Optimising Fracture Assessment of Welded Structures Using BS 7910, R6 and FEA

2019 ◽  
Author(s):  
Isabel Hadley ◽  
Tyler London
Keyword(s):  
Residual Stress ◽  
Assessment Methods ◽  
Welding Residual Stress ◽  
Weld Strength ◽  
Materials Testing ◽  
Welded Structures ◽  
Analytical Formulae ◽  
Fracture Assessment ◽  
Crack Tip Constraint ◽  
Assessment Procedures

Abstract The fracture clauses of BS 7910 and R6 present a hierarchy of assessment methods. Depending on the data available, the user may adopt the simplest approach (Option 1), or the higher Options (2 & 3), allowing increasing accuracy and decreasing conservatism. Additional assessment procedures are available via the Annexes of BS 7910 and via Chapter III of R6, which address the inclusion of welding residual stress, crack tip constraint, weld strength mismatch and warm prestress. This paper illustrates the application of both basic and advanced fracture assessment procedures to a set of welded wide plate test data. The tests featured extensive materials testing, along with detailed characterisation of welding residual stress both in the as-welded condition and after a warm prestress treatment. The study shows how the accuracy of the assessment increases as the more advanced assessment methods are employed. A tailored assessment of the uniaxial tests using elastic-plastic FEA was also carried out, allowing a comparison between the analytical formulae given in BS 7910/R6, numerical analysis and experimental results.

BS 7910: History and Future Developments

2009 ◽  
Author(s):  
Isabel Hadley
Keyword(s):  
Assessment Methods ◽  
Weld Strength ◽  
Assessment Procedure ◽  
Metallic Structures ◽  
History Of ◽  
Fracture Assessment ◽  
Offshore Industry ◽  
Crack Tip Constraint ◽  
The Uk ◽  
Assessment Procedures

BS 7910, the UK procedure for the assessment of flaws in metallic structures, was first published almost 30 years ago in the form of a fracture/fatigue assessment procedure, PD6493. It provided the basis for analysing fabrication flaws and the need for repair in a rational fashion, rather than relying on long-established (and essentially arbitrary) workmanship rules. The UK offshore industry in particular embraced this new approach to flaw assessment, which is now widely recognised by safety authorities and specifically referred to in certain design codes, including codes for pressure equipment. Since its first publication in 1980, PD6493/BS 7910 has been regularly maintained and expanded, taking in elements of other publications such as the UK power industry’s fracture assessment procedure R6 (in particular the Failure Assessment Diagram approach), the creep assessment procedure PD6539 and the gas transmission industry’s approach to assessment of locally thinned areas in pipelines. The FITNET European thematic network, run between 2002 and 2006, has further advanced the state of the art, bringing in assessment methods from SINTAP (an earlier European research project), R6, R5 and elsewhere. In particular, the FITNET fracture assessment methods represent considerable advances over the current BS 7910 methods; for example, weld strength mismatch can be explicitly analysed by using FITNET Option 2, and crack tip constraint through Option 5. Corrosion assessment methods in FITNET are also more versatile than those of BS 7910, and now include methods for vessels and elbows as well as for pipelines. In view of these recent advances, the BS 7910 committee has decided to incorporate many elements of the FITNET procedure into the next edition of BS 7910, to be published c2012. This paper summarises the history of the development of BS 7910, its relationship with other flaw assessment procedures (in particular FITNET and R6) and its future.

