ISO 27306:2016

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
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ISO/DIS 27306:2015(E)
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PROVIDE SUPPORTING DOCUMENTATION. ISO 2015

ISO/DIS 27306:2015(E)
© ISO 2015
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ii © ISO 2015 – All rights reserved

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 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

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

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
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
2 © ISO 2008 – All rights reserved

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
V mm Reference volume defined for Weibull stress
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
-

(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-
 -
a
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
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 ≤ 0,5
...