CEN TR 15235-2005 Welding - Methods for assessing imperfections in metallic structures《焊接 金属结构中缺陷评估方法》.pdf

1、PUBLISHED DOCUMENT PD CEN/TR 15235:2005 Welding Methods for assessing imperfections in metallic structures ICS 25.160.40 PD CEN/TR 15235:2005 This Published Document was published under the authority of the Standards Policy and Strategy Committee on 21 December 2005 BSI 21 December 2005 ISBN 0 580 4

2、7133 0 National foreword This Published Document is the official English language version of CEN/TR 15235:2005. The UK participation in its preparation was entrusted to Technical Committee WEE/37, Acceptance levels for welds, which has the responsibility to: aid enquirers to understand the text; pre

3、sent to the responsible international/European committee any enquiries on the interpretation, or proposals for change, and keep UK interests informed; monitor related international and European developments and promulgate them in the UK. A list of organizations represented on this committee can be o

4、btained on request to its secretary. Cross-references The British Standards which implement international or European publications referred to in this document may be found in the BSI Catalogue under the section entitled “International Standards Correspondence Index”, or by using the “Search” facili

5、ty of the BSI Electronic Catalogue or of British Standards Online. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a Published Document does not of itself confer immunity from legal obligation

6、s. Summary of pages This document comprises a front cover, an inside front cover, the CEN/TR title page, pages 2 to 13 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Amendments issued since publication Amd. No. Date CommentsTECHNICA

7、LREPORT RAPPORTTECHNIQUE TECHNISCHERBERICHT CEN/TR15235 October2005 ICS25.160.40 EnglishVersion WeldingMethodsforassessingimperfectionsinmetallic structures SoudageMthodesdvaluationdesdfautsdansles constructionsmtalliques SchweienVerfahrenzurBeurteilungvon UnregelmigkeitenbeimetallischenBauteilen Th

8、isTechnicalReportwasapprovedbyCENon22September2005.IthasbeendrawnupbytheTechnicalCommitteeCEN/TC121. CENmembersarethenationalstandardsbodiesofAustria,Belgium,Cyprus,CzechRepublic,Denmark,Estonia,Finland,France, Germany,Greece,Hungary,Iceland,Ireland,Italy,Latvia,Lithuania,Luxembourg,Malta,Netherland

9、s,Norway,Poland,Portugal, Slovakia, Slovenia,Spain,Sweden,SwitzerlandandUnitedKingdom. EUROPEANCOMMITTEEFORSTANDARDIZATION COMITEUROPENDENORMALISATION EUROPISCHESKOMITEEFRNORMUNG ManagementCentre:ruedeStassart,36B1050Brussels 2005CEN Allrightsofexploitationinanyformandbyanymeansreserved worldwidefor

10、CENnationalMembers. Ref.No.CEN/TR15235:2005:ECEN/TR 15235:2005 2 Contents Page Foreword 3 Introduction4 1 Scope .5 2 Terms and definitions.5 3 Symbols and abbreviations5 4 ECA principles 6 5 Safety considerations.6 6 Existing ECA-procedures.9 Bibliography.13 CEN/TR 15235:2005 3 Foreword This CEN Tec

11、hnical Report (CEN/TR 15235:2005) has been prepared by Technical Committee CEN/TC 121 “Welding”, the secretariat of which is held by DIN. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN shall not be held responsible for identi

12、fying any or all such patent rights. 4 Introduction European provisions for assessing imperfections in metallic structures are needed to meet the requirements of industry. The technology is being applied by many industries for materials selection, design and fabrication and in-service assessment usi

13、ng existing methods. Engineering Critical Assessment (ECA) methods for the assessment of imperfections have received further support by the EC directive 97/23/EC concerning pressure equipment (PED) which permits such methods as an alternative to conventional methods. The present Technical Report giv

14、es guidance to the application of BS 7910 and the European SINTAP Report. Some further documents are also mentioned. Experience from the application should, in a few years, provide enhanced technology in the subject and eventually permit standardisation at the European level. Conventional design pro

