Non-linear elastic-plastic techniques have been used to study the level A operating conditions of a relief valve subject to a small number of severe thermal shock transients during operation. The analysis uses a detailed FE mesh verified to capture the elastic-plastic behaviour of the valve. In addition, short duration transient heat loading on the valve was calculated using computational fluid dynamics using a conjugate heat transfer approach. The non-linear plasticity behaviour of the model was simulated using a kinematic hardening model incorporating non-linear hardening. Due to the very localised plasticity around small radius fillets in the model, a highly refined mesh strategy was needed. An innovative meshing strategy was therefore incorporated utilising a similar methodology to that used for fluid dynamics meshing; the fluid facing surfaces and nozzles were finely meshed with hexahedral elements, while parts of the internal bulk material were meshed using high order tetrahedral elements. Primary strength was analysed using traditional elastic methods. The progressive distortion check was based on the elasto-plastic through-wall strain distribution and the fatigue analysis based on the equivalent strain range.
This paper describes some of the outcomes of the development of finite element modelling guidelines for the stress analysis of bolted joints in pressure vessels and piping. The modelling methods originally developed at Rolls-Royce typically used 2D axisymmetric models as this was deemed adequate at the time. However, computing software and hardware improvements have subsequently been made which enable more realistic 3D bolted joint models to be solved where a greater level of geometric detail is required. For example the bolts, nuts and perforated flanges can now be represented more realistically reducing the degree of geometric abstraction that is required. Also, modern finite element codes such as ABAQUS and ANSYS now offer gasket elements which enable the initial compression, in-service performance and unloading of the joint to be modelled more realistically. Additionally, contact techniques can also be used to simulate the axial and radial distribution of thread load in the joint which will affect the stress distribution remote from the threaded region. Consequently, the modelling guidelines have been updated and provide guidance for stress engineers to decide which degree of model complexity is warranted.
This paper describes the outcome of a fracture study to ascertain the benefits of using modern three dimensional finite element techniques. The defects considered are postulated to be present at the root of a threaded fastener. The defects are explicitly modelled using three dimensional finite element analysis to extract the relevant fracture parameters. A large number of thumbnail defects of varying size and aspect ratio have been incorporated into the root of the first engaged thread of studs typically used in pressure vessels. Stress intensity factors at the defect surface and depth locations have been extracted for a number of thermal transient events using a domain integration technique within the general finite element code ANSYS. The above stress intensity factors were then used to generate fracture calculations using an R6 based procedure [1] for: • Toughness-based fracture margins for each defect modelled; • Critical defect size; • Comparisons with a fully extended defect.
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