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.

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.

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.

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.

Fracture Assessment of Flaws in Weldments using FITNET FFS Procedure: An Overview

2007 ◽  
Vol 345-346 ◽  
pp. 401-409 ◽  
Author(s):  
Mustafa Koçak ◽  
Stephen Webster ◽  
Isabel Hadley
Keyword(s):  
European Community ◽  
Structural Integrity ◽  
Assessment Procedure ◽  
Metallic Structures ◽  
Fracture Assessment ◽  
Basic Equations ◽  
Network Project

Recently a European community funded thematic network project (participation of 17 countries) FITNET (www.eurofitnet.org) has completed a new and unified engineering assessment procedure (FITNET FFS Procedure) of flaws in metallic structures and welds. This newly developed procedure (under CEN Workshop Agreement WA22) provides assessment rules for flaws or damage due to fracture, fatigue, creep and corrosion to demonstrate the structural integrity of the component. This paper gives an overview of the FITNET Fitness-for-Service (FFS) Procedure and specifically presents the features and basic equations of the Fracture Module. It also presents two brief examples for the validation of the procedure using laser welded specimens.

Recent Studies Towards Updating the BS7910 Flaw Interaction Rule

2018 ◽  
Author(s):  
Bostjan Bezensek ◽  
Harry E. Coules
Keyword(s):  
Aspect Ratio ◽  
Rule Based ◽  
Crack Driving Force ◽  
Reference Stress ◽  
Surface Flaws ◽  
Close Proximity ◽  
Fracture Assessment ◽  
Assessment Procedures ◽  
Work Done ◽  
Interaction Rule

Fitness for service assessment procedures rely on flaw interaction rules for assessment of multiple flaws in close proximity. Such rules are aimed at avoiding excessive amplification of the crack driving force that may result in a non-conservative fracture assessment. In BS7910, the 2013 edition [1] introduced a new flaw interaction rule for the co-planar flaws where the proximity of adjacent flaws is judged based on flaw height (i.e. s = 0.5*max(a1,a2) for surface flaws). The rule was introduced for flaws with aspect ratio of a/c < 1 for both flaws, while for other flaw shapes and combinations the earlier rule from the predecessor document PD6493:1993 [2] was retained. This paper summarises the recent work done by the authors and work from literature to examine the applicability of the s = 0.5*max(a1,a2) rule to flaws with aspect ratio a/c ≥ 1 and dissimilar flaw combinations. It is shown that the current BS7910 rule based on s = 0.5*max(a1,a2) produces a conservative flaw assessment with the use of BS7910 solutions for stress intensity factor and reference stress. An exception are cases of two deep surface flaws where the rule is proposed to change to: s ≤ max(a1, a2) for two surface flaws with a1/t & a2/t > 0.5

Validation of Plastic Collapse Assessments Using BS 7910:2013 and R6 Procedures

2013 ◽  
Author(s):  
Şefika Elvin Eren ◽  
Tyler London ◽  
Yang Yang ◽  
Isabel Hadley
Keyword(s):  
Finite Element Analysis ◽  
Limit Load ◽  
Plastic Collapse ◽  
Element Analysis ◽  
Metallic Structures ◽  
British Standard ◽  
Reference Stress ◽  
Fracture Assessment ◽  
The Difference ◽  
Reference Stress Approach

The British Standard, BS 7910 Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures is currently under revision [1]. Major changes have been undertaken, especially in the fracture assessment routes, and this paper specifically addresses the assessment of proximity to plastic collapse, usually expressed as the parameter Lr via either a reference stress or limit load approach. In the new edition of BS 7910, the reference stress approach has been retained for the assessment of many geometries, mainly for reasons of continuity. However, new limit load solutions (originating in the R6 procedure) are given for use in the assessments of strength mismatched structures or clad plates. In general, a reference stress solution and a limit load solution for the same geometry should deliver the same value of Lr. However, recent comparative studies have shown differences in the assessment of plastic collapse depending on whether the reference stress solutions in BS 7910:2013 or the limit load solutions in R6 are used for the calculation of Lr. In this paper, the extent of the difference in the assessment results with respect to the choice of solutions and boundary conditions are discussed. The results of the assessments in accordance with BS 7910 and R6 are compared with the results of numerical assessments obtained via Finite Element Analysis (FEA). The collapse loads observed in various wide plate tests conducted in the last 20 years are also compared with the collapse loads predicted by BS 910:2013, R6 and FEA. Finally, observations regarding the accuracy of different Codes and FEA are discussed.

