Overview of European Fitnet Fitness-for-Service Procedure

by Isabel Hadley, Structural Integrity Group, TWI Ltd, Cambridge, UK and Mustafa Koçak, GKSS Research Centre, Geesthacht, Germany

Paper presented at OMAE 2008, 27th international Conference on Offshore Mechanics and Arctic Engineering, Estoril, Portugal, 15 - 20 June 2008. Paper OMAE 2008 - 57741.

Abstract

The European FITNET consortium was convened in 2002 with the remit of preparing a procedure for fitness-for-service (FFS) evaluation of flawed engineering components. The procedure is intended to be used by a broad range of industries across Europe, and can be used at any stage in the life of an engineering component, eg design, fabrication, operation, failure analysis or life extension.

This paper presents an overview of the structure of the procedure. There are four main modules, each covering a particular failure or damage mechanism: fracture, fatigue, creep and corrosion (including environmentally-assisted cracking). These are linked by the use of a common terminology and a single set of reference compendia (annexes), eg for stress intensity, plastic collapse and residual stress, so that a particular flaw can be rapidly analysed for more than one failure mechanism.

The FITNET fracture assessment procedures in particular represent a significant advance compared with current published FFS procedures such as API 579-1/ASME FFS-1 and BS 7910. There is a hierarchy of different approaches, designated Options 0 to 5, the choice between them depending on the quality of information (in particular, materials property data) available to the user. This could range from Charpy and tensile data only (Option 0) through to the constraint-dependence of fracture toughness (Option 5). Other Options allow crack driving force to be calculated directly from FEA (Option 4), or permit weld metal strength mismatch to be taken into account (Option 2).

The fatigue analysis module likewise contains several alternative approaches, termed Routes. Some (Routes 1-3) are based on the concept of a nominally flaw-free structure, whilst Route 4 is based on cycle-by-cycle integration of the Paris law, and Route 5 addresses non-planar flaws.

FITNET also set itself the goal of providing training in FFS techniques, both through a series of seminars held during the project (2002-2006) and through provision of lasting training material (slides, tutorials, case studies and a validation document), which are now publicly available.

1. Introduction

Flaws (such as cracks, welding defects and corrosion damage) can arise during the manufacture and/or use of metallic components. For safety-critical items such as pressure vessels and pipelines, the failure of even a single component due to the presence of a flaw could threaten human life, as well as having severe economic and environmental consequences. On the other hand, some flaws are harmless, as they will not lead to failure during the lifetime of the component; replacement and/or repair of such flaws would be economically wasteful. A fitness-for-service (FFS) procedure allows flaws to be evaluated consistently and objectively, using fracture mechanics principles. Although several reputable fitness-for-service procedures already exist (e.g. BS 7910, API 579-1/ASME FFS-1, SINTAP, R6) ^[1-4], they tend to be aimed at a particular industry sector, or a single failure mode, or are national documents. There is therefore a need for a unified European procedure covering a range of industry sectors.

With this in mind, the European FITNET (FITness-for-service NETwork) consortium was convened in 2002. The vehicle for the project was a Thematic Network, a mechanism which promotes international collaboration on a particular topic, primarily to collate and formalize existing research work. This was part-funded by the European Union (EU), with the balance of the funding provided by the members (representatives of industry, academia, safety bodies and research organizations) themselves. Participants outside the EU were also sought, providing self-funded contributions from organizations in Switzerland, Korea, Japan and the USA.

2. Overview of FITNET

2.1 Structure of the Network

The management structure of the network is shown in Figure 1. A series of Working Groups (WGs) and Work Packages (WPs) were set up under the overall management of GKSS (Germany), each led by a different contractor. Whilst the WGs covered a particular failure/damage mechanism (eg fracture, corrosion), the Work Packages comprised a particular activity cutting across all WGs, for example validation and case studies, training and education. Additional sub-groups and task groups were set up as the network progressed, to pursue specialist areas of the procedure.

A full list of participants, their country of origin and their principal role in the consortium, is shown in Table 1.

