Three-dimensional finite element analysis of creep continuum damage growth and failure in weldments

• Main Author: Wong, M. T.
• Published: University of Manchester 1999
• Subjects: 671.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.542702

This thesis describes the development of computational analysis techniques for weldments which contain three different material regions (Parent Material, Weld Material and Heat Affected Zone) and which exhibit different creep behaviour. The different strain rate behaviour of the three regions and the growth of creep continuum damage lead to local stress redistribution and to complex states of stress which can greatly enhance the accumulation of creep damage. A review of literature is presented which covers weldment design methodology and the associatedm etallurgy. The shortcomingso f design and assessmenct odes for pipe vessels, and the need for approximate methods of lifetime estimation, are revealed, indicating the need for further research in these areas. The development of the Continuum Damage Mechanics (CDM) method is presented, which is a physically based technique for the analysis of the creep behaviour of materials and engineering structures. Previous works are also reviewed showing that the CDM method can be used to accurately describe the behaviour of weldments providing the constitutive equations are available which describe the deformation and rupture of materials. Since the research is concerned with the analysis of the creep rupture of welded pressure vessels and pipes using the CDM method, previous research is reviewed regarding the creep behaviour of weldments and 'the determination of constitutive equations for different weldment material phases. A review is also presented of different solution techniques for solving systems of equations and for minimising the bandwidth and profile of matrix. The modification is described of the two-dimensional (2D) solver, Damage XX- 2D, to extend its capability to three-dimensional (3D), together with the techniques required to satisfy plastic incompressibility using a special brick arrangement of tetrahedral elements. The Three-dimensional CDM Finite Element Solver is known as Damage XX-3D. The 3D finite element theory and the co-ordinate transformation techniques are outlined for the solution of axi-symmetricengineering problems. The technique for the removal of failed finite elements is described for both of the two-dimensional and the three-dimensional analyses. The restart facility and the associated data output strategy are developed to help to minimise the loss of result data files, in the case of an accidental power cut to computers. These methods also allow a complete analysis to be divided into individual smaller analyses. Two numerical solution methods, with different storage schemes, for sets of linear algebraic equations have been developed and validated against results obtained independently using a commercial Finite Element package Abaqus (version 5.6-1). A damage averaging technique is developed to reduce the number of iterations required for the solution of three-dimensional problems which have large number of degrees of freedom; and also to preserve the symmetry of creep CDM solutions for axi-symmetric two-dimensional analyses. The creep CDM solutions obtained using Damage XX-3D are compared with the solutions obtained using Damage XX-2D Axi-symmetric analysis, and good agreement has been obtained for lifetimes and failure mechanisms. Applications of Damage XX-3D, are presented for the analysis of the high temperaturec reep behaviour of a CrossweldedT estpiece,t he Cylinder-SphereP ipe Intersection (Flank Section) subjected to an internal pressure, and Butt-welded ferritic steel pipe subjected to a combined internal pressure and a global bending moment. Finally, a three-dimensional Finite Element CDM Solver has been developed which is computationally fast and efficient, and which yields predictions which have been validated against independent solutions. IV