ISO 27306:2016

Metallic materials — Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components

In fracture assessments of steel structures containing cracks, it has generally been assumed that the fracture resistance of fracture toughness specimens is equal to the fracture resistance of structural components. However, such an assumption often leads to excessively conservative fracture assessments. This is due to a loss of plastic constraint in structural components, which are subjected mainly to tensile loading. By contrast, fracture toughness specimens hold a constrained stress state near the crack-tip due to bending mode. The loss of constraint is significant for high strength steels with high yield-to-tensile ratios (= yield stress/tensile strength) which have been extensively developed and widely applied to structures in recent years. ISO 27306:2016 specifies a method for converting the CTOD (crack-tip opening displacement) fracture toughness obtained from laboratory specimens to an equivalent CTOD for structural components, taking constraint loss into account. This method can also apply to fracture assessment using the stress intensity factor or the J-integral concept (see Clause 9). ISO 27306:2016 deals with the unstable fracture that occurs from a crack-like defect or fatigue crack in ferritic structural steels. Unstable fracture accompanied by a significant amount of ductile crack extension and ductile fractures are not included in the scope hereof. The CTOD fracture toughness of structural steels is measured in accordance with the established test methods, ISO 12135[1] or BS 7448-1. The fracture assessment of a cracked component is done using an established method such as FAD (Failure Assessment Diagram) in the organization concerned, and reference is not made to the details thereof in ISO 27306:2016. It can be used for eliminating the excessive conservatism frequently associated with the conventional fracture mechanics methods and accurately assessing the unstable fracture initiation limit of structural components from the fracture toughness of the structural steel. This is also used for rationally determining the fracture toughness of materials to meet the design requirements of performance of structural components.

Matériaux métalliques — Méthode de correction de perte de contrainte du CTOD de la ténacité à la rupture pour l'évaluation de la rupture des composants en acier

General Information

• Status: Published
• Publication Date: 18-Sep-2016
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: ISO/TC 164/SC 4 - Fatigue, fracture and toughness testing
• Drafting Committee: ISO/TC 164/SC 4/WG 3 - Fracture toughness
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 03-Oct-2022

Relations

• Revises: ISO 27306:2009 - Metallic materials — Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components
• Effective Date: 05-Nov-2015

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 27306
Second edition
2016-09-15
Metallic materials — Method of
constraint loss correction of CTOD
fracture toughness for fracture
assessment of steel components
Matériaux métalliques — Méthode de correction de perte de
contrainte du CTOD de la ténacité à la rupture pour l’évaluation de la
rupture des composants en acier
Reference number
ISO 27306:2016(E)
©
ISO 2016

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ISO 27306:2016(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
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Tel. [phone]
Fax [phone]
[e-mail]
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ii © ISO 2016 – All rights reserved

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ISO 27306:2016(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units . 3
5 Principle . 3
6 Structural components of concern . 4
7 Conditions for use . 5
8 Assessment levels I, II, and III . 6
8.1 General . 6
8.2 Level I: Simplified assessment . 6
8.3 Level II: Normal assessment . 7
8.4 Level III: Material specific assessment . 7
9 Equivalent CTOD ratio, β. 7
9.1 General . 7
9.2 Factors influencing the equivalent CTOD ratio, β .7
9.3 Procedure for calculating the equivalent CTOD ratio, β, at assessment levels I to III . 8
9.3.1 General. 8
9.3.2 Surface crack cases (CSCP and ESCP) . 8
9.3.3 Through-thickness crack cases (CTCP and ETCP) . 9
Annex A (informative) Procedure for the selection of Weibull parameter, m, at level
II assessment .17
Annex B (informative) Analytical method for the determination of Weibull parameter, m, at
level III assessment .19
Annex C (informative) Guidelines for the equivalent CTOD ratio, β .24
Annex D (informative) Examples of fracture assessment using the equivalent CTOD ratio, β .31
Bibliography .47
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ISO 27306:2016(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the
Technical Barriers to Trade (TBT) see the following URL: www.iso.org/iso/foreword.html.
The committee responsible for this document is ISO/TC 164 Mechanical Testing of Metals, Subcommittee
SC 4, Toughness testing — Fracture (F), Pendulum (P), Tear (T).
This second edition cancels and replaces the first edition (ISO 27306:2009), which has been technically
revised.
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INTERNATIONAL STANDARD ISO 27306:2016(E)
Metallic materials — Method of constraint loss correction
of CTOD fracture toughness for fracture assessment of steel
components
1 Scope
In fracture assessments of steel structures containing cracks, it has generally been assumed that
the fracture resistance of fracture toughness specimens is equal to the fracture resistance of
structural components. However, such an assumption often leads to excessively conservative fracture
assessments. This is due to a loss of plastic constraint in structural components, which are subjected
mainly to tensile loading. By contrast, fracture toughness specimens hold a constrained stress state
near the crack-tip due to bending mode. The loss of constraint is significant for high strength steels with
high yield-to-tensile ratios (= yield stress/tensile strength) which have been extensively developed and
widely applied to structures in recent years.
This International Standard specifies a method for converting the CTOD (crack-tip opening
displacement) fracture toughness obtained from laboratory specimens to an equivalent CTOD for
structural components, taking constraint loss into account. This method can also apply to fracture
assessment using the stress intensity factor or the J-integral concept (see Clause 9).
This International Standard deals with the unstable fracture that occurs from a crack-like defect or
fatigue crack in ferritic structural steels. Unstable fracture accompanied by a significant amount of
ductile crack extension and ductile fractures are not included in the scope hereof.
The CTOD fracture toughness of structural steels is measured in accordance with the established test
1)
methods, ISO 12135 or BS 7448-1. The fracture assessment of a cracked component is done using an
established method such as FAD (Failure Assessment Diagram) in the organization concerned, and
reference is not made to the details thereof in this International Standard.
This International Standard can be used for eliminating the excessive conservatism frequently
associated with the conventional fracture mechanics methods and accurately assessing the unstable
fracture initiation limit of structural components from the fracture toughness of the structural steel.
This is also used for rationally determining the fracture toughness of materials to meet the design
requirements of performance of structural components.
2 Normative references
The following referenced documents are indispensable for the application of this International
Standard. For dated references, only the edition cited applies. For updated references, the latest edition
of the referenced document (including any amendments) applies.
ISO 12135, Metallic materials — Unified method of test for the determination of quasistatic fracture
toughness
BS 7448-1, Fracture mechanics toughness tests —Part 1: Method for determination of K , critical CTOD
Ic
and critical J values of metallic materials
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 12135 and the following apply.
1) To be published.
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ISO 27306:2016(E)