 

Table 1. Members of the FITNET consortium

 

Name	Country/body	Type of organisation	Role in FITNET
GKSS	Germany	Research and Technology Organisation (RTO)	Network Co-ordinator and Leader, WP3 (procedure development)
Joint Research Centre of the European Commission	European Union	RTO	Leader, WP2 (state of the art review, strategy)
VTT	Finland	RTO	Leader, WP4 (validation and case studies)
TWI	UK	RTO	Leader, WP5 (dissemination and IPR)
University of Cantabria	Spain	University	Leader, WP6 (training and education)
CESI	Italy	RTO	Leader, WP7 (standardization)
Corus	UK/Netherlands	Industry	Leader, WG1 (fracture)
Caterpillar	France	Industry	Leader, WG2 (fatigue)
British Energy	UK	Industry	Leader, WG3 (creep)
Shell	Netherlands/UK	Industry	Leader, WG4 (corrosion)
IWT	Germany	RTO	Member
BZF	Hungary	RTO	Member
CSM	Italy	RTO	Member
HSE	UK	Safety authority	Member
ALSTOM Power	UK	Industry	Member
FHG/IWM	Germany	RTO	Member
University of Maribor	Slovenia	University	Member
SCK·CEN	Belgium	RTO	Member
Advantica	UK	RTO	Member
Centro Ricerche Fiat	Italy	RTO	Member
CEIT	Spain	RTO	Member
FORCE Technology	Denmark	RTO	Member
University of Gent	Belgium	University	Member
InnospeXion	Denmark	Industrial services	Member
Kielce Uni of Technology	Poland	University	Member
Rolls-Royce	UK	Industry	Member
DNV	Sweden	RTO/safety authority	Member
Technical University of Darmstadt	Germany	University	Member
IIS	Italy	RTO	Member
MPA	Germany	RTO	Member
University of Aveiro	Portugal	University	Member
Institut de Soudure	France	RTO	Member
Skoda Vyzkum s.r.o	Czech Republic	RTO	Member
Bureau Veritas	France	RTO/safety authority	Member
Deutsches Zentrum für Luft- und Raumfahrt. e.V.	Germany	RTO	Member
National Physical Laboratory	UK	RTO	Member
CETIM	France	RTO	Member
EMPA	Switzerland	RTO	Member
ICOM-ENAC-EPFL	Switzerland	RTO	Member
ALCAN	France	Industry	Participant (in-kind contributor)
IRSID, USINOR	France	RTO	Participant
Inasmet	Spain	RTO	Participant
Schweißtechnische Zentralanstalt	Austria	RTO	Participant
EDF Electricite de France	France	Industry	Participant
Hitachi	Japan	Industry	Participant
Battelle Columbus	USA	RTO	Participant
BiSAFE	Czech Republic	Industrial services	Participant
Tecnatom	Spain	Industrial services	Participant
Korea University	Korea	University	Participant
Note: WG=Working Group    WP=Work Package

2.2 Structure of the procedure

The principles of FITNET were agreed early in the life of the network, namely that the procedure should:

• cover all major failure/damage modes, ie fracture, fatigue, creep and corrosion,
• allow fitness-for-service analysis at any stage in the life of a component, including design, fabrication, operation, life extension and failure investigation,
• be applicable to any industry sector,
• be adopted throughout Europe, eg by publication of a CEN procedure

The scope of FITNET is therefore seen to be wider than that of the existing FFS procedures, which tend to be used by a particular industry sector, or address just one failure mechanism, or are national documents, as shown in Table 2.

Table 2. Comparison of FITNET with other major FFS procedures

Procedure	Status	Failure/damage mechanism(s) addressed	Industry sector
FITNET	European document, intended for publication by CEN	Fracture, fatigue, creep, corrosion	General
SINTAP	European document (output of a research project), freely available	Fracture	General
BS7910	UK national procedure, published by BSI	Fracture, fatigue, creep, corrosion	General
R6	Industry-specific procedure	Fracture and fatigue	Nuclear power
R5	Industry-specific procedure	Creep	Nuclear power
API579-1/ASME FFS-1	Joint API/ASME standard	Fracture, fatigue, creep, corrosion	To date, used mainly for in-service assessment of downstream oil and gas facilities

The structure of the procedure was intended to ensure the use of common concepts and terminology, whilst facilitating editing and maintenance. The first five sections of the procedure therefore cover the common concepts: the overall philosophy of the FFS approach, definitions of terms and abbreviations, and details of the type of information required. Four self-contained modules (Sections 6-9, covering fracture, fatigue, creep and corrosion) follow - these are discussed in more detail later in the paper. Section 10 then covers assessment and reporting of results, mainly emphasising fracture assessment; for example, the use of sensitivity calculations, reserve (safety) factors and partial safety factors. Sections 11 and 12 address more specialist and/or tentative fracture assessment topics, for example, proof testing and warm prestressing, leak-before-break (LBB) analysis and crack arrest.

