ISO 12135:2021
(Main)Metallic materials — Unified method of test for the determination of quasistatic fracture toughness
Metallic materials — Unified method of test for the determination of quasistatic fracture toughness
This document specifies methods for determining fracture toughness in terms of K, δ, J and R-curves for homogeneous metallic materials subjected to quasistatic loading. Specimens are notched, precracked by fatigue and tested under slowly increasing displacement. The fracture toughness is determined for individual specimens at or after the onset of ductile crack extension or at the onset of ductile crack instability or unstable crack extension. In cases where cracks grow in a stable manner under ductile tearing conditions, a resistance curve describing fracture toughness as a function of crack extension is measured. In some cases in the testing of ferritic materials, unstable crack extension can occur by cleavage or ductile crack initiation and growth, interrupted by cleavage extension. The fracture toughness at crack arrest is not covered by this document. Special testing requirements and analysis procedures are necessary when testing weldments, and these are described in ISO 15653 which is complementary to this document. Statistical variability of the results strongly depends on the fracture type, for instance, fracture toughness associated with cleavage fracture in ferritic steels can show large variation. For applications that require high reliability, a statistical approach can be used to quantify the variability in fracture toughness in the ductile-to-brittle transition region, such as that given in ASTM E1921. However, it is not the purpose of this document to specify the number of tests to be carried out nor how the results of the tests are to be applied or interpreted.
Matériaux métalliques — Méthode unifiée d'essai pour la détermination de la ténacité quasi statique
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INTERNATIONAL ISO
STANDARD 12135
Third edition
2021-07
Metallic materials — Unified method
of test for the determination of
quasistatic fracture toughness
Matériaux métalliques — Méthode unifiée d'essai pour la
détermination de la ténacité quasi statique
Reference number
©
ISO 2021
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ii © ISO 2021 – All rights reserved
Contents Page
Foreword .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 General requirements . 5
5.1 General . 5
5.2 Fracture parameters. 7
5.3 Fracture toughness symbols . 8
5.4 Test specimens . 8
5.4.1 Specimen configuration and size . 8
5.4.2 Specimen preparation .13
5.5 Pre-test requirements .19
5.5.1 Pre-test measurements .19
5.5.2 Crack shape/length requirements .19
5.6 Test apparatus .19
5.6.1 Calibration .19
5.6.2 Force application .20
5.6.3 Displacement measurement .20
5.6.4 Test fixtures .20
5.7 Test requirements .24
5.7.1 Three-point bend testing .24
5.7.2 Compact tension testing .24
5.7.3 Specimen test temperature.24
5.7.4 Recording .25
5.7.5 Testing rates .25
5.7.6 Test analyses .25
5.8 Post-test crack measurements .25
5.8.1 General.25
5.8.2 Initial crack length, a .25
5.8.3 Stable crack extension, Δa .30
5.8.4 Unstable crack extension .30
6 Determination of fracture toughness for stable and unstable crack extension .31
6.1 General .31
6.2 Determination of plane strain fracture toughness, K .32
lc
6.2.1 General.32
6.2.2 Interpretation of the test record for F .32
Q
6.2.3 Calculation of K .33
Q
6.2.4 Qualification of K as K .34
Q lc
6.3 Determination of fracture toughness in terms of δ .34
6.3.1 Determination of F and V , F and V , or F and V .34
c c u u uc uc
6.3.2 Determination of F and V .35
m m
6.3.3 Determination of V .36
p
6.3.4 Calculation of δ .36
6.3.5 Qualification of δ fracture toughness value .37
6.4 Determination of fracture toughness in terms of J .38
6.4.1 Determination of F and V or q , F and V or q , or F and V or q .38
c c c u u u uc uc uc
6.4.2 Determination of F and q .38
m m
6.4.3 Determination of U .38
p
6.4.4 Calculation of J .39
6.4.5 Qualification of J fracture toughness value .40
7 Determination of resistance curves δ-Δa and J-Δa and initiation toughness δ and
0,2BL
J and δ and J for stable crack extension .41
0,2BL i i
7.1 General .41
7.2 Test procedure .41
7.2.1 General.41
7.2.2 Multiple-specimen procedure .41
7.2.3 Single-specimen procedure .41
7.2.4 Final crack front straightness .42
7.3 Calculation of J and δ .42
7.3.1 Calculation of J .42
7.3.2 Calculation of δ .42
7.4 R-curve plot .43
7.4.1 Plot construction .44
7.4.2 Data spacing and curve fitting .45
7.5 Qualification of resistance curves .46
7.5.1 Qualification of J-Δa resistance curves .46
7.5.2 Qualification of δ−Δa resistance curves .46
7.6 Determination and qualification of J and δ .47
0,2BL 0,2BL
7.6.1 Determination of J .47
0,2BL
7.6.2 Determination of δ .48
0,2BL
7.7 Determination of initiation toughness J and δ by scanning electron microscopy (SEM) .49
i i
8 Test report .49
8.1 Organization .49
8.2 Specimen, material and test environment .50
8.2.1 Specimen description .50
8.2.2 Specimen dimensions .50
8.2.3 Material description . .50
8.2.4 Additional dimensions .50
8.2.5 Test environment .50
8.2.6 Fatigue precracking conditions .50
8.3 Test data qualification .51
8.3.1 Limitations .51
8.3.2 Crack length measurements .51
8.3.3 Fracture surface appearance .51
8.3.4 Pop-in .51
8.3.5 Resistance curves .51
8.3.6 Checklist for data qualification .51
8.4 Qualification of K .52
lc
8.5 Qualification of δ , δ , δ or δ .52
c(B) u(B) uc(B) m(B)
8.6 Qualification of J , J , J or J .53
c(B) u(B) uc(B) m(B)
8.7 Qualification of the δ-R Curve .53
8.8 Qualification of the J-R Curve .53
8.9 Qualification of δ as δ .53
0,2BL(B) 0,2BL
8.10 Qualification of J as J .53
0,2BL(B) 0,2BL
Annex A (informative) Determination of δ and J .55
i i
Annex B (normative) Crack plane orientation .60
Annex C (informative) Example test reports .62
Annex D (informative) Stress intensity factor coefficients and compliance relationships .71
Annex E (informative) Measurement of load-line displacement q in the three-point bend test .75
Annex F (informative) Derivation of pop-in formulae .80
Annex G (informative) Analytical methods for the determination of V and U .82
p p
Annex H (informative) Guidelines for single-specimen methods .83
Annex I (normative) Power-law fits to crack extension data (see Reference [42]) .97
iv © ISO 2021 – All rights reserved
Bibliography .98
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
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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
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals,
Subcommittee SC 4, Fatigue, fracture and toughness testing.