3.1
CTOD of standard fracture toughness specimen
crack-tip opening displacement of standard fracture toughness specimen
δ
CTOD, as the fracture driving force, for the standard fracture toughness specimen (three-point bend or
compact specimen) with 0,45 ≤ a /W ≤ 0,55, where a and W are the initial crack length and specimen
0 0
width, respectively
3.2
CTOD fracture toughness
crack-tip opening displacement fracture toughness
δ
cr
critical CTOD at the onset of brittle fracture in the standard fracture toughness specimen [δ (B) as
c
defined in ISO 12135] with 0,45 ≤ a /W ≤ 0,55
0
3.3
CTOD of structural component
crack-tip opening displacement of structural component
δ
WP
CTOD, as the fracture driving force, for a through-thickness crack or a surface crack existing in a
structural component regarded as a wide plate
Note 1 to entry: The CTOD of a surface crack is defined at the maximum crack depth.
3.4
critical CTOD of structural component
critical crack-tip opening displacement of structural component
δ
WP,cr
critical CTOD at the onset of brittle fracture in structural components
3.5
equivalent CTOD ratio
equivalent crack-tip opening displacement ratio
β
CTOD ratio defined by δ/δ , where δ and δ are CTODs of the standard fracture toughness specimen
WP WP
and the structural component, respectively, at the same level of the Weibull stress σ
W
Note 1 to entry: See Figure 1.
Note 2 to entry: See Reference [1].
3.6
Weibull stress
σ
W
fracture driving force defined with the consideration of statistical instability of microcracks in the
fracture process zone against brittle fracture
Note 1 to entry: See Reference [2].
3.7
critical Weibull stress
σ
W,cr
Weibull stress at the onset of unstable fracture
3.8
Weibull shape parameter
m
material parameter used in the definition of the Weibull stress; one of two parameters describing the
statistical distribution of the critical Weibull stress, σ
W, cr
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ISO 27306:2016(E)

3.9
yield-to-tensile ratio
R
Y
ratio of yield strength, σ , (lower yield point, R , or 0,2% proof strength, R ) to tensile strength, R
Y eL p0,2 m
4 Symbols and units
For the purposes of this document, the following symbols, units, and designations are applied in
addition to those in ISO 12135.
Symbol Unit Designation
a mm Depth of surface crack or half-length of through-thickness crack in structural component
c mm Half-length of surface crack in structural component
m — Weibull shape parameter
t mm Plate thickness
3
V mm Reference volume defined for Weibull stress
0
3
V mm Volume of fracture process zone
f
R — Yield-to-tensile ratio (= σ /R )
Y Y m
β — Equivalent CTOD ratio
Equivalent CTOD ratio for reference crack length
β —
0
(In cases of surface crack panel, β is defined for plate thickness t = 25 mm.)
0
Equivalent CTOD ratio for target length of centre surface crack or double-edge surface crack
β —
2c, t
on target plate thickness
Equivalent CTOD ratio for target length of centre through-thickness crack or double-edge
β —
2a
through-thickness crack
β — Equivalent CTOD ratio for target length of single-edge surface crack on target plate thickness
c, t
β — Equivalent CTOD ratio for target length of single-edge through-thickness crack
a
δ mm CTOD of standard fracture toughness specimen
Critical CTOD of standard fracture toughness specimen at onset of brittle fracture (CTOD
δ mm
cr
fracture toughness)
δ mm CTOD at small-scale yielding limit for standard fracture toughness specimen
SSY limit
δ mm CTOD of structural component
WP
δ mm Critical CTOD of structural component at onset of brittle fracture
WP, cr
σ MPa Effective stress used for the calculation of Weibull stress
eff
σ MPa Lower yield point, R , or 0,2 % proof strength, R
Y eL p0,2
σ MPa Weibull stress
W
σ MPa Critical Weibull stress at onset of brittle fracture
W, cr
5 Principle
This International Standard deals with the initiation of unstable fracture due to cleavage of structural
steels. It presents a method for converting the CTOD fracture toughness obtained from the standard
fracture toughness specimen [three-point bend or compact specimen with 0,45 ≤ a /W ≤ 0,55 and B
0
(specimen thickness) = t (plate thickness of structural component)], which are characterized by an
extremely severe plastic constraint in the vicinity of the crack-tip, to an equivalent critical CTOD for
structural components, which are generally characterized by less constraint. The reverse procedure
is also possible with this method. Thus, this method links fracture toughness tests and fracture
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ISO 27306:2016(E)

