Fatigue. Prepared by European Fitness-for-Service Thematic Network - FITNET

Transcription

1 FITNET FFS MK 7 All rights reserved Fitness-for-Service Fatigue Fracture Creep Corrosion FITNET Fitness-for-Service PROCEDURE FINAL DRAFT MK7 Prepared by European Fitness-for-Service Thematic Network - FITNET Edited by M Koçak, GKSS RESEARCH CENTER, Germany (Coordinator-FITNET Thematic Network) S Webster, CORUS, UK JJ Janosch, CATERPILLAR, France RA Ainsworth, BRITISH ENERGY, UK R Koers, SHELL, The Netherlands Volume I: FITNET FFS Procedure

2

3 FITNET FFS MK 7 All rights reserved FITNET FFS PROCEDURE- Final Draft Date: Reference number of document: Committee identification: FITNET/WP3 FITNET FITNESS-FOR-SERVICE (FFS) PROCEDURE FINAL DRAFT Edited by M Koçak, GKSS RESEARCH CENTER, Germany (Coordinator-FITNET Thematic Network) S Webster, CORUS, UK JJ Janosch, CATERPILLAR, France RA Ainsworth, BRITISH ENERGY, UK R Koers, SHELL, The Netherlands Warning This final draft (version MK7) of the FITNET FFS Procedure has been developed within the European Fitness for Service Thematic Network (FITNET TN) and is not a Standard. It is distributed for information and to be a basis for a CEN document. When finalised it will provide guidelines for assessing the structural significance of flaws postulated or detected in service with respect to FRACTURE, FATIGUE, CREEP and CORROSION damage in metallic structures and hence serve as a basis for development of a European and International FFS Standard. Recipients of this draft FITNET FFS are invited to submit, with their comments, notification of any relevant patent and intellectual property rights of which they are aware and to provide supporting documentation.

4 Copyright notice This final draft FITNET FFS Procedure is prepared by four Working Groups of the FITNET Thematic Network and is copyright-protected by FITNET TN. Mustafa Koçak Coordinator European Thematic Network FITNET GKSS Research Centre Institute for Materials Research, Dep. of Joining and Assessment (WMF) Max-Planck-Str GEESTHACHT, GERMANY Tel / 2503 Fax: mustafa.kocak@gkss.de iv FITNET 2006 All rights reserved

5 Contents Page 1 Scope References Terms and definitions Symbols and abbreviated terms Information Required for Assessment (Input) Introduction Flaw Information Postulated Flaws Flaws in Component in-service Stresses Material Properties Flaw Information Single Planar Flaws Types of Planar Cracks Stresses Dynamic Analysis Load Levels Stress Categorisation Stress Linearisation Polynomial Representations of Stress Fields Plastic Collapse away from a Flaw Finite-Element Stress Analysis Performing Finite-Element Stress Analysis Heat transfer analysis Material properties Mesh design Boundary conditions Calculation control Mixed Mode Loading FITNET 2006 All rights reserved v

6 5.3.9 Warm Pre-stressing Fluctuating Stress Relaxation of Residual Stress Plastic Limit Load Analysis Definition Determination of the limit load e) Use of Plastic Limit Analysis Solutions Stress Analysis for Fatigue Assessment Load/stress history Stress concentrations Structural stress approach Material properties Introduction Elastic and Physical Properties Deformation and Strength Properties Yield Strength and Tensile Strength Engineering and True Stress-Strain Strain Hardening Coefficient Lüders Strain Fracture Toughness Toughness Data Format Fracture Initiation Toughness Fracture Data Collection Analysis of Data for Initiation of Fracture Brittle Fracture Preliminary Steps Maximum Likelihood Method Determination of Characteristic Values Bi-modal Master Curve analysis method Ductile Tearing Use of Maximum Load Fracture Toughness Data Charpy impact energy - fracture toughness correlations Definition of the applicability regimes (lower shelf / transition/ upper shelf) Lower shelf and early transition (prevalently brittle behaviour) Transition region (mixed brittle/ductile behaviour) Incomplete transition curves Upper shelf (ductile behaviour) Estimation of point values Estimation of the crack resistance (J-R) curve Treatment of sub-size Charpy data vi FITNET 2006 All rights reserved

