A distortional hardening model for finite plasticity

Plastic anisotropy may strongly affect the stress and strain response in metals subjected to multiaxial cyclic loading. This anisotropy evolves due to various microstructural features. We first use simple models to study how such features result in evolving plastic anisotropy. A subsequent analysis of existing distortional hardening models highlights the difference between stress- and strain-driven models. Following this analysis, we conclude that the stress-driven approach is most suitable and propose an improved stress-driven model. It is thermodynamically consistent and guarantees yield surface convexity. Many distortional hardening models in the literature do not fulfill the latter. In contrast, the model proposed in this work has a convex yield surface independent of its parameter values. Experimental results, considering yield surface evolution after large shear strains, are used to assess the model\u27s performance. We carefully analyze the experiments in the finite strain setting, showing how the numerical results can be compared with the experimental results. The new model fits the experimental results significantly better than its predecessor without introducing additional material parameters

A COMPARISON OF TWO FRAMEWORKS FOR KINEMATIC HARDENING IN HYPERELASTO-PLASTICITY

In this work we compare two frameworks for thermodynamically consistent hyperelasto-plasticity with kinematic hardening. The first was formulated by Dettmer and Reese (2004), inspired by Lion (2000), and has been used to model sheet metal forming. The second, formulated by Wallin et al. (2003), has been used to model large shear strains and cyclic ratcheting behavior of pearlitic steel (Johansson et al. 2006). In this paper we show that these frameworks can result in equivalent models for certain choices of free energies. Furthermore, it is shown that the choices of free energy found in the literature only result in minor differences. These differences are discussed theoretically and investigated numerically

Efficient 3d finite element modeling of cyclic elasto-plastic rolling contact

Railway rails accumulate large plastic deformations due to cyclic rolling contact loading. The plastic deformations alter the rail geometry, affect material behavior, and cause crack formation and growth. The complex interactions between these phenomena require high fidelity simulations to be understood. 3d finite element simulations are accurate, but their computational cost limits the possible number of simulated cycles. We propose a cyclic finite element simulation in which the wheel and rail remain in contact throughout the simulation. It uses periodic boundary conditions, shadow elements, and model reductions. Compared to previous work, it is 25 times faster. The method is available as an open-source plugin to Abaqus, enabling other researchers to study rolling contact loading coupled with large plastic deformations

Investitionen, Weiterbildung und betriebliche Reorganisation (Investments, further training and company reorganization)

"The correlations between investments, reorganization and further training existing at company level are examined in this paper. A production theory approach in conjunction with an optimisation calculation of the companies shows that the variables focused on here are mutually dependent on one another. If the model analysis is expanded by a few plausibility considerations on possible company developments, the initially simple and clearly manageable structure becomes more complex and counter-effects can occur. This is why it is not possible to formulate any clear hypotheses on the interdependency effects of the variables of investments, reorganization and further training as a point of departure for an empirical analysis. The empirical study is based on the data of the IAB establishment panel of the years 1997-2000. The focus of research is a simultaneous triangular equation model with further training, reorganization and investments as the variables to be studied. The key results are: there is a mutually interdependent relationship between reorganization measures and investments. Although further training is encouraged by real capital investments, the reverse correlation cannot be found. Clear relationships between further training and reorganization measures only exist if individual measures are analysed. All in all, simultaneous relationships between investments, reorganization measures and further training schemes can be empirically proven. Delayed adjustment processes partly interfere with them." (Author's abstract, IAB-Doku) ((en))organisatorischer Wandel, IAB-Betriebspanel, Unternehmensorganisation, Arbeitsorganisation, Investitionen, betriebliche Weiterbildung, Produktinnovation, Westdeutschland, Ostdeutschland, Bundesrepublik Deutschland

