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 ≤ 0,55 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,  ,
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.
4 © ISO 2008 – All rights reserved

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;
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
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 .

6 © ISO 2008 – All rights reserved

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);
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 Determine the equivalent CTOD ratio,  , for a reference size of the surface crack on 25 mm thick plate
from the nomographs shown in Figures 7 and 8 as a function of m and R . Figures 7 and 8 provide 
Y 0
for the crack depth ratios, a/t = 0,04, 0,12 and 0,24 (a = 1, 3 and 6 mm and t = 25 mm).
Step 4 Calculate the equivalent CTOD ratio,  , for the target length, 2c, and the target plate thickness, t,
2c, t
with Equation (5) or (6), depending on the type of crack:
k m 2
( ) 1
CSCP
b =b · 25 / t · 2c 40 , k m = (5)
2c, t (CSCP) 0(CSCP) ( ) CSCP( )
exp 0,1 m - 33 +1
{ ( )}
k m 2
( ) 1
ESCP
(6)
b =b · 25 / t · 2c 30 , k m =
( ) ( )
2c, t (ESCP) 0(ESCP) ESCP
exp 0,1 m- 40 +1
{ ( )}
NOTE Equations (5) and (6) hold under a given crack depth ratio, a/t.
In the case of single-edge surface crack of length c, the equivalent CTOD ratio,  =  , is given in the form
c, t
k m 2
( )
ESCP
(7)
b =b · 1 2
c, t (ESCP) 2c, t (ESCP) ( )
Level III:  is calculated, as shown in Figure 6, with a statistically determined m-value.
8 © ISO 2008 – All rights reserved

ISO 27306
9.3.3 Through-thickness crack cases (CTCP and ETCP)
The procedure for calculating the equivalent CTOD ratio, , for the through-thickness 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 length, 2a, 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: Determine the equivalent CTOD ratio,  , for a reference length of the through-thickness crack from the
nomographs shown in Figures 9 and 10 as a function of m and R .
Y
Step 4: Calculate the equivalent CTOD ratio,  , for the target crack length, 2a, with Equation (8) or (9),
2a
depending on the type of crack:
0.4
(8)
b =b · 2a 13,8
( )
2a(CTCP) 0(CTCP)
k m, R
ETCP( Y) -0,57+ 3,1R -1,45R
Y Y
b =b · 2a 11 , k m,R = (9)
( ) ( )
2a(ETCP) 0(ETCP) ETCP Y
exp -0,35 m-10 +1
{ }
( )
In the case of single-edge through-thickness crack of length a, the equivalent CTOD ratio,  =  , is given in the
a
form
b =b 2 (10)
a(ETCP) 2a(ETCP)
The equivalent CTOD ratio, , of through-thickness cracks shows no dependence on the plate thickness.
Level III:  is calculated, as shown in Figure 6, with a statistically determined m-value.
1/2
In the case of the fracture assessment using the stress intensity factor K,  can be used for the constraint loss
correction. For the assessment based on the J-integral,  may be used as it is.
FE-analysis of the Weibull stress for the fracture toughness specimen is required for determining the m-value at level
III assessment. A recommended procedure for the analytical determination of the m-value is described in Annex B.
Annex C describes the guidelines for application of the equivalent CTOD ratio, , at assessment levels I to III. In
cases where the crack size in structural components, yield-to-tensile ratio, R , and the shape parameter, m, of the
Y
material being assessed are not in the range of the nomographs in Figures 7 to 10, and are also outside the
applicable range of Equations (5), (6), (8) and (9), an equivalent CTOD ratio, , obtained by a suitable method, e.g.
FE analysis of the target component, may be used.
Annex D presents examples of fracture assessments of structural components using the equivalent CTOD ratio, .
[6] [7]
Fracture assessment methods, such as FAD (Failure Assessment Diagram) or CTOD design curve , which have
been duly authorized in the organization concerned, may be used.
ISO 27306:2009 (E)
Figure 1 – Definition of the equivalent CTOD ratio, , based on the Weibull stress fracture criterion

Figure 2 – Method of constraint loss correction to link fracture toughness tests and structural performance
evaluation
10 © ISO 2008 – All rights reserved

ISO 27306
Figure 3 – Flow of fracture assessment of structural components from fracture toughness test results,
where three assessment levels of the equivalent CTOD ratio, , are included for constraint loss correction

Figure 4 – Standard fracture toughness specimens and wide plate components linked by the equivalent
CTOD ratio, 
ISO 27306:2009 (E)
Figure 5 – Assessment levels I, II and III of  for correcting constraint loss in wide plate components