7 5.4.9 Fatigue strength and fatigue crack growth Fatigue Resistance Fatigue Crack Growth Properties Paris Law Forman-Mettu Law Creep strength and creep crack growth data Creep rupture Creep deformation Creep ductility Creep crack initiation Creep crack growth Cyclic Creep Crack Growth Corrosion properties K ISCC determination Stress corrosion crack growth determination Bibliography Fracture module Introduction Analysis FAD and CDF Routes FAD / CDF The FAD Approach The CDF Approach Primary and Secondary Stress Treatment Evaluation of K r, J and CTOD Evaluation of K r From Primary and Secondary Stresses Evaluation of J e and J from Primary and Secondary Stresses Evaluation of from Primary And Secondary Stresses Analysis Options Guidance on Option Selection Introduction Selection of Failure Assessment Diagram - Option 0 to Aspects of Fracture Toughness Selection of Analysis Methods: Initiation and Tearing Plastic Yield Load Analysis Determination of Stress Intensity Factors Probabilistic Fracture Mechanics Weld Residual Stresses Load-History Effects Constraint Effects FITNET 2006 All rights reserved vii

8 Weld Mismatch Analysis Procedures Preliminary stages: assessment of objectives and available data Objectives: Available Assessment Procedures Structural Data and Characterisation of Flaws Loads and Stresses on the Structure Material s Tensile Properties Material s Fracture Properties The Standard Option Applicability Procedure Analysis Procedures Analysis Option 2 (Mismatch Procedure) Applicability Both Base and Weld Metal Exhibit Continuous Yielding Behaviour Both Base and Weld Metal Exhibit Discontinuous Yielding One of the Constituents has a Continuous Stress Strain Curve and the Other has a Discontinuous One Limit Load Solutions for Material Mis-match Analysis Option 3 (Known Stress-Strain Curves) Applicability Calculation Steps, Homogeneous Material Calculational Steps for Mismatch Material Analysis Option 4 (J-Integral Analysis) Analysis Option 5 (Constraint Analysis) Special options Basic Level of Analysis (Option 0) Applications Tensile Properties Determination of Type of Stress-Strain Curve Estimation of Lower Yield Strength Estimation of Ultimate Tensile Stress Determination of Fracture Toughness for Use at Default Level Introduction Other Guidance/ Limitations Determination of Failure Assessment Diagram Flaw Characterisation Determination of Loads and Stresses Assessment of Results viii FITNET 2006 All rights reserved

9 Unacceptable Result Ductile tearing analysis FAD analysis CDF(J) analysis CDF(δ) analysis Allowance for Constraint Effects Introduction Scope Procedures FAD Procedure I: Modification to the FAD FAD Procedure II: Modification to K r CDF Route Procedure Background Notes and Guidance on Using the Procedure Definition of Loads Evaluation of Structural Constraint, β Influence of Constraint on Material Resistance to Fracture Construction of Modified FAD Calculation of Parameter ρ I Assessment of the Significance of Results Bibliography Fatigue Module Introduction Definitions and Input Parameters Definition of Fatigue Service Conditions Description of Variable Loads Partial Safety Factors Fatigue Actions Cumulative Fatigue Assessment Fatigue Limit Assessment Environmental Issues Exemption from Fatigue Assessment Fatigue Assessment Routes Route 1 - Fatigue Damage Assessment using Nominal Stresses Route 1 (Welded Structures) Step 1: No Postulated or Detected Flaw is Present in the Structure Step 2: Service Condition Step 3: Environmental Issues Step 4: Thresholds for Fatigue Assessment FITNET 2006 All rights reserved ix