This third edition cancels and replaces the second edition (ISO 12135:2016), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— formulae to calculate CTOD have been replaced with those based on rigid rotation assumption
throughout; replacing the previous R-curve formulae based on CTOD from J. CTOD formulae for
SENBs are now those based on recent research to include the material yield to tensile strength ratio
in the CTOD formulae;
— the determination of J directly from displacement defined in terms of CMOD has been included, in
addition to the methods based on load line displacement;
— where fatigue precrack straightness requirements cannot be met due to internal residual stresses,
the application of modification techniques, originally developed for weld specimens, is now
permitted;
— the rotation correction factor for compact specimens has been revised with a new formula;
— editorial changes have been made to improve consistency of terms and definitions used throughout
the document.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
vi © ISO 2021 – All rights reserved
INTERNATIONAL STANDARD ISO 12135:2021(E)
Metallic materials — Unified method of test for the
determination of quasistatic fracture toughness
1 Scope
This document specifies methods for determining fracture toughness in terms of K, δ, J and R-curves for
homogeneous metallic materials subjected to quasistatic loading. Specimens are notched, precracked
by fatigue and tested under slowly increasing displacement. The fracture toughness is determined for
individual specimens at or after the onset of ductile crack extension or at the onset of ductile crack
instability or unstable crack extension. In cases where cracks grow in a stable manner under ductile
tearing conditions, a resistance curve describing fracture toughness as a function of crack extension
is measured. In some cases in the testing of ferritic materials, unstable crack extension can occur
by cleavage or ductile crack initiation and growth, interrupted by cleavage extension. The fracture
toughness at crack arrest is not covered by this document. Special testing requirements and analysis
procedures are necessary when testing weldments, and these are described in ISO 15653 which is
complementary to this document.
Statistical variability of the results strongly depends on the fracture type, for instance, fracture
toughness associated with cleavage fracture in ferritic steels can show large variation. For applications
that require high reliability, a statistical approach can be used to quantify the variability in fracture
toughness in the ductile-to-brittle transition region, such as that given in ASTM E1921. However, it is
not the purpose of this document to specify the number of tests to be carried out nor how the results of
the tests are to be applied or interpreted.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 3785, Metallic materials — Designation of test specimen axes in relation to product texture
ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Calibration and verification of the force-measuring system
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
stress intensity factor
K
magnitude of the elastic stress-field singularity for a homogeneous, linear-elastic body
Note 1 to entry: The stress intensity factor is a function of applied force, crack length, specimen size and specimen
geometry.
3.2
crack-tip opening displacement
δ
relative opening displacement of the crack surfaces normal to the original (undeformed) crack plane at
the tip of the fatigue precrack, evaluated using the rotation point formula
3.3
J-integral
line or surface integral that encloses the crack front from one crack surface to the other and
characterizes the local stress-strain field at the crack tip
3.4
J
loading parameter, equivalent to the J-integral (3.3), the specific values of which, experimentally
determined by this method of test (J , J , J ,…), characterize fracture toughness under elastic-plastic
c i u
conditions
3.5
stable crack extension
crack extension which stops or would stop when the applied displacement is held constant as a test
progresses under displacement control
3.6
unstable crack extension
abrupt crack extension occurring with or without prior stable crack extension (3.5)
3.7
pop-in
abrupt discontinuity in the force versus displacement record, featured as a sudden increase in
displacement and, generally, a decrease in force followed by an increase in force
Note 1 to entry: Displacement and force subsequently increase beyond their values at pop-in.
Note 2 to entry: When conducting tests by this method, pop-ins can result from unstable crack extension
(3.6) in the plane of the precrack and are to be distinguished from discontinuity indications arising from: i)
delaminations or splits normal to the precrack plane; ii) roller or pin slippage in bend or compact specimen
load trains, respectively; iii) improper seating of displacement gauges in knife edges; iv) ice cracking in low-
temperature testing; v) electrical interference in the instrument circuitry of force and displacement measuring
and recording devices.
3.8
crack extension resistance curves
R-curves
variation in δ (3.2) or J (3.4) with stable crack extension (3.5)
4 Symbols and abbreviated terms
Symbol Unit Designation
a mm Nominal crack length (for the purposes of fatigue precracking, an assigned value less
than a )
a mm Final crack length (a + Δa)
f 0
a mm Instantaneous crack length
i
a mm Length of machined notch
m
a mm Initial crack length
NOTE 1 This is not a complete list of parameters. Only the main parameters are given, other parameters are referred to in
the text.
NOTE 2 The values of all parameters used in calculations are assumed to be those measured or calculated for the
temperature of the test, unless otherwise specified.
