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

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Published
Publication Date
18-Sep-2016
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9060 - Close of review
Start Date
03-Dec-2021
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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
---------------------- Page: 1 ----------------------
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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CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2016 – All rights reserved
---------------------- Page: 2 ----------------------
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

© ISO 2016 – All rights reserved iii
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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.
iv © ISO 2016 – All rights reserved
---------------------- Page: 4 ----------------------
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

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

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.
© ISO 2016 – All rights reserved 1
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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

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

critical CTOD at the onset of brittle fracture in the standard fracture toughness specimen [δ (B) as

defined in ISO 12135] with 0,45 ≤ a /W ≤ 0,55
3.3
CTOD of structural component
crack-tip opening displacement of structural component

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
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 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

Note 1 to entry: See Figure 1.
Note 2 to entry: See Reference [1].
3.6
Weibull stress

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
W,cr
Weibull stress at the onset of unstable fracture
3.8
Weibull shape parameter

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
2 © ISO 2016 – All rights reserved
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ISO 27306:2016(E)
3.9
yield-to-tensile ratio

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
V mm Reference volume defined for Weibull stress
V mm Volume of fracture process zone
R — Yield-to-tensile ratio (= σ /R )
Y Y m
β — Equivalent CTOD ratio
Equivalent CTOD ratio for reference crack length
β —
(In cases of surface crack panel, β is defined for plate thickness t = 25 mm.)

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 —
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

δ mm CTOD of standard fracture toughness specimen

Critical CTOD of standard fracture toughness specimen at onset of brittle fracture (CTOD

δ mm
fracture toughness)
δ mm CTOD at small-scale yielding limit for standard fracture toughness specimen
SSY limit
δ mm CTOD of structural component
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
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

(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

© ISO 2016 – All rights reserved 3
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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

constraint near the crack-tip than that with 0,45 ≤ a /W ≤ 0,55. The equivalent CTOD ratio β defined in this

International Standard leads to a conservative fracture assessment, if the user employs a deep cracked specimen

with a /W > 0,55.

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

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

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
4 © ISO 2016 – All rights reserved
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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)

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,

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

may also be effective in cases where R , m and the crack size are not within the above range, provided

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.

© ISO 2016 – All rights reserved 5
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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
— Crack type in structural
— Yield-to-tensile ratio, R
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
For a long crack ,
For a long crack and R < 0,8,
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

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.
6 © ISO 2016 – All rights reserved
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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

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

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

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;

— crack type (CSCP, ESCP, CTCP, ETCP) and crack size (crack depth of surface crack and crack

length of through-thickness crack);
© ISO 2016 – All rights reserved 7
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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
NATIONAL REGULATIONS.
ISO/DIS 27306:2015(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2015
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ISO/DIS 27306:2015(E)
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
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2015 – All rights reserved
---------------------- Page: 2 ----------------------
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

© ISO 2008 – All rights reserved iii
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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.

iv © ISO 2009– All rights reserved
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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

J values of metallic materials
© ISO 2006 – All rights reserved 1
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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

critical CTOD at the onset of brittle fracture in the standard fracture toughness specimen [ (B) as defined in ISO

12135:2002] with 0,45 ≤ a / W ≤ 0,55
3.3
CTOD of structural component
crack-tip opening displacement of structural component

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 
See Figure 1.
NOTE See Reference [1].
3.6
Weibull stress

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
2 © ISO 2008 – All rights reserved
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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

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
V mm Reference volume defined for Weibull stress
V mm Volume of fracture process zone
R - Yield-to-tensile ratio (= / R )
Y m
Equivalent CTOD ratio
 -
Equivalent CTOD ratio for reference crack length
(In cases of surface crack panel, is defined for plate thickness t = 25 mm.)

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-

 -
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
 -
 mm CTOD of standard fracture toughness specimen

Critical CTOD of standard fracture toughness specimen at onset of brittle fracture

(CTOD fracture toughness)
CTOD at small-scale yielding limit for standard fracture toughness specimen
 mm
SSY limit
 mm CTOD of structural component
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
Weibull stress
 MPa
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

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

the crack- tip than that with 0,45 ≤ a / W ≤ 0,55. The equivalent CTOD ratio  defined in this International Standard leads to a

conservative fracture assessment, if the user employs a deep cracked specimen with a / W > 0,55.

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,  ,

where and  are CTODs of the standard fracture toughness specimen and the structural component,

respectively, at the same level of the Weibull stress  . The equivalent CTOD ratio, , is in the range 1 >  > 0.

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

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

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;

– 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

effective in cases where R , m and the crack size are not within the above range, provided that, , is obtained by an

appropriate procedure.

R and m at the temperature of the target component shall be employed for the determination of .

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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
- Yield-to-tensile ratio, R
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,
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
: 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

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 .
6 © ISO 2008 – All rights reserved
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ISO 27306

In cases of fracture assessment of structural components from fracture toughness results:

m = 10 for d ≤ 0,05 (mm) ï
cr,ave -25
ý (3)
m = 20 for d > 0,05 (mm)
cr,ave -25

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

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

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;

– 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 .

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
...

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