performance assessments of structural components by taking account of loss of plastic constraint in
structural components, as shown in Figure 2.
NOTE 1 The fracture toughness specimen with a deep crack such as a /W = 0,7 presents somewhat higher
0
constraint near the crack-tip than that with 0,45 ≤ a /W ≤ 0,55. The equivalent CTOD ratio β defined in this
0
International Standard leads to a conservative fracture assessment, if the user employs a deep cracked specimen
with a /W > 0,55.
0
NOTE 2 This International Standard does not intend to address size and temperature effects nor influence of
[3]
data scatter on the results. Refer to ASTM E1921-13a for guidance.
The CTOD fracture toughness (critical CTOD) of the standard fracture toughness specimen is
determined in accordance with the established test methods (ISO 12135 or BS 7448-1). The fracture
assessment of a cracked component can be done using established methods at the user’s discretion such
as Failure Assessment Diagram (FAD) and CTOD design curve in the organization concerned.
The critical CTOD of the standard fracture toughness specimen is converted to the critical CTOD of the
structural component using the equivalent CTOD ratio, β. The equivalent CTOD ratio, β, is defined as a
CTOD ratio, δ/δ , where δ and δ are CTODs of the standard fracture toughness specimen and the
WP WP
structural component, respectively, at the same level of the Weibull stress σ . The equivalent CTOD
W
ratio, β, is in the range 1 > β > 0.
The critical CTOD, δ , of the fracture toughness specimen is converted to the critical CTOD, δ , of
cr WP,cr
the structural component using β in the form of
δδ= /β (1)
WP,crcr
Furthermore, when the CTOD performance, δ , for the structural component is required, the
WP,req
material fracture toughness, δ , needed to meet the performance requirement is specified as
req
δβ=⋅δ (2)
reqWP,req
Formulae (1) and (2) transfer the CTOD fracture toughness to the equivalent CTOD of the structural
component at the same fracture probability. The CTOD fracture toughness to be used for fracture
assessments shall be determined by agreement of the parties concerned, for instance, a minimum of
three test results.
The equivalent CTOD ratio, β, is dependent on the yield-to-tensile ratio, R , of the material, the Weibull
Y
shape parameter m, and the type and size of a crack in the structural component. In addition, β also
depends on the deformation level of the structural component, but its dependence is rather small in the
deformation range beyond small-scale yielding (SSY). The equivalent CTOD ratio, β, in this International
Standard is specified in this large deformation range and given in nomographs. The β-nomographs are
physically effective in cases where both the standard fracture toughness specimen and the structural
component show unstable fracture.
Three assessment levels (level I, level II and level III) for β are included in this method, as shown in
Figure 3. The details are described in Clause 8. The assessment level to be applied depends upon the
agreement of the parties concerned.
6 Structural components of concern
The structural components concerned in this International Standard are of the following four types
regarded as wide plates under tensile loading, as shown in Figure 4. The crack in the components should
be sufficiently small in comparison with the component dimensions (length, width) so as to ensure that
the plate width effect on the stress intensity factor is negligibly small.
— CSCP (Centre surface crack panel): Wide plate component with a surface crack at the centre of the
plate under tensile loading
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ISO 27306:2016(E)

— ESCP (Edge surface crack panel): Wide plate component with double-edge or single-edge surface
crack at the edge of the plate under tensile loading
— CTCP (Centre through-thickness crack panel): Wide plate component with a through-thickness
crack at the centre of the plate under tensile loading
— ETCP (Edge through-thickness crack panel): Wide plate component with double-edge or single-edge
through-thickness crack at the edge of the plate under tensile loading
NOTE These represent some important structural configurations. For instance, CSCP represents a shell or
pipe component with a flaw induced by crane scratch. ESCP is related to a beam or box component including a
crack originated from geometrical discontinuity by fatigue or seismic loading. CTCP and ETCP may correspond
to an extreme case of CSCP and ESCP where the surface crack grows in thickness direction to a large extent. Weld
cracks such as lack of fusion, incomplete penetration, undercut, cold crack (hydrogen induced crack) and slag
inclusion, etc. are more likely in weldments. But this International Standard does not deal with the welded joints,
because further investigation is necessary on the effects of strength mismatch, residual stress and the crack-tip
location with respect to welds. Embedded cracks are not considered in this International Standard on the ground
that embedded cracks are less likely in normal structural components than surface cracks.
The loading condition is assumed to be substantially uni-axial and perpendicular to the crack plane.
The surface crack is assumed to be semi-elliptical, and the half-length, c, of the crack should be larger
than the crack depth, a (shallow surface crack). Surface cracks existing in structural components are
not necessarily of semi-elliptical type, but they should be idealized as semi-elliptical cracks by flaw
assessment methods duly authorized in the organization concerned.
Other components can be assessed if the equivalent CTOD ratio β is derived by a suitable method.
7 Conditions for use
This International Standard allows β to be applied for the fracture assessment of ferritic steel
components under the following conditions:
— Brittle fracture beyond SSY (Small-Scale Yielding) is assessed. The assessment of brittle fracture
preceded by a significant stable crack growth is not recommended;
— The fracture toughness specimen (three-point bend or compact specimen with 0,45 ≤ a /W ≤ 0,55)
0
shall have the same thickness as the structural component;
— No significant differences in fracture toughness through the thickness of the steel being assessed;
— β -nomographs for a reference crack size are presented in Clause 9, where the yield-to-tensile ratio,
0
R , Weibull shape parameter, m, are in the range 0,6 ≤ R ≤ 0,98 and 10 ≤ m ≤ 50;
Y Y
— The crack size, c and a, and the plate thickness, t, covered by this International Standard are as
follows:
a) CSCP: 2c ≥ 16 mm, 0,04 ≤ a/t ≤ 0,24, 12,5 ≤ t ≤ 50 mm;
b) ESCP: 2c ≥ 24 mm, 0,04 ≤ a/t ≤ 0,24, 12,5 ≤ t ≤ 50 mm;
c) CTCP: 5 ≤ 2a ≤ 50 mm;
d) ETCP: 5 ≤ 2a ≤ 30 mm.
R and m for ferritic structural steels are generally in the above range. The constraint correction by β
Y
may also be effective in cases where R , m and the crack size are not within the above range, provided
Y
that β is obtained by an appropriate procedure.
R and m at the temperature of the target component shall be employed for the determination of β.
Y
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ISO 27306:2016(E)