2 © ISO 2021 – All rights reserved
Symbol Unit Designation
Δa mm Stable crack extension including blunting
Δa mm Crack extension limit for δ or J controlled crack extension
max
B mm Specimen thickness
B mm Specimen net thickness between side grooves
N
C m/N Specimen elastic compliance
CMOD mm Crack-mouth opening displacement, V
CTOD mm Crack tip opening displacement, δ
E GPa Modulus of elasticity at the pertinent temperature
F kN Applied force
F kN Applied force at the onset of unstable crack extension or pop-in when Δa is less than
c
0,2 mm offset from the construction line (Figure 2)
F kN Force value corresponding to the intersection of the test record with the secant line
d
(Figure 18)
F kN Maximum fatigue precracking force
f
F kN Limiting collapse load estimated for a given specimen type
L
F kN Maximum force for a test which exhibits a maximum force plateau preceding fracture
m
with no significant prior pop-ins (Figure 2)
F kN Provisional force value used for the calculation of K
Q Q
F kN Applied force at the onset of unstable crack extension or pop-in when Δa is equal to or
u
greater than the 0,2 mm offset from the construction line (Figure 2)
J MJ/m Experimental equivalent to the J-integral
J MJ/m Size sensitive fracture resistance J at onset of unstable crack extension or pop-in when
c(B)
stable crack extension is less than 0,2 mm offset from the construction line (B = spec-
imen thickness in mm)
J MJ/m J at upper limit of J-controlled crack extension
g
J MJ/m Size-insensitive fracture resistance J at initiation of stable crack extension
i
J MJ/m Size sensitive fracture resistance J at the first attainment of a maximum force plateau
m(B)
for fully plastic behaviour (B = specimen thickness in mm)
J MJ/m Limit of J-R material behaviour defined by this method of test
max
J MJ/m Size sensitive fracture resistance J at the onset of unstable crack extension or pop-in
u(B)
when the event is preceded by stable crack extension equal to or greater than 0,2 mm
offset from the construction line (B = specimen thickness in mm)
J MJ/m Size sensitive fracture resistance J at the onset of unstable crack extension or pop-in
uc(B)
when stable crack extension cannot be measured (B = specimen thickness in mm)
J MJ/m J unclassified, and uncorrected for stable crack extension
J MJ/m Size insensitive fracture resistance J at 0,2 mm stable crack extension offset from the
0,2BL
construction line
J MJ/m Size sensitive fracture resistance J at 0,2 mm stable crack extension offset from the
0,2BL(B)
construction line (B = specimen thickness in mm)
0,5
K MPa m Stress intensity factor
0,5
K MPa m Maximum value of K during the final stage of fatigue precracking
f
0,5
K MPa m Plane strain linear elastic fracture toughness
lc
0,5
K MPa m Plane strain linear elastic fracture toughness equivalent to J
J0,2BL 0,2BL
0,5
K MPa m A provisional value of K
Q lc
NOTE 1 This is not a complete list of parameters. Only the main parameters are given, other parameters are referred to in
the text.
NOTE 2 The values of all parameters used in calculations are assumed to be those measured or calculated for the
temperature of the test, unless otherwise specified.
Symbol Unit Designation
M M
M — Where M appears as a superscript designation (such as J or δ ), it indicates that re-
sidual stress modification techniques have been applied to the specimen prior to test.
q mm Load-line displacement. q equals V in compact specimens (Figure 14).
R MPa Ultimate tensile strength perpendicular to crack plane at the test temperature
m
R MPa 0,2 % offset yield strength perpendicular to crack plane at the test temperature
p0,2
S mm Span between outer loading points in a three-point bend test
T °C Test temperature
U J Area under plot of force F versus crack-mouth opening displacement V, or load-line
displacement q
U J Elastic component of U
e
U J Plastic component of U (Figure 20)
p
V mm In bend specimens, V is the crack-mouth opening displacement (CMOD), which is the
opening displacement at the notch edges (Figure 13). In compact specimens, the open-
ing displacement, V, is determined at the load-line. V equals q in compact specimens
(Figure 14).
V mm Elastic component of V
e
V mm Displacement measured by clip gauges mounted on knife edges at a distance z from the
g
crack -mouth. Where integral knife edges are used, V =V (Figure 13).
g
V mm Plastic component of V
p
W mm Width of the test specimen
z mm For bend and straight-notch compact specimens, z is the initial distance of the crack-
mouth opening gauge measurement position from the notched edge of the specimen,
either further from the crack tip [+z in Figure 8 b)] or closer to the crack tip (−z); or, for
a stepped-notch compact specimen, z is the initial distance of the crack-mouth opening
gauge measurement position either beyond (+z) or before (−z) the initial load-line.
δ mm Crack-tip opening displacement (CTOD)
δ mm Size sensitive fracture resistance δ at the onset of unstable crack extension or pop-in
c(B)
when stable crack extension is less than 0,2 mm crack offset from the construction line
(B = specimen thickness in mm)
δ mm δ at the limit of δ-controlled crack extension
g
δ mm Fracture resistance δ at initiation of stable crack extension
i
δ mm Size sensitive fracture resistance δ at the first attainment of a maximum force plateau
m(B)
for fully plastic behaviour (B = specimen thickness in mm)
δ mm Limit of δ-R curve defined by this method of test
max
δ mm Size sensitive fracture resistance δ at the onset of unstable crack extension or pop-in
u(B)
when the event is preceded by stable crack extension equal to or greater than 0,2 mm
offset from the construction line (B = specimen thickness in mm)
δ mm Size sensitive fracture resistance δ at the onset of unstable crack extension or pop-in
uc(B)
when stable crack extension Δa cannot be measured (B = specimen thickness in mm)
δ mm δ unclassified, and uncorrected for stable crack extension
δ mm Size insensitive fracture resistance δ at 0,2 mm crack extension offset from construc-
0,2BL
tion line
δ mm Size sensitive fracture resistance δ at 0,2 mm stable crack extension offset from con-
0,2BL(B)
struction line (B = specimen thickness in mm)
η — Dimensionless function of geometry used to calculate J
p
ν — Poisson's ratio
NOTE 1 This is not a complete list of parameters. Only the main parameters are given, other parameters are referred to in
the text.