15、cedures involve application of mathematical models such as the theory of elasticity. Actions are described by characteristics such as stress and strain. Resistance described by characteristics such as yield stress and ultimate limit stress. The designer has to assure that the resistance of the struc

16、ture is adequate, using adequate safety factors, partial coefficients, etc. The mathematical models presuppose a homogenous material. Many failure modes involve cracks. Failure may originate from a crack and/or failure may propagate (slow or fast) as a crack. Application of the conventional theory o

17、f elasticity to a structure with a crack leads to a singularity at the crack tip because the stresses approach infinity. To this should be added that a closer study of the fracture processes shows that in-homogeneities such as grain structure and even the atomic structure may influence the mode of f

18、racture. Conventional design procedures can, for these reasons, not be applied in situations where an analysis of the significance of a crack-like imperfection is necessary and they cannot be applied for an analysis of the propagation of fatigue cracks, creep cracks, stress corrosion cracks, etc. Al

19、ternative methods termed fracture mechanics have been developed in order to model the behaviour of structures containing cracks. Fracture mechanics interpret crack driving force and materials resistance by an alternative set of parameters such as stress intensity factor, crack tip opening displaceme

20、nt, etc. Engineering critical assessments use a combination of conventional design procedures and fracture mechanics calculations, depending on the nature of the imperfection and the likely type of failure. General corrosion results for example in a reduction in cross section and may be analysed by

21、conventional design procedures whereas propagation of fatigue cracks has to be analysed by fracture mechanics methods. CEN/TR 15235:20055 1 Scope This Technical Report provides guidance on the selection and application of methods for assessing the significance of imperfections in all types of struct

22、ures and components. The guidance is primarily tailored to welded structures and components in steel or aluminium alloys. Some of the methods may also be applied for other types of metals and for non-welded structures and components. 2 Terms and definitions For the purposes of this Technical Report,

23、 the following definitions apply: ECA Engineering Critical Assessment methods for the assessment of the significance of imperfections for the strength and usability of structures (see also clause 4) FAD Failure Assessment Diagram combines the analysis of the safety against plastic instability and fi

24、nal fracture in a single diagram 3 Symbols and abbreviations CDF Crack Driving Force plot ETM Engineering Treatment Model FITNET European Fitness-for-service Network HIDA High Temperature Defect Assessment SINTAP Structural Integrity assessment procedures for European industry The following symbols

25、are used to characterise the local stress-strain field around the crack front. They are (usually with subscripts) used for crack driving force as well as resistance. K stress intensity factor J a line or surface integral that encloses the crack front from one crack surface to the other CTOD Crack Ti

26、p Opening Displacement See the publications listed in the clause “Bibliography“ (in particular references 1 and 2) for further detail. CEN/TR 15235:20056 4 ECA principles ECA is a designation for methods used for the assessment of the acceptability of imperfections. Assessment of the acceptability i

27、nvolves consideration of: a) Legal requirements Legal requirements and/or provisions in the code(s) for the structure in question or contractual requirements may restrict the acceptance. Mandatory acceptance criteria, to be used for fabrication of new structures may e.g. be specified in the code or

28、contract covering the structure. b) Contractual requirements The application of ECA methods should be acceptable to the parties concerned in each particular case. c) Commercial requirements Costs and market position may influence the benefits or disadvantages of an application d) Requirements to fab

29、rication. A key consideration is maintenance of proper quality control. 5 Safety considerations 5.1 Conventional provisions for acceptance of welded structures Standards for design and fabrication of welded structures do, as a general rule, include provisions for inspection and testing of the welded

30、 joints. The standards usually specify: a) Acceptance levels for imperfections, normally by reference to a quality level in standards such as EN ISO 5817. b) Methods for non-destructive testing by reference to the comprehensive system of EN standards for NDT, at least by reference to EN 12062. c) Th

31、e amount of testing (100% or examination of only a part of the welds). d) Procedures for action when non-conformity is detected, typically requirements for repair, re-examination and some supplementary non-destructive testing. e) Appropriate safety factors. Conventional non-destructive testing metho

32、ds involve an element of subjective judgement and the output of the testing is considered to be an evaluation and not a measurement (even though figures may be reported). The evaluation has two final outcomes: Accepted or not accepted. CEN/TR 15235:20057 Table 1 Outcomes of conventional inspections