10 Step 5: Fatigue Resistance Data Specification Step 6: R-Ratio effects Step 7: Thickness Reduction Factor Effects Step 8: Fatigue Assessment using S-N Curves Route 1 for Non-Welded Locations Step 1: No Postulated or Detected Flaw is Present in Component Step 2: Service Conditions Step 3: Environmental Issues Step 4: Influence of geometric parameters Step 5: Permissible stress σa and mean stress effects Step 6: Results Route 2 - Fatigue Damage Assessment using Structural or Notch Stresses Route 2 for Welded Structures Step 1: No Postulated or Detected Flaw is Present in the Structure Step 2: Service Conditions Step 3: Environmental issues Step 4: Thresholds for fatigue assessment Step 5: Fatigue Data Specifications Step 6: Fatigue assessment using S-N Curves Route 2 for Non-Welded Parts Step 1: No Postulated or Detected Flaw is Present in the Component or Structure Step 2: Service Conditions Step 3: Issues, Corrosion and High Temperatures Step 4: Thresholds for Fatigue Assessment Step 5: Fatigue Resistance Data Step 6: Fatigue Assessment using Δσ e -N Curves Route 3 - Fatigue Damage Assessment using Local Stress-Strain Approach Step 1: No Postulated or Detected Flaw is Present in the Component or Structure Step 2: Service Condition Constant amplitude loading Variable amplitude loading Step 3: Environmental issues chapter Step 4: Thresholds for fatigue assessment Step 5: Fatigue resistance data for elasto-plastic loading Material elasto-plastic behaviour Step 5: Cumulative Fatigue life calculation Route 4 - Fatigue Crack Growth Assessment Step 1: Detected or Postulated Planar Flaw Step 2: Establish Service Conditions and Cause of Cracking Step 3: Exclude Environmental or Creep Effects x FITNET 2006 All rights reserved

11 Step 4: Collect Materials Data and Perform Stress Analysis Step 5: Pre-Checks Step 6: Calculate Crack Growth Paris Equation Forman-Mettu Approach Crack Growth under Variable Amplitude Loading Route 5 - Non-Planar Flaw Assessment Step 1: Postulated or Detected Non Planar Flaw is Present in the Component or Structure Step 2: Service Condition Step 3: Environmental Issues Step 4: Types of Imperfections Step 5: Effects and Assessment of Imperfections Undercut Porosity and Inclusions Crack-like Imperfections (Route 4) Life Improvement [7.22] Introduction Applicability of Improvement Methods Procedures for some improvement techniques Burr Grinding TIG dressing Hammer Peening Needle Peening Fatigue design data for improved welds Fatigue strength of joints improved by weld toe grinding or TIG dressing Fatigue strength of joints improved by hammer peening or needle peening Special Options Dang Van criterion The macroscopic-microscopic approach Link between macroscopic and mesoscopic scales Failure criterion Multiaxial analysis Local effective equivalent stress Local stress for fatigue assessment Effective equivalent stress Failure Criteria Rolling contact fatigue Introduction Analytic determination of the limit thermo-mechanical state for fatigue assessment FITNET 2006 All rights reserved xi

12 Numerical methods for treatment of fatigue Example of application to analysis of rail damage Fatigue creep Fatigue corrosion Growth of Short Cracks Additional information Variable amplitude loading Loading description Time history sequences description Histogram description Power or energy density spectrum description Data reduction Presentation of results Miner sum Equivalent stress range Gassner curve Bibliography Creep module Introduction Overall Procedure Establish Cause of Cracking Define Service Conditions Collect Materials data Creep Rupture Data Creep Deformation Data Creep Ductility Data Creep Crack Initiation/Incubation Data Creep Crack Growth Data Cyclic Crack Growth Data Method I Method II Other Data Elastic and Physical Constants Stress-strain Data Fracture Toughness Data Perform Basic Calculations Stress Intensity Factors Reference Stress xii FITNET 2006 All rights reserved