NOTE 2 The values of all parameters used in calculations are assumed to be those measured or calculated for the
temperature of the test, unless otherwise specified.
4 © ISO 2021 – All rights reserved
5 General requirements
5.1 General
The fracture toughness of metallic materials can be characterized in terms of either specific (single
point) values (see Clause 6), or a continuous curve relating fracture resistance to crack extension over
a limited range of crack extension (see Clause 7). The procedures and parameters used to determine
fracture toughness vary depending upon the level of plasticity realized in the test specimen during
the test. Under any given set of conditions, however, any one of the fatigue-precracked test specimen
configurations specified in this method may be used to measure any of the fracture toughness
parameters considered. In all cases, tests are performed by applying slowly increasing displacements
to the test specimen and measuring the forces and displacements realized during the test. The
forces and displacements are then used in conjunction with certain pre-test and post-test specimen
measurements to determine the fracture toughness that characterizes the material’s resistance to
crack extension. Details of the test specimens and general information relevant to the determination
of all fracture parameters are given in this method. A flow-chart illustrating the way this method can
be used is presented in Figure 1. Characteristic types of force versus displacement records obtained in
fracture toughness tests are shown in Figure 2.
Figure 1 — General flowchart showing how to use the standard method of test
6 © ISO 2021 – All rights reserved
Key
X crack-mouth opening displacement (V) or load-line displacement (q)
Y force (F)
NOTE 1 The classifications of F , F and F are described in 6.3.1 and 6.4.1.
c u m
NOTE 2 Pop-in behaviour is a function of the material toughness and parameters of the test setup such as the
testing machine/specimen compliance and the recorder response rate.
a
Fracture.
b
Pop-in.
Figure 2 — Characteristic types of force versus displacement records in fracture tests
5.2 Fracture parameters
Specific (point) values of fracture toughness are determined from individual specimens to define the
onset of unstable crack extension or describe stable crack extension.
NOTE K characterizes the resistance to extension of a sharp crack so that i) the state of stress near the
lc
crack front closely approximates plane strain, and ii) the crack tip plastic zone is small compared with the
specimen crack size, thickness and ligament ahead of the crack.
K is considered a size-insensitive measurement of fracture toughness under the above conditions.
lc
Certain test criteria shall be met in order to qualify measurements of K .
lc
The parameters δ , J , δ , J , δ and J also characterize the resistance of a material to unstable
c c u u uc uc
extension of a sharp crack. However, these measurements are regarded as size-sensitive and as such
characterize only the specimen thickness tested. The specimen thickness is thus noted in millimetre
units in parentheses appended to the parameter symbol when reporting a test result.
When stable crack extension is extensive, a test procedure and fracture toughness measurement shall
be performed as specified in Clause 7. Stable crack extension is characterized either in terms of crack
tip opening displacement δ and fracture toughness J parameters, or of a continuous δ- and
0,2BL 0,2BL
J-resistance curve. The values δ and J , regarded as specimen size insensitive, are engineering
0,2BL 0,2BL
estimates of the onset of stable crack extension, not to be confused with the actual initiation toughness
δ and J . Measurement of δ and J is described in Annex A.
i i i i
Two procedures are available for determining δ and J . The multiple specimen procedure
0,2BL 0,2BL
requires several nominally identical specimens to be monotonically loaded, each to different amounts
of displacement. Measurements of force and displacement are made and recorded. Specimen crack
fronts are marked (e.g. by heat tinting or post-test fatiguing) after testing, thus enabling measurement
of stable crack extension on the specimen halves after each specimen is broken open. Post-test cooling
of ferritic material specimens to ensure brittle behaviour can be helpful in preserving crack front
markings prior to breaking open the specimens.
A minimum of six specimens is required by the multiple-specimen method. When material availability
is limited, a single-specimen procedure based on either unloading compliance or the potential drop
technique may be used. There is no restriction on the single-specimen procedure providing sufficient
accuracy can be demonstrated. In all cases, certain criteria are to be met before δ or J values
0,2BL 0,2BL
and δ- or J-resistance curves are qualified by this standard method of test.
5.3 Fracture toughness symbols
Fracture toughness symbols identified in this document are given in Table 1.
Table 1 — Fracture toughness symbols
Size sensitive quantities Qualifying limits
Parameter Size insensitive quantities
(specific to thickness B tested) to R-curves
K
lc
K
K
J0,2BL
δ
c(B)
δ
i
δ δ δ , δ (Δa )
0,2BL(B) g g max
δ
0,2BL
δ , δ , δ
u(B) uc(B) m(B)
J
c(B)
J
i
J J J J (Δa )
0,2BL(B) g, g max
J
0,2BL
J , J , J
u(B) uc(B) m(B)
5.4 Test specimens
5.4.1 Specimen configuration and size
Dimensions and tolerances of specimens shall conform to Figures 3 to 5.
8 © ISO 2021 – All rights reserved
The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top
and bottom edges of the specimen to within 0,005 W.
NOTE 1 Integral or attachable knife edges for clip gauge attachment can be used (see Figures 8 and 9).
NOTE 2 For starter notch and fatigue crack configuration, see Figure 6.
NOTE 3 1,0 ≤ W/B ≤ 4,0 (W/B = 2 preferred).
NOTE 4 0,45 ≤ a/W ≤ 0,70. For K determination, 0,45 ≤ a/W ≤ 0,55.
lc
NOTE 5 Surface roughness Ra in micrometres.
a
See Figures 6 to 8 and 5.4.2.3.
Figure 3 — Proportional dimensions and tolerances for bend specimen
The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top
and bottom edges of the specimen to within 0,005 W.
