Large natural strains and some special difficulties due to non-linearity and incompressibility in finite elements

The finite element method is now a well established tool for the routine treatment of large linear problems, but the treatment of non-linear problems by the method is yet at the beginning.Section 1 of the present work extends the idea of natural strains and stresses to large strains in simplex finite elements.Section 2 applies some algorithms developed for structural dynamics to the problem of non-linear wave propagation including a case of shock development.Section 3 discusses with numerical examples special difficulties when displacement finite elements are used to solve problems with incompressible or nearly incompressible material.     

On large displacement-small strain analysis of structures with rotational degrees of freedom

The matrix displacement analysis of geometrically nonlinear structures becomes an intricate task as soon as finite elements in space with rotational degrees of freedom are considered. The fundamental reason for these difficulties lies in the non-commutativity of successive finite rotations about fixed axes with different directions. In order to circumvent this difficulty, a new definition of rotations — the so-called semitangential rotations — is introduced in this paper. Our new definition leads to a reformulation of the theory of [1,2]which in itself is clearly consistent and correct.In contrast to rotations about fixed axes these semitangential rotations which correspond to the semitangential torques of Ziegler [3]possess the most important property of being commutative. In this manner, all complexities involved in the standard definition of rotations are avoided ab initio.A specific aspect of this paper is a careful exposition of semitangential torques and rotations, as well as the consequences of the semitangential definitions for the geometrical stiffness of finite elements. In fact, these new definitions permit a very simple and consistent derivation of the geometrical stiffness matrices. Moreover, the semitangential definition automatically leads to a symmetric geometrical stiffness which clearly expresses that the nonlinear strain-displacement relations must satisfy the condition of conservativity of the structure itself — independently of any loading.The general theory of geometrical stiffness matrices as evolved in this paper is applied to beams in space. The consistency of the theory is demonstrated by a large number of numerical examples not only of straight beams but also of the lateral and torsional buckling and post-buckling behaviour of stiff-joined frames. Most of the former developments appear to be inadequate.     

Finite element analysis of inelastic structural behaviour

The paper examines computational aspects of the numerical solution of dissipation and diffusion problems which describe inelastic mechanical processes and transport mechanisms of transient environments. Both phenomena are rate processes which lead in general to a system of coupled first order differential equations after finite element idealization of the spatial dependence.     

A new iterative solution for structures and continua with very stiff or rigid parts

Structures with widely different orders of stiffness or nearly incompressible continua are not uncommon in structural analysis and give considerable computational difficulties when using the matrix displacement method on a 32 or 36 bit word computer. This paper presents three approaches to overcome these problems. The first one is based on the idea of the natural method. The second one makes exclusively use of the widely used cartesian notation. Finally the third one considers structures with only a few rigid members. One central aim of this paper is to implement the developed theoretical tools in standard finite element packages. The paper has illustrative hand and computer solutions and the appendix presents the underlying rigorous mathematical proofs.     

The natural factor formulation of the stiffness for the matrix displacement method

The natural formulation of the elemental stiffness has proved a most significant interpretation of the elastic behaviour of an element, both from the physical and mathematical point of view. It yields not only concise and elegant matrix expressions for the elements — especially for simplex elements — but leads effortlessly to the non-linear effects associated with large displacements. In the present paper the basic idea is brought to a logical conclusion by the direct construction of the factorised form of the elemental and overall stiffness matrices from their natural stiffness roots. This technique is denoted as the natural factor formulation. It is believed that the new approach leads to a better understanding of the relevant error analysis so powerfully initiated by Rosanoff et al.; see e.g. [20]. The natural factor formulation may also be shown to retain more numerical information of the idealised physical system [21, 28]. Both static and dynamic problems are considered here. For additional information the reader is referred to [16].     

Unified concepts of constitutive modelling and numerical solution methods for concrete creep problems

In the first part of the paper an internal variable model is developed for concrete creep with the following features: creep strain is a linear functional of stress history; application or removal of stress produces instantaneous elastic response; upon removal of stress, creep deformations partially recover, approaching in the limit an irrecoverable part. The mechanism of each creep component is affected be temperature, humidity and loading age.In the second part the internal state variables are derived from concrete creep data with loading ages ranging from 7 to 4560 days and temperatures from 20° to 95°C. Several aging models are examined for which the parameters are determined via least squares optimization. A root mean square criterion is used to assess the approximation.In the third part a step by step algorithm is developed for incremental solution of the time-dependent creep process, noting that the governing equation of evolution lends itself to an economic computer solution without storing previous results. The one-dimensional constitutive statements are extended to multiaxial conditions and subsequently projected from the local level onto the structural level via finite elements (incremental initial load method).In the fourth part the creep behaviour of a prestressed concrete reactor vessel is examined for a loading history which includes hygrothermal effects due to drying and heating as well as prestress and cycling pressure.     

Sustainable energy policy indicators: Review and recommendations

Nowadays, the development of a sustainable indicators’ framework towards the sustainable energy policy making should be characterized by clarity and transparency. Even though the energy policy making has been the subject of many researchers, studies proposing an appropriate framework of sustainable indicators that have to be used are not present in the international literature. The purpose of the current paper is to present an integrated review of the methodologies and the related activities of the energy indicators and to recommend an operational framework of appropriate indicators supporting thus the policy makers/analysts/citizens towards a sustainable energy policy making.     

欧盟可再生能源现状、未来发展与挑战

据估计，欧盟的可再生能源在欧盟内部总能耗中的份额将从1992年的5．3％提高到2020年的6．2％。到2030年这个比例仍将小于7％。而白皮书中规定的战略和行动计划将力图使欧盟的2010年可再生能源份额达到12％的目标。另外各国国内的可再生能源发展目标应该和欧盟目标中可再生能源电力份额从1997年的13，9％增加到2010年的22．1％的目标一致。发展可再生能源是欧盟能源政策的中心目标，因为可再生能源在欧盟的主要目标——减少二氧化碳的排放中扮演着一个重要的角色。而且它还将通过减少欧盟对进口能源的依赖增进能源供应安全性。从中长期来看，和常规能源相比，可再生能源在经济上具备竞争力。文章就可再生能源在欧盟的发展现状和未来发展进行论述     

The policy perspective for RES development in the new member states of the EU

The energy sector of the European Union (EU) is based mainly on fossil fuels and almost two-thirds of the energy demand is imported. In the future, fossil fuels are expected to strengthen their participation in the European energy balance and imports will amount to 70% of overall energy needs. On the other hand, the development of Renewable Energy Sources (RES) is a central aim of the European Commission's energy policy. RES hold a significant share in the 10 new countries recently joined the EU and the adoption of RES-related policies is of imperative importance for the joint cooperation of the new member states with the EU. This paper includes a review of RES status, related policies and prospects in each of the 10-new member states of EU.