13 8.6.3 C* Parameter Redistribution Time, t red C(t) Parameter Check Significance of Creep and Fatigue Insignificant Creep Insignificant Fatigue Insignificant Creep-Fatigue Interactions Perform Assessment Calculations Calculate Rupture Life, Calculate Crack Incubation Time, t i Calculate Crack Size after Growth, a g Assess Significance of Results Sensitivity Analysis Remedial Action Report Results Additional Information Treatment of Defects in Weldments Introduction Scope Significance of Welding Residual Stresses Simplified Assessment Detailed Assessment Specific Modes of Cracking Treatment of Secondary Loading Failure Assessment Diagram Methods Introduction Creep Crack Initiation Assessment Procedures TDFAD Approach Two Criteria Diagram Comparison of Parameters Comparison of the TDFAD and the Two Criteria Diagram The Approach Bibliography Corrosion damage module Assessment of stress corrosion cracking and corrosion fatigue Introduction Procedural approach assessment of stress corrosion cracking and fatigue Step 1: Characterise the nature of the crack FITNET 2006 All rights reserved xiii

14 Step 2: Establish cause of cracking Define service condition Stress Service environment Development of local environments (crevicing, hideout/evaporation, deposits) Excursions Corrosion (or system) monitoring Step 3: Define the material characteristics Surface finish and cold work Welding Thermal ageing Irradiation damage Material processing and microstructural and microchemical orientation Step 4: Establish data for stress corrosion cracking assessment K ISCC determination Stress corrosion crack growth determination Establish data for corrosion fatigue assessment ΔK th determination Crack growth determination Corrosion fatigue crack growth data Step 5: Undertake structural integrity assessment Assessment of Local Thin Areas (LTA) Introduction Step 1: Establish cause of wall thinning Step 2: Define service condition Step 3: Collect material properties Step 4: Analysis Applicable flaws Exclusions Cylindrial body Safe working pressure estimate cylindrical body Safe working system stress estimate cylindrical body Minimum allowable remaining wall thickness cylindrical body Sphere and vessel end Elbow Safe working pressure estimate Safe moment estimate Minimum allowable wall thickness elbow Nozzles Determine minimum allowable wall thickness of the vessel shell xiv FITNET 2006 All rights reserved

15 Determine minimum allowable wall thickness of the nozzle Interaction rules Interacting flaws General Safe working pressure estimate Bibliography Assessment and Reporting of Results (Output) Assessment of Results Introduction Input Data Loads and Stresses Tensile Properties Fracture Toughness Significance of Result Reserve Factors Sensitivity Analysis Initiation Analyses Tearing Analyses Guidance on Determining Acceptable Reserve Factors Partial Safety Factors Reporting of Results Introduction Loading Conditions Material Properties Definition of Postulated and Detected Flaw Welding / Microstructural Issues Failure Assessment Diagram / Crack Driving Force (For Fracture Module only) Limit Load (For Fracture Module only) Stress Intensity Factor Solution Significance of Results Probabilistic Analysis Summary of Assessment / Further Action Bibliography Alternative and Specific Assessments for Fracture Introduction Leak Before Break (LBB) Introduction FITNET 2006 All rights reserved xv

16 Scope Leak-Before-Break Procedure Background Notes and Guidance on Using the Procedures Flaw characterisation Calculation of limiting flaw lengths Calculation of flaw length at breakthrough Calculation of crack-opening areas Leak rate calculations Leak detection and flaw stability following breakthrough Assessment of Results Bibliography Crack arrest Introduction Crack Arrest Phenomena Scope Crack Arrest Assessment Procedure Determination of Enhancement Factor, f s Notes on Specific Applications Crack Propagating in a Temperature Gradient Resulting from a Thermal Shock Crack Propagation from an Enclosed Brittle Region Crack Propagating in a Region of Varying Toughness Short Duration Loads Determination of Toughness Values Used in Crack Arrest Assessment Static Crack Arrest Toughness, KIa CAT Determination Additional Information Bibliography Consideration of proof testing and warm prestressing Introduction Proof or overload testing Warm prestressing Simplified WPS argument Full WPS procedure The WPS effect Bibliography Evaluation under Mode I, II and III Loads Introduction Failure Assessment Diagrams for use in Modes I, II and III Evaluation of K r Linear Elastic Stress Intensity Factor xvi FITNET 2006 All rights reserved