A second volume of the procedure contains the annexes - compendia of stress intensity factor, limit load and residual stress solutions, plus additional annexes covering flaw idealisation, re-characterisation and interaction, reliability-based methods, NDE and constraint-based analysis amongst other topics.

Two further sections addressing the validation of the procedure, case studies and tutorials, will form a separate volume of the FITNET procedure, since they are supplemental to the main procedure rather than an essential part of it.

Further general information on the FITNET project is given in a series of overview documents published throughout the course of the project.^[5-8]

2.3 The fracture module

The fracture analysis module of FITNET draws on a number of existing sources, including SINTAP, BS 7910, R6 and the GKSS ETM (Engineering Treatment Method) procedure.^[9] The basic concept is that the driving force for the cracked body under load (K_I) is compared with the materials fracture toughness (K_mat), whilst the load on the uncracked ligament (the reference stress) is compared with the limit load for the cracked body. This can be done using either a Crack Driving Force (CDF) curve or a Failure Analysis Diagram (FAD). The two approaches are equivalent, although the FAD approach is perhaps more well-established through its use in other fracture procedures such as BS 7910, R6 and API 579-1/ASME FFS-1.

The fracture module of FITNET allows users to assess the integrity of metallic structures at several different levels (known as Options), depending on the complexity of the materials property data available to them. On the one hand, they may have very simple information to hand, such as Charpy impact results and specified values of tensile properties. Under these circumstances, a very simple approach based on empirical correlations between Charpy energy and fracture toughness may be adopted, known as Option 0. On the other hand, if detailed stress-strain data for both parent metal and weld metal are available, along with fracture toughness data for the weldment, it may be possible to use higher options, eg Option 3. A list of Options is shown in Table 3, along with the corresponding materials data requirements.

Table 3. Relationship between FITNET fracture Options and type of materials information required

Option No.	Type of tensile data required	Type of fracture toughness data required	Other information
0 (basic)	YS or SMYS only	None; Charpy energy only	Relies on correlations; applicable to ferritic steels only
1 (standard)	YS and UTS	Single-point fracture toughness data or tearing resistance curves	Based on tensile properties of the weaker material (typically the PM) and the fracture toughness of the material in which the flaw is located
2 (mismatch)	YS and UTS of PM and WM	Single-point fracture toughness data or tearing resistance curves	Takes account of strength mismatch; typically worth applying only if M≥1.1
3 (stress-strain)	Full stress-strain curves for PM and WM	Single-point fracture toughness data or tearing resistance curves	Can take into account both strength mismatch and the shape of the stress-strain curve
4 (J-integral)	Full stress-strain curves for PM and WM	Single-point fracture toughness data or tearing resistance curves	CDF approach only; elastic-plastic FEA is used to calculate the driving force for the cracked body
5 (constraint)	Full stress-strain curves for PM and WM	Relationship between fracture toughness and crack-tip constraint, eg J as a function of T-stress	Can take into account constraint effects, by matching crack-tip constraint in the test specimen and the cracked structure
YS: yield (or proof) strength SMYS: Specified Minimum Yield Strength PM: Parent Metal WM: Weld Metal M: mismatch ratio (ratio of WM yield strength to PM yield strength)

The effects of materials property data (and hence the Option employed) on the accuracy of the analysis are comprehensively addressed in the FITNET document and several background publications^[10-12] and will therefore not be considered further in this paper. However, it should be borne in mind that other factors can significantly influence the accuracy of a fracture assessment. For example, the precision of the stress analysis will also affect the result of the analysis according to a given Option, and a hypothetical case is examined below in order to illustrate this. If only a code maximum value of applied stress is available, along with the information that the component is in the 'as-welded' condition, then both primary and secondary stress would be input as uniform values of membrane stress. If more detailed stress analyses are available, it might be possible to resolve the stress into membrane and bending components, or to describe the stress distribution in terms of a polynomial, using weight function methods for the calculation of stress intensity. The FITNET K-solution compendium (Annex A) supports the use of all three types of input. An example is illustrated in Figure 2. This shows a cross-section through a hypothetical flat plate containing a finite surface flaw with a height (a) of 20% of the plate thickness (B). The actual stress distribution (derived, for example, from Finite Element Analysis) across the whole plate section is shown by the polynomial stress distribution. In many cases, the user will know only the maximum stress across the section, shown in Figure 2 as a membrane stress of 210N/mm². Alternatively the actual stress distribution could be linearised across the whole cross section as shown, producing a membrane stress of 165N/mm² and a bending stress of 45N/mm². This linearisation conservatively describes the actual stress distribution, but is more accurate than the membrane stress assumption. A fourth method of treating the stress distribution, also permitted by FITNET, is to linearise the stress distribution across the flaw, for the purposes of calculating applied stress intensity, K_I. [For the calculation of the reference stress, σ_ref and the normalised limit load, L_r, this approach could be non-conservative, so the stress distributions used for K_r and L_r need to be separated on this occasion.] Linearisation across the flaw produces still lower values of membrane stress (P_m=110N/mm²) and a higher bending stress (P_b=100N/mm²). The consequences of these different treatments of the stress input can be seen in the FAD in Figure 3. All analyses have been carried out using Option 1, but the analysis point moves away from the Failure Assessment Line, towards the origin of the FAD, as the stress input becomes more accurate, the membrane stress assumption producing the highest values of L_r, K_r and the weight function method the lowest value of K_r.