Progress Towards the Revision of BS 7910

2011 ◽  
Author(s):  
Isabel Hadley
Keyword(s):  
Residual Stress ◽  
Weld Strength ◽  
Metallic Structures ◽  
Fracture Assessment ◽  
Crack Tip Constraint ◽  
The Uk ◽  
Assessment Procedures ◽  
Industry Sectors ◽  
First Time ◽  
Non Destructive

BS 7910, the UK procedure for the assessment of flaws in metallic structures, is being revised with a view to publication in 2012. Like the existing procedure, the new procedure will address all major failure/damage mechanisms, namely fracture, fatigue, creep and corrosion, and is intended to be used across a range of industry sectors and component types. There are several major proposed changes, which draw mainly on the existing BS 7910 procedures, the UK nuclear industry’s R6 document and the European FITNET procedure. The most far-reaching changes are in Section 7 (fracture) and related annexes. Here, the modifications include: • a re-structuring of the fracture assessment procedures from their present form (Levels 1–3) to a new hierarchy based on Options 1–3, which are more compatible with the current R6 and FITNET approaches, • revised treatment of flaw interaction, • a new annex (Annex N) permitting analysis under conditions of reduced crack tip constraint, • a new annex (I) addressing analysis of weld strength mismatch, • a revised residual stress compendium (Annex Q). As part of the revision, all annexes will be reviewed and edited where necessary, and a new annex on non-destructive examination (NDE) will be included for the first time. In view of the fact that many of the major changes concern the fracture assessment clauses, this paper presents a case study based on the analysis of a fully-circumferential flaw in a pipeline girth weld. The basic assessment Options (1 and 2) given in the new procedure are used to analyse the flaw, and three more advanced techniques (constraint-based assessment, assessment using an idealised residual stress distribution and analysis based on weld strength mismatch) are also applied.

The Future of the BS 7910 Flaw Assessment Procedures

2010 ◽  
Author(s):  
Isabel Hadley ◽  
Bob Ainsworth ◽  
Peter Budden ◽  
John Sharples
Keyword(s):  
Weld Strength ◽  
Assessment Procedure ◽  
Metallic Structures ◽  
Damage And Failure ◽  
Reference Stress ◽  
Fracture Assessment ◽  
Crack Tip Constraint ◽  
The Uk ◽  
Assessment Procedures ◽  
Industry Sectors

BS 7910, the UK procedure for the assessment of flaws in metallic structures, was first published some 30 years ago in the form of a fracture/fatigue assessment procedure, PD6493. Since then it has been regularly maintained and expanded, taking in elements of other publications such as the UK power industry’s ‘R6’ procedure (in particular the Failure Assessment Diagram or FAD approach), the creep assessment procedure PD6539 and the UK gas transmission industry’s approach to corrosion assessment of locally thinned areas in pipelines. Work is currently underway to prepare another major revision, this time incorporating many elements of the European flaw assessment procedure FITNET. Like its predecessor, the new BS 7910 is intended for use by a range of industry sectors for virtually any type of metallic structure or component. The procedures will cover damage and failure by fatigue crack growth, fracture, creep and corrosion, including Environmentally Assisted Corrosion. The objective in revising the procedures is to support the use of more advanced assessment methods, whilst preserving compatibility with previous editions of BS 7910 and retaining methods for preliminary analyses based on simple, conservative inputs. Features of the new BS 7910 will include adoption of new advanced fracture assessment procedures (taking account of crack tip constraint and weld strength mismatch where appropriate), revision of the residual stress annex, preparation of a new annex covering guidance on NDE, an enhanced library of K-solutions and reference stress solutions and greater compatibility with procedures such as R6 and FITNET.

Validation of the European FITNET Fitness-for-Service Fracture Assessment Procedure

2008 ◽  
Author(s):  
Isabel Hadley ◽  
Liwu Wei
Keyword(s):  
Large Scale ◽  
Crack Tip ◽  
Direct Calculation ◽  
Weld Strength ◽  
Assessment Procedure ◽  
Fracture Assessment ◽  
Fracture Tests ◽  
The Many ◽  
Crack Tip Constraint ◽  
Assessment Procedures

