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.

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.

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.

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.

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.

Crack-Tip Fields of Short Cracks in Bending As Characterized by Two Parameter Criterion

1993 ◽  
Author(s):  
J. F. Zarzour ◽  
Y. Dah-Wei ◽  
M. J. Kleinosky
Keyword(s):  
Fracture Toughness ◽  
Large Scale ◽  
Stress Triaxiality ◽  
Crack Tip ◽  
Significant Loss ◽  
Integral Approach ◽  
Rigorous Framework ◽  
Crack Tip Constraint ◽  
Two Parameter ◽  
Crack Tip Stress

Abstract Single edge notched bars (SENB), in the bending mode, with a/W ratios ranging from 0.05 to 0.5 were examined for fracture toughness in terms of the J-integral approach. The results indicate that for a/W ratios less than 0.3, there is a significant loss of J-dominance. This loss is attributed to the effect of plastic deformation on the cracked face. For a/W ratios greater than 0.3, J-dominance is maintained into the large scale yielding regime. According to the recently developed two-parameter criterion (J,Q), compressive Q-stress was interpreted as an indication of low crack-tip stress triaxiality for shallow cracks, while positive Q-stress was associated with high crack-tip stress triaxiality for deep cracks. For the material properties and specimen geometries considered herein, a fracture toughness locus was constructed in terms of the (J,Q) parameters for each of the a/W ratios. The overall fracture data are in agreement with those predicted by other approaches and provide a rigorous framework for interpreting the effect of loss of crack-tip constraint in elastic-plastic fracture analyses.

Shallow Flaws Under Biaxial Loading Conditions—Part I: The Effect of Specimen Size on Fracture Toughness Values Obtained From Large-Scale Cruciform Specimens1

2000 ◽  
Vol 123 (1) ◽  
pp. 10-24 ◽  
Author(s):  
Wallace J. McAfee ◽  
B. Richard Bass ◽  
Paul T. Williams
Keyword(s):  
Fracture Toughness ◽  
Transition Temperature ◽  
Temperature Region ◽  
Large Scale ◽  
Biaxial Loading ◽  
Crack Tip ◽  
Specimen Size ◽  
Biaxial Stress ◽  
Lower Transition ◽  
Crack Tip Constraint

A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed. This technology is for application to the safety assessment of RPVs containing postulated shallow-surface flaws. It has been shown that relaxation of crack-tip constraint causes shallow-flaw fracture toughness of RPV material to have a higher mean value than that for deep flaws in the lower transition temperature region. Cruciform beam specimens developed at Oak Ridge National Laboratory (ORNL) introduce far-field, out-of-plane biaxial stress components in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock (PTS) loading of an RPV. The biaxial stress component has been shown to increase stress triaxiality (constraint) at the crack tip, and thereby reduce the shallow-flaw fracture toughness enhancement. The cruciform specimen permits controlled application of biaxial loading ratios, resulting in controlled variation of crack-tip constraint. An extensive matrix of intermediate-scale cruciform specimens with a uniform depth surface flaw was previously tested and demonstrated a continued decrease in shallow-flaw fracture toughness with increasing biaxial loading. This paper describes the test results for a series of large-scale cruciform specimens with a uniform depth surface flaw. These specimens were all of the same size with the same depth flaw and were tested at the same temperature and biaxial load ratio (1:1). The configuration is the same as the previous set of intermediate-scale tests, but has been scaled upward in size by 150 percent. These tests demonstrated the effect of biaxial loading and specimen size on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials. For specimens tested under full biaxial (1:1) loading at test temperatures in the range of 23°F (−5°C) to 34°F (1°C), toughness was reduced by approximately 15 percent for a 150-percent increase in specimen size. This decrease was slightly greater than the predicted reduction for this increase in specimen size. The size corrections for 1/2T C(T) specimens did not predict the experimentally determined mean toughness values for larger size shallow-flaw specimens tested under biaxial (1:1) loading in the lower transition temperature region.