Figure 6 – Flow chart for calculating the equivalent CTOD ratio, 

12 © ISO 2008 – All rights reserved

ISO 27306
a) CSCP (a/t = 0,04; t = 25 mm):  versus m     b) CSCP (a/t = 0,04; t = 25 mm):  versus R
Y
0 0
c) CSCP (a/t = 0,12; t = 25 mm):  versus m     d) CSCP (a/t = 0,12; t = 25 mm):  versus R
Y
0 0
e) CSCP (a/t = 0,24; t = 25 mm):  versus m     f) CSCP (a/t = 0,24; t = 25 mm):  versus R
Y
0 0
Figure 7 – Nomographs of equivalent CTOD ratio,  , for centre surface crack panel (CSCP) with plate
thickness t = 25mm
ISO 27306:2009 (E)
a) ESCP (a/t = 0,12; t = 25 mm):  versus m     b) ESCP (a/t = 0,24; t = 25 mm):  versus R
Y
0 0
c) ESCP (a/t = 0,12; t = 25 mm):  versus m     d) ESCP (a/t = 0,12; t = 25 mm):  versus R
Y
0 0
e) ESCP (a/t = 0,24; t = 25 mm):  versus m     f) ESCP (a/t = 0,24; t = 25 mm):  versus R
Y
0 0
Figure 8 – Nomographs of equivalent CTOD ratio,  , for double-edge surface crack panel (ESCP) with plate
thickness t = 25mm
14 © ISO 2008 – All rights reserved

ISO 27306
a) CTCP (2a = 13,8 mm):  versus m           b) CTCP (2a = 13,8 mm):  versus R
Y
0 0
Figure 9 – Nomographs of equivalent CTOD ratio,  , for centre through-thickness crack panel (CTCP)
a) ETCP (2a = 11 mm):  versus m             b) ETCP (2a = 11 mm):  versus R
Y
0 0
Figure 10 – Nomographs of equivalent CTOD ratio,  , for double-edge through-thickness crack panel
(ETCP)
ISO 27306:2009 (E)
Annex A (Informative) Procedure for the selection of Weibull parameter, m,
at level II asessment
A.1 General
The procedure for the selection of the Weibull shape parameter, m, at level II assessment is described. The shape
parameter m is selected on the basis of the average CTOD fracture toughness at the assessment temperature.
A.2 Determination of average CTOD fracture toughness
In selecting the shape parameter, m, the average (arithmetic mean) value of the CTOD fracture toughness,  ,
cr,ave
at the assessment temperature obtained with 25mm thick test specimens should be used.
In cases where no test data with 25mm thick test specimens are available, the CTOD fracture toughness for 25mm
[3] [8]
thick specimen,  , calculated by Equations (A.1) and (A.2) , may be used.
cr,ave-25
1/4
ì ü
ï æ B ö ï
d = d + d - d × (A.1)
í ý
cr,ave-25 min ( cr,ave-B min)
ç ÷
è ø
ï ï
î þ
500×1-n
( )
d = ×K (A.2)
min min
s ×E
Y
where
B is the test specimen thickness, in mm;
 is the average CTOD fracture toughness with test specimen thickness B, in mm;
cr,ave-B
 is the lower yield strength or 0,2 % proof strength, in MPa;
Y
E is Young’s modulus of elasticity, in MPa;
 is Poisson’s ratio;
K is equal to 20 MPa .
m
min
Note that Equations (A.1) and (A.2) are valid for the CTOD fracture toughness at brittle fracture initiation without a
significant amount of stable crack extension.
A.3 Determination of Weibull shape parameter, m
A.3.1 Assessment of brittle fracture initiation of steel structure components from fracture
toughness of structural steel
In cases where the brittle fracture limit of steel components is to be assessed from the fracture toughness of the
structural steel, the shape parameter m is selected as shown in Equation (3) depending on the average CTOD
fracture toughness,  .
cr,ave -25
The m-value in Equation (3) is a lower-bound value in the diagram for m and  (Figure A.1) exhibited with
cr,ave -25
data from Reference [1] and References [9] to [26], where m was determined statistically with fatigue pre-cracked
toughness specimens. The use of the lower-bound m-value leads to a conservative fracture assessment.