8 Assessment levels I, II, and III
8.1 General
This International Standard proposes three levels for the assessment of the equivalent CTOD ratio, β.
The choice of level depends on the agreement of the parties concerned. The detail of the assessments
and required information are summarized in Table 1.
Assessment levels I to III are applied in loading conditions beyond SSY. The δ described in Figure 5
SSY limit
is the crack-tip opening displacement, δ, of the standard fracture toughness specimen corresponding
to the SSY limit specified in ISO 12135. When stress fields in a wide plate structural component are
focused to build the same level of the Weibull stress as in the fracture toughness specimen beyond
δ , constraint loss can be significant in the structural component. This International Standard
SSY limit
provides the equivalent CTOD ratio, β, under such stress conditions.
Table 1 — Assessment levels I, II and III of β and required information
Level I
Level II Level III
(Simplified
(Normal assessment) (Material specific assessment)
assessment)
— Yield-to-tensile ratio, R
Y
— Crack type in structural
— Yield-to-tensile ratio, R
Y
component
— Crack type in structural com-
Information
— Crack size (length, depth)
ponent
needed for None
assessment
— Stress-strain curve for
— Crack size (length, depth)
FE-analysis
— Lower-bound m-value
— Statistically determined
m-value
0 < β < 1 (in most case, 0 < β < 0,5) 0 < β (Level III) < β (Level II)
Equivalent CTOD
β = 0,5 β = f (R , a, c, t, m) for CSCP, ESCP β = f (R , a, c, t, m) for CSCP, ESCP
Y Y
ratio β
β = f (R , a, m) for CTCP, ETCP β = f (R , a, m) for CTCP, ETCP
Y Y
Constitutive equation and finite
a
For a long crack ,
a
For a long crack and R < 0,8,
Y
element size ahead of the crack-
Remarks
level II is
tip should be well defined in FE
level III is recommended.
recommended.
analysis.
CSCP, ESCP: Centre and edge surface crack panels
CTCP, ETCP: Centre and edge through-thickness crack panels
a
Surface crack: 2c > 50 mm, Through-thickness crack: 2a > 25 mm, (2c: surface crack length, 2a: through-thickness
crack length, t: plate thickness, m: Weibull shape parameter).
8.2 Level I: Simplified assessment
Level I assessment is applicable to cases where the information necessary for calculating β, such as the
mechanical properties of the structural component being assessed, the type and size of the assumed
crack, etc., is not fully available. At level I assessment, β =0,5 is used as an upper-bound engineering
approximation.
However, for a structural component that potentially includes a long crack (surface crack length 2c > 50
mm or through-thickness crack length 2a > 25 mm), level II assessment is recommended because β may
exceed 0,5 with a low shape parameter, m.
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ISO 27306:2016(E)

8.3 Level II: Normal assessment
Level II assessment is applicable to cases where the yield-to-tensile ratio, R , of the material and the
Y
type and size of the crack being assessed are known, but the Weibull shape parameter, m, is unknown. A
lower-bound value for m is assumed for the assessment of β.
In cases of fracture assessment of structural components from fracture toughness results:

m = 10  for δ ≤ 0,05 (mm)  (3)
cr,ave -25

m = 20  for δ > 0,05 (mm)
cr,ave --25 

where δ is the average CTOD fracture toughness at the assessment temperature obtained with
cr,ave-25
25 mm thick specimen. Annex A can be referred to when selecting the lower-bound m-value depending
on the CTOD toughness level, δ . Annex A includes a procedure for estimating δ , when the
cr,ave-25 cr,ave-25
thickness of the fracture toughness specimen is not 25 mm.
In cases of fracture toughness determination needed to meet design requirement of performance of
structural components:
m = 10 (4)
At level II, β-values are derived from nomographs as a function of the yield-to-tensile ratio, R , and the
Y
Weibull parameter m of the material.
The use of a lower-bound m-value may lead to an excessive overestimation of β for a long crack (surface
crack length 2c > 50 mm or through-thickness crack length 2a > 25 mm) with R < 0,8. Level III
Y
assessment is recommended in such cases.
8.4 Level III: Material specific assessment
Level III assessment is applicable to cases where the information for the assessment of β is fully known.
At level III, β-values are also derived from nomographs, but with a statistically determined m-value from
a sufficient number of fracture toughness test results. A recommended procedure for the determination
of the m-value is described in Annex B.
Generally, β at level III is smaller than that at level II.
9 Equivalent CTOD ratio, β
9.1 General
This section describes a method for converting the CTOD of the standard fracture toughness specimen
[4]
to the equivalent CTOD of structural components by using the equivalent CTOD ratio, β.
9.2 Factors influencing the equivalent CTOD ratio, β
The equivalent CTOD ratio, β, based on the Weibull stress criterion, depends on the shape parameter, m,
of the material.
In addition, β is also influenced by the following factors, although the strength class and uniform
[4] [5]
elongation of the material have virtually no influence on β:
a) factors affecting plastic constraint in the vicinity of the crack-tip:
— yield-to-tensile ratio, R , of the material;
Y
— crack type (CSCP, ESCP, CTCP, ETCP) and crack size (crack depth of surface crack and crack
length of through-thickness crack);
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ISO 27306:2016(E)

— plate thickness, t;
b) factor exerting a volumetric effect:
— length of surface crack.
NOTE The equivalent CTOD ratios, β, for CTCP and ETCP do not depend on the plate thickness because the
plate thickness plays the same role in the evolution of the Weibull stresses for the CTCP (ETCP) and the fracture
toughness specimen, where the crack is of through-thickness type.
9.3 Procedure for calculating the equivalent CTOD ratio, β, at assessmen
...

