Martin Muscat

A work criterion for plastic collapse

A new criterion for evaluating limit and plastic loads in pressure vessel design by analysis is presented. The proposed criterion is based on the plastic work dissipated in the structure as loading progresses and may be used for structures subject to a single load or a combination of multiple loads. Example analyses show that limit and plastic loads given by the plastic work criterion are robust and consistent. The limit and plastic loads are determined purely by the inelastic response of the structure and are not influenced by the initial elastic response: a problem with some established plastic criteria.

Evaluating shakedown under proportional loading by non-linear static analysis

Abstract A lower bound method for calculating shakedown loads under proportional loading by static non-linear finite element analysis is presented. Stress fields obtained by static analysis and stress superposition are substituted into Melan’s lower bound shakedown theorem. The proposed method is applied to two sample problems: a thick cylinder under internal pressure and a square plate with a central hole under proportional biaxial loading. The results indicate that the method gives accurate lower bound shakedown loads for these problems.

Elastic shakedown analysis of axisymmetric nozzles

An investigation of the shakedown behavior of axisymmetric nozzles under internal pressure is presented. The analysis is based on elastic-plastic finite element analysis and Melans lower bound shakedown theorem. Calculated shakedown pressures are compared with values from the literature and with the ASME Boiler and Pressure Vessel Code Section VIII Division 2 primary plus secondary stress limits. Results obtained by the lower bound method are also verified by cyclic elastic-plastic finite element analysis.

Shakedown analysis for complex loading using superposition

Bounding techniques for calculating shakedown loads are of great importance as design criteria since these eliminate the need for performing full cyclic loading programs either numerically or experimentally. The classical Melan theorem provides a way to recognize whether or not elastic shakedown occurs under a specified loading. Polizzotto extended Melans theorem to the case where a combination of steady and cyclic loads are acting on the structure. The purpose of this paper is to present a finite element method, based on Polizzottos theorem, to estimate elastic shakedown for a structure subjected to loads resulting from a combination of steady and cyclic mechanical loads. This method, called non-linear superposition, is then applied to investigate the shakedown behaviour of a biaxially loaded square plate with a central hole. Results obtained for the plate with a hole problem are compared with those available in the literature and are verified by means of cyclic elastoplastic finite element analysis.

Elastic Shakedown in Pressure Vessel Components Under Non-Proportional Loading

Bounding techniques for calculating shakedown loads are of great importance in design since this eliminates the need for performing full elasto-plastic cyclic loading analyses. The classical Melans lower bound theorem is widely used for calculating shakedown loads of pressure vessel components under proportional loading. Polizzotto extended the Melans theorem to the case of non-proportional loading acting on a structure. This paper presents a finite element method, based on Polizzottos theorem, to estimate the elastic shakedown load for a structure subjected to a combination of steady and cyclic mechanical loads. This method, called non-linear superposition, is then applied to investigate the shakedown behaviour of a pressure vessel component - a nozzle/cylinder intersection and that of a biaxially loaded square plate with a central hole. Results obtained for both problems are compared with those available in the literature and are verified by means of cyclic elasto-plastic finite element analysis.

Design of an Underwater Towfish Using Design by Rule and Design by Analysis

The European unfired pressure vessel code EN13445-3 [1] has been used to design a preliminary prototype of a towfish. The towfish is essentially an underwater vessel equipped with various sensors, cameras, hydroplanes and control systems that are used to capture data on the levels of pollutants in the sea and at the same time monitor plankton and jellyfish levels. The towfish is towed behind a surface ship and is designed to dive to a depth of 50m below sea level. The depth of dive can be controlled by means of hydroplanes. Data, signals and electrical power are transferred from the towfish to the surface ship and vice versa via the towing line. From a structural point of view the towfish is a vessel acted upon by external pressure and local loads. Design by rule (DBR) was first used to calculate some of the various dimensions and thicknesses of the towfish components. The various components were designed mainly to prevent failure due to buckling. Design by analysis (DBA) based on Annex B of the pressure vessel code EN13445-3 [1] was then used to carry out further buckling checks that were not possible to do using design by rule. At the end of the paper the results from the two design approaches are compared and any major differences are highlighted.Copyright