2.4 The fatigue module

The fatigue module (Section 7 of FITNET) comprises five different approaches to fatigue analysis, termed Routes 1-5. The first three are essentially fatigue damage assessment approaches, which assume that there is no pre-existing flaw in the structure, whilst Routes 4 and 5 are based on the assumption that a flaw exists. A brief summary of each of the Routes is given below, and summarised in Figure 4.

Route 1 is also known as the nominal stress method and is compatible with other stress-based approaches such as the IIW rules^[13] and BS 7608.^[14] The welded joint is assigned to a particular weld class based on its geometry and the direction of loading. Its fatigue life is then estimated from statistical data on geometrically similar joints, using S-N curves.

Route 2, the structural stress or notch stress method, recognises that fatigue can be associated with very small hot-spot areas of the component. Stresses in the relevant area are calculated, eg by FEA, and the appropriate S-N curve (see Route 1) is then followed.

Route 3 (local stress-strain approach) is applicable mainly to non-welded components, and addresses crack initiation only (subsequent crack growth could be modelled using Route 4 - see below). A relationship between cyclic strain range and the number of cycles to initiation is derived using, for example, the Coffin-Manson law.

Route 4 is based on the assumption of a planar flaw, which can grow on a cycle-by-cycle basis as described by the Paris law; the fatigue crack growth constant and the effects of stress ratio are given in the procedure for a range of materials and environments. As such, Route 4 is very similar to the fatigue crack growth approach used in BS 7910.

Route 5 addresses non-planar flaws in welded joints, and is currently restricted to undercut, slag inclusions and porosity in steel and aluminium butt welds. Non-planar flaws can, of course, be analysed using Route 4 and treating them as planar, but this treatment is likely to be highly conservative. Route 5 therefore gives an empirically-based alternative; the maximum flaw size is tabulated as a function of required fatigue class, flaw type and material.

2.5 The creep module

The creep module (Section 8 of FITNET) provides methods for calculating creep crack growth and creep-fatigue interaction. It draws in particular on the R5 high temperature assessment procedure, developed by the UK nuclear industry [15]. The procedure is presented in the form of 13 steps, some of which are covered by other sections of FITNET, as shown below:

STEP 1.	Establish Cause of Cracking and Characterise Initial Defect; for example, determine the relative contributions of creep and fatigue, and ensure that there are no other sources of cracking, eg environmentally assisted cracking (EAC)
STEP 2.	Define Service Conditions; load and temperature cycles need to be drawn up
STEP 3.	Collect Materials Data; these could include creep rupture, creep deformation, creep ductility, creep crack growth and cyclic crack growth data, along with fracture toughness (in the creep-damaged condition, if appropriate), stress-strain data and elastic constants. Some indicative data are given in Annex K of the procedure.
STEP 4.	Perform Basic Stress Analysis; this is typically an elastic stress analysis of the uncracked component
STEP 5.	Check Stability under Time-Independent Loads; this is basically a fracture analysis using Section 6 of the procedure to ensure that failure will not occur instantaneously
STEP 6.	Check Significance of Creep and Fatigue; the relative contributions of creep and fatigue are assessed, and a further test determines whether creep-fatigue interactions need to be allowed for.
STEP 7.	Calculate Rupture Life based on the Initial Defect Size; if this is less than the required service life, the calculation can be stopped at this point, as creep crack growth need not then be taken into account.
STEP 8.	Calculate Initiation Time; this is the time between high-temperature operation and the start of significant crack growth. It is conservative to assume a zero initiation time.
STEP 9.	Calculate Crack Size after Growth; this requires integration of the appropriate creep crack and/or fatigue crack growth laws.
STEP 10.	Re-Calculate Rupture Life after Crack Growth; the rupture life is re-calculated based on the flaw size derived from Step 9.
STEP 11.	Check Stability under Time-Independent Loads after Crack Growth; this is essentially a repeat of Step 5, but based on conditions after crack growth has occurred.
STEP 12.	Assess Significance of Results; this step requires sensitivity calculations and some consideration of the margins associated with the result, which are left to the user to decide.
STEP 13.	Report Results; as in all aspects of a FITNET assessment, the input data, assumptions, route used, results and implications/recommendations should all be documented in the interests of traceability and repeatability.