DRAFT INTERNATIONAL STANDARD
ISO/DIS 27306
ISO/TC 164/SC 4 Secretariat: ANSI
Voting begins on: Voting terminates on:
2015-07-29 2015-10-29
Metallic materials — Method of constraint loss correction
of CTOD fracture toughness for fracture assessment of
steel components
Matériaux métalliques — Méthode de correction de perte de contrainte du CTOD de la ténacité à la rupture
pour l’évaluation de la rupture des composants en acier
ICS: 77.040.10
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
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ISO/DIS 27306:2015(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
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COPYRIGHT PROTECTED DOCUMENT
© ISO 2015
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
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Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
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ISO 27306
Contents
Foreword. iv
1. Scope . 1
2. Normative references . 1
3. Terms and definitions . 2
4. Symbols and units . 3
5. Principle . 4
6. Structural components of concern. 5
7. Range of use . 5
8. Assessment levels I, II, and III . 6
8.1 General . 6
8.2 Level I: Simplified assessment . 6
8.3 Level II: Normal assessment . 6
8.4 Level III: Material specific assessment . 7
9. Equivalent CTOD ratio,  . 7
9.1 General . 7
9.2 Factors influencing the equivalent CTOD ratio,  . 7
9.3 Procedure for calculating the equivalent CTOD ratio, , at assessment levels I to III . 8
9.3.1 General . 8
9.3.2 Surface crack cases (CSCP and ESCP) . 8
9.3.3 Through-thickness crack cases (CTCP and ETCP) . 9
Annex A (Informative) Procedure for the selection of Weibull parameter, m, at level II assessment . 16
Annex B (Informative) Analytical method for the determination of Weibull parameter, m, at level III
assessment . 18
Annex C (Informative) Guidelines for the equivalent CTOD ratio,  . 23
Annex D (Informative) Examples of fracture assessment using the equivalent CTOD ratio,  . 29
Bibliography . 43

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ISO 27306:2009 (E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards adopted
by the technical committees are circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting a vote.
ISO/DIS 27306 was prepared by Technical Committee ISO/TC 164 Mechanical Testing of Metals.
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ISO 27306

Metallic materials - Method of constraint loss correction of CTOD
fracture toughness for fracture assessment of steel components
1. Scope
In fracture assessments of steel structures containing cracks, it has generally been assumed that the fracture
resistance of fracture toughness specimens is equal to the fracture resistance of structural components. However,
such an assumption often leads to excessively conservative fracture assessments. This is due to a loss of plastic
constraint in structural components, which are subjected mainly to tensile loading. By contrast, fracture toughness
specimens hold a constrained stress state near the crack-tip due to bending mode. The loss of constraint is
significant for high strength steels with high yield-to-tensile ratios (= yield stress / tensile strength) which have been
extensively developed and widely applied to structures in recent years.
This International Standard specifies a method for converting the CTOD (Crack-Tip Opening Displacement) fracture
toughness obtained from laboratory specimens to an equivalent CTOD for structural components, taking constraint
loss into account. This method can also apply to fracture assessment using the stress intensity factor or the J-
integral concept (see Clause 9).
This International Standard deals with the unstable fracture that occurs from a crack-like defect or fatigue crack in
ferritic structural steels. Unstable fracture accompanied by a significant amount of ductile crack extension and ductile
fractures are not included in the scope hereof.
The CTOD fracture toughness of structural steels is measured in accordance with the established test methods, ISO
12135:2002 or BS7448-1:1999. The fracture assessment of a cracked component is done using an established
method such as FAD (Failure Assessment Diagram) in the organization concerned, and reference is not made to the
details thereof in this International Standard.
This International Standard can be used for eliminating the excessive conservatism frequently associated with the
conventional fracture mechanics methods and accurately assessing the unstable fracture initiation limit of structural
components from the fracture toughness of the structural steel. This is also used for rationally determining the
fracture toughness of materials to meet the design requirements of deformability of structural components.
2. Normative references
The following referenced documents are indispensable for the application of this International Standard. For dated
references, only the edition cited applies. For updated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 12135:2002(E), Metallic materials – Unified method of test of the determination of quasistatic fracture toughness
BSI, BS7448-1:1991, Fracture mechanics toughness tests, Method for determination of K , critical CTOD and critical
Ic
J values of metallic materials


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ISO 27306:2009 (E)
3. Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 12135:2002 and the following apply.
3.1
CTOD of standard fracture toughness specimen
crack-tip opening displacement of standard fracture toughness specimen

CTOD, as the fracture driving force, for the standard fracture toughness specimen (three point bend or compact
specimen) with 0,45 ≤ a / W ≤ 0,55, where a and W are the initial crack length and specimen width, respectively
0 0
3.2
CTOD fracture toughness
crack-tip opening displacement fracture toughness

cr
critical CTOD at the onset of brittle fracture in the standard fracture toughness specimen [ (B) as defined in ISO
c
12135:2002] with 0,45 ≤ a / W ≤ 0,55
0
3.3
CTOD of structural component
crack-tip opening displacement of structural component

