Finite strain viscoplasticity with nonlinear kinematic hardening: phenomenological modeling and time integration

This article deals with a viscoplastic material model of overstress type. The model is based on a multiplicative decomposition of the deformation gradient into elastic and inelastic part. An additional multiplicative decomposition of inelastic part is used to describe a nonlinear kinematic hardening of Armstrong-Frederick type. Two implicit time-stepping methods are adopted for numerical integration of evolution equations, such that the plastic incompressibility constraint is exactly satisfied. The first method is based on the tensor exponential. The second method is a modified Euler-Backward method. Special numerical tests show that both approaches yield similar results even for finite inelastic increments. The basic features of the material response, predicted by the material model, are illustrated with a series of numerical simulations.Comment: 29 pages, 7 figure

El viejo y el joven en Tucídides: experiencia, aprendizaje y Temístocles

Thucydides composed his work in order to provide political education to his future readers. He wrote in a period which was fond of expert knowledge and appreciated intellectual progress. Knowledge-based education was available especially to young people. On the other hand, in his work he presents people learning from experience, a process which privileges people of a certain age. An exceptional figure is Themistocles who is characterized in a way which points to the limits of education and conveys an additional message to Thucydides’ contemporaries.Tucídides compuso su obra con el fin de proveer de educación política a sus futuros lectores. Escribió en un periodo afecto al conocimiento técnico y que valoraba el progreso intelectual. La educación basada en el conocimiento estaba disponible especialmente para la juventud. Por otro lado, en su obra presenta a los sujetos aprendiendo de la experiencia, un proceso que privilegia a gente de una cierta edad. Una figura de excepción es Temístocles, al que se representa en un modo que señala los límites de la educación y transmite un mensaje extra a los contemporáneos de Tucídides

Modelling of crack propagation in ductile materials

Crack propagation in ductile materials is a major objective of fracture mechanics with numerous applications in structural analysis and engineering mechanics. The thesis is concerned with two different methods of modelling such issues, which have been employed successfully for problems in elasticity and have drawn attention in the fracture mechanics community in the recent years. The first one utilises the configurational forces approach and considers possible extensions to elastic-plastic materials with isotropic and kinematic hardening. The analysis of calculated examples makes clear that this method is generally not an appropriate one. Mathematically, this becomes noticeable by the fact that resulting J-integral expressions are path-dependent and physically, through the knowledge that it is not an easy matter to define a crack driving force unambiguously. Hence, a second method is applied, which addresses crack propagation for ductile materials in the frameworks of phase field theories and non-conventional thermodynamics. Again, plasticity with isotropic and kinematic hardening is supposed. Simulated examples and physical considerations suggest to employ the methods and ideas of traditional continuum damage mechanics as basis for the constitutive modelling. Accordingly, a new phase field model for ductile crack propagation is proposed. The capabilities of the model are verified with reference to one-, two- and three-dimensional examples, calculated with the finite element method. The analysis of these examples reveals that the proposed model is well suited for predicting crack propagation in ductile materials, subject to complex and in particular to cyclic loading conditions

Old and Young in Thucydides: Experience, Learning, and Themistocles

Although Thucydides does not shed light on the reasons and the historical facts relating to his departure from Athens, he is certainly far clearer on the effects that this event had on his work. He explicitly recognizes that the condition of exile offered him the rather unique possibility to observe the conflict from the Peloponnesian side as well and to follow the course of events without being distracted by troubles or other activities (καθ’ ἡσυχίαν). In this study I emit the hypothesis that in 5.26 Thucydides makes an indirect allusion to the fact that a life of quietude, which liberates the eminent thinker from engagement in political life, is the path leading to intellectual production. This reading can help us recognize the intellectual affinities between Thucydides and the Athenian thinkers of the end of the fifth century BC and more particularly with Socrates’s circle

System Component Modelling of Electric Vehicles and Charging Infrastructure

The objective of this research is to develop a model for the electrical components that are involved in charging and discharging of an electric vehicle (EV). This will enable testing differ-ent energy management strategies that improve energy efficiency, battery lifetime, and ener-gy availability. Furthermore, the model will enable the investigation of vehicle to grid (V2G), thermal preconditioning of vehicles, and an economic analysis and optimization. In order to achieve the above goals, the effects that determine the performance of the infra-structure, rectifier, and battery are investigated and included as a second step in a parameter-ized model. Implementing an open loop control enables sample times of one minute and, by simplifying the process for a low runtime, multiple EVs can be included in the simulation of a smart grid. The structure is designed in a way as to support both uncontrolled and controlled charging with variable charging power and variable charging current. For the battery, a simple model approach was developed that limits the computational com-plexity and the effort for parameterization. It was found that the energy management of an electric vehicle is a complex process with battery handling being a key issue. The performance is dependent on various parameters that involve battery temperature, depth of discharge, and charge and discharge rates

Ναυτική ισχύς, ναυτικές επιχειρήσεις και η λογοτεχνική τους έκφραση από τις αρχές ως τα τέλη του 5ου αι. π.Χ.

The perception of the sea as a source of power is associated with Athens at the time of the Peloponnesian War. This paper examines the emergence, the development and the transformations of this concept from Homer to the end of the 5th century BC. A special focus is on the idea of expediency which is progressively associated with seafaring and naval activity and of the terms applied to this positive evaluation of the sea. In both literary and historical texts from the archaic and classical periods, the idea of the appropriation of the sea is pointedly formulated and specified in various ways: at an initial stage, familiarity with the sea is treated as a pre-requisite for cultural development (shipbuilding, seafaring, trade, marine fighting) and the scope of the discussion is mankind at large; after the Persian Wars, naval excellence becomes a distinct mark of the Greeks and distinguishes them from the barbarians; thus, discourse about the sea reflects an ideological stance, as national characteristics are related to a metaphysically founded moral order which goes along the geographical separation of Asia and Europe. Finally, from the Athenians’ point of view, naval progress is a constituent of their particular identity and becomes a tool for supremacy and domination in the context of their hegemony.The story of the conquest of the sea is a story of problem-solving which required the development of technical means and boosted up specific moral and intellectual qualities. Coping with the sea entails a set of skills, technical knowledge, experience and ability for self-reflection which produces a specific mentality and attitude towards the sea. Thus, the idea of domination over the sea enhances abstract thinking, given that – apart from its reference to a world reality – its construction and imposition is rooted in mental representations and evaluations

Thermodynamics and Analysis of Predicted Responses of a Phase Field Model for Ductile Fracture

The fundamental idea in phase field theories is to assume the presence of an additional state variable, the so-called phase field, and its gradient in the general functional used for the description of the behaviour of materials. In linear elastic fracture mechanics the phase field is employed to capture the surface energy of the crack, while in damage mechanics it represents the variable of isotropic damage. The present paper is concerned, in the context of plasticity and ductile fracture, with a commonly used phase field model in fracture mechanics. On the one hand, an appropriate framework for thermodynamical consistency is outlined. On the other hand, an analysis of the model responses for cyclic loading conditions and pure kinematic or pure isotropic hardening are shown