JEDI: A Code for the Calculation of J for Cracks Inserted in Initial Strain Fields and the Role of J and Q in the Prediction of Crack Extension and Fracture

2008 ◽  
Author(s):  
David W. Beardsmore
Keyword(s):  
Fracture Toughness ◽  
Strain Field ◽  
Crack Extension ◽  
Crack Tip ◽  
Initial Strain ◽  
Small Scale ◽  
Assessment Procedure ◽  
Crack Tip Fields ◽  
Crack Tip Constraint ◽  
Inelastic Strains

When crack tip constraint is high, the crack tip contour integral J characterises the asymptotic stress, strain and displacement fields of a stationary crack in an elastic-plastic material. In other cases, the crack tip fields can be related to J and a second parameter Q which governs the crack tip constraint. These observations are the basis of J-Q fracture mechanics assessments. In the most usual procedure J is compared to an effective, constraint-corrected fracture toughness Jc which is derived from Q and the fracture toughness of the material. The difference Jc – J is a measure of the margin of safety. The assessment procedure assumes there are no initial inelastic strains in the component or the fracture toughness specimen prior to introducing the crack and subsequent loading. However, plant components may contain inelastic strains prior to cracking arising from welding and other manufacturing or fit-up processes. This initial strain field can be established by a finite element analysis that simulates the welding and/or manufacture sequence. Weld residual stresses develop due to the accumulation of incompatible, inelastic strains, including thermal, plastic and transformation strains in the material. If a crack is inserted into an initial strain field, a procedure is required to calculate J by analysis of the resulting crack tip fields. Moreover, for the fracture assessment method to remain valid, it must be demonstrated that the values of J and Q continue to govern the onset of crack extension or fracture so that a meaningful comparison of J with Jc can be made. This paper describes a domain integral for calculating J when inelastic strains exist prior to cracking, and its implementation in the JEDI computer code. The code is used to determine J for a crack inserted into a three-point bend specimen containing an initial inelastic strain field representative of one that might develop during welding. The extent to which the crack tip stress field is characterised by J and Q is examined by comparing it to the field for high constraint, small-scale yielding conditions.

Constraint Quantification of Single Edge Notched Bend Specimens Under Large-Scale Yielding

2003 ◽  
Author(s):  
Yuh J. Chao ◽  
Xian-Kui Zhu ◽  
Yil Kim ◽  
M. J. Pechersky ◽  
M. J. Morgan ◽  
...  
Keyword(s):  
Large Scale ◽  
Crack Tip ◽  
Bending Moment ◽  
Additional Term ◽  
Small Scale ◽  
Bending Stress ◽  
Single Edge ◽  
Crack Tip Fields ◽  
Scale Yielding ◽  
Crack Tip Constraint

Because crack-tip fields of single edge notched bend (SENB) specimens are significantly affected by the global bending moment under the conditions of large-scale yielding (LSY), the classical crack tip asymptotic solutions fail to describe the crack-tip fields within the crack tip region prone to ductile fracture. As a result, existing theories do not quantify correctly the crack-tip constraint in such specimens under LSY conditions. To solve this problem, the J-A2 three-term solution is modified in this paper by introducing an additional term derived from the global bending moment in the SENB specimens. The J-integral represents the intensity of applied loading, A2 describes the crack-tip constraint level, and the additional term characterizes the effect of the global bending moment on the crack-tip fields of the SENB specimens. The global bending stress is derived from the strength theory of materials, and proportional to the applied bending moment and the inverse of the ligament size. Results show that the global bending stress near the crack tip of SENB specimens is very small compared to the J-A2 three-term solution under small-scale yielding (SSY), but becomes significant under the conditions of LSY or fully plastic deformation. The modified J-A2 solutions match well with the finite element results for the SENB specimens at all deformation levels ranging from SSY to LSY, and therefore can effectively model the effect of the global bending stress on the crack-tip fields. Consequently, the crack-tip constraint of such bending specimens can now be quantified correctly.

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