Fracture Assessment of Through-Wall and Surface Cracked Pipes by BS 7910 and API 579 Assessment Procedures: A Comparative Study

2004 ◽  
Author(s):  
Afshin K. Motarjemi
Keyword(s):  
Experimental Data ◽  
Comparative Study ◽  
Pressure Vessels ◽  
Plastic Collapse ◽  
Four Point Bending ◽  
Reference Stress ◽  
Fracture Assessment ◽  
Pipe Geometry ◽  
Assessment Procedures ◽  
The Comparative Study

Fracture assessment procedures such as BS 7910 and API 579 are formulated based on the Fracture Mechanics concept for assessing integrity of structures such as pipelines, pressure vessels, etc. In the current study those procedures are applied to through-wall and surface cracked pipe geometry under four-point bending. The predicted maximum tolerable applied loads are then compared with pipe full-scale fracture testing results published by Miura et al (2002). Other assessment schemes namely, GE/EPRI, Net-section plastic collapse, LBB.NRC and finally LBB.ENG2, as reported in the same publication are also included in the current paper for sake of comparison. The comparative study showed that BS 7910 and API 579 predict similar maximum tolerable load for through-wall pipes but different value for surface-cracked pipes. Difference in predictions for the latter geometry is owing to the use of different stress intensity factor/reference stress solution by BS 7910 than API 579. However, both procedures provided conservative results compared with the experimental data as well as other engineering routes mentioned in Miura et al (2002).

The Effect of Bi-Axial Stress on Limit Loads of Structures Containing Surface-Breaking Flaws and its Influence on Structural Integrity Assessments

2012 ◽  
Author(s):  
Liwu Wei ◽  
Isabel Hadley
Keyword(s):  
Surface Crack ◽  
Structural Integrity ◽  
Axial Loading ◽  
Limit Load ◽  
Plastic Limit ◽  
Integrity Assessment ◽  
Reference Stress ◽  
Fracture Assessment ◽  
Plastic Limit Load ◽  
Assessment Procedures

Fracture assessment diagram (FAD) based fracture assessment procedures are universally adopted by standards/documents including BS7910, R6, API579-1/ASME FFS-1 and FITNET. In the use of a FAD for structural integrity assessment, one important consideration is to determine the load ratio (Lr) which is defined by two equivalent definitions: Lr is either defined as the ratio of reference stress (σref) to yield strength (σY) as in BS7910, or as the ratio of applied load to plastic limit load as in R6. The solutions of reference stress or limit load are given in the assessment procedures for commonly encountered flawed structures such as a plate containing a surface crack and a cylinder containing an external surface crack. Although the solutions given in the various standards are not all the same, they were invariably derived on the basis of analysis of the force and moment equilibrium with regard to a flawed section and none of them has taken into account the effects of bi-axial stressing on a flawed section, thus leading to the likelihood of an overly conservative assessment. In this work, finite element analysis (FEA) of various flawed geometries (plate and cylinder containing surface cracks) was performed to compute plastic limit load, with the focus on understanding the effects of bi-axial stressing on plastic limit load. The geometries assessed include a plate with a surface crack subjected to both uni-axial and bi-axial loading, and a cylinder with circumferentially internal and external surface cracks sustaining a combination of axial loading and internal pressure. The investigation of these cases has demonstrated a significant increase in plastic limit load arising from bi-axial stressing. Comparison of the results of plastic limit load obtained from FEA with those derived from BS 7910 reference stress solutions was carried out to assess the extent of conservatism when the standard solutions are used in the applications containing bi-axial stresses. The implication for structural integrity assessment due to bi-axial stressing was also addressed. A comparison between BS 7910 Level 2B (material-specific FAD) and Level 3C (based on a FAD generated with FEA) procedures was also made and it was shown that whether the Level 3C procedure can reduce the conservatism in an assessment is dependent on individual cases.

Assessment Procedure for Multiple Crack-Like Flaws in Fracture Assessment Diagram (FAD)

2008 ◽  
Author(s):  
Shinji Konosu
Keyword(s):  
Body Force ◽  
Pressure Vessels ◽  
The Body ◽  
Assessment Procedure ◽  
Multiple Crack ◽  
Reference Stress ◽  
Discrete Crack ◽  
Fracture Assessment ◽  
Common Problems ◽  
Shielding Effects

Assessment of multiple discrete crack-like flaws is one of the most common problems relating to pressure vessels and piping components. Under the current Fitness for Service (FFS) rules, such as ASME, BS and so on, multiple crack-like flaws are usually recharacterized as an enveloping crack (defined as a single larger crack), following their assessment rules. The procedure, however, varies significantly in these FFS codes. In this paper, the interaction between non-aligned multiple unequal cracks is clarified by applying the body force method. Based on the interaction which indicates the magnification and shielding effects and the reference stress solutions, a newly developed assessment procedure for multiple discrete crack-like flaws in the Fracture Assessment Diagram (FAD) is proposed.

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