17 The Effective Stress Intensity Factor Evaluation of K r for an Initiation Analysis Procedure for the Evaluation of ρ Evaluation of L r Further Information Bibliography Master curve Introduction Homogeneous Material Inhomogeneous Material Bibliography Additional Information for Fracture Assessment Local approach Introduction Models Beremin model of cleavage Gurson Damage Mechanics Model Procedure Discussions and Limitations Bibliography Thin walled structures Introduction Pronounced Stable Crack Extension Prior to Failure Low Constraint Fracture Toughness Buckling as a Failure Mechanism Competitive to Fracture The Procedure Bibliography Loading rate effects on fracture toughness Introduction Experimental determination of fracture toughness at high loading rates Lower shelf and early transition region (mainly brittle behaviour Ductile-to-Brittle Transition region Upper shelf (fully ductile behaviour) Use of dynamic tensile properties Nomenclature Bibliography Bi-modal Master Curve Introduction FITNET 2006 All rights reserved xvii

18 Principles of Bi-Modal Master Curve Analysis Method Bibliography Treatment of non-sharp defects Introduction Stress distribution at notches Failure Criteria Flaws at the tip of a notch Failure Assessment Diagram For Notches Bibliography xviii FITNET 2006 All rights reserved

19 Foreword European Fitness-for-Service Network (FITNET) is an four year thematic network developed within the Competitive and Sustainable Growth (GROWTH) research programme of the European Community under the contract of GIRT-CT This thematic network started to work in February 2002 with the objective of developing and extending the use of fitness-for-service procedures to assess postulated or real damage due to FRACTURE, FATIGUE, CREEP and CORROSION in metallic structures. About 60 experts from 16 European countries (as well as from Japan, Korea and USA) covering universities, research and technology organisations and a wide range of industrial sectors have provided unique contributions for the development of this FITNET FFS Procedure which addresses the analysis of four major failure modes. The FITNET FFS Procedure is designed to assess the structural integrity of metallic welded or non-welded structures transmitting loads. In particular it embodies techniques for dealing with defects known or postulated to be present, in a structure together with the possible growth of such defects by a range of mechanisms and the assessment techniques required to evaluate the failure risk. This unified Fitness-for-Service Procedure, adopted by the European standardisation body of CEN via a CEN Workshop Agreement W22,covering four major structural failure modes, universally applicable to all major industries, able to be used at all stages of the life cycle of structures, aims to reach wider use in Europe and in the world for safer structures. The Italian standardisation organisation UNI has worked with the FITNET experts within the framework of the CEN W22. This final draft (MK7) is the 7 th version of the FITNET FFS document developed during the last four years. It consists three volumes; Volume I: FITNET FFS Procedure, Volume II: Case Studies and Tutorials, Volume III: Annexes Flaws (such as cracks, welding defects and corrosion damage etc.) can arise during the manufacture and/or use of metallic components in engineering structures. For safety-critical structures such as aircraft, pressure vessels and pipelines, the failure of a single component due to the presence of a flaw can threaten human life, as well as having severe economic and environmental consequences. Other flaws can be harmless, as they will not lead to failure during the lifetime of the structure and hence repair of such flaws or replacement of the respective component is economically wasteful. The FITNET FFS Procedure can be used by expert engineers working in the field of structural safety, advanced manufacturing and design to assess the structural significance of such defects or postulated cracks or damage. The use of the FITNET FFS Procedure at the design and fabrication stages of advanced metallic structures working under static or cyclic loading conditions is also covered to provide an effective engineering tool for decisions with respect to material selection and fabrication route for an expected applied stress. This FITNET FFS Procedure is developed by the expert members of the four Working Groups (WG): WG 1: FRACTURE: Coordinated by S. Webster, CORUS, UK WG 2: FATIGUE: Coordinated by JJ. Janosch, CARTEPILLAR, France WG 3: CREEP: Coordinated by RA. Ainsworth, BRITISH ENERGY, UK WG 4: CORROSION: Coordinated by R. Koers, SHELL, The Netherlands A wide range of technical sources in the field of Fitness-for-Service technology, such as existing international or national standards, codes, in-house procedures of various industries, results of completed and on-going research projects are used to develop this FITNET FFS procedure. A number of well-documented case studies are used for validation of assessment steps and routes to avoid non-conservatism in the procedure and check the sensitivity of an assessment to the selection of input parameters, assumptions and correlations. Further improvements and revisions in this Final Draft FITNET FFS (version MK7) Procedure will be carried out within the CEN standardisation process as well as after having users experience. This document is available at both web sites of FITNET TN ( and CEN ( FITNET 2006 All rights reserved xix