16 © ISO 2008 – All rights reserved

ISO 27306
Figure A.1 – Relationship between Weibull parameter, m, and average CTOD fracture toughness, 
cr,ave -25
A.3.2 Determination of fracture toughness needed to meet design requirement of deformability of
structural components
In cases where the fracture toughness needed to meet the design requirement of deformability of structural
components is to be determined, the use of the lower-bound value in Equation (4) is recommended for estimation
of the required fracture toughness.
In cases where the level of CTOD fracture toughness of the material can be estimated from the Charpy impact test
results or other properties, m may be selected as shown in Equation (3).
For additional information on the relationship between the Weibull parameter and the fracture toughness of
structural steels, see Reference [27].

ISO 27306:2009 (E)
Annex B (Informative) Analytical method for the determination of Weibull
parameter, m, at level III assessment
B.1 General
This annex describes the analytical procedure for the determination of the Weibull shape parameter m that is
[9]
needed at level III failure assessment. A common procedure is shown in Figure B.1, and the recommended
methods in the Steps 1 to 3 are described in the following.
B.2 Fracture toughness test (Step 1)
Fracture toughness tests should be performed using the three-point bend test specimen or the compact specimen
in accordance with ISO 12135. However, the initial crack length of the test specimen should be within the range of
0,45 ≤ a / W ≤ 0,55. The toughness tests (Table B.1) should be performed with an adequate number of
specimens to determine the parameter m, and statistical data of the critical CTOD should be obtained. After
testing, the fracture surface should be observed, and the fact that brittle fracture occurred without stable crack
extension larger than 0,2 mm should be confirmed.
[28], [29]
NOTE 1 In R6 Revision 4 – Section III.9 in Chapter III, a minimum number of 30 tests is recommended .
NOTE 2 The fracture toughness specimen with a deep crack such as a0 / W = 0,7 presents a 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
[30]
conservative fracture assessment, if the user employs the deep cracked specimen with a / W > 0,55 .
NOTE 3 One set of fracture toughness data obtained by the above specimen may give a non-unique m-value, if the stress
fields near the crack-tip show the singularity controlled by K or HRR (Hutchinson, Rice and Rosengren) field within the range of
the fracture toughness level measured. This non-uniqueness is related to the statistical characteristics of toughness values,
which follow a two-parameter Weibull distribution with a constant shape parameter (=2 for the critical CTOD), under the singular
stress fields. In such case, the use of two sets of specimens with high and low constraints, e.g. deep-cracked and shallow-
cracked specimens, is recommended to get a unique solution for m. The detail of the calibration procedure for m using two data
sets is described in Reference [31].
Table B.1 – Fracture toughness testing
Large enough for determination of Weibull parameter, m,
Number of test specimens
with statistical reliability
Force, P, and crack mouth opening displacement, V
Items measured
g
Fracture toughness parameter Critical CTOD, 
cr
B.3 FE-analysis of stress fields ahead of crack-tip in fracture toughness specimen
(Step 2)
B.3.1 General
The stress fields ahead of the crack-tip in the fracture toughness specimen should be analyzed by a finite element
method (FEM) that incorporates large deformation analysis. A guideline for obtaining sound FE-results in terms of
the stress-strain curve of material and the FE-model is described in the following subclauses.
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ISO 27306
B.3.2 Stress-strain curve for FE-analysis
B.3.2.1 Round-bar tensile testing
In order to obtain the stress-strain curve of the material for use in the FE-analysis, a round-bar tensile test shall be
[32]
performed in accordance with established International Standard for testing, such as ISO 6892-1 and ISO 15579
[33]
.
The test should be performed at the same temperature as that of the fracture toughness test in Step 1. During the
test, force and elongation between gauge marks should be measured and recorded.
B.3.2.2 Equivalent stress – equivalent plastic strain curve for FE-analysis
Based on the results of the round-bar tensile test, the relationship between equivalent stress and equivalent plastic
strain to be used in the FE-analysis should be determined in accordance with the following procedure.
a) Calculate the nominal stress–nominal plastic strain relationship, excluding the elastic strain component, from
the nominal stress–nominal strain curve measured in the strain range up to uniform elongation.
b) Convert the nominal stress–nominal plastic strain relationship to the true stress–true plastic strain relationship
(equivalent stress - equivalent plastic strain relationship) using the following equations.
s = (1+ ) (B.1)
e = ln (1+ ) (B.2)
where
s is the true stress, in MPa;
e is the true plastic strain;
 is the nominal stress, in MPa;
 the nominal plastic strain.
NOTE Lüder’s strain, if observed in the round-bar tension test, would be included in Equations (B.1) and (B.2).
c) Constitute the equivalent stress–equivalent plastic strain relationship beyond uniform elongation  in the form:
T
n
(B.3)
s =s 1+e /a
( )
Y p
where
is the equivalent stress, in MPa;
s
e is the equivalent
...