33、Structure Result of inspection Safe Unsafe Accepted This should be the normal outcome. Really dangerous situation where a potentially unsafe structure is accepted by mistake, neglect or inefficient inspection procedures. Usually termed customers and societys risk. Design codes, etc. aim for reductio

34、n of this risk to very low levels for critical structures. Not accepted This outcome represents an expense due to unnecessary scrapping or repair in order to make the structure formally acceptable. One possible application for ECA (see below) is to document the inherent safety of the structure and t

35、hus avoid scrapping or repair. Usually termed producers risk. Rejection of an unsafe structure saves the customer and the society from a potential risk. However, it necessitates scrapping or repair in order to make the structure safe and it results in expenses and also a waste of resources. Experien

36、ce has shown that the system results in structures characterised by acceptable risks of failure (customers and societys risk). The actual risk depends on the nature of the structure and on the failure mode. The acceptable risk for sudden, catastrophic failure may be of the order 10 -6or even lower f

37、or critical structures. The acceptable risk of having substantial fatigue cracks prior to expiration of the stipulated life time of the structure may be much higher, for example of the order 10 -2 . 5.2 Application of ECA for new products Application of ECA as a tool for specification of quality cri

38、teria for new structures is feasible in theory but difficult in practice. ECA shall not be invoked as an excuse for acceptance of poor workmanship. Application of ECA involves several requirements: a) Fracture toughness and other relevant materials data for weld metal, parent metal and heat affected

39、 zones have to be determined. This is usually performed as part of the welding procedure qualification. However, strict process control of welding operations is required in order to assure that materials data obtained during procedure testing are truly representative. If not, testing of production t

40、est plates may be required. b) The welds have to be inspected by one or more procedures for non-destructive testing able to: Detect all potentially dangerous imperfections. Determine the type of the imperfections, at least to distinguish between planar and non-planar imperfections. Measure imperfect

41、ion size, position and orientation. c) All procedures for non-destructive testing have to be validated on representative samples and the inspection uncertainties determined. d) Safety factors have to be calculated in order to counteract inspection uncertainties and other uncertainties. This may invo

42、lve application of advanced probabilistic methods. CEN/TR 15235:20058 e) Acceptance criteria, extents of testing and other quality criteria have to be specified. The common procedure for measurement of imperfections in the weldments is ultrasonic testing (UT), standardised in several European test s

43、pecifications. UT requires in general a high quality of the weld metal as regards porosity and slag inclusions which may mask more serious imperfections. The principles for evaluation of a measurement of dimensions are specified in EN ISO 14253-1. A measured value Y is associated with a measuring un

44、certainty U. U is usually determined from the measurement standard deviation multiplied by a safety factor. The real value may be any value in the interval Y U (with a confidence determined by the safety factor). For a largest acceptable imperfection A, the acceptance limit consequently becomes: Y +

45、 U A This illustrates the key role of the uncertainty U. The results of ECAs should only be used with a clear knowledge of the uncertainties involved in detection, sizing and identification, of imperfections. It should be noted that virtually none of the published NDT standards include provisions fo

46、r the determination of uncertainties. Specification of adequate safety factors or partial coefficients is not simple because standards for design and fabrication of structures and products rarely specify safety factors or partial coefficients for ECA. Safety considerations are covered in an annex to

47、 BS 7910, Annex K: “Reliability, partial safety factors, number of tests and reserve factors”. However, compatibility with the relevant design code can be assured with expert knowledge. ECA may, therefore be used for new structures. One case, where the application may be most useful, is for design o

48、f fatigue loaded structures. Fatigue cracks are likely to initiate at welds in areas of high structural stress concentrations. Finite element analysis + ECA may be the only alternative to full-scale fatigue testing in such cases. Inspection uncertainties are of less importance because the safety mai

49、nly depends on visual examination of weld surface quality and in particular the occurrence of undercut in the critical areas. Non- conformity may be removed by grinding. Specification of safety factors is also comparatively simple when growth of fatigue cracks is the dominating failure mode. Another case involves intentional intr