20 The FITNET TN consortium consists of following member and participant organisations: GKSS RESEARCH CENTER (Coordinator), Germany; JRC, The Netherlands; VTT, Finland; TWI, UK; UNIVERSITY OF CANTABRIA, Spain; CESI, Italy; CORUS, UK; CATERPILLAR, France; BRITISH ENERGY, UK; SHELL, The Netherlands; IWT, Germany; BAY ZOLTAN INSTITUTE, Hungary; CSM, Italy; HSE, UK; ALSTON POWER, UK; IWM, Germany; UNIVERSITY OF MARIBOR, Slovenia; NRC, Belgium; ADVENTICA, UK; CRF-FIAT, Italy; CEIT, Spain; FORCE INSTITUTE, Denmark; UNIVERSITY OF GENT, Belgium; INNOSPEXION, Denmark; KIELCE UNIVERSITY OF TECHNOLOGY, Poland; ROLLS-ROYCE, UK; DNV, Sweden; TECHNICAL UNIVERSITY OF DARMSTADT, Germany; IIS, Italy; MPA, Germany; UNIVERSITY OF AVEIRO, Portugal; IdS, France; EMPA, Switzerland; ALCAN/PECHINEY, France; IRSID- USINOR, France; CETIM, France; SKODA, Czech Republic; BUREAU VERITAS, France; DLR Köln, Germany;NPL, UK; EDF, France; ICOM-EPFL, Switzerland; HITACHI, Japan; BATTELLE,USA; TECNATOM, Spain; BiSAFE, Czech Republic; UNIVERSITY OF OSAKA, Japan; KOREAN UNIVERSITY, Korea. This document has been developed with valuable contributions of various experts. I would like to acknowledge all contributions of the FITNET TN consortium members, particularly RA Ainsworth, S Webster, JJ Janosch, R Koers, I Hadley, K Wallin, N Taylor, F Solana, S Szavai, A Laukkanen, Y Tkach, E Lucon, W De Waele, K Hasegawa, H.J. Schmidt, B. Schmidt-Brandecker, Editors M Koçak, GKSS RESEARCH CENTER, Germany S Webster, CORUS, UK JJ Janosch, CATERPILLAR, France RA Ainsworth, BRITISH ENERGY, UK R Koers, SHELL, The Netherlands 17 th May 2006, Amsterdam xx FITNET 2006 All rights reserved