WP
CTOD, as the fracture driving force, for a through-thickness crack or a surface crack existing in a structural
component regarded as a wide plate
NOTE The CTOD of a surface crack is defined at the maximum crack depth.
3.4
critical CTOD of structural component
critical crack-tip opening displacement of structural component

WP,cr
critical CTOD at the onset of brittle fracture in structural components
3.5
equivalent CTOD ratio
equivalent crack-tip opening displacement ratio

CTOD ratio defined by / , where  and  are CTODs of the standard fracture toughness specimen and the
WP WP
structural component, respectively, at the same level of the Weibull stress 
W
See Figure 1.
NOTE See Reference [1].
3.6
Weibull stress

W
fracture driving force defined with the consideration of statistical instability of microcracks in the fracture process zone
against brittle fracture
NOTE See Reference [2].
3.7
critical Weibull stress

W,cr
Weibull stress at the onset of unstable fracture
3.8
Weibull shape parameter
m
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ISO 27306
material parameter used in the definition of the Weibull stress; one of two parameters describing the statistical
distribution of the critical Weibull stress, 
W, cr
3.9
yield-to-tensile ratio
R
Y
ratio of yield strength,  , (lower yield point, R , or 0,2% proof strength, R ) to tensile strength, R
Y eL p0,2 m
4. Symbols and units
For the purposes of this document, the following symbols, units, and designations are applied in addition to those in
ISO 12135:2002.

Symbol Unit Designation
a mm Depth of surface crack or half length of through-thickness crack in structural component
c mm Half length of surface crack in structural component
m 1 Weibull shape parameter
t mm Plate thickness
3
V mm Reference volume defined for Weibull stress
0
3
V mm Volume of fracture process zone
f
R - Yield-to-tensile ratio (= / R )
Y m
Y
Equivalent CTOD ratio
 -
Equivalent CTOD ratio for reference crack length
-

0
(In cases of surface crack panel, is defined for plate thickness t = 25 mm.)
0
Equivalent CTOD ratio for target length of centre surface crack or double-edge surface
 -
2c, t
crack on target plate thickness
Equivalent CTOD ratio for target length of centre through-thickness crack or double-
 -
a
2
edge through-thickness crack
Equivalent CTOD ratio for target length of single-edge surface crack on target plate
-

c, t
thickness
Equivalent CTOD ratio for target length of single-edge through-thickness crack
 -
a
 mm CTOD of standard fracture toughness specimen
Critical CTOD of standard fracture toughness specimen at onset of brittle fracture
mm

cr
(CTOD fracture toughness)
CTOD at small-scale yielding limit for standard fracture toughness specimen
 mm
SSY limit
 mm CTOD of structural component
WP
mm Critical CTOD of structural component at onset of brittle fracture

WP, cr
Effective stress used for the calculation of Weibull stress
 MPa
eff
MPa Lower yield point, R , or 0,2 % proof strength, R
  eL p0,2
Y
Weibull stress
 MPa
W
Critical Weibull stress at onset of brittle fracture
 MPa
W, cr



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ISO 27306:2009 (E)
5. Principle
This International Standard deals with the initiation of unstable fracture due to cleavage of structural steels. It
presents a method for converting the CTOD fracture toughness obtained from the standard fracture toughness
specimen [three-point bend or compact specimen with 0,45 ≤ a / W ≤ 0,55 and B (specimen thickness) = t (plate
0
thickness of structural component)], which are characterized by an extremely severe plastic constraint in the vicinity
of the crack-tip, to an equivalent critical CTOD for structural components, which are generally characterized by less
constraint. The reverse procedure is also possible with this method. Thus, this method links fracture toughness tests
and fracture performance assessments of structural components by taking account of loss of plastic constraint in
structural components, as shown in Figure 2.
NOTE 1 The fracture toughness specimen with a deep crack such as a / W = 0,7 presents somewhat higher constraint near
0
the crack- tip than that with 0,45 ≤ a / W ≤ 0,55. The equivalent CTOD ratio  defined in this International Standard leads to a
0
conservative fracture assessment, if the user employs a deep cracked specimen with a / W > 0,55.
0
NOTE 2  This International Standard does not intend to address size and temperature effects nor influence of data scatter on
[3]
the results.  Refer to ASTM E1921 for guidance.
The CTOD fracture toughness (critical CTOD) of the standard fracture toughness specimen is determined in
accordance with the established test methods, ISO 12135:2002 or BS7448-1:1991. The fracture assessment of a
cracked component can be done using established methods at the user’s discretion such as FAD (Failure
Assessment Diagram) and CTOD design curve in the organization concerned.
The critical CTOD of the standard fracture toughness specimen is converted to the critical CTOD of the structural
component using the equivalent CTOD ratio, . The equivalent CTOD ratio, , is defined as a CTOD ratio,  ,
WP
where and  are CTODs of the standard fracture toughness specimen and the structural component,
WP
respectively, at the same level of the Weibull stress  . The equivalent CTOD ratio, , is in the range 1 >  > 0.
W
The critical CTOD,  , of the fracture toughness specimen is converted to the critical CTOD,  , of the structural
cr WP,cr
component using  in the form
d =d b               (1)
WP,cr cr

Furthermore, if the deformability,  , required for the structural component is given, the material fracture
WP,req
toughness needed to meet the deformability requirement,  , can be calculated as
req
d = b ·d (2)
req WP, req

