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Kadkhodapour J.,Isfahan University of Technology | Butz A.,Fraunhofer Institute for Mechanics of Materials | Ziaei Rad S.,Isfahan University of Technology
Acta Materialia | Year: 2011

A detailed analysis of the microstructure and failure mechanism of a dual-phase steel material as a function of strain was conducted. Accordingly, three tensile tests were performed and interrupted at different strain levels in order to investigate void nucleation, void growth and void coalescence. Scanning electron microscopy analysis revealed that void nucleation occurs by ferrite grain-boundary decohesion in the neighborhood of martensite grains. Further, void initiation could be observed between closely situated martensite grains. Martensite morphology and distribution has a significant impact on the accumulation of damage. The mechanism of failure was found to be influenced by deformation localization due to microstructural inhomogeneity. Based on the experimental observations and simulation results, a model describing the failure mechanism is proposed for dual-phase steel material. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Greve L.,Volkswagen AG | Fehrenbach C.,Fraunhofer Institute for Mechanics of Materials
Journal of Power Sources | Year: 2012

A quasi-static mechanical abuse test program on cylindrical Lithium ion battery cells has been performed at a state of charge (SoC) of 0%. The investigated load cases involved radial crushing, local lateral indentation and global three-point bending of the cell. During the tests, the punch load, the punch displacement, the cell voltage and the temperature development of the cell have been monitored using an infrared camera and temperature sensors. After the test, the cells have been analysed using computer tomography. It is indicated that macroscopic jelly roll fracture on a global scale initiates the internal short circuits, revealed by a sudden decrease of the global mechanical load due to the rupture, followed by a drop of the measured voltage and immediate increase in cell temperature. A macro-mechanical finite element crash simulation model has been established for the cell housing and the jelly roll. The classical stress-based criterion after Mohr and Coulomb (MC) has been applied to predict fracture and the initiation of an internal short circuit of the jelly roll. The MC criterion correctly represents the punch displacement to fracture, where the predicted fracture locations correspond to the observed locations of the internal short circuits of the cells. © 2012 Elsevier B.V. All rights reserved.

Schmidt I.,Fraunhofer Institute for Mechanics of Materials
Computational Mechanics | Year: 2011

This article presents the details of a numerical technique for computing the macroscopic response of a material with a given micro-structure to arbitrary prescribed loading histories. The method uses classical concepts of homogenisation theory in combination with the finite element method and focusses on the computation of macroscopic yield surfaces and inelastic strain rates. It places no restrictions on themagnitude of deformation and allows arbitrary combinations of stress- or strain control including the prescription of histories of the Cauchy stress. The method is illustrated by analysing a model material, consisting of a non-linearly elastic matrix with stiff elasto-plastic inclusions, which exhibits macroscopically associative elastoplastic material behaviour with finite elastic strains. Yield surfaces and the directions of plastic flow after a prior finite simple shear deformation are computed for this material and are shown to be consistent with an additive decomposition of the Eulerian strain rate into elastic and plastic parts and asuitable formulation of the normality rule in Cauchy stress space; a novel version of the latter is derived which is valid for an arbitrary reference configuration. © 2011 Springer-Verlag.

Korner W.,Fraunhofer Institute for Mechanics of Materials | Elsasser C.,Fraunhofer Institute for Mechanics of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We present a first-principles density functional theory study of doped ZnO with focus on its application as a transparent conducting oxide, having both high optical transparency and high electrical conductivity. Investigated is the impact of grain boundaries on the physics of atomic defects, and especially the formation energies of oxygen vacancies, cation dopants Al and Ga, and anion dopants N and P are determined. The main goal is to obtain information about the positions of the defect levels generated by the different dopants in the electronic band gap. Because of the known deficiency of the local density approximation (LDA) to yield accurate values for band gap energies for insulators such as ZnO a self-interaction correction (SIC) to the LDA is employed. As atomistic supercell models which contain grain boundaries and dopants are quite large in size we implemented the SIC by means of SIC pseudopotentials which merely increase the computational costs, as compared to the LDA. The main result of our study is that grain boundaries do affect the formation energies for substitutional dopants significantly. Furthermore the position and shape of dopant-induced electronic energy levels at the grain boundaries are changed considerably with respect to the single crystal. This may help us to explain, for example, why N doping can lead to p conductivity at room temperature or why Al or Ga doping can increase the transparency. © 2010 The American Physical Society.

Seifert T.,Fraunhofer Institute for Mechanics of Materials | Riedel H.,Fraunhofer Institute for Mechanics of Materials
International Journal of Fatigue | Year: 2010

In the present paper, mechanism-based models are developed to describe the time and temperature dependent cyclic plasticity and damage of cast iron materials. The cyclic plasticity model is a combination of a viscoplastic model with kinematic hardening and a porous plasticity model to take the effect of graphite inclusions into account. Thus, the model can describe creep, relaxation and the Bauschinger-effect as well as the tension-compression asymmetry often observed for cast iron. The model for thermomechanical fatigue life prediction is based on a crack growth law, which assumes that the crack growth per cycle, da/dN, is correlated with the cyclic crack-tip opening displacement, Δ CTOD. The effect of the graphite inclusions on crack growth is incorporated with a scalar factor into the crack growth law. Both models can describe the essential phenomena which are relevant for cast iron materials. © 2010 Elsevier Ltd. All rights reserved.

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