21 Introduction Presence or occurrence of damage in engineering components may have different origins and growth mechanisms depending on the application area, type of the component and loading conditions etc. Four major failure modes; fracture, fatigue, creep and corrosion have generally been identified as most frequent failure modes of engineering structures and hence different Fitness-for-Service (FFS) methodologies developed to cover these failure mechanisms. Certainly, failure of a component may include numbers of these four basic mechanisms and their interactions at different stages of the damage process in service. Numbers of FFS procedures (include analytical methods) have been specifically developed and used to address the components of a particular industrial sector. A number of industrial sectors, such as nuclear power, petrochemical, offshore, aerospace or pipeline girth weld applications have partly established FFS standards in place for the assessment of flaws found in-service. However, their use in design and fabrication of new engineering components has been limited to some areas. Furthermore, some methods for design and remaining life assessments of metallic structures with or without welds are still unduly conservative in different loading regimes. Hence, there is still a need to generate a general purpose, unified, comprehensive and updated FFS methodology endorsed by CEN for standardisation in Europe. Therefore, European Community funded project FITNET in the form of Thematic Network (TN) organisation has started to review the existing FFS procedures and develop an updated, unified and verified European FITNET FFS Procedure to cover structural integrity analysis to avoid failures due to fracture, fatigue, creep and corrosion. FITNET Fitness for Service (FFS) Procedure aims to provide better design principles, support for fabrication of new components and advanced analysis routes for prevention of failures in-service due to fracture, fatigue, creep and corrosion damages (no coverage of structural instability due to buckling). FITNET FFS Procedure can be used to establish the size limits for defects in various metallic engineering structures can provide substantial cost savings in operating such structures. The use of FITNET FFS Procedure involves making a critical engineering assessment of a component containing a defect to ensure its structural integrity for its intended design life or its next inspection period. The outcome of the assessment of a component in service is a decision to operate as is, repair, monitor (including re-setting of inspection intervals), or replace. Therefore, engineering assessment procedures containing analytical expressions have been developed to assess (primarily to provide conservative estimation of the critical condition) the structural significance of the flaws or damage. Conventional design approach and operation principles implicitly assume that the component (loadcarrying) is defect-free. However, even components fabricated by good workmanship principles may contain or develop cracks and hence need to be assessed using modern FFS methodologies for provision of structural safety, improvement of in-service inspection intervals and/or for establishment of life extension measures. The FITNET FFS flaw assessment methodology in the form of a step-by-step procedure is set out for assessing a welded or non-welded metallic component containing a known or postulated flaw under static, fatigue, creep loading conditions or component subject to a corrosion damage (e.g. metal loss). Figure 1 is showing the overall structure of the FITNET FFS procedure. The technical content of the procedure will contain prescriptive (defined, mandatory methodology for decision making analysis) and informative (guidance for user as recommendations) sections. Prescriptive parts aims to provide an engineering methodology for assessing flaws to reach a decision about the component. The informative parts serve to provide best-of-knowledge recommendations to the user. These parts may contain information without sufficient validation. When a defect has been detected in a component that has been in service, the conservative assumption for the analysis of continuum damage is that the crack initiated early in life. This approach should be adopted unless there is evidence to the contrary. However, the FITNET FFS procedure is not being developed only for the assessment of damage of structures in-service, but also aims to be applied a component that has not yet seen operation as well as should be applied to a failed component to clarify the cause of the failure. Therefore, FITNET FFS Procedure is applicable to four major stages of a typical component life: 1) Design of New Component, 2) Fabrication Support and Quality Assurance, 3) Assessment of In-Service Component, 4) Failure Analysis The overall flow-chart of the FITNET FFS Procedure is shown below to demonstrate its basic features. FITNET 2006 All rights reserved xxi

22 Design of New Structures Application Areas of FITNET FFS Procedure Fabrication Support In-Service Assessment Failure Analysis Information Required for Assessment (Inputs) Flaw Information Stresses Material Properties Assessment Modules Fracture Fatigue Creep Corrosion Assessment and Reporting of Results Alternative Approaches and Specific Applications (LBB, Local Approach, Crack Arrest, Mixed-Mode, Bi-Material etc.) Additional Information Compendia (K-Solutions etc.) Validation and Case Studies For the application of this FFS procedure to optimise the design of a new component, it is usually a postulated defect is used to assess the critical condition of a new component for a given material, load/stress conditions and geometry of the component. In this context, a postulated initial defect size will be based on the non-destructive evaluation (NDE) detection limit. Depending on the design philosophy (safe-life or damage tolerant (fail-safe) design etc.) in combination of the NDE technique is used as well as good description of the loading conditions of a new component, FITNET FFS can provide information for re-selection of material, design&fabrication route and NDE technique. The FITNET FFS assessment modules require, in general, for the components in-service the following interdisciplinary inputs: Description/knowledge (mechanism) of damage Determination of operating conditions, load/stress analysis Flaw characterisation (location, sizing via NDE) Material properties (incl. environmental effects) FITNET FFS assessment results aim to provide information on material selection and hence the most suitable fabrication route for safe and economical performance. Analysis results can yield, for example, the required minimum fracture toughness for a given loading conditions and postulated defect size or can provide maximum tolerable defect size (e.g., weld imperfection) for a given material, loading conditions and fabrication route. Again an applicability of the FFS analysis in an efficient manner to support the fabrication route and quality assurance in addition to the conventional good workmanship principles will depend on the capability of the applied NDE technique and its probability of detection of a flaw. According to the practical workmanship xxii FITNET 2006 All rights reserved

