Failure Characterization of Hot Formed Boron Steels with Tailored Mechanical Properties

This thesis presents the results from characterization of the failure behaviour of hot stamped USIBOR® 1500-AS steel sheet with tailored properties. A phenomenological approach is used in which failure strain is characterized as a function of stress state and as-hot stamped condition, based on the results of an extensive experimental campaign. A range of material quench conditions are investigated, resulting in a multitude of material microstructures. Considering the framework for tailored hot stamping using in die heating to achieve different material responses, six different material quench conditions were considered in this work, ranging from fully martensitic (Vickers microhardness of 485HV) to a mixed ferrite-bainite microstructure (185HV). Four of the material quench conditions investigated were produced using laboratory equipment, while the remainder were obtained from tailored axial crush components that were quenched with die temperatures of 400 and 700 °C. Miniature shear, butterfly, hole expansion, hole tensile, and hemispherical dome tests were developed for fracture characterization of sheet metal and digital image correlation (DIC) techniques were used extensively in order to obtain fracture strains and strain paths for the different experiments. Notched tensile specimens were also tested, however these specimens were not used for fracture characterization, due to their non-proportional loading paths and indeterminate fracture locations. Considering fracture strain to be a function of stress state and assuming the material investigated in this work to be isotropic and von Mises yielding, the equivalent strain at fracture and stress triaxiality of each experiment was determined from DIC-measured major and minor strains. Fracture loci were then calibrated for different material quench conditions. The validity of the experimental fracture locus was probed using a number of non-proportional loading experiments, in which specimens were initially pre-strained in equi-biaxial tension, before being subjected to loading states of simple shear or uniaxial tension. Additionally, work was done to adapt and apply the experimental fracture loci to impact simulations of hot stamped components with tailored properties, requiring development of “model dependent fracture loci”. There was found to be an inverse relationship between material hardness and measured fracture strain, with the fully martensitic material quench condition (485HV) possessing the lowest ductility, while the greatest fracture strains were measured in the samples produced through die quenching at 700 °C (185HV). For the range of material conditions considered, the lowest fracture strains corresponded to a plane strain loading condition, while the fracture strains measured in simple shear were considerably greater. The material quench conditions, ordered in increasing ductility, are as follows (fracture strains for simple shear and plane-strain tension included in parentheses): fully martensitic (0.54, 0.15), intermediate forced air quench (0.68, 0.22), fully bainitic (0.90, 0.36), 400 °C die quench (1.01, 0.38), and 700 °C die quench (1.05, 0.44)

A realistic model for battery state of charge prediction in energy management simulation tools

In this paper, a comprehensive model for the prediction of the state of charge of a battery is presented. This model has been specifically designed to be used in simulation tools for energy management in (smart) grids. Hence, this model is a compromise between simplicity, accuracy and broad applicability. The model is verified using measurements on three types of Lead-acid (Pb-acid) batteries, a Lithium-ion Polymer (Li-Poly) battery and a Lithium Iron-phosphate (LiFePo) battery. For the Pb-acid batteries the state of charge is predicted for typical scenarios, and these predictions are compared to measurements on the Pb-acid batteries and to predictions made using the KiBaM model. The results show that it is possible to accurately model the state of charge of these batteries, where the difference between the model and the state of charge calculated from measurements is less than 5%. Similarly the model is used to predict the state of charge of Li-Poly and LiFePo batteries in typical scenarios. These predictions are compared to the state of charge calculated from measurements, and it is shown that it is also possible to accurately model the state of charge of both Li-Poly and LiFePo batteries. In the case of the Li-Poly battery the difference between the measured and predicted state of charge is less than 5% and in the case of the LiFePo battery this difference is less than 3%

A comprehensive model for battery State of Charge prediction

In this paper the relatively simple model for State of Charge prediction, based on energy conservation, introduced in [1] is improved and verified. The model as introduced in [1] is verified for Pb-acid, Li-ion and Seasalt batteries. The model is further improved to accommodate the rate capacity effect and the capacity recovery effect, the improvements are verified with lead-acid batteries. For further verification the model is applied on a realistic situation and compared to measurements on the behavior of a real battery in that situation. Furthermore the results are compared to results of the well-established KiBaM model. Predictions on the SoC over time done using the proposed model closely follow the SoC over time calculated from measured data. The resulting improved model is both simple and effective, making it specially useful as part of smart control, and energy usage simulations

Water-splitting electrocatalysis in acid conditions using ruthenate-iridate pyrochlores

The pyrochlore solid solution (Na0.33Ce0.67)(2)-(Ir1-xRux)(2)O-7 (0<x<1), containing B-site Ru-IV and Ir-IV is prepared by hydrothermal synthesis and used as a catalyst layer for electrochemical oxygen evolution from water at pH<7. The materials have atomically mixed Ru and Ir and their nanocrystalline form allows effective fabrication of electrode coatings with improved charge densities over a typical (Ru, Ir)O-2 catalyst. An in situ study of the catalyst layers using XANES spectroscopy at the Ir L-III and Ru K edges shows that both Ru and Ir participate in redox chemistry at oxygen evolution conditions and that Ru is more active than Ir, being oxidized by almost one oxidation state at maximum applied potential, with no evidence for ruthenate or iridate in + 6 or higher oxidation states

Redox-Dependent Stability, Protonation, and Reactivity of Cysteine-Bound Heme Proteins

Cysteine-bound hemes are key components of many enzymes and biological sensors. Protonation (deprotonation) of the Cys ligand often accompanies redox transformations of these centers. To characterize these phenomena, we have engineered a series of Thr78Cys/Lys79Gly/Met80X mutants of yeast cytochrome c (cyt c) in which Cys78 becomes one of the axial ligands to the heme. At neutral pH, the protonation state of the coordinated Cys differs for the ferric and ferrous heme species, with Cys binding as a thiolate and a thiol, respectively. Analysis of redox-dependent stability and alkaline transitions of these model proteins, as well as comparisons to Cys binding studies with the minimalist heme peptide microperoxidase-8, demonstrate that the protein scaffold and solvent interactions play important roles in stabilizing a particular Cys–heme coordination. The increased stability of ferric thiolate compared with ferrous thiol arises mainly from entropic factors. This robust cyt c model system provides access to all four forms of Cys-bound heme, including the ferric thiol. Protein motions control the rates of heme redox reactions, and these effects are amplified at low pH, where the proteins are less stable. Thermodynamic signatures and redox reactivity of the model Cys-bound hemes highlight the critical role of the protein scaffold and its dynamics in modulating redox-linked transitions between thiols and thiolates