NOTE 1 Integral or attachable knife edges for clip gauge attachment can be used (see Figures 8 and 9).
NOTE 2 For starter notch and fatigue crack configuration, see Figure 6.
NOTE 3 0,8 ≤ W/B ≤ 4,0 (W/B = 2 preferred).
NOTE 4 0,45 ≤ a/W ≤ 0,70. For K determination, 0,45 ≤ a/W ≤ 0,55.
lc
+0,004W
NOTE 5
Alternative pin hole diameter, φ 0,188 W .
NOTE 6 Surface roughness Ra in micrometres.
a
See Figures 6 to 8 and 5.4.2.3.
Figure 4 — Proportional dimensions and tolerances for straight-notch compact specimen
10 © ISO 2021 – All rights reserved
The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top
and bottom edges of the specimen to within 0,005 W.
Second step may not be necessary for some clip gauges; configuration optional providing fatigue crack starter notch
and fatigue crack fit within the envelope represented in Figure 6.
NOTE 1 Integral or attachable knife edges for clip gauge attachment can be used (see Figures 8 and 9).
NOTE 2 For starter notch and fatigue crack configuration, see Figure 6.
NOTE 3 0,8 ≤ W/B ≤ 4,0 (W/B = 2 preferred).
NOTE 4 0,45 ≤ a/W ≤ 0,70. For K determination, 0,45 ≤ a/W ≤ 0,55.
lc
+0,004W
Alternative pin hole diameter, φ 0,188 W . When this pin size is used, notch opening can be
NOTE 5 0
increased to 0,21 W maximum.
NOTE 6 Surface roughness Ra in micrometres.
a
See Figures 6 to 8.
Figure 5 — Proportional dimensions and tolerances for stepped-notch compact specimen
The choice of specimen design shall take into consideration the likely outcome of the test (see Figure 1),
any preference for δ or J fracture toughness values, the crack plane orientation of interest (Annex B)
and the quantity and condition of test material available.
NOTE 1 All specimen designs (Figures 3 to 5) are suitable for determining K , δ and J values, although there
lc
are special procedural requirements for J values calculated from measurements made away from the load line.
Table 2 provides guidance on specimen size for K measurement.
lc
Table 2 — Minimum recommended thickness for K testing
lc
B
R
p0,2
mm
E
R
p0,2
0,005 0 ≤ < 0,005 7 75
E
R
p0,2
0,005 7 ≤ < 0,006 2 63
E
R
p0,2
0,006 2 ≤ < 0,006 5 50
E
R
p0,2
0,006 5 ≤ < 0,006 8 4
...
INTERNATIONAL ISO
STANDARD 12135
Third edition
2021-07
Corrected version
2022-08
Metallic materials — Unified method
of test for the determination of
quasistatic fracture toughness
Matériaux métalliques — Méthode unifiée d'essai pour la
détermination de la ténacité quasi statique
Reference number
© ISO 2021
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Published in Switzerland
ii
Contents Page
Foreword . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms.2
5 General requirements . 5
5.1 General . 5
5.2 Fracture parameters . 7
5.3 Fracture toughness symbols . 8
5.4 Test specimens. 8
5.4.1 Specimen configuration and size . 8
5.4.2 Specimen preparation .13
5.5 Pre-test requirements . 19
5.5.1 Pre-test measurements . 19
5.5.2 Crack shape/length requirements. 19
5.6 Test apparatus. 19
5.6.1 Calibration . . . 19
5.6.2 Force application . 20
5.6.3 Displacement measurement . 20
5.6.4 Test fixtures . 20
5.7 Test requirements. 24
5.7.1 Three-point bend testing . 24
5.7.2 Compact tension testing . 24
5.7.3 Specimen test temperature . 24
5.7.4 Recording . 25
5.7.5 Testing rates . 25
5.7.6 Test analyses . 25
5.8 Post-test crack measurements . 25
5.8.1 General . 25
5.8.2 Initial crack length, a .25
5.8.3 Stable crack extension, Δa .30
5.8.4 Unstable crack extension .30
6 Determination of fracture toughness for stable and unstable crack extension .31
6.1 General . 31
6.2 Determination of plane strain fracture toughness, K . 32
lc
6.2.1 General . 32
6.2.2 Interpretation of the test record for F . 32
Q
6.2.3 Calculation of K .33
Q
6.2.4 Qualification of K as K .34
Q lc
6.3 Determination of fracture toughness in terms of δ .34
6.3.1 Determination of F and V , F and V , or F and V .34
c c u u uc uc
6.3.2 Determination of F and V . 35
m m
6.3.3 Determination of V . 36
p
6.3.4 Calculation of δ .36
6.3.5 Qualification of δ fracture toughness value . 37
6.4 Determination of fracture toughness in terms of J .38
6.4.1 Determination of F and V or q , F and V or q , or F and V or q .38
c c c u u u uc uc uc
6.4.2 Determination of F and q .38
m m
6.4.3 Determination of U .38
p
6.4.4 Calculation of J . 39
6.4.5 Qualification of J fracture toughness value .40
iii
7 Determination of resistance curves δ-Δa and J-Δa and initiation toughness δ
0,2BL
and J and δ and J for stable crack extension .41
0,2BL i i
7.1 General . 41
7.2 Test procedure . 41
7.2.1 General . 41
7.2.2 Multiple-specimen procedure . 41
7.2.3 Single-specimen procedure . 41
7.2.4 Final crack front straightness . 42
7.3 Calculation of J and δ . 42
7.3.1 Calculation of J . . 42
7.3.2 Calculation of δ . 42
7.4 R-curve plot . 43
7.4.1 Plot construction .44
7.4.2 Data spacing and curve fitting . 45
7.5 Qualification of resistance curves . .46
7.5.1 Qualification of J-Δa resistance curves .46
7.5.2 Qualification of δ−Δa resistance curves .46
7.6 Determination and qualification of J and δ . 47
0,2BL 0,2BL
7.6.1 Determination of J . . 47
0,2BL
7.6.2 Determination of δ .48
0,2BL
7.7 Determination of initiation toughness J and δ by scanning electron microscopy
i i
(SEM) .49