2.6 The corrosion module

Section 9 of the FITNET procedure addresses two main types of analysis:

• crack propagation by Environmentally Assisted Cracking (EAC), including Stress Corrosion Cracking (SCC) and Corrosion Fatigue (CF)
• analysis of locally thinned areas (LTAs), in which the most likely failure mechanism is plastic collapse

Analysis of EAC (see Figure 5) follows the same general principles as those for other types of growing flaw, eg fatigue and creep crack growth. When assessing the integrity of structures with cracks or crack-like defects, it is necessary to consider whether sub-critical crack growth is a potential factor. If so, an estimate of the amount of tolerable growth during the design lifetime or between in-service inspections is required. In that context, structural integrity assessment has to take into account the distinct characteristics of the damage processes associated with environmental assisted cracking (EAC). The conditions in which EAC occurs involves a combination of stress, environment and microstructural susceptibility, although it should be emphasised that this interaction occurs at a highly localised level and it is the local characteristics of these variables rather than the nominal bulk values that are critical.

The corrosion module considers subcritical crack growth due to stress corrosion cracking (predominantly static load or slow rising load) and corrosion fatigue (predominantly cyclic load), with crack growth rate prediction in service based principally on the application of fracture mechanics in terms of either stress intensity factor (K) in the case of stress corrosion cracking, or the range of stress intensity factor (ΔKem>K), in corrosion fatigue. Underlying that assumption is the presumption that the flaws or cracks are of a dimension that allows a description of the mechanical driving force by linear elastic fracture mechanics (LEFM). In practice, for some systems, a significant amount of life may occur in the short crack regime that, if known, should be taken into account in the assessment.

So far as LTA analysis is concerned, FITNET encompasses damage in pressure piping, pipelines and pressure vessels that have been designed to a recognised code such as BS PD5500, EN13445, ASME VIII Divisions I and II or ASME B31.3. The types of geometry covered by the procedure include spheres, cylinders, elbows, ends (hemispherical, to rispherical and elliptical) and integrally reinforced nozzles. The principles of the method are that the corrosion-damaged vessel or pipework should be capable of undergoing a hydrotest without failure, and that the stresses in the thinned area of the component should not exceed the yield strength of the material under design pressure. The safe working pressure of the equipment may need to be reduced in order to retain a safety factor similar to that for the undamaged equipment; a summary of the approach is shown in Figure 6.

Various conditions exclude the use of the LTA approach - for example, there must be no crack-like flaws, no mechanical damage combined with corrosion, no cyclic loading and no flaws with depth greater than 80% of the original wall thickness. These exclusions are intended to ensure that the damage is truly LTA, not crack-like, and that the extent of damage lies within the envelope for which experimental validation exists.

2.7 Validation, case studies and tutorials

FITNET also set itself the goal of providing training in FFS techniques, both through a series of seminars held during the project (2002-2006) and through provision of lasting training material (slides, tutorials, case studies and a validation document). An additional volume of FITNET brings together information on validation, case studies and tutorials.

Validation typically consist of the application of FITNET principles to a set of experimental data, for example a large-scale laboratory test or a real structural failure. A failure event is unambiguous; for example, if failure of a laboratory specimen has in practice occurred, then analysis of the failure conditions in accordance with FITNET should predict failure conservatively - if it predicts survival, then the method is non-conservative. Extensive validation of all four major failure/damage modes is documented in this additional volume, in the FITNET final conference proceedings and elsewhere. An example of the validation of the fracture procedure is shown in Figure 7. This summarises the results of over 300 full-scale and large-scale tests, including wide plate, pressure and bend tests on welded and plain materials.^[11] A range of materials (structural, linepipe and stainless steels and aluminium alloys) is included in the database. All tests ended in failure; consequently, all analysis points fall outside the Failure Analysis Line, in the 'potentially unsafe' area of the FAD. All the analyses shown in Figure 7 are based on Option 1, ie the simplest fracture mechanics-based analysis route, although selected cases have subsequently been re-analysed using more advanced Options.^[16,17]

Case studies and tutorials consist of the application of FITNET principles either to equipment that has not failed in service, or to hypothetical situations devised for teaching and illustration. As such, these examples illustrate certain features of the procedure (for example, how to analyse creep-fatigue interaction) but do not necessarily test the procedure critically. The document includes examples of the application of FITNET to a broken forklift truck, to strength mismatched welded components, to ship structures and to a hip implant, amongst other things.

