Detlef Kuhl

Energy‐conserving and decaying Algorithms in non‐linear structural dynamics

A generalized formulation of the Energy-Momentum Methodwill be developed within the framework of the GeneralizedMethodwhich allows at the same time guaranteed conservation or decay of total energy and controllable numerical dissipation of unwanted high frequency response. Furthermore, the latter algorithm will be extended by the consistently integrated constraints of energy and momentum conservation originally derived for the Constraint Energy-Momentum Algorithm. The goal of this general approach of implicit energyconserving and decaying time integration schemes is, to compare these algorithms on the basis of an equivalent notation by the means of an overall algorithmic design and hence to investigate their numerical properties. Numerical stability and controllable numerical dissipation of high frequencies will be studied in application to non-linear structural dynamics. Among the methods considered will be the Newmark Method, the classical -methods, the Energy-Momentum Methodwith and without numerical dissipation, the Constraint EnergyMomentum Algorithm and the Constraint Energy Method. Copyright ? 1999 John Wiley & Sons, Ltd.

Generalized Energy–Momentum Method for non-linear adaptive shell dynamics

In the present study, a generalization of the Energy‐Momentum Method, denoted by Generalized Energy‐Momentum Method, applied to the non-linear dynamics of shells will be developed within the framework of the Generalized-a Method. This algorithmic environment contains the unconditionally stable Energy‐Momentum Method and its numerically damped version as well as the classical Newmark and a-methods as special cases. In order to control the size of the time steps of the integration scheme with respect to accuracy and eAciency, an adaptive time stepping procedure based on local a posteriori error estimation will be improved for non-linear dynamical systems and applied to the proposed class of algorithms. The spatial discretization is realized by an eight noded finite shell element of Reissner/Mindlin type including an extensible shell director field permitting the application of three-dimensional material laws. The original formulation of this finite element will be developed for non-linear dynamic analysis and adapted for the employment within the introduced energy conserving/decaying time integration scheme. ” 1999 Elsevier Science S.A. All rights reserved.

Environmentally induced deterioration of concrete: physical motivation and numerical modeling

This paper is concerned with the effects of moisture, heat and chemical dissolution processes on the long-term behavior of concrete structures. Motivated by experimental findings described in this paper, numerical models for durability analysis of concrete structures, taking hygrally, thermally and chemically induced degradation processes into account, are described. As an example for chemically corrosive mechanisms in concrete structures, the material degradation due to calcium leaching and mechanical loading is considered in a coupled chemo-mechanical model. Furthermore, the interactions between mechanically induced damage, moisture and heat transport are taken into account in a coupled hygro-thermo-mechanical model for concrete. Results from two-dimensional simulations of a concrete panel subjected to coupled mechanical, hygral and thermal loading and coupled chemo-mechanical loading, respectively, are included as representative numerical examples.

Thermomechanical Analysis and Optimization of Cryogenic Liquid Rocket Engines

A couplednite elementuid -structure interaction analysis of regeneratively cooled rocket combustion cham- bers, which allows the computation of the coolantow and the heat conduction between the coolant and the combustion chamber structure, is presented. Furthermore, the resulting elasto-plastic deformation of the combus- tion chamber under cyclic thermal and mechanical loading is analyzed. The developed solution strategy is applied to the prediction of the heat transfer and thermomechanical load-induced deformation process of the European rocket engine Vulcain. Based on the results, the failure mechanism of the combustion chamber and its governing parameters are identied. It is demonstrated that this mechanism signicantly reduces the lifetime of the rocket engine. Besides the conceptual design by the engineer, a mathematical optimization procedure based on thenite element model of the combustion chamber is investigated. This optimization method allows the improvement of an initial design with respect to anite number of design variables such that the stress, plastic strain, or temperature levels are decreased, and accordingly, the lifetime will be increased.

