, ferritic CR5 mild steel and austenitic stainless steel 201 were also tested under the same conditions. Though the differences in the steel chemistries were not taken into account, the results obtained here suggest a strong relationship between the phase-content of the steel and its response to the changes in the loading rate. The relationships between the observed mechanical behavior and the phases present in the microstructure are discussed

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

Cure History Dependent Viscoelastic Modeling of Adhesively Bonded Joints using MAT_277 in LS-DYNA®

The effects of Coefficient of Thermal Expansion (CTE) mismatch in multi-material adhesive joints, induced during the manufacturing process, are expected to hinder the peak performance of the adhesive in the service life of the vehicle. With a goal to estimate these effects, this paper attempts to model the curing phenomenon of an adhesive and predict its mechanical properties using MAT_277 material model available in LS-DYNA, which serves as a good starting point towards modeling the cure history dependent viscoelastic behavior of adhesives. The adhesive is used to join two substrates of dissimilar metals and tested to capture the relative displacement of substrates. The experiments are performed on a specialized setup, which is built to perform experiments on lap shear joints. The curing kinetics model is calibrated using the results obtained by advanced experimental techniques like Differential Scanning Calorimetry (DSC); the mechanical properties are modeled by Generalized Maxwell model using Dynamic Mechanical Analysis (DMA) results. The fitted parameters are fed into MAT_277 to perform simulations of the lap shear joints tests. Finally, the calibrated model is validated by comparing the relative displacement in the steel-aluminum lap shear joint on a full curing cycle, similar to automotive paint baking oven, to experimentally obtained measurements using digital image correlation (DIC). The results of this work provide insights that will help in predicting the adhesive behavior over varying temperature-time histories during the manufacturing and in the service life of the vehicle