The implementation of finite element methods (FEM) for fatigue analysis of complex structures in industry are becoming an increasingly effective and accepted practice. In the case of large plate-like structures, such as Load Haul Dumper (LHD) equipment, constructional frames and supports in plants and heavy vehicle trailers to name but a few, modeling can take place by implementation of either two dimensional shell elements or three dimensional solid elements. It is, however, not clear which shell element modeling procedure is the most realistic. Solid elements are accepted to give the closest resemblance since the element itself is the closest to reality in terms of geometry and also due to the fact that it is a three dimensional element. Due to economical and practical considerations, however, shell elements are used in industry - especially in large, plate-like structures. Another primary source of uncertainty lies with the definition of nominal stress in complex structures and the correct determination and extraction thereof from finite element obtained stress distributions. This situation occurs as a consequence of the absence of clear and distinct guidelines in the nominal stress based fatigue design codes such as BS 7608:1993; ECCS 6: 1985 and IIW XIII-1965-03 on weld modeling and nominal stress extraction procedures in conjunction with FEM. Explicit guidelines for finite element modeling and fatigue relevant stress determination do exist in the IIW fatigue design recommendations on top of the nominal stress guidelines, but focus primarily on the implementation of the hot spot stress fatigue assessment procedures. This dissertation consequently entails the development of a nominal stress extraction procedure for fatigue design and analysis of plate-like structures, utilizing shell elements. Firstly, the integrity of shell elements as concerned with the accurate capturing of the stiffness properties and stress distribution in the vicinity of welds are investigated, with the aim of establishing a set of guidelines and recommendations for the correct meshing and modeling procedure of welds in plate-like structures. Secondly, an extensive numerical investigation into the stress concentration characteristics of various T-piece and stiffener configurations is performed, resulting in a nominal stress extraction procedure. The developed methodology is applied on a complex plate-like structure for verification purposes. The structure is modeled by means of a finite element model, compiled according to the meshing recommendations developed. The stress distribution due to static loading is investigated and compared with measured values. Furthermore, the stress response of the structure due to stochastic dynamic loading is investigated and also validated in terms of the suitability for assessment by static equivalent design criteria, in particular the Fatigue Equivalent Static Load (FESL) methodology. A nominal stress and hot spot stress fatigue life prediction under stochastic loading is made, based on measured stresses in conjunction with the developed stress extraction methodology and the IIW guidelines respectively. Furthermore the finite element stresses are implemented in conjunction with the FESL procedure to repeat the nominal stress and hot spot stress life predictions. The viability and integrity of the FESL methodology is also critically assessed. The actual fatigue life of the structure under the particular loading characteristics is then determined and compared to the predicted lives. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / MEng / unrestricted

Il s'agit dans ce travail de développer une démarche multiéchelles, afin d'identifier puis de modéliser le comportement mécanique d'un matériau à gradient de propriétés (un cordon de soudure) en relation avec les hétérogénéités microstructurales responsables de la déformation et de l'endommagement par fluage des aciers P91, matériaux retenus pour des applications de tenue mécanique à chaud des centrales thermiques à flamme. Cette étude porte sur l'utilisation des techniques de mesure de champs cinématiques par corrélation d'images couplées aux simulations numériques par éléments finis. Les différentes zones d'intérêt sont : le métal de base, l'ICHAZ (intercritical heat affected zone), la FGHAZ (fine grain heat affected zone), la CGHAZ (coarse grain heat affected zone) et le métal fondu. L'identification a porté dans un premier temps sur le comportement en traction et traction–relaxation à l'ambiante et à 625°C. Cette phase a permis d'établir une loi de comportement pour chacune des zones du joint soudé et de prédire la zone de localisation de déformation pour une gamme assez large de vitesse de sollicitation.Une attention particulière a ensuite été portée aux essais de fluage, pour lequel le glissement intergranulaire joue un rôle essentiel dans le comportement du cordon de soudure et notamment dans l'ICHAZ. Afin de mettre en évidence le glissement intergranulaire, une technique de microlithographie a été adoptée. L'effet de ce mécanisme sur le comportement macroscopique et à l'échelle locale a été étudié au moyen de simulations numériques par éléments finis avec introduction de zones cohésives. Pour cela une loi cohésive a été élaborée et validée sur une configuration simplifiée à quatre grains. Ce modèle permet également de rendre compte de la transition fluage dislocation – fluage diffusion en retardant le déclenchement du glissement aux joints de grains quand la vitesse de sollicitation augmente.Cette étude est financée par la Chaire EDF-GDF-SUEZ-GRT gaz - ENSMP –ENPC sur la «durabilité des matériaux et des structures pour l'énergie ». / The goal of this study is to develop a micromechanical approach, to identify and model the elementary mechanisms in connection with the microstructural heterogeneities accountable for the deformation and for the creep damage of P91 steels, material used for applications at high temperature in thermal power plants. This study deals with the use of kinematic full-fields measurements by digital image correlation coupled with finite element numerical simulations for the identification of constitutive equation parameters of materials presenting a strongly heterogeneous microstructure, leading to gradients of mechanical properties. The welded joint in P91 presents various microstructures along the joint: base metal, ICHAZ (intercritical heat affected zone), FGHAZ (fine grain heat affected zone), CGHAZ (coarse grain heat affected zone) and weld metal, which will be characterized by uniaxial tensile tests and tensile-relaxation tests performed at room temperature and at 625°C. These experimental tests enable to establish a constitutive law for each part of the welded joint and for a wide range of strain rate.A particular attention has then been focused on the behaviour at high temperature (creep) for which the intergranular glide plays an essential role, in particular within the ICHAZ. To characterize the grain boundary gliding mechanisms, a technique of microlithography is used. This mechanism is modelled with cohesive zones. A cohesive constitutive equation has been proposed and validated with a simplified four grain configuration. This model is able to take into account the dislocation creep – diffusion creep transition by delaying the beginning of the grain boundaries sliding when the strain rate increases.This study is granted by the EDF-GDF-SUEZ-GRT gas – Mines ParisTech – Ecole des Ponts ParisTech chair on the "durability of materials and structures for the energy ".