Time integration in the context of energy control and locking free finite elements

SummaryIn the present paper two main research areas of computational mechanics, namely the finite element development and the design of time integration algorithms are reviewed and discussed with a special emphasis on their combination. The finite element techniques are designed to prevent locking and the time integration schemes to guarantee numerical stability in non-linear elastodynamics. If classical finite element techniques are used, their combination with time integration schemes allow to avoid any modifications on the element or algorithmic level. It is pointed out, that on the other hand Assumed Stress and Enhanced Assumed Strain elements have to be modified if they are combined with energy conserving or decaying time integration schemes, especially the Energy-Momentum Method in its original and generalized form. The paper focusses on the necessary algorithmic formulation of Enhanced Assumed Strain elements which will be developed by the reformulation of the Generalized Energy-Momentum Method based on a classical one-field functional, the extension to a modifiedHu-Washizu three-field functional including enhanced strains and a suitable time discretization of the additional strain terms. The proposed method is applied to non-linear shell dynamics using a shell element which allows for shear deformation and thickness change, and in which the Enhanced Assumed Strain Concept is introduced to avoid artificial thickness locking. Selected examples illustrate the locking free and numerically stable analysis.

Lifetime-Oriented Design Concepts

Structures deteriorate during their lifetimes, e.g. their original quality decreases. In terms of structural safety, this reduces the original safety margin, a process, which also can be described as an increase of structural damage. If, in such deterioration, the safety parameter decreases below the admissible safety limit, or the structural damage parameter increases beyond the admissible damage limit, then the structural service life will be terminated. If the failure safety value or the structural damage parameter both reach unity, the structure (theoretically) will fail.

Deterioration of Materials and Structures: Phenomena, Experiments and Modelling

Reliable computational prognoses of the structural integrity and serviceability throughout the lifetime of structures require the realistic consideration of the damage behaviour of the construction materials for various loading scenrios including static and cyclic loading, environmental loading processes such as moisture and heat transport, corrosion processes, freeze-thaw actions and possible interactions between these long- and short-term processes. Both, load-induced damage mechanisms such as evolving microcracks and physically and chemically induced deterioration originate from mechanical, physical and chemical processes starting at lower scales of the microstructure of the materials. Investigating and understanding these processes acting at various scales is a prerequisite for the development of adequate and suitable material models suitable for life-time oriented simulations.

Damage-Oriented Actions and Environmental Impact on Materials and Structures

Mechanical loading and ambient actions on civil engineering structures and components cause lifetime-related deteriorations. Not the rare extreme loading events are in the first place responsible for the evolution of structural degradation but the ensemble of load effects during the life-time of the structure. It is of major importance to have models at hand which adequately reflect the experienced time histories of impacts, and which can include justified predictions of future trends. Leading types of loading and load-effects with relation to mechanical fatigue as well as damages due to hygro-thermal and chemical impacts are considered in this chapter. Selected contributions from wind and temperature effects with certain meteorological characteristics as well as from traffic loads on roads and railway lines are modeled as typical examples of contributions to mechanically induced degradations of structures. A specific aspect is the permanent settlement of soil due to high-cyclic, longterm loading, for which novel representations are developed. The attack of freeze-thaw circles in different environments and of chemical impacts leading to solving, swelling and leaching processes in concrete including principle interactions are discussed as examples for the main types of non-mechanically induced degradations.

Computational Solution of the Inverse Heat Conduction Problem of Rocket Combustion Chambers

A method enabling the calculation of transient rocket combustion chamber hot gas - and coolant side heat transfer based on the temperature measurement by thermocouples, positioned within the combustion chamber wall, is described. The applied novel computational concept specifies the boundary conditions of the combustion chamber inverse heat conduction problem. Therefore, a finite element discretization of the energy balance equation of the combustor, a finite difference discretization of the related semidiscrete equation of transient heat conduction in the time domain and a conjugate direction optimization method are used. The uniqueness of the solution, the error sensitivity and the performance of the proposed parameter identification method will be investigated by an analysis of the stead state as well as transient heat transfer of DLRs model combustor B, a high pressure H2/O2 model rocket engine operated at the test facility P8 in Lampoldshausen, Germany.

Quantifying Cytoskeletal Morphology in Endothelial Cells to Enable Mechanical Analysis

Wall shear stress induced remodelling of endothelial cell morphology is a focal cause of atherosclerosis. While the force distribution within endothelial cells has been quantified using computational modelling, no studies have included an image-informed cytoskeleton, nor have any studies examined the effect of population variation of the cytoskeleton. In this paper we quantified the spatial variation of the cytoskeleton and primary cilium in a population of endothelial cells to enable future mechanical analysis.