Integrated material modelling on the crashworthiness of automotive high strength steel sheets

The aim of this study is to investigate the impact of microstructure features on the crashworthiness for automotive high-strength steel sheets by using multiscale modelling approach ondifferent length scales, which provides a toolkit for the further microstructure design to meet the desired improvement of component performance. An extensive experimental program is designed involving various sample geometries that cover a wide range of stress states and tests are performed under quasi-static and high strain rate conditions and up to 2500 s-1 for an automotive dual-phase steel sheet (DP1000). The modified Bai-Wierzbicki (MBW) damage model is extended to a non-local formulation to cope with the simulations for lab and component levels. For the linking between the microstructure and mechanical properties, the representative microstructure model which considers the distributions of grain size, grain shape, crystallographic orientation and misorientation etc., is employed. The bridging between the models at different levels are powered by the virtual experiments and the entire approach is validated by lab-scale experiments and the crash box tests

A Numerical Study of the Influence of Void Growth on Ductile Fracture and Metal Extrusion

176 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987.Large deformation finite element analysis is used to study the near crack tip growth of long cylindrical holes aligned parallel to the plane of a mode I plane strain crack. The near crack tip stress and deformation fields are analyzed. The results show that the holes are pulled towards the crack tip and change their shape to approximately elliptical with the major axis radial to the crack. Several crack-hole coalescence criteria are discussed and estimates for the conditions for fracture initiation are given and compared with experimental results. The range of estimates now available from finite element calculations coincides quite well with the range of experimental data for materials containing inclusions which are only loosely bonded to the matrix.Using large deformation finite element analysis together with Gurson's constitutive model, we also study the behavior of microvoids nucleated at second phase particles during direct axisymmetric extrusion. Two different die-designs are analyzed. Comparison of the stress fields of the two die-designs provides a possible explanation of how central bursting initiates and why it appears after several steps of multi-step extrusions. The finite element results are in agreement with experimental observation and show that the finite element method can be successfully used to predict the formation of central bursts during extrusion.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Recent developments in the study of hydrogen embrittlement at the University of Illinois

This paper summarizes recent work at the University of Illinois on the fundamental mechanisms of hydrogen embrittlement. Our approach combines experimental and theoretical methods. We describe the theoretical work on hydride formation and its application to hydrogen embrittlement of titanium alloys through the stress-induced hydride formation and cleavage mechanism, the localization of shear due to solute hydrogen, and finally, we present experimental evidence that favors the decohesion mechanism of hydrogen embrittlement in a ??-Ti alloy.published or submitted for publicationis peer reviewe

Hydrogen induced shear localization of plastic flow in metals and alloys

It is well known that hydrogen enhanced localized plasticity (HELP) is a viable mechanism for hydrogen embrittlement supported by experimental observations. The objective of this work is to reveal the role of hydrogen in possibly localizing the macroscopic deformation into bands of intense shear from a solid mechanics point of view. The hydrogen effect on material deformation is modeled through the hydrogen induced volume dilatation and the reduction in the local flow stress upon hydrogen dissolution into the lattice. Hydrogen in reversible traps associated with the plastic deformation is always in local equilibrium with interstitial hydrogen that is also assumed to be always in equilibrium with local stress. The analysis of the plastic deformation and the conditions for localization are carried out in plane strain uniaxial tension. The localization criteria employed are those established by the work of Rice (1976) and Rudnicki and Rice (1975).Energy Department DEFGO2-91ER4543

Static and dynamic properties of helical spin chains

A detailed quantitative study of a classical Heisenberg chain with nearest- and next-nearest-neighbor interaction is presented. For a wide range of interaction strength, this model leads to helical structures and the effects of this peculiar structure on the static and dynamic properties is extensively discussed. The most striking feature is the presence of both a central resonance and a spin-wave peak at fairly low temperatures.

A strategy for synthetic microstructure generation and crystal plasticity parameter calibration of fine-grain-structured dual-phase steel

This study aims to establish a strategy for bridging the microstructure and mechanical properties of fine-grain-structured dual-phase steel. A complete workflow is built up commencing with the microstructure observations and characterization in both phase and grain levels by assorted experimental techniques. An assessment criterion is proposed to quantitatively examine the representativeness of synthetic microstructure models in terms of the refined microstructural features including phase fraction, grain size, grain shape, and texture for each phase of the steel. The criterion is employed to define a two-step optimization procedure for building the synthetic microstructure model for the dual-phase steel with nanoscale grain size. The crystal plasticity model is employed to describe the material deformation behavior. The corresponding material parameters are calibrated by an inverse approach combining the nanoindentation test and the macroscopic uniaxial tensile test. The simulation with the calibrated parameters and the synthetic microstructure model gives an excellent prediction of the Lankford coefficient of the dual-phase steel. Benefiting from the strategy, a virtual laboratory is conducted to investigate the micro-structure sensitivity on the mechanical properties, which serves a basis for the microstructure design with desired properties.Peer reviewe