The new European fitness-for-service procedure FITNET includes a hierarchy of different approaches for fracture assessment, designated Option 0 to Option 5. The choice of Option depends on the information available to the user, and can include direct calculation of crack driving force by FEA, allowance for weld strength mismatch and allowance for crack-tip constraint. The fracture assessment procedures have been extensively validated by analysing the results of large-scale fracture tests and engineering failures in accordance with the procedure, and comparing actual with predicted behaviour. This paper presents a selection of the many hundreds of analyses carried out, with particular emphasis on the more advanced Options incorporating weld strength mismatch and crack-tip constraint. Failure of the testpieces was correctly predicted by the FITNET procedure in all cases (ie, the analysis point corresponding to failure of the specimen lay outside the failure analysis line of the FAD). The safety factor associated with the analysis is, however, shown to decrease as the more advanced Options are invoked, ie the analysis becomes more accurate and less conservative, as intended.

Fracture Assessment of Structural Component With Residual Stress on the Basis of the Weibull Stress Criterion

2008 ◽  
Author(s):  
Yoichi Yamashita ◽  
Fumiyoshi Minami
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Structural Component ◽  
Welding Residual Stress ◽  
Fracture Toughness Test ◽  
Stress Criterion ◽  
Conventional Procedure ◽  
Calculation Results ◽  
Fracture Assessment ◽  
Weibull Stress

This paper studies the method for estimating the residual stress effects on brittle fracture of structural component based on the Weibull stress criterion. Experiments show that the critical CTOD and the critical load of wide plate with welding residual stress are apparently smaller than those of wide plate without residual stress. It has been found that the critical CTODs of wide plate with and without residual stress can be predicted from the 3PB fracture toughness test results based on the Weibull stress criterion. Constraint loss effects on CTOD of wide plate with residual stress can be assessed by the equivalent CTOD ratio. The equivalent CTOD ratio β is defined as the ratio, β = δ/δWP, where δ and δWP, are CTODs of the standard fracture toughness specimen and wide plate, respectively, at the same level of the Weibull stress. Calculation results of beta are also shown for various residual stress levels and crack lengh based on the Weibull stress criterion. Fracture assessment results using β are shown within the context of CTOD design curve. An excessive conservatism observed in the conventional procedure is reasonably reduced by applying the equivalent CTOD ratio, β.

Effects of Mechanical Loading on Residual Stress and Fracture: Part I — Background to the BS 7910:2013 Rules

2014 ◽  
Author(s):  
Isabel Hadley ◽  
Simon Smith
Keyword(s):  
Residual Stress ◽  
Stress Relaxation ◽  
Mechanical Loading ◽  
Mechanical Load ◽  
Welding Residual Stress ◽  
Assessment Procedure ◽  
Welded Structures ◽  
Interaction Factor ◽  
Welded Structure ◽  
The Uk

Failure of welded structures due to the presence of flaws is typically driven by a mixture of applied and residual stresses, yet in most cases only the former are known accurately. In as-welded structures, a typical assumption is that the magnitude of welding residual stress is bounded by the room temperature yield strength of the parent material. The UK flaw assessment procedure BS 7910:2013 also assumes that mechanical loading (either as a result of proof testing or during the initial loading of an as-welded structure) will bring about a relaxation in residual stress. Conversely, the UK structural assessment code for nuclear structures, R6, contains a warning on the ‘limited validation’ of the BS 7910 approaches for stress relaxation and suggests that they should be used ‘with caution’. The aim of this study was therefore to review the basis of the BS 7910 clauses on stress relaxation with a view to harmonising the BS 7910 and R6 rules for cases in which the original welding residual stress distribution is not known. The residual stress relaxation clauses of BS 7910:2013 date back to the 1991 edition of PD 6493 and have not changed substantially since then. A considerable programme of work was carried out by TWI at the time to justify and validate the clause, but the full underlying details of the work have not hitherto been available in the public domain, and are described in a separate companion paper. The approach proposed in BS 7910 combines ‘global’ relaxation of residual stress (Qm) under high mechanical load with ‘local’ enhancement of crack tip driving force through the adoption of a simplified primary/secondary stress interaction factor, ρ.

