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

ISO 27306:2009 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 toughness assessment using the stress intensity factor or the J-integral concept. ISO 27306:2009 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 is not included in the scope hereof. ISO 27306:2009 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.

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

General Information

Status
Withdrawn
Publication Date
10-May-2009
Withdrawal Date
10-May-2009
Current Stage
9599 - Withdrawal of International Standard
Start Date
19-Sep-2016
Completion Date
19-Sep-2016
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INTERNATIONAL ISO
STANDARD 27306
First edition
2009-05-15
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 27306:2009(E)
ISO 2009
---------------------- Page: 1 ----------------------
ISO 27306:2009(E)
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ii © ISO 2009 – All rights reserved
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ISO 27306:2009(E)
Contents Page

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 Assessment levels I, II and III .............................................................................................................. 6

7.1 General................................................................................................................................................... 6

7.2 Level I: Simplified assessment............................................................................................................ 6

7.3 Level II: Normal assessment ............................................................................................................... 7

7.4 Level III: Material specific assessment............................................................................................... 7

8 Equivalent CTOD ratio, β...................................................................................................................... 7

8.1 General................................................................................................................................................... 7

8.2 Factors influencing the equivalent CTOD ratio, β ............................................................................. 7

8.3 Procedure for calculating the equivalent CTOD ratio, β, at assessment levels I to III .................. 8

8.3.1 General................................................................................................................................................... 8

8.3.2 Surface crack case (CSCP or ESCP) .................................................................................................. 8

8.3.3 Through-thickness crack case (CTCP or ETCP) ............................................................................... 8

Annex A (informative) Procedure for the selection of Weibull parameter m at level II assessment........ 18

Annex B (informative) Analytical method for the determination of Weibull parameter m at level III

assessment ......................................................................................................................................... 21

Annex C (informative) Guidelines for the equivalent CTOD ratio, β............................................................ 26

Annex D (informative) Examples of fracture assessment using the equivalent CTOD ratio, β................ 32

Bibliography ..................................................................................................................................................... 53

© ISO 2009 – 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.

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.

ISO 27306 was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals, Subcommittee

SC 4, Toughness testing — Fracture(F), Pendulum(P), Tear(T).
iv © ISO 2009 – All rights reserved
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INTERNATIONAL STANDARD ISO 27306:2009(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 loading. 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 toughness assessment using the

stress intensity factor or the J-integral concept (see Clause 8).

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 is not included in the scope hereof.

The CTOD fracture toughness of structural steels is measured in accordance with any one of the established

test methods, ISO 12135:2002, BS 7448-1:1991 or ASTM E1290-99. 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 document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments) applies.

ISO 12135:2002, Metallic materials — Unified method of test for the determination of quasistatic fracture

toughness

BS 7448-1:1991, Fracture mechanics toughness tests — Method for determination of KIc, critical CTOD and

critical J values of metallic materials
© ISO 2009 – All rights reserved 1
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ISO 27306:2009(E)

ASTM E1290-99 , Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness

measurement
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 u (a /W) u 0,55, where a and W are the initial crack length and specimen width,

0 0
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 12135:2002] with 0,45 u a /W u 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 δ/δ , where δ and δ are CTODs of the standard fracture toughness specimen and

WP WP

the structural component, respectively, at the same level of the Weibull stress, σ

See Figure 1.
NOTE See Reference [1].

1) The procedure for calculating CTOD in ASTM E1290-08 is different from the one in ASTM E1290-1999. The new

ASTM E1290 procedure gives a somewhat different CTOD value compared to those calculated by ISO 12135:2002 and

BS 7448-1:1991. This International Standard employs ASTM E1290-99, which specifies the CTOD calculation procedure

similar to ISO 12135:2002 and BS 7448-1:1991.
2 © ISO 2009 – All rights reserved
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ISO 27306:2009(E)
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

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
3.9
yield-to-tensile ratio
ratio of yield strength (or 0,2 % proof strength), R , to tensile strength, R
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 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 /R )
Y p0,2 m
β — Equivalent CTOD ratio
β — Equivalent CTOD ratio for reference crack size

β — Equivalent CTOD ratio for target size of centre surface crack or double-edge surface

crack

β — Equivalent CTOD ratio for target size of centre through-thickness crack or double-edge

through-thickness crack
β — Equivalent CTOD ratio for target size of single-edge surface crack
β — Equivalent CTOD ratio for target size of single-edge through-thickness crack
© ISO 2009 – All rights reserved 3
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ISO 27306:2009(E)
Symbol Unit Designation
δ mm CTOD of standard fracture toughness specimen

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

(CTOD 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 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

specimens [three-point bend or compact specimens with 0,45 u a /W u 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 a higher constraint near the

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

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

NOTE 2 This International Standard does not intend to address size and temperature effects nor the influence of data

[3]
scatter on 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 any one of the established test methods, ISO 12135:2002, BS 7448-1:1991 or

ASTM E1290-99. 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, 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 the

cr WP,cr
structural component using β in the form
δ =δβ/ (1)
WP,cr cr

Furthermore, if the deformability, δ , required for the structural component is given, the material fracture

WP,req
toughness needed to meet the deformability requirement, δ , can be calculated as
req
δβ=⋅δ (2)
req WP, req
4 © ISO 2009 – All rights reserved
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ISO 27306:2009(E)

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, 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. The nomographs are presented in Clause 8, where the

yield-to-tensile ratio, R , and the Weibull shape parameter, m, are in the range 0,6 u R u 0,95 and

Y Y

10 u m u 50 (R and m for structural steels are generally in this range). They are prepared on the conditions

that the thickness, B, of the fracture toughness specimen is equal to the plate thickness, t, of the structural

component, and that there are no significant differences in fracture toughness through the thickness of the

steel being assessed. This procedure may also be applicable in cases where the crack size, yield-to-tensile

ratio, R , etc. of the structural component concerned are not within the range of the nomographs, provided

that, β, is obtained by an appropriate procedure.