Design-by-Analysis Methods for Asymmetric or Unbalanced Cylindrical Composite Pressure Vessels

The combination of fibre volume fraction, fibre orientation and lay-up sequence in composite materials makes it possible to design a multitude of composite pressure vessels and pipes. Analytical models, based on the classical laminate theory and numerical predictive techniques offer a means to optimize the lay-up sequence in order to maximize the strength to weight / cost ratio. This study looks at the validity of using analytical models prescribed in the design by analysis filament wound composite standards and compares the results with realistic test and numerical models. The results show that the classical laminate theory accurately establishes the design load in symmetric and balanced lay-up laminates when appropriate material properties are assigned. However in the case of asymmetric or unbalanced lay-up sequences, the bending and twisting stiffness geometrically strengthens the pipes such that the classical hoop and axial loading conditions based on isotropic material properties, no longer apply. In such instances the analytical solutions can underestimate the design load by more than 33%. An analytical solution that accurately establishes the loading configuration and magnitude is required. On the other hand numerical models gave good agreement with the experimental test results immaterial of the lay-up sequences, when appropriate end coupling, pressure loading and material properties are applied.Copyright

Comparison Between Different Approaches for the Evaluation of the Hot Spot Structural Stress in Welded Pressure Vessel Components

Fatigue cracks in welds often occur at the toe of a weld where stresses are difficult to calculate at the design stage. To circumvent this problem the ASME Boiler and PV code Section VIII Division 2 Part 5 [1] uses the structural stress normal to the expected crack to predict fatigue life using elastic analysis and as welded fatigue curves. The European Unfired Pressure Vessel Code [2] uses a similar approach. The structural stress excludes the notch stress at the weld toe itself. The predicted fatigue life has a strong dependency on the calculated value of structural stress. This emphasizes the importance of having a unique and robust way of extracting the structural stress from elastic finite element results. Different methods are available for the computation of the structural hotspotstress at welded joints. These are based on the extrapolation of surface stresses close to the weld toe, on the linearisation of stresses in the through-thickness direction or on the equilibrium of nodal forces. This paper takes a critical view on the various methods and investigates the effects of the mesh quality on the value of the structural stress. T-shaped welded plates under bending are considered as a means for illustration.

Design and development of a towfish to monitor marine pollution

AbstractMonitoring of marine pollution is currently being given a lot of importance as an aid to legislators for when it comes to protecting the environment. A clean marine environment is very important in order to sustain healthy life for all creatures and plants that live in the sea and beyond. The work presented in this paper includes work done within the project BIODIVALUE (Biodiversity and Sustainable Development in the Straits of Sicily) funded through the European Union Regional Development funds (ERDF) Italia-Malta 2007–2013. One of the project aims was to support the monitoring of pollution at sea in the Straits of Sicily, hence contributing to drawing future legislation at the national and European level. The project involved the study and analysis of maritime traffic in the Straits of Sicily and the water pollution produced by it. In particular, this paper presents the design, fabrication, and preliminary functionality testing of an underwater vehicle known as a towfish that can be used to monitor seaborne pollutants such as hydrocarbons and nitrates. The towfish can also be used to measure seawater temperatures and conductivity at different depths and also to monitor jellyfish populations in the open sea.

An Investigation of Different Offshore Wind Turbine Jacket Support Foundation Models Designed for Central Mediterranean Deep Waters

The trend for improving the cost effectiveness of offshore wind turbines is to maximise wind turbine size while minimising support structure costs. Minimising the support structure costs in deeper waters is undoubtedly more challenging. Studies have shown that the type of foundation model used at the base of the support structure has a significant influence on the overall design result. Two foundation models are studied independently: the pinned foundation and the rigid foundation. The effect of the foundation type on the modeled overall support structure characteristics, are investigated. The objective of this study is to produce an optimised jacket design for a 70 metre water depth in the central Mediterranean region. The study shows that different foundation types influence the internal loads of the individual jacket members and consequently affect the structures natural frequencies. The final result shows that the jacket structure with a rigid foundation design was 5.4 tonnes lighter than its pinned counterpart.