COLAB : a hybrid knowledge representation and compilation laboratory

Knowledge bases for real-world domains such as mechanical engineering require expressive and efficient representation and processing tools. We pursue a declarative-compilative approach to knowledge engineering. While Horn logic (as implemented in PROLOG) is well-suited for representing relational clauses, other kinds of declarative knowledge call for hybrid extensions: functional dependencies and higher-order knowledge should be modeled directly. Forward (bottom-up) reasoning should be integrated with backward (top-down) reasoning. Constraint propagation should be used wherever possible instead of search-intensive resolution. Taxonomic knowledge should be classified into an intuitive subsumption hierarchy. Our LISP-based tools provide direct translators of these declarative representations into abstract machines such as an extended Warren Abstract Machine (WAM) and specialized inference engines that are interfaced to each other. More importantly, we provide source-to-source transformers between various knowledge types, both for user convenience and machine efficiency. These formalisms with their translators and transformers have been developed as part of COLAB, a compilation laboratory for studying what we call, respectively, "vertical\u27; and "horizontal\u27; compilation of knowledge, as well as for exploring the synergetic collaboration of the knowledge representation formalisms. A case study in the realm of mechanical engineering has been an important driving force behind the development of COLAB. It will be used as the source of examples throughout the paper when discussing the enhanced formalisms, the hybrid representation architecture, and the compilers

Modeling and experimental characterization of large biaxial strains and induced anisotropy in pearlitic rail steel

Large shear strains accumulate in the near-surface region under the running band of railway rails. In this region, rolling contact fatigue cracks often initiate, causing major problems for the railway industry. However, characterization of the constitutive and fatigue behavior of the material in this region is difficult due to the high strain gradient. The solution proposed in this thesis is to produce highly deformed cylindrical test bars: An axial-torsion test rig is used to predeform the bars in torsion while subjected to axial compressive loading. The obtained material state is found to be similar to that of field samples of rails at a depth between 50 and 100 μm. Using this predeformation method, the evolution of the yielding behavior is evaluated. The predeformed test bars are re-turned and drilled out to form thin-walled test bars, which can be used to measure yield surfaces. It is found that the degree of anisotropy quickly evolves with increasing predeformation and then saturates. Furthermore, the quadratic Hill yield criterion describes the anisotropic yield surfaces well. To better optimize rail maintenance and material selection, there is an industrial need for a model capable of predicting rail deterioration. An important component of such a model is an accurate material model that captures the relevant physical phenomena. A hyperelasto-plastic framework for finite strain material models is adopted in this thesis. As a first study, the predeformation method was simulated using 2D axisymmetric elements. It is shown that very good results can be achieved by using material models with advanced kinematic hardening laws. Next, an improved simulation methodology for axial, torsional and pressure loading is developed, resulting in an efficient 1D formulation. This methodology includes material removal to simulate the re-machining of the test bars into tubular bars. Using this methodology, 3 different distortional hardening models are evaluated in terms of how well they fit and predict the experimental data. The two phenomenological models perform better than the crystal plasticity model. However, these models should be further developed to improve their predictive abilities

A method for in-field railhead crack detection using digital image correlation

Railway infrastructure managers must decide when and how to maintain rails. However, they often have insufficient information about railhead cracks. Therefore, we propose a new method for rail crack detection using a train-mounted digital image correlation (DIC) camera system. The measurement train\u27s weight cause rail bending, allowing the DIC to measure strain concentrations caused by surface-breaking cracks. In this study, we evaluate the method under laboratory conditions. The detected cracks correlate to the actual crack network in the analysed rail field sample. Furthermore, finite element simulations show the method\u27s high sensitivity to crack depths. Existing methods, such as ultra-sonic and eddy-current, produce damage severity indications. The proposed method complements these techniques by providing a discrete description of the surface-breaking cracks and their depth. This information enables infrastructure managers to optimize rail maintenance. Additionally, such detailed measurements can be valuable for research in railhead damage evolution

Effects of Dwell Time on the Deformation and Fatigue Behaviour of A356-T7 Cast Aluminium Alloys Used in High Specific Power IC Engine Cylinder Heads