Residual Stress Management in Welding: Residual Stress Measurement and Improvement Treatments

2012 ◽  
Author(s):  
Jacob Kleiman ◽  
Yuri Kudryavtsev
Keyword(s):  
Residual Stress ◽  
Fatigue Life ◽  
Residual Stresses ◽  
Stress Management ◽  
Stress Measurement ◽  
Industrial Applications ◽  
Welding Residual Stress ◽  
Residual Stress Measurement ◽  
Engineering Properties ◽  
Welded Structures

Residual stress (RS) can significantly affect engineering properties of materials and structural components, notably fatigue life, distortion, dimensional stability, corrosion resistance etc. Residual stresses play an exceptionally significant role in fatigue of welded elements. The influence of residual stresses on the multi-cycle fatigue life of butt and fillet welds can be compared with the effects of stress concentration. Even more significant are the effects of residual stresses on the fatigue life of welded elements in the case of relieving harmful tensile residual stresses and introducing beneficial compressive residual stresses in the weld toe zones. Residual stress management is a concept that addresses major aspects of residual stresses in welds and welded structures. According to the concept three major stages, i.e. RS determination, RS analysis and RS redistribution are considered and evaluated, either experimentally or theoretically to achieve the optimum performance of welded structures. All three stages as well as a number of new engineering tools such as ultrasonic computerized complex for residual stress measurement, UltraMARS, software for analysis of the effect of residual stresses on the fatigue life of welded elements and new technology and, based on it, compact system for beneficial redistribution of residual stresses by ultrasonic peening, UltraPeen will be discussed. Examples of industrial applications of the developed engineering tools for residual stress analysis and fatigue life improvement of welded elements and structures will be given.

Effects of Mechanical Loading on Residual Stress and Fracture: Part II — Validation of the BS 7910:2013 Rules

2014 ◽  
Author(s):  
Isabel Hadley ◽  
Simon Smith
Keyword(s):  
Residual Stress ◽  
Stress Relaxation ◽  
Stress Distribution ◽  
Mechanical Loading ◽  
Welding Residual Stress ◽  
Numerical Work ◽  
Welded Structures ◽  
Interaction Factor ◽  
Welded Structure ◽  
The Uk

Failure of welded structures due to the presence of flaws is typically driven by a mixture of applied and residual stresses, yet in most cases only the former are known accurately. In as-welded structures, a typical assumption is that the magnitude of welding residual stress is bounded by the room temperature yield strength of the parent material. The UK flaw assessment procedure BS 7910:2013 also assumes that mechanical loading (either as a result of proof testing or during the initial loading of an as-welded structure) will bring about a relaxation in residual stress. Conversely, the UK structural assessment code for nuclear structures, R6, contains a warning on the ‘limited validation’ of the BS 7910 approaches for stress relaxation and suggests that they should be used ‘with caution’. The aim of this study was therefore to review the basis of the BS 7910 clauses on stress relaxation with a view to harmonising the BS 7910 and R6 rules for cases in which the original welding residual stress distribution is not known. A companion paper describes the history of the residual stress relaxation clauses of BS 7910. A considerable programme of work was carried out in the late 1980s to justify and validate the clauses, using a range of experimental and numerical work. This included analysis of work carried out by the UK power industry and used in the validation of the R6 procedure. The full underlying details of the work have not hitherto been available in the public domain, although the principles were published in 1988. The approach proposed in BS 7910 combines ‘global’ relaxation of residual stress (Qm) under high mechanical load with ‘local’ enhancement of crack tip driving force through the adoption of a simplified primary/secondary stress interaction factor, ρ. This is different from the method adopted by R6, but appears to be equivalent to allowing negative values of ρ under conditions of high primary stress. A re-analysis of the original TWI work, using the current version of BS 7910, has shown nothing to contradict the approach, which represents a workable engineering solution to the problem of how to analyse residual stress effects in as-welded structures rapidly and reasonably realistically when the as-welded stress distribution is unknown.

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