Equations (1) and (2) transfer the CTOD fracture toughness to the equivalent CTOD of the structural component at
the same fracture probability. The CTOD fracture toughness to be used for fracture assessments shall be
determined by agreement of the parties concerned, for instance, a minimum of three test results.
The equivalent CTOD ratio, , is dependent on the yield-to-tensile ratio, R , of the material, the Weibull shape
Y
parameter m, the type and size of a crack in the structural component. In addition,  also depends on the
deformation level of the structural component, but its dependence is rather small in the deformation range beyond
small-scale yielding (SSY). The equivalent CTOD ratio, , in this International Standard is specified in this large
deformation range, and given in nomographs. The -nomographs are physically effective in cases where both the
standard fracture toughness specimen and the structural component show unstable fracture.
Three assessment levels (level I, level II and level III) for  are included in this method, as shown in Figure 3. The
details are described in Clause 8. The assessment level to be applied depends upon the agreement of the parties
concerned.


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ISO 27306
6. Structural components of concern
The structural components concerned in this International Standard are of the following four types regarded as wide
plates under tensile loading, as shown in Figure 4. The crack in the components should be sufficiently small in
comparison with the component dimensions (length, width) so as to ensure that the plate width effect on the stress
intensity factor is negligibly small.
CSCP (Centre surface crack panel): Wide plate component with a surface crack at the centre of the plate under
tensile loading
ESCP (Edge surface crack panel): Wide plate component with double-edge or single-edge surface crack at the edge
of the plate under tensile loading
CTCP (Centre through-thickness crack panel): Wide plate component with a through-thickness crack at the centre of
the plate under tensile loading
ETCP (Edge through-thickness crack panel): Wide plate component with double-edge or single-edge through-
thickness crack at the edge of the plate under tensile loading
NOTE These represent some important structural configurations. For instance, CSCP represents a shell or pipe component
with a flaw induced by crane scratch. ESCP is related to a beam or box component including a crack originated from geometrical
discontinuity by fatigue or seismic loading. CTCP and ETCP may correspond to an extreme case of CSCP and ESCP where the
surface crack grows in thickness direction to a large extent. Weld cracks such as lack of fusion, incomplete penetration, undercut,
cold crack (hydrogen induced crack) and slag inclusion etc. are more likely in weldments. But this International Standard does not
deal with the welded joints, because further investigation is necessary on the effects of strength mismatch, residual stress and the
crack-tip location with respect to welds. Embedded cracks are not considered in this International Standard on the ground that
embedded cracks are less likely in normal structural components than surface cracks.
The loading condition is assumed to be substantially uni-axial and perpendicular to the crack plane. The surface
crack is assumed to be semi-elliptical, and the half-length, c, of the crack should be larger than the crack depth, a
(shallow surface crack). Surface cracks existing in structural components are not necessarily of semi-elliptical type,
but they should be idealized as semi-elliptical cracks by flaw assessment methods duly authorized in the organization
concerned.
Other components can be assessed if the equivalent CTOD ratio  is derived by a suitable method.
7. Range of use
This International Standard allows  to be applied for the fracture assessment of ferritic steel components under the
following conditions:
– Brittle fracture beyond SSY is assessed. The assessment of brittle fracture preceded by a significant stable crack
growth is not recommended;
– The fracture toughness specimen (three-point bend or compact specimen with 0,45 ≤ a / W ≤ 0,55) shall have the
0
same thickness as the structural component;
– No significant differences in fracture toughness through the thickness of the steel being assessed;
–  -nomographs for a reference crack size are presented in Clause 9, where the yield-to-tensile ratio, R , Weibull
0 Y
shape parameter, m, are in the range, 0,6 ≤ R ≤ 0,98 and 10 ≤ m ≤ 50;
Y
– The crack size, c and a, and the plate thickness, t, covered by this International Standard are as follows:
   CSCP: 2c ≥ 16 mm, 0,04 ≤ a/t ≤ 0,24, 12,5 ≤ t ≤ 50 mm
   ESCP: 2c ≥ 24 mm, 0,04 ≤ a/t ≤ 0,24, 12,5 ≤ t ≤ 50 mm
   CTCP: 5 ≤ 2a ≤ 50 mm
   ETCP: 5 ≤ 2a ≤ 30 mm
R and m for ferritic structural steels are generally in the above range. The constraint correction by  may also be
Y
effective in cases where R , m and the crack size are not within the above range, provided that, , is obtained by an
Y
appropriate procedure.
R and m at the temperature of the target component shall be employed for the determination of .
Y
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ISO 27306:2009 (E)
8. Assessment levels I, II, and III
8.1 General
This International Standard proposes three levels for the assessment of the equivalent CTOD ratio, . The choice of
level depends on the agreement of the parties concerned. The detail of the assessments and required information
are summarized in Table 1.
Assessment levels I to III are applied in loading conditions beyond small-scale yielding (SSY). The  described
SSY limit
in Figure 5 is the crack-tip opening displacement, , of the standard fracture toughness specimen corresponding to
the SSY limit specified in ISO 12135. When stress fields in a wide plate structural component are focused to build
the same level of the Weibull stress as in the fracture toughness specimen beyond  , constraint loss can be
SSY limit
significant in the structural component. This International Standard provides the equivalent CTOD ratio, , under
such stress conditions.
Table 1 – Assessment levels I, II and III of  and required information

Level I Level II Level III
(Simplified assessment) (Normal assessment) (Material specific assessment)
- Yield-to-tensile ratio, R
Y
- Yield-to-tensile ratio, R
Y
Information - Crack type in structural component
- Crack type in structural component
needed for None - Crack size (length, depth)
- Crack size (length, depth)
assessment - Stress-strain curve for FE-analysis
- lower-bound m-value
- Statistically determined m-value
0 <  < 1 (in most case, 0 <  < 0,5) 0 < (Level III) < (Level II)
Equivalent CTOD
 = 0,5 = f (R , a, c, t, m) for CSCP, ESCP = f (R , a, c, t, m) for CSCP, ESCP
  
