Richard Charles

The effect of phase transformations on predicted values of residual stresses in welded ferritic components

The accurate prediction of the residual stresses present in welded structures can be of great importance to the fracture assessment of such components. Therefore, a large amount of benefit can be gained from improving techniques for measuring and numerically analysing these stresses. In recent years many advances have been made in the field of analysing residual stresses using finite element methods. That said, very little work has been conducted on the accurate modelling of welded ferritic components. This is largely due to the added complication of phase transformations that occur during the heating and cooling of such steels. The objective of the work presented in this paper was to improve understanding of the effect that phase transformations have on residual stresses present within welded ferritic structures. This was conducted by simulating such welding processes using the finite element package SYSWELD. An investigation was conducted to determine how phase transformations, and therefore residual stresses, are affected by the welding process used. Phase transformation and material property data available within SYSWELD were used for this analysis. An autogenously welded beam provided a simple basis for this qualitative investigation. In the future the manufacture and measurement of suitable test-pieces will enable these simulations to be validated.

Further Studies to Evaluate Crack Opening Areas for Through-Wall Cracks in the Vicinity of Welded Attachments

A paper was presented at the 2009 ASME PVP Conference on evaluating, by finite element techniques, crack opening area (COA) and stress intensity factor, KI , values for through-wall cracks located in the region where an attachment is welded to a plate geometry. Both membrane and bend loads were considered. In addition, based on the stress profile in the un-cracked complex geometry over the region where the cracks would be introduced, COA and KI values were evaluated for the same crack sizes located in a simple plate geometry. This enabled information to be established on the conservatism, or otherwise, of using simple plate solutions to evaluate COA and KI for cracks in the complex geometry. The present paper reports on further studies that have been undertaken to investigate the effect on the previous COA and KI results of considering (i) large displacement theory which may be important for combined membrane and bend loading, and (ii) contact elements in the finite element models since in the previous studies, the mesh was allowed to “overlap on itself” when crack closure was evident due to compressive stresses during bend loading.Copyright

Load History Effects on Crack Driving Force for Cracks in Residual Stress Fields

Fracture mechanics assessments of engineering components and structures containing defects are made by comparing an estimate of the crack driving force KJ with an effective fracture toughness KJc . The assessments must account for the combined effect of primary loads, such as internal pressure in pressurised components, and secondary stresses arising from welding and/or thermal loading. Elastic-plastic finite element analysis, or simplified methods set out in standard assessment procedures, can be used to estimate the crack driving force KJ as a function of the applied primary load on the component. The effective fracture toughness KJc should take account of the material fracture toughness and the crack tip constraint. For the assessment of defects in weld residual stress fields, it is usually assumed that the defect is inserted into the as-welded stress distribution in such a way that traction free crack surfaces are created simultaneously at all positions on the crack faces. However, it may be beneficial to take account of any relaxation in the residual stress field that might arise during proof-testing or in-service cyclic loading, and to consider a more gradual, progressive introduction of the defects. These benefits could, in principle, result in a reduction in the crack driving force. This paper describes work that has been undertaken to provide estimates of the crack driving force KJ for a fully-circumferential defect in a circumferential repair weld in a cylindrical pipe. Calculations have been carried out to establish KJ for a number of cases where different pressure overloads are applied to the uncracked pipe and different methods of crack insertion are applied. Estimates of the margin of safety on fracture toughness and pressure loading were calculated. At the outset, it was assumed that the fracture toughness of relevance for the defects is the material fracture toughness KJc * derived from strain free, high constraint fracture toughness specimens. No allowance was made for constraint effects associated with the finite geometry or initial strains in the pipe. The values of KJ were derived from values of J calculated using the JEDI post-processing code; this allows for initial inelastic strains present in the model prior to the start of the crack insertion process.© 2008 ASME

“First Estimate” Plastic Collapse Solutions for Flat and Torispherical Ended Pressure Vessels Subjected to Extensive Corrosion Wall Thinning

Plants in safety-critical industries comprise of a number of different components, including various sizes of pressure vessels, and their integrity is often paramount to the safe running of the plant. In many cases these vessels can contain highly corrosive liquids, which, over time can lead to a degree of thinning in the vessel walls. With corrosion wall-thinning come a number of issues, including a reduced plastic collapse load of the vessel and possible fatigue initiation from corrosion pits. Since failure of these vessels could lead to catastrophic failure and the escape of highly corrosive material, the assessment and maintenance of these vessels is of high importance.Current Fitness for Service Codes and Standards provide guidance on the assessment of cylinders and pipes with localized wall thinning and general corrosion, with respect to plastic collapse. This guidance, however, is not currently applicable to components with geometric discontinuities (the pressure vessel base-wall junction).This paper follows a preceding paper presented at the ASME PVP Conference 2010 which derived “First Estimate” plastic collapse solutions for flat-based, corroded, pressure vessels. These solutions have been expanded through further finite element studies to accommodate the estimation of plastic collapse in tori-spherical headed pressure vessels subjected to extensive corrosion wall-thinning. Results from this study are presented in graphical form to enable a quick and efficient first estimate of the effect of wall thinning on the collapse pressure to be attained.© 2012 ASME

