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Presented at the I Mech E seminar, 'Flaw assessment in pressure equipment and welded structures - PD 6493 to BS 7910', London, 8 June 1999 SynopsisThe British Standards guidance document PD 6493 ('Guidance on methods for assessing the acceptability of flaws in fusion welded structures'), first published in 1980 and revised in 1991, has now been revised again and published as a British Standard Guide, BS 7910. The new document incorporates additional structural and defect geometries, a number of new structural integrity tools, and industry-specific application guides, which are briefly reviewed in this paper. 1 IntroductionDesign and fabrication codes ensure that welded structures such as pressure vessels, pipelines, offshore structures, bridges and buildings are safe, provided all code requirements are met during the lifetime of the structure. The margins of safety inherent in such codes can be somewhat arbitrary, and the codes cannot be applied to situations where the structure, build quality, materials or service conditions do not meet the strict requirements of the code.An alternative philosophy is based on fitness-for-purpose principles and states that a component is acceptable provided the conditions for failure are not reached in the service lifetime. A number of possible failure mechanisms must be considered, including fracture, plastic collapse and fatigue. In order to ensure that such analyses are carried out in a systematic manner, the British Standards Institution (BSI) published guidance for the assessment of structural integrity of welded structures containing flaws, designated PD (Published Document) 6493 '(Guidance on methods for assessing the acceptability of flaws in fusion welded structures'). This document first appeared in 1980, and was extensively revised in 1991, incorporating more advanced fracture assessment methods from the nuclear industry.
Since 1980, PD 6493 has become very widely used, in terms of both geography and industry sector.Recent advances in the application of fracture mechanics, along with developments in other fitness-for-purpose procedures and changes in BS fracture mechanics test standards provided a driving force for a further revision of PD 6493. A decision was also taken to combine PD 6493 with the high-temperature assessment procedure PD 6539, to produce a single document. The result was published in December 1999 as a BS guide, designated BS 7910 ('Guide on methods for assessing the acceptability of flaws in metallic structures'). The fracture and fatigue assessments parts of BS 7910 only are covered in this paper.The overall structure of BS 7910 is shown in Fig.1. Clauses 1-6 of the procedure cover the principles of the method and the type of input required.
Clause 7 covers fracture assessment, and has been extensively modified since the 1991 edition of PD 6493. Relatively minor modifications have been made to the fatigue assessment procedures (Clause 8). A completely new chapter (Clause 9) has also been added, covering assessment of flaws in plant operating at high temperature. Finally, there are some 20 annexes, many of which introduce new methods and concepts.
2 Fracture assessmentClause 7 (the fracture clause) of BS 7910 has been completely re-written to improve clarity and usability, and to incorporate modern defect assessment technology, including a number of methods originally published in the CEGB R6 procedure. Flow diagrams have also been included to guide users through the three fracture assessment levels.As in PD 6493, there is a three-level fracture assessment procedure in BS 7910. All three levels use the concept of a Failure Assessment Diagram (FAD). The basic assumption is that the flawed body could fail by one of two extreme failure modes - fracture or plastic collapse (overload). The propensity to failure by plastic collapse is shown on the x-axis as a dimensionless number (L r or S r ) showing the ratio of applied 'reference' stress (net section stress) to the yield strength or flow strength of the material.
The proximity to fracture is calculated independently, and shown on the y-axis as the ratio of applied stress intensity to materials toughness (K r. Note that the BS 7910 Level 2 FADs are more conservative than the old PD 6493 Level 2 diagram, especially in the 'knee' region (high K r, high L r ).
Whilst this will lead to smaller tolerable flaw dimensions, assessment of large-scale validation tests has shown that there were cases where the PD 6493 level 2 method resulted in non-conservative predictions unless partial safety factors were applied to the input data. By contrast, BS 7910 predictions of the same tests produces points which are all outside the failure assessment line (see Figure 3). Level 3 of BS 7910 (ductile tearing instability assessment) remains unchanged with respect to PD 6493, except for the addition of the R6 Option 3 method, which becomes Level 3c in BS 7910.
In this approach, the FAD and driving force may be derived from detailed elastic-plastic finite element analysis to give more accurate predictions of structural behaviour. 3 Fatigue assessmentThe fatigue assessment clauses in PD 6493:1991 were also reviewed in the light of new information and experience gained from their use in practice.
The main change was the introduction of new fatigue crack growth laws, based on an extensive review and analysis of published data for steels. These include more precise two-branch Paris laws and allowance for applied stress ratio, R. In PD 6493, the assumption was made that fatigue crack growth could be represented by a single Paris law between the limits of the threshold stress intensity factor range for fatigue crack growth ( ΔK o ) and fracture (K Ic ). Although this simplification is satisfactory for many assessments, it is seen to be over-conservative at low values of ΔK, approaching the threshold ( Figure 4).
However, new conservative (upper-bound) single Paris laws, which result in higher growth rates than those in PD 6493, are still provided, for convenience. More attention is paid to environmental influences and the new recommendations cover marine corrosion, with and without cathodic protection, and fatigue crack growth at elevated temperature.
The specific cases covered include ferritic steels in air, freely-corroding in seawater and in seawater with cathodic protection (-850mV and -1100mV Ag/AgCl). The simplified laws also refer to ferritic steels, including at elevated temperature. They relate to high R values or to welded material (assumed to contain high tensile residual stresses and hence to experience a high effective R value under any fatigue loading). Austenitic steels can be treated using the simplified law for ferritic steels in air, but no advice is given for other environments. New data did not justify any changes to the recommended stress intensity factor threshold values in PD 6493. However, it is now strongly recommended that crack growth rate and threshold values for high R values, as detailed in Table 2, are assumed when assessing a flaw in a welded structure, to allow for the influence of high tensile residual stresses.Table 2 Recommended fatigue crack growth laws for steel (upper bound, R ≥0.5)Advice on the derivation of fatigue crack growth laws and threshold values for non-ferrous metals is also given, using correlations based on the relative E values.
EnvironmentΔK o,Nmm -3/2Stage A constantsStage B constantsΔK (Nmm -3/2 ) at transition from A to BAmAmAir632.10x10 -175.101.29x10 -122.88144Marine, free corrosion01.72x10 -133.427.48x10 -71.11748Marine, -850mV CP632.10x10 -175.102.02x10 -112.67290Marine, -1100mV CP632.10x10 -175.101.02x10 -71.40415The constants given above give fatigue crack growth rate (da/dN) in mm/cycle, as a function of ΔK in Nmm -3/2;da/dN=A( ΔK) m for ΔK o.
Bs 7910 Pdf Maker
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.
Bs 7910 Pdf File
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.