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 7. 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

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, undercut, cold cracking (hydrogen-induced cracking) 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 uniaxial 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.
© ISO 2009 – All rights reserved 5
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ISO 27306:2009(E)

Other components can be assessed if the equivalent CTOD ratio, β, is derived by a suitable method.

7 Assessment levels I, II and III
7.1 General

This International Standard proposes three levels for the assessment of the equivalent CTOD ratio, β.

Applicable assessment levels can be selected by agreement of the parties concerned. The details of the

assessments and required information are summarized in Table 1.
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 ⎯ Yield-to-tensile ratio, R
Y Y
⎯ Crack type in structural ⎯ Crack type in structural
component component
Information
needed for None ⎯ Crack size (length, depth) ⎯ Crack size (length, depth)
assessment
⎯ Reference m-value (lower-bound ⎯ Stress-strain curve for finite
value) element (FE) analysis
⎯ Statistically determined m-value
0 < β < 1 (in most cases, 0 < β < 0,5) 0 < β (Level III) < β (Level II)
Equivalent
β = 0,5 β = f (R , a, c, m) for CSCP, ESCP β = f (R , a, c, m) for CSCP, ESCP
Y Y
CTOD 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-tip
level II is
Remarks
level III is recommended. should be well defined in FE-
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; m: Weibull shape parameter

Assessment levels I to III are applied in loading conditions beyond small-scale yielding (SSY). The δ

SSY limit

described in Figure 5 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 to build the same level of the

Weibull stress as in the fracture toughness specimen beyond δ are considered in a wide plate

SSY limit

structural component, constraint loss can be significant in the structural component. This International

Standard presents the equivalent CTOD ratio, β, under such loading conditions.
7.2 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 small shape parameter, m.
6 © ISO 2009 – All rights reserved
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ISO 27306:2009(E)
7.3 Level II: Normal assessment

Level II assessment is applicable to cases where the mechanical properties (yield-to-tensile ratio, R ) of the

structural component being assessed and the type and size of the assumed crack are known, but the Weibull

shape parameter, m, is unknown. A lower-bound value for m is assumed for the assessment of β.

At level II, β-values are derived from nomographs as a function of the component crack type and size, material

yield-to-tensile ratio and the parameter m.

The use of a lower-bound m-value may lead to an excessive overestimation of β for cases where the

yield-to-tensile ratio R < 0,8, and the surface crack length 2c > 50 mm or the through-thickness crack length

2a > 25 mm. Level III assessment is recommended in such cases.
7.4 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.
Generally, the β-value at level III is smaller than that at level II.
8 Equivalent CTOD ratio, β
8.1 General

This clause describes a method for converting the CTOD of a standard fracture toughness specimen to the

[4]
equivalent CTOD of structural components by using the equivalent CTOD ratio, β .
8.2 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 elongation of

[4], [5]
the material have virtually no influence on β :

a) factors mainly controlling 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);
⎯ plate thickness (in the case of a deep surface crack);
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.
© ISO 2009 – All rights reserved 7
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ISO 27306:2009(E)

8.3 Procedure for calculating the equivalent CTOD ratio, β, at assessment levels I to III

8.3.1 General

The procedure for calculating the equivalent CTOD ratio, β, at assessment levels I to III is described below.

Equations (3), (4), (6) and (7) are applicable for the following crack sizes:
CSCP: 2c W 16 mm, 1 u a u 6 mm, t W 25 mm
ESCP: 2c W 24 mm, 1 u a u 6 mm, t W 25 mm
CTCP: 5 u 2a u 50 mm
ETCP: 5 u 2a u 30 mm
8.3.2 Surface crack case (CSCP or ESCP)

The procedure for calculating the equivalent CTOD ratio, β, for the surface crack is as follows.

Level I: β = 0,5

Level II: β is calculated, as shown in Figure 6, according to the following steps.

Step 1: Define the crack size (crack length 2c, depth a) and the material yield-to-tensile ratio, R .

Step 2: Set the reference value (lower-bound value) of the shape parameter, m. Annex A can be

referred to when selecting the lower-bound m-value.

Step 3: Determine the equivalent CTOD ratio, β , for a reference crack size from the nomographs

shown in Figures 7 and 8 as a function of the m-value, crack depth, a, and the yield-to-tensile ratio, R .

Step 4: Calculate the equivalent CTOD ratio, β = β , for the target crack length, 2c, using

Equation (3) or Equation (4), depending on the type of crack:
km 2
CSCP
ββ==i24ck0 , m (3)
() ()
2c(CSCP) 0(CSCP) CSCP
exp⎡⎤0,1 m − 33 +1
km() 2
ESCP
ββ==i()23ck0 , ()m (4)
2c(ESCP) 0(ESCP) ESCP
exp⎡⎤0,1 m − 40 +1

In the case of single-edge surface crack of length c, the equivalent CTOD ratio, β = β , is given in the form

km( ) 2
ESCP
ββ= i()12 (5)
cc(ESCP) 2 (ESCP)

For t W 25 mm and 1 u a u 6 mm, the equivalent CTOD ratio, β, shows virtually no dependence on the

plate thickness, t.

Level III: β is calculated, as shown in Figure 6, with a statistically determined m-value.

8.3.3 Through-thickness crack case (CTCP or ETCP)

The procedure for calculating the equivalent CTOD ratio, β, for the through-thickness crack is as follows.

Level I: β = 0,5
8 © ISO 2009 – All rights reserved
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