Developments in the Treatment of Residual Stresses in Welded Components

A long-term UK research programme on environmentally assisted cracking (EAC), residual stresses [1, 2] and fracture mechanics [3, 4] was launched in 2004. It involves Rolls-Royce plc and Serco Technical Services, supported by UK industry and academia. The residual stress programme is aimed at progressing the understanding of residual stresses and on the basis of this understanding manage how residual stresses affect the structural integrity of plant components. Improved guidance being developed for the treatment of residual stresses in fracture assessments includes the use of stress intensity factor solutions for displacement controlled loading as opposed to the more commonly used load controlled solutions. Potential reductions in crack driving force are also being investigated in relation to (i) utilizing a residual stress field that has “shaken-down” due to operational loads, (ii) introducing a crack progressively as opposed to instantaneously, and (iii) allowing for the fact that a crack may have been initiated during the life of a component as opposed to being present from the start-of-life. This paper describes some of these latest developments in relation to residual stress effects

The Evaluation of Crack Opening Areas for Through-Wall Cracks in the Vicinity of Pipe Branch Connections

A number of papers have been presented at previous ASME PVP conferences, which have evaluated the crack opening areas (COA) and stress intensity factors (K), using elastic finite element analysis techniques, for through-wall cracks in a region where an attachment is welded to a plate. This was a simplified geometry aimed at representing a more complicated geometry of a pipe-branch connection. A number of analyses were considered and conclusions made on the estimation of COA and K using simple handbook solutions. More recently the analyses included the application of nonlinear geometry and the addition of crack face contact when applying bending loads. This paper is a continuation of these previous studies, assessing through-wall cracks in a more realistic pipe-branch connection geometry. The calculated COA and K values for the more complex geometry are compared to values from pipe models with no branch connections, in a similar manner to that applied in the previous work on the simplified plate geometry. Judgments are made on the conservatism, or otherwise, of the estimated COA and K for the more complex geometry solutions compared to the simple geometry solutions.Copyright

Assessment of Lower Regions of Pressure Vessels Subjected to Corrosion Wall Thinning

Pressure vessels are common components of plant currently in operation within the nuclear industry and elsewhere. Their assessment and maintenance is of paramount importance to the integrity and safe working of the plant. Often these pressure vessels contain highly corrosive substances, which over time, cause deterioration in the integrity of the vessel. Current codes and standards such as BS7910, API579 and FITNET provide guidance on the assessment of cylinders and pipes with localised and general corrosion, but to date, no such guidance is available for complete vessels, where the corrosion occurs in the lower region of the vessel, e.g. where the base of a vessel is connected to the vessel wall. This paper investigates the resistance to plastic collapse, from internal pressure loading, of flat based cylindrical pressure vessels, where the base and wall thicknesses are considerably different due to corrosion damage. A ‘First Estimate’ or conservative lower bound collapse pressure solution for such vessels is described. The solution has been derived using axi-symmetric finite element models with varying degrees of general corrosion thinning.Copyright

Crack Opening Area Solutions for Through-Wall Cracks in the Vicinity of Welded Attachments

This paper presents the latest results of a finite element study undertaken to evaluate crack opening areas (COA) and stress intensity factors (KI ) for through-wall cracks located in the region where an attachment is welded to a plate geometry. Both membrane and bend loads have been considered. In addition, COAs and stress intensity factors have been evaluated for the same crack sizes located in a simple plate geometry. These values have been determined by applying both membrane and bend stresses to the plain plate, the magnitudes of which correspond to those for the stress profile in the un-cracked complex geometry in the vicinity of where the cracks would be introduced. This has enabled information to be established on the conservatism or otherwise of using simple plate solutions to evaluate COAs and stress intensity factors for cracks in the complex geometry.Copyright

Recent Developments to Improve Crack Opening Areas for R6 Leak-Before-Break Procedures

This paper provides an overview of developments that have been undertaken in recent years in order to extend the Crack Opening Area (COA) solutions for Leak-Before-Break (LBB) within the R6 Flaw Evaluation Procedures. For high temperature operations, the effect of creep strains on COA has been studied using finite element (FE) models of a centre-cracked plate with combined primary and secondary stresses. Primary creep, as well as secondary creep, has been included in the studies. The ultimate aim of this work is to further develop and validate by FE analysis, simplified expressions for the prediction of COA under realistic loading conditions for high temperature operations. For COA evaluations in general, work has been undertaken by FE modelling of the following aspects: • Straight-fronted cracks under combined loading. • Converging/diverging cracks whereby the length on one surface is different to that on the other surface (i.e. cases where there is a through-wall variation in crack length). • Weld strength mis-match effects. • Component features effects.Copyright

Crack Opening Area Solutions for Through-Wall Cracks in a Complex Geometry

This paper presents the results of a finite element study that has been undertaken to evaluate crack opening areas (COA) for through-wall flaws located in the region where an attachment is “welded” to a plate geometry. This represents the first stage of a study to evaluate crack opening area solutions for flaws situated in complex geometries such as pipe elbows, nozzles and other attachments. The work has been performed by way of 3D finite element methods. In addition to COA evaluations, stress intensity factors have been determined in the study. The crack opening areas and stress intensity factors evaluated for this complex geometry have been compared with those for the same flaw sizes located in a simple plate geometry. This has led to an initial understanding of how conservative or otherwise the use of plate solutions is for representing the more complex geometry cases.Copyright