The electrification of automotive powertrains in recent years has been driving the development of internal combustion engines towards reduced volumes with higher power outputs. These changes place extreme demands on engine materials. Engineers employ the computer-aided engineering approach to design reliable and cost-effective engines. However, this approach relies on accurate knowledge of the material deformation and fatigue characteristics during service-like loading. The present study seeks to investigate the effect of dwell times on the deformation and fatigue behaviour of the A356-T7 + 0.5 wt.% Cu alloy used to cast cylinder heads. In particular, we study the effect of dwell time duration at various temperatures. A combined fatigue-dwell testing procedure, with the dwell at the maximum compressive strain, replicates the service conditions. It is found that the material exhibits a stress relaxation behaviour with a decreasing relaxation rate. At lower temperatures, the load level influences the relaxation more than at elevated temperatures. However, the dwell does not significantly affect the hardening behaviour or the life of the tested alloy. Finally, we model the time-dependent material behaviour numerically. The Chaboche model, combined with a Cowper–Symonds power-law, is found to capture the visco-plastic deformation behaviour accurately

Berichte aus dem Ökolandbau

In diesem Heft der Schriftenreihe des Landesamtes für Umwelt, Landwirtschaft und Geologie sind folgende drei Beiträge enthalten: Einfluss des Klimawandels auf Humus- und Stickstoffvorräte im Boden sowie Kompensationsmöglichkeiten durch den Ökologischen Landbau. Am Beispiel von drei Agrarstrukturgebieten in Sachsen wurden zunächst mit Hilfe mehrerer Methoden der Humusbilanzierung die Auswirkungen des Klimawandels bis zum Jahr 2050 ermittelt. Die Ergebnisse zeigen, dass die Humusgehalte auf den Diluvial- und Lößböden leicht und den Verwitterungsstandorten der Bergregionen deutlich abnehmen können. Durch verschiedene Formen des ökologischen Landbaus (Ist-Situation, Marktfrucht, Futterbau, organische Düngung, Umwidmung zu Grünland) kann diesem Trend in unterschiedlichem Ausmaß entgegengewirkt werden. Nähr- und Schadstoffgehalte von Wirtschaftsdüngern in Sachsen. In den Jahren 2006 und 2007 wurde eine umfassende Erhebung an Wirtschaftsdüngern von ökologisch wirtschaftenden Betrieben in Sachsen durchgeführt. Von den Hauptnährstoffen wurden die Gehalte an Stickstoff, Phosphor, Kalium, Magnesium, Calcium, Natrium und Schwefel ermittelt. Durch die Ergebnisse wurden im Vergleich zu konventionellen Standardwerten niedrigere N- und K-Gehalte in den Wirtschaftsdüngern ökologischen Ursprungs bestätigt. Auch reicht das Datenmaterial erstmals aus, um die NH4-N-Anteile in den Düngemitteln genauer abschätzen zu können. Die hier präsentierten Untersuchungsergebnisse können zur weiteren Verbesserung von ökologischen Richtwerten genutzt werden. Gleichfalls durchgeführte Untersuchungen zum Schwermetall- und Arsengehalt der Wirtschaftsdünger ergaben für die überwiegende Mehrzahl der untersuchten Proben lediglich geringe Belastungen. Eignung nichtlegumer Zwischenfrüchte vor dem Anbau von Körnerleguminosen. Im Jahr 2008 wurden an drei Standorten in Sachsen Feldversuche zur Eignung nichtlegumer Zwischenfrüchte für Systeme der Mulch- und Direktsaat von Körnerleguminosen durchgeführt. Von den geprüften Zwischenfrüchten Sommerroggen, Schwarzhafer, Sonnenblume, Weißer Senf, Buchweizen und Hanf waren nahezu unabhängig vom Standort insbesondere Sommerroggen und Hafer aufgrund guter Unkrautunterdrückung und hohem Sprossertrag am besten geeignet