Y Y
ratio 
= f (R , a, m) for CTCP, ETCP = f (R , a, m) for CTCP, ETCP
 
Y Y
Constitutive equation and finite element
a a
For a long crack , For a long crack and R < 0,8,
Y
Remarks size ahead of the crack-tip should be well
level II is recommended. level III is recommended.
defined in FE-analysis.
CSCP, ESCP: Centre and edge surface crack panels
CTCP, ETCP: Centre and edge through-thickness crack panels
a
: Surface crack: 2c > 50 mm, Through-thickness crack: 2a > 25mm,
2c: Surface crack length, 2a: Through-thickness crack length, t: Plate thickness, m: Weibull shape parameter

8.2 Level I: Simplified assessment
Level I assessment is applicable to cases where the information necessary for calculating , such as the mechanical
properties of the structural component being assessed, the type and size of the assumed crack, etc. is not fully
available. At level I assessment,  =0,5 is used as an upper-bound engineering approximation.
However, for a structural component that potentially includes a long crack (surface crack length 2c > 50mm or
through-thickness crack length 2a > 25mm), level II assessment is recommended because  may exceed 0,5 with a
low shape parameter, m.
8.3 Level II: Normal assessment
Level II assessment is applicable to cases where the yield-to-tensile ratio, R , of the material and the type and size of
Y
the crack being assessed are known, but the Weibull shape parameter, m, is unknown. A lower-bound value for m is
assumed for the assessment of .

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ISO 27306
In cases of fracture assessment of structural components from fracture toughness results:
ü
m = 10  for d ≤ 0,05 (mm) ï
cr,ave -25
 ý (3)
m = 20  for d > 0,05 (mm)
ï
cr,ave -25
þ

where  is the average CTOD fracture toughness at the assessment temperature obtained with 25 mm thick
cr,ave-25
specimen, in mm. Annex A can be referred to when selecting the lower-bound m-value depending on the CTOD
toughness level,  . Annex A includes a procedure for estimating  , when the thickness of the fracture
cr,ave-25 cr,ave-25
toughness specimen is not 25 mm.
In cases of fracture toughness determination needed to meet design requirement of deformability of structural
components:
m = 10 (4)
At level II, -values are derived from nomographs as a function of the yield-to-tensile ratio, R , and the Weibull
Y
parameter m of the material.
The use of a lower-bound m-value may lead to an excessive overestimation of  for a long crack (surface crack
length 2c > 50 mm or through-thickness crack length 2a > 25 mm) with R < 0,8. Level III assessment is
Y
recommended in such cases.
8.4 Level III: Material specific assessment
Level III assessment is applicable to cases where the information for the assessment of  is fully known.
At level III, -values are also derived from nomographs, but with a statistically determined m-value from a sufficient
number of fracture toughness test results. A recommended procedure for the determination of the m-value is
described in Annex B.
Generally,  at level III is smaller than that at level II.
9. Equivalent CTOD ratio, 
9.1 General
This section describes a method for converting the CTOD of the standard fracture toughness specimen to the
[4]
equivalent CTOD of structural components by using the equivalent CTOD ratio,  .
9.2 Factors influencing the equivalent CTOD ratio, 
The equivalent CTOD ratio, , based on the Weibull stress criterion, depends on the shape parameter, m, of the
material.
In addition,  is also influenced by the following factors, although the strength class and uniform elongation of the
[4], [5]
material have virtually no influence on  :
a) factors affecting plastic constraint in the vicinity of the crack-tip:
– yield-to-tensile ratio, R , of the material;
Y
– crack type (CSCP, ESCP, CTCP, ETCP) and crack size (crack depth of surface crack and crack length of
through-thickness crack);
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ISO 27306:2009 (E)
– plate thickness, t.
b) factor exerting a volumetric effect:
– length of surface crack.
NOTE The equivalent CTOD ratios, , for CTCP and ETCP do not depend on the plate thickness, because the plate
thickness plays the same role in the evolution of the Weibull stresses for the CTCP (ETCP) and the fracture toughness specimen,
where the crack is of through-thickness type.
9.3 Procedure for calculating the equivalent CTOD ratio, , at assessment levels I to III
9.3.1 General
The procedure for calculating the equivalent CTOD ratio, , at assessment levels I to III is described below.
Equations (5) to (9) are applicable for the following crack sizes:
   CSCP: 2c ≥ 16 mm, 0,04 ≤ a/t ≤ 0,24, 12,5 ≤ t ≤ 50 mm
   ESCP: 2c ≥ 24 mm, 0,04 ≤ a/t ≤ 0,24, 12,5 ≤ t ≤ 50 mm
   CTCP: 5 ≤ 2a ≤ 50 mm
   ETCP: 5 ≤ 2a ≤ 30 mm
9.3.2 Surface crack cases (CSCP and ESCP)
The procedure for calculating the equivalent CTOD ratio, , for the surface crack is as follows.
Level I:  = 0,5
Level II:  is calculated, as shown in Figure 6, according to the following steps.
Step 1 Define the crack size (crack length 2c, depth a), plate thickness, t, and the yield-to-tensile ratio, R .
Y
Step 2 Set a lower-bound value of the shape parameter, m: 10 or 20 depending on the material toughness
level and cases of the fracture assessment [Equations (3) and (4)].
Step 3
...