23 criteria, no crack or defect may occur during the fabrication of a component is acceptable if size of the defect exceeds the detection limit of the NDE method used (i.e., some small flaws smaller than this limit may exist in the component). At this stage, FITNET FFS methodology can provide an engineering analysis to predict the critical condition of a new component using either postulated larger defect or defect size defined by the NDE detection limit and hence can give opportunity to the designer for possible reselection of failure criteria (i.e., against crack initiation or allowance for crack growth with respective inspection intervals), design load, material type or fabrication route. It is often necessary to critically examine the integrity of new or existing constructions by the use of nondestructive testing methods, it is also necessary to establish acceptance levels for the flaws detected. In the present procedure, the derivation of acceptance levels for flaws is based on the concept of 'fitness-forservice'. By this principle a particular fabrication is considered to be adequate for its purpose, provided the conditions to cause failure are not reached, after allowing for some measure of abnormal use or degradation in service. A distinction has to be made between acceptance based on quality control and acceptance based on fitness-for-service. Quality control levels are, of necessity, both arbitrary and usually conservative and are of considerable value in the monitoring of quality during production. Flaws which are less severe than such quality control levels as given, for example, in current application standards, are acceptable without further consideration. If flaws more severe than the quality control levels are revealed, rejection is not necessarily automatic. In such situations decisions on whether rejection and/or repairs are justified may be based on fitness-for-purpose, either in the light of previously documented experience with similar material, stress and environmental combinations or on the basis of 'engineering critical assessment; (ECA). It is with the latter that this FITNET FFS Procedure document is concerned. It is emphasised, however, that a proliferation of flaws, even if shown to be acceptable by ECA, should be regarded as indicating that quality is in need of improvement. The quality of the major input data (flaw size, stresses and material properties) has a significant effect on the FFS analysis results, and hence efforts should be made to generate data or use a good data bank. After conducting an assessment for a given component with in-service flaw, a sensitivity analysis with modified or new set of input data can provide accurate description of the limiting conditions of the component. If flaw indications are detected during in-service inspections of a component, the indications are carefully examined to determine the dimensions of these flaws. If multiple discrete flaws are in the same cross-section, the flaws may lead to flaw interaction and hence are treated as a single flaw, using the distance between adjacent flaws as the combination criteria. Before performing a FFS analysis for flaws detected in in-service components, an investigation should be carried out to determine the most likely cause of cracking. At least, this should involve a combination of non destructive testing, visual examination and metallurgical examination (e.g., fatigue crack growth tends to be transgranular whereas creep crack growth is generally intergranular). If it is possible to identify the mode and mechanism of cracking, this should provide qualitative information on the relative contributions of the different mechanisms and phases to the overall damage process. Based on the results, a selection of the analysis module should be made. Training and education of young engineers in Europe to conduct Fitness-for-Service analysis of engineering structures is essential to maintain safety of structures with and without welds. FITNET thematic network has developed training and education documents and conducted various seminars in Europe. FITNET FFS Volume II includes therefore a special section for tutorials purpose. The tutorials part covers the use of the different modules of the procedure in a set of examples related with fracture, fatigue, creep, stress corrosion cracking and local thinned areas, as well as some crossed examples where different modules have to be used to assess the dominant failure conditions. The procedure is completed with a Training Package, in electronic version, covering all four Modules (Fracture, Fatigue, Creep and Corrosion) with four different sections: The basic concepts of FFS analysis The structure of the FFS procedures and its link with the basic concepts. A summary of the FITNET FFS modules. Some practical examples. FITNET 2006 All rights reserved xxiii