Eckart Weiß

Finite element analyses concerning the fatigue strength of nozzle-to-spherical shell intersections

This paper deals with the problem of the fatigue strength of nozzle-to-spherical shell intersections subjected to internal pressure, axial force, bending and torsional moments. General aspects of the proof of fatigue strength and stress analysis are discussed first. Finite element analyses have been done for the whole range of practically important geometries. The results for flush and protruded nozzles are given as stress concentration factors (SCF) representing the maximum von Mises stress in the intersection region referred to an appropriate nominal stress. Comparisons are made with the curves given in BS 5500 and with analytical results.

Linear and nonlinear finite-element analyses of pipe bends

Pipe bends are widely used as connecting elements in chemical plants. They are often submitted to high pressure loading and supplementary bending and torsional moments. The present paper deals with the load carrying capacity and fatigue strength of bends based on finite-element analyses. The resulting diagrams for internal pressure loading and supplementary loads can easily be used for design purposes. The effect of superposition is analysed with respect to fatigue strength.

Fatigue lifetime assessment procedures for welded pressure vessel components

Design against fatigue of welded pressure vessel components is usually based on structural or notch stress approaches. Post weld treatment methods such as grinding or TIG dressing provide significant improvements of the fatigue behaviour by blunting of the weld toe notches and by using materials of higher strength. Although well established on the level of welding technologies, fatigue design rules for improvement methods are not yet available within design codes. The local strain approach can be applied as a method of lifetime assessment in these cases. The methodology is demonstrated for cylinder-to-cylinder intersections and butt welded joints as typical pressure vessel structures. Concept-conforming modelling and numerical stress and strain analysis are relevant modules of the procedure. The derivation of fatigue curves for special weld details is shown under the aspect of practical application. Once determined, these fatigue curves can be used in the same sense as the traditional nominal stress approach.

Failure modes of pressure vessel components and their consideration in analyses

Abstract Metallic pressure vessel components such as dished ends show different modes of failure depending upon the geometrical and loading conditions. These are mainly gross plastic deformation under static load, loss of stability (buckling), fatigue crack initiation at highly stressed locations under cyclic loading (especially in the low cycle regime), progressive plastic deformation (ratcheting) and creep at high temperatures. Existing codes are mostly used to determine necessary wall thickness (design by rule) based on the static load-carrying capacity. Guidelines for other failure modes are given in a more or less general way for complex structures. In this article it is shown that all the failure modes mentioned can be estimated by carrying out linear and non-linear finite elements analyses. The possibility of using the finite elements method as a basis for design by rule and design by analysis is discussed.

Modeling of welded joints for design against fatigue

Historically, design against fatigue of welded joints is widely based on the nominal stress approach with categorized structural details and appropriate S-N-curves without any need of detailed numerical analysis. Recently, local approaches based on structural and notch stresses are of growing importance for the lifetime assessment of components without post-weld treatment for dominant elastic material behavior in high cycle fatigue (HCF), and for design against the endurance limit. Finally, high quality weld seams submitted to post-weld treatment such as grinding and TIG or plasma dressing and/or operating conditions in low cycle fatigue (LCF) with high plastic deformations are advantageously considered within a strain-based approach. A concept-conforming modeling strategy as a relevant module of lifetime assessment is required to ensure reliable and reproducible analysis results for all local approaches of design against fatigue. A unique modeling strategy based on an adaptive nominal weld seam contour is derived covering a wide range of typical welded pressure vessel joints for stress and strain based approaches. An alternative method for the determination of structural stresses is proposed to avoid the uncertainties of extrapolation and description of structural stiffness.

Strategie zur festigkeitsmäßigen Auslegung von Druckbehälterkomponenten

Das Sicherheitsgebot ist fur Ausrustungen der Chemie- und Kraftwerkstechnik nach wie vor gultig. Der strukturelle Wandel in der apparatebauenden Branche und die gegenwartige Umstellung auf europaische und internationale Normen bergen Gefahren in sich, die sich letztlich in der Anlagensicherheit niederschlagen konnen. Die verfahrenstechnisch determinierten Ausrustungen sind in der Regel kein Massenprodukt mit einem festgeschriebenen Sicherheitsstandard. Sie sind im Gegenteil spezifizierten Belastungen ausgesetzt und beanspruchungsmasig als Unikat zu betrachten. Der Festigkeitsnachweis ist ein absolutes Gebot. Der Beitrag vermittelt einige allgemeine Hinweise, was in der Auslegungspraxis beachtet werden sollte und auf welche Tendenzen sich die Praxis einstellen muss. Die Differenz zwischen Forschung und Praxis muss durch anwenderbezogene Ergebnisdarstellung uberwunden werden, wofur einige Beispiele dargelegt werden. Strength-orientated Design Strategy for Pressure Vessel Component Safety of equipment destined for the chemical and power generation industries is still mandatory. Structural changes in the equipment manufacturing industry and the current switch-over to European and international standards harbour potential dangers which can ultimately manifest themselves in plant safety. Items of equipment used in process engineering are generally not mass produced articles with a generally valid safety standard. In contrast, they are subjected to specified loads and in this respect each one to be regarded as one of a kind. Strength testing is a mandatory requirement. This article considers a number of general points demanding attention in design work and future trends affecting design practice. The gap between research and everyday practice has to be bridged by applications-orientated presentation of results. The paper presents a number of illustrative examples.

Fatigue behaviour of oblique nozzles on cylindrical shells submitted to internal pressure and axial forces

Abstract In detailed analytical checks against fatigue failure, the local notch stress approach can be considered as one of the most advanced methods for predicting the fatigue behaviour of a mechanical structure. However, it is based on a detailed strength analysis, including macroscopic notch effects, of the real structure, e.g., at weld seams. In this paper, the fatigue behaviour of oblique nozzles on cylindrical shells submitted to internal pressure, axial force and combined loading is analysed according to these principles. Detailed three-dimensional, parametric, finite element models based on brick elements are applied to describe the mechanical behaviour of the structure for a wide range of relevant geometrical parameters. The results of the serial finite element analyses are approximated analytically and thus can be offered to potential users to determine the stresses at arbitrary points of the nozzle-to-vessel junction and to predict the most probable location of fatigue failure.

Fatigue damage of weld seams with and without postweld treatment under multiaxial loading

Abstract The service durability of weld seams is one of the most challenging problems of component fatigue. Hereby, the complexity is due to several peculiarities of welded structures: the detailed weld seam geometry with macroscopic and microscopic notch effects and/or defects, the properties of the welded material, residual stresses and multiaxial proportional or non-proportional loading conditions. Non-experimental approaches to design against fatigue have to account for the difference between weld seams with and without post-weld treatment as well as proportional and non-proportional loading. The application of fracture mechanics parameters allows for a more realistic description of damage progression. This paper is intended to explain possible design approaches based on numerical fracture mechanics and their combination with stress and strain based approaches using clear and classified damage criteria (engineering crack size, fracture). All numerical approaches are presented with special regard to complexity and modeling requirements from the point of view of the practicing design engineer. Non-proportional loading is considered by the identification of a damage critical plane which requires special post-processing routines within the numerical analysis. Again, the question of practicability is raised.