Welding - Methods for assessing imperfections in metallic structures

This Technical Report provides guidance on the selection and application of methods for assessing the significance of imperfections in all types of structures 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.

Schweißen - Verfahren zur Beurteilung von Unregelmäßigkeiten bei metallischen Bauteilen

Dieser Technische Bericht liefert einen Leitfaden für die Auswahl und Anwendung von Verfahren zur Beurteilung der Bedeutung vor Unregelmäßigkeiten für alle Arten von Bauwerken und -teilen. Der Leitfaden ist ursprünglich auf geschweißte Bauwerke und -teile aus Stahl- oder Aluminiumlegierungen zugeschnitten. Einige der Verfahren können auch für andere Werkstoffarten und für nicht geschweißte Bauwerke und -teile angewendet werden.

Soudage - Méthodes d'évaluation des défauts dans les constructions métalliques

Le présent Rapport technique fournit des lignes directrices pour le choix et l?application de méthodes
permettant l?évaluation de la signification des défauts dans tous les types de constructions et de composants.
Les lignes directrices sont essentiellement destinées aux constructions et composants soudés en acier et en
alliages d?aluminium. Certaines des méthodes peuvent également etre appliquées a d?autres types de métaux
ainsi que pour des constructions et des composants non soudés.

Varjenje – Metode ocenjevanja nepopolnosti v strukturi kovin

General Information

Status
Published
Publication Date
28-Feb-2006
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Mar-2006
Due Date
01-Mar-2006
Completion Date
01-Mar-2006

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SLOVENSKI STANDARD
SIST-TP CEN/TR 15235:2006
01-marec-2006
Varjenje – Metode ocenjevanja nepopolnosti v strukturi kovin
Welding - Methods for assessing imperfections in metallic structures
Schweißen - Verfahren zur Beurteilung von Unregelmäßigkeiten bei metallischen
Bauteilen
Soudage - Méthodes d'évaluation des défauts dans les constructions métalliques
Ta slovenski standard je istoveten z: CEN/TR 15235:2005
ICS:
25.160.40 Varjeni spoji in vari Welded joints
SIST-TP CEN/TR 15235:2006 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CEN/TR 15235:2006

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SIST-TP CEN/TR 15235:2006
TECHNICAL REPORT
CEN/TR 15235
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
October 2005
ICS 25.160.40

English Version
Welding - Methods for assessing imperfections in metallic
structures
Soudage - Méthodes d'évaluation des défauts dans les Schweißen - Verfahren zur Beurteilung von
constructions métalliques Unregelmäßigkeiten bei metallischen Bauteilen
This Technical Report was approved by CEN on 22 September 2005. It has been drawn up by the Technical Committee CEN/TC 121.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15235:2005: E
worldwide for CEN national Members.

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SIST-TP CEN/TR 15235:2006
CEN/TR 15235:2005 (E)
Contents Page
Foreword .3
Introduction.4
1 Scope .5
2 Terms and definitions.5
3 Symbols and abbreviations.5
4 ECA principles .6
5 Safety considerations.6
6 Existing ECA-procedures.9
Bibliography.13

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Foreword
This CEN Technical 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 identifying any or all such patent rights.
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CEN/TR 15235:2005 (E)
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 using 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 gives 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 procedures 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 structure 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 of 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 fracture. 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.
Alternative 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 displacement, 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 conventional design
procedures whereas propagation of fatigue cracks has to be analysed by fracture mechanics methods.
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1 Scope
This Technical Report provides guidance on the selection and application of methods for assessing the
significance of imperfections in all types of structures 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, 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 final 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 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 Tip Opening Displacement
See the publications listed in the clause "Bibliography" (in particular references [1] and [2]) for further detail.
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4 ECA principles
ECA is a designation for methods used for the assessment of the acceptability of imperfections.
Assessment of the acceptability involves 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 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 fabrication.
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 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) The 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 methods 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.
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Table 1 — Outcomes of conventional inspections
Structure
Result of
inspection
Safe Unsafe
Accepted This should be the normal Really dangerous situation where a
outcome. potentially unsafe structure is accepted
by mistake, neglect or inefficient
inspection procedures. Usually termed
customer’s and society’s risk. Design
codes, etc. aim for reduction of this risk
to very low levels for critical structures.
Not accepted This outcome represents an Rejection of an unsafe structure saves
expense due to unnecessary the customer and the society from a
scrapping or repair in order to potential risk. However, it necessitates
make the structure formally scrapping or repair in order to make the
acceptable. One possible structure safe and it results in
application for ECA (see below) expenses and also a waste of
is to document the inherent resources.
safety of the structure and thus
avoid scrapping or repair.
Usually termed producer’s risk.

Experience has shown that the system results in structures characterised by acceptable risks of failure
(customer’s and society’s risk). The actual risk depends on the nature of the structure and on the failure mode.
-6
The acceptable risk for sudden, catastrophic failure may be of the order 10 or even lower for critical
structures. The acceptable risk of having substantial fatigue cracks prior to expiration of the stipulated life time
-2
of the structure may be much higher, for example of the order 10 .
5.2 Application of ECA for new products
Application of ECA as a tool for specification of quality criteria 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 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 test 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 imperfection 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 involve application of advanced probabilistic methods.
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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 specifications. 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 uncertainty 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 + 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 for 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 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 of 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 mainly 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 introduc
...

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