2.8 Current status of the FITNET procedure

The FITNET procedure is currently available to interested parties (members of the consortium and selected standards bodies) in the form of a final draft, MK8.^[18,19] The ultimate aim remains to publish the procedure (Volumes 1 and 2) as a CEN document, via a CEN workshop agreement (CWA). It is likely that the volume containing validation, case studies and tutorials will remain the intellectual property of the FITNET consortium, and will be published separately by them. In the meantime, plans are underway to adopt relevant parts of FITNET into a future edition of the BS 7910, the UK national procedure.

3. References

1. BSI, 2005: BS 7910:2005 (incorporating Amendment 1); 'Guide to methods for assessing the acceptability of flaws in metallic structures'.
2. API 579-1/ASME FFS-1 2007, 'Fitness-for-service'.
3. SINTAP: www.eurofitnet.org/sintap_index.html
4. R6 - Assessment of the Integrity of Structures containing Defects, Revision 4.
5. Koçak, M.; 'FITNET fitness-for-service procedure: an overview', International Institute of Welding, Welding in the World, vol.51, no.5-6. May-June 2007. pp.94-105.
6. Koçak, M.; 'FITNET fitness-for-service procedure: an overview', FITNET 06-04, in Proceedings of the International Conference on Fitness-for-service (FITNET 2006): 17-19 May, Shell Global Solutions, Amsterdam, The Netherlands (ISBN 978-3-00-021084-6).
7. Koçak, M.: 'Fitness for service analysis of structures using the FITNET procedure: an overview'. In: Offshore Mechanics and Arctic Engineering (OMAE 2005). Proceedings, 24th International Conference, Halkidiki, Greece, 12-17 June 2005. Publ: New York, NY 10016, USA; American Society of Mechanical Engineers; 2005.
8. Koçak, M.; 'FITNET fitness-for-service procedure: an overview', European Seminar on Pressure Equipement (ESOPE), Paris, 9-11 Oct, 2007
9. Schwalbe, K-H. et al, 'EFAM ETM 97 - The ETM method for assessing the significance of crack-like defects in engineering structures', comprising versions ETM 97/1 and ETM 97/2. GKSS report 98/E/6.
10. Koçak, M., Seib, E. and Motarjemi, A., 2006, 'Treatments of structural welds using FITNET fitness-for-service procedure: FITNET 06-013', as Ref.^[6]
11. Hadley, I. and Moore, P., 'Validation of fracture assessment procedures through full-scale testing: FITNET 06-018', as Ref.^[6]
12. Seib, E., Volkan Uz, M. and Koçak. M., 'Fracture analysis of thin-walled laser beam and friction stir welded Al-alloys using the FITNET procedure', FITNET 06-019, as Ref.^[6]
13. Hobbacher, A., 2004, Recommendations for fatigue design of welded joints and components, IIW document XIII-1965-03/XV-1127-03, February.
14. BS 7608:1993, 'Code of practice for fatigue design and assessment of steel structures'.
15. R5 - 'Assessment procedure for the high temperature response of structures, Issue 3'.
16. Hadley, I., 2007, 'Validation of the European FITNET fitness-for-service procedure: incorporation of weld strength mismatch into fracture assessment (Options 2 and 3)'. TWI Industrial Members' report 890/2007.
17. Hadley, I., 2008, 'Validation of the European FITNET fitness-for-service procedure: Use of fracture assessment Option 4'. TWI Industrial Members' report 893/2008.
18. FITNET Fitness-for-Service (FFS) - Procedure (Volume 1) ISBN 978-3-940923-00-4, Koçak, M., Webster, S., Janosch, J.J., Ainsworth, R.A., Koers, R., printed by GKSS Research Center, Geesthacht, 2008
19. FITNET Fitness-for-Service (FFS) - Annex (Volume 2) ISBN 978-3-940923-01-1, Koçak, M., Hadley, I., Szavai, S., Tkach, Y., Taylor, N., printed by GKSS Research Center, Geesthacht, 2008