8 Test report .50
8.1 Organization .50
8.2 Specimen, material and test environment .50
8.2.1 Specimen description .50
8.2.2 Specimen dimensions.50
8.2.3 Material description .50
8.2.4 Additional dimensions .50
8.2.5 Test environment .50
8.2.6 Fatigue precracking conditions.50
8.3 Test data qualification . 51
8.3.1 Limitations . 51
8.3.2 Crack length measurements . 51
8.3.3 Fracture surface appearance . 51
8.3.4 Pop-in. 51
8.3.5 Resistance curves . 51
8.3.6 Checklist for data qualification . 51
8.4 Qualification of K . 52
lc
8.5 Qualification of δ , δ , δ or δ . 52
c(B) u(B) uc(B) m(B)
8.6 Qualification of J , J , J or J . 53
c(B) u(B) uc(B) m(B)
8.7 Qualification of the δ-R Curve . 53
8.8 Qualification of the J-R Curve . 53
8.9 Qualification of δ as δ . 53
0,2BL(B) 0,2BL
8.10 Qualification of J as J . 53
0,2BL(B) 0,2BL
Annex A (informative) Determination of δ and J .55
i i
Annex B (normative) Crack plane orientation .60
Annex C (informative) Example test reports .62
Annex D (informative) Stress intensity factor coefficients and compliance relationships .71
Annex E (informative) Measurement of load-line displacement q in the three-point bend test.75
Annex F (informative) Derivation of pop-in formulae .80
Annex G (informative) Analytical methods for the determination of V and U .82
p p
Annex H (informative) Guidelines for single-specimen methods .83
iv
Annex I (normative) Power-law fits to crack extension data (see Reference [42]) .97
Bibliography .98
v
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
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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
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For an explanation of the voluntary nature of standards, 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
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals,
Subcommittee SC 4, Fatigue, fracture and toughness testing.
This third edition cancels and replaces the second edition (ISO 12135:2016), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— formulae to calculate CTOD have been replaced with those based on rigid rotation assumption
throughout; replacing the previous R-curve formulae based on CTOD from J. CTOD formulae for
SENBs are now those based on recent research to include the material yield to tensile strength ratio
in the CTOD formulae;
— the determination of J directly from displacement defined in terms of CMOD has been included, in
addition to the methods based on load line displacement;
— where fatigue precrack straightness requirements cannot be met due to internal residual stresses,
the application of modification techniques, originally developed for weld specimens, is now
permitted;
— the rotation correction factor for compact specimens has been revised with a new formula;
— editorial changes have been made to improve consistency of terms and definitions used throughout
the document.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
This corrected version of ISO 12135:2021 incorporates the following corrections:
— in Figure 6 a) the envelope tip angle was corrected from 60° to 30°;
vi
— in 7.3.1, Formula (35) was corrected, with the addition of "Δ" before "a", to read:
2 η U γ ⋅Δa
a
FS⋅ 1−ν
pp p
J= g ⋅⋅+ 11− ;
05,
15, WE BW−a Wa−
() ()
BB⋅ W
() N 0 0
N
— in 7.3.2, Formula (38) was corrected, with the deletion of "+z", to read:
2 1−raΔ ++rB
a ()
FS⋅ 1−ν p pN
g +
δ = ⋅V ;
1 p
05,
15, WmRE
1−raΔ ++rB a
()BB⋅ W p02, ()
pp N 0
N
— in 7.3.2, Formula (43) was corrected, with the deletion of "+z", to read:
a 05,,40Δa+ 46 WWa−
()
F 1−ν
0 0
δ = g ⋅ + ⋅V ;
2 p
05,
WR2 E 05,,40aa+Δ + 46W
()
BB⋅⋅W p02, 0
()
N
— in Table C.3 the small "v" was corrected to capital "V";
2 2
a a
— in Annex D, Formula (D.7) was corrected, with the replacement of 1− with 1− , to read:
W W
2 34
a 15,8 a a a a
g = 0,,121+−1210,,159 −14771+ ,30 ;
W W W W W
a
1−
W
— in Annex H, Formula (H.13) was corrected, with the replacement of "g " with "g ", to read:
6 4
a
g
W a
coefficient λ= and the function to read: g .
a
W
0,est
g
W
vii
INTERNATIONAL STANDARD ISO 12135:2021(E)
Metallic materials — Unified method of test for the
determination of quasistatic fracture toughness
1 Scope
This document specifies methods for determining fracture toughness in terms of K, δ, J and R-curves for
homogeneous metallic materials subjected to quasistatic loading. Specimens are notched, precracked
by fatigue and tested under slowly increasing displacement. The fracture toughness is determined for
individual specimens at or after the onset of ductile crack extension or at the onset of ductile crack
instability or unstable crack extension. In cases where cracks grow in a stable manner under ductile
tearing conditions, a resistance curve describing fracture toughness as a function of crack extension
is measured. In some cases in the testing of ferritic materials, unstable crack extension can occur
by cleavage or ductile crack initiation and growth, interrupted by cleavage extension. The fracture
toughness at crack arrest is not covered by this document. Special testing requirements and analysis
procedures are necessary when testing weldments, and these are described in ISO 15653 which is
complementary to this document.
Statistical variability of the results strongly depends on the fracture type, for instance, fracture
toughness associated with cleavage fracture in ferritic steels can show large variation. For applications
that require high reliability, a statistical approach can be used to quantify the variability in fracture
toughness in the ductile-to-brittle transition region, such as that given in ASTM E1921. However, it is
not the purpose of this document to specify the number of tests to be carried out nor how the results of
the tests are to be applied or interpreted.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 3785, Metallic materials — Designation of test specimen axes in relation to product texture
ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Calibration and verification of the force-measuring system
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
stress intensity factor
K
magnitude of the elastic stress-field singularity for a homogeneous, linear-elastic body
Note 1 to entry: The stress intensity factor is a function of applied force, crack length, specimen size and specimen
geometry.
3.2
crack-tip opening displacement
δ
relative opening displacement of the crack surfaces normal to the original (undeformed) crack plane at
the tip of the fatigue precrack, evaluated using the rotation point formula
3.3
J-integral
line or surface integral that encloses the crack front from one crack surface to the other and
characterizes the local stress-strain field at the crack tip
3.4
J
loading parameter, equivalent to the J-integral (3.3), the specific values of which, experimentally
determined by this method of test (J , J , J ,…), characterize fracture toughness under elastic-plastic
c i u
conditions
3.5
stable crack extension
crack extension which stops or would stop when the applied displacement is held constant as a test
progresses under displacement control
3.6
unstable crack extension
abrupt crack extension occurring with or without prior stable crack extension (3.5)
3.7
pop-in
abrupt discontinuity in the force versus displacement record, featured as a sudden increase in
displacement and, generally, a decrease in force followed by an increase in force
Note 1 to entry: Displacement and force subsequently increase beyond their values at pop-in.
Note 2 to entry: When conducting tests by this method, pop-ins can result from unstable crack extension
(3.6) in the plane of the precrack and are to be distinguished from discontinuity indications arising from: i)
delaminations or splits normal to the precrack plane; ii) roller or pin slippage in bend or compact specimen
load trains, respectively; iii) improper seating of displacement gauges in knife edges; iv) ice cracking in low-
temperature testing; v) electrical interference in the instrument circuitry of force and displacement measuring
and recording devices.
3.8
crack extension resistance curves
R-curves
variation in δ (3.2) or J (3.4) with stable crack extension (3.5)
4 Symbols and abbreviated terms
Symbol Unit Designation
a mm Nominal crack length (for the purposes of fatigue precracking, an assigned value less
than a )
a mm Final crack length (a + Δa)
f 0
a mm Instantaneous crack length
i
a mm Length of machined notch
m
a mm Initial crack length
NOTE 1 This is not a complete list of parameters. Only the main parameters are given, other parameters are referred to in
the text.
NOTE 2 The values of all parameters used in calculations are assumed to be those measured or calculated for the
temperature of the test, unless otherwise specified.
Symbol Unit Designation
Δa mm Stable crack extension including blunting
Δa mm Crack extension limit for δ or J controlled crack extension
max
B mm Specimen thickness
B mm Specimen net thickness between side grooves
N
C m/N Specimen elastic compliance
CMOD mm Crack-mouth opening displacement, V
CTOD mm Crack tip opening displacement, δ
E GPa Modulus of elasticity at the pertinent temperature
F kN Applied force
F kN Applied force at the onset of unstable crack extension or pop-in when Δa is less than
c
0,2 mm offset from the construction line (Figure 2)
F kN Force value corresponding to the intersection of the test record with the secant line
d
(Figure 18)
F kN Maximum fatigue precracking force
f
F kN Limiting collapse load estimated for a given specimen type
L
F kN Maximum force for a test which exhibits a maximum force plateau preceding fracture
m
with no significant prior pop-ins (Figure 2)
F kN Provisional force value used for the calculation of K
Q Q
F kN Applied force at the onset of unstable crack extension or pop-in when Δa is equal to or
u
greater than the 0,2 mm offset from the construction line (Figure 2)
J MJ/m Experimental equivalent to the J-integral
J MJ/m Size sensitive fracture resistance J at onset of unstable crack extension or pop-in when
c(B)
stable crack extension is less than 0,2 mm offset from the construction line (B = spec-
imen thickness in mm)
J MJ/m J at upper limit of J-controlled crack extension
g
J MJ/m Size-insensitive fracture resistance J at initiation of stable crack extension
i
J MJ/m Size sensitive fracture resistance J at the first attainment of a maximum force plateau
m(B)
for fully plastic behaviour (B = specimen thickness in mm)
J MJ/m Limit of J-R material behaviour defined by this method of test
max
J MJ/m Size sensitive fracture resistance J at the onset of unstable crack extension or pop-in
u(B)
when the event is preceded by stable crack extension equal to or greater than 0,2 mm
offset from the construction line (B = specimen thickness in mm)
J MJ/m Size sensitive fracture resistance J at the onset of unstable crack extension or pop-in
uc(B)
when stable crack extension cannot be measured (B = specimen thickness in mm)
J MJ/m J unclassified, and uncorrected for stable crack extension
J MJ/m Size insensitive fracture resistance J at 0,2 mm stable crack extension offset from the
0,2BL
construction line
J MJ/m Size sensitive fracture resistance J at 0,2 mm stable crack extension offset from the
0,2BL(B)
construction line (B = specimen thickness in mm)
0,5
K MPa m Stress intensity factor
0,5
K MPa m Maximum value of K during the final stage of fatigue precracking
f
0,5
K MPa m Plane strain linear elastic fracture toughness
lc
0,5
K MPa m Plane strain linear elastic fracture toughness equivalent to J
J0,2BL 0,2BL
0,5
K MPa m A provisional value of K
Q lc
NOTE 1 This is not a complete list of parameters. Only the main parameters are given, other parameters are referred to in
the text.
NOTE 2 The values of all parameters used in calculations are assumed to be those measured or calculated for the
temperature of the test, unless otherwise specified.
Symbol Unit Designation
M M
M — Where M appears as a superscript designation (such as J or δ ), it indicates that re-
sidual stress modification techniques have been applied to the specimen prior to test.
q mm Load-line displacement. q equals V in compact specimens (Figure 14).
R MPa Ultimate tensile strength perpendicular to crack plane at the test temperature
m
R MPa 0,2 % offset yield strength perpendicular to crack plane at the test temperature
p0,2
S mm Span between outer loading points in a three-point bend test
T °C Test temperature
U J Area under plot of force F versus crack-mouth opening displacement V, or load-line
displacement q
U J Elastic component of U
e
U J Plastic component of U (Figure 20)
p
V mm In bend specimens, V is the crack-mouth opening displacement (CMOD), which is the
opening displacement at the notch edges (Figure 13). In compact specimens, the open-
ing displacement, V, is determined at the load-line. V equals q in compact specimens
(Figure 14).
V mm Elastic component of V
e
V mm Displacement measured by clip gauges mounted on knife edges at a distance z from the
g
crack -mouth. Where integral knife edges are used, V =V (Figure 13).
g
V mm Plastic component of V
p
W mm Width of the test specimen
z mm For bend and straight-notch compact specimens, z is the initial distance of the crack-
mouth opening gauge measurement position from the notched edge of the specimen,
either further from the crack tip [+z in Figure 8 b)] or closer to the crack tip (−z); or, for
a stepped-notch compact specimen, z is the initial distance of the crack-mouth opening
gauge measurement position either beyond (+z) or before (−z) the initial load-line.
δ mm Crack-tip opening displacement (CTOD)
δ mm Size sensitive fracture resistance δ at the onset of unstable crack extension or pop-in
c(B)
when stable crack extension is less than 0,2 mm crack offset from the construction line
(B = specimen thickness in mm)
δ mm δ at the limit of δ-controlled crack extension
g
δ mm Fracture resistance δ at initiation of stable crack extension
i
δ mm Size sensitive fracture resistance δ at the first attainment of a maximum force plateau
m(B)
for fully plastic behaviour (B = specimen thickness in mm)
δ mm Limit of δ-R curve defined by this method of test
max
δ mm Size sensitive fracture resistance δ at the onset of unstable crack extension or pop-in
u(B)
when the event is preceded by stable crack extension equal to or greater than 0,2 mm
offset from the construction line (B = specimen thickness in mm)
δ mm Size sensitive fracture resistance δ at the onset of unstable crack extension or pop-in
uc(B)
when stable crack extension Δa cannot be measured (B = specimen thickness in mm)
δ mm δ unclassified, and uncorrected for stable crack extension
δ mm Size insensitive fracture resistance δ at 0,2 mm crack extension offset from construc-
0,2BL
tion line
δ mm Size sensitive fracture resistance δ at 0,2 mm stable crack extension offset from con-
0,2BL(B)
struction line (B = specimen thickness in mm)
η — Dimensionless function of geometry used to calculate J
p
ν — Poisson's ratio
NOTE 1 This is not a complete list of parameters. Only the main parameters are given, other parameters are referred to in
the text.
NOTE 2 The values of all parameters used in calculations are assumed to be those measured or calculated for the
temperature of the test, unless otherwise specified.
5 General requirements
5.1 General
The fracture toughness of metallic materials can be characterized in terms of either specific (single
point) values (see Clause 6), or a continuous curve relating fracture resistance to crack extension over
a limited range of crack extension (see Clause 7). The procedures and parameters used to determine
fracture toughness vary depending upon the level of plasticity realized in the test specimen during
the test. Under any given set of conditions, however, any one of the fatigue-precracked test specimen
configurations specified in this method may be used to measure any of the fracture toughness
parameters considered. In all cases, tests are performed by applying slowly increasing displacements
to the test specimen and measuring the forces and displacements realized during the test. The
forces and displacements are then used in conjunction with certain pre-test and post-test specimen
measurements to determine the fracture toughness that characterizes the material’s resistance to
crack extension. Details of the test specimens and general information relevant to the determination
of all fracture parameters are given in this method. A flow-chart illustrating the way this method can
be used is presented in Figure 1. Characteristic types of force versus displacement records obtained in
fracture toughness tests are shown in Figure 2.
Figure 1 — General flowchart showing how to use the standard method of test
Key
X crack-mouth opening displacement (V) or load-line displacement (q)
Y force (F)
NOTE 1 The classifications of F , F and F are described in 6.3.1 and 6.4.1.
c u m
NOTE 2 Pop-in behaviour is a function of the material toughness and parameters of the test setup such as the
testing machine/specimen compliance and the recorder response rate.
a
Fracture.
b
Pop-in.
Figure 2 — Characteristic types of force versus displacement records in fracture tests
5.2 Fracture parameters
Specific (point) values of fracture toughness are determined from individual specimens to define the
onset of unstable crack extension or describe stable crack extension.
NOTE K characterizes the resistance to extension of a sharp crack so that i) the state of stress near the
lc
crack front closely approximates plane strain, and ii) the crack tip plastic zone is small compared with the
specimen crack size, thickness and ligament ahead of the crack.
K is considered a size-insensitive measurement of fracture toughness under the above conditions.
lc
Certain test criteria shall be met in order to qualify measurements of K .
lc
The parameters δ , J , δ , J , δ and J also characterize the resistance of a material to unstable
c c u u uc uc
extension of a sharp crack. However, these measurements are regarded as size-sensitive and as such
characterize only the specimen thickness tested. The specimen thickness is thus noted in millimetre
units in parentheses appended to the parameter symbol when reporting a test result.
When stable crack extension is extensive, a test procedure and fracture toughness measurement shall
be performed as specified in Clause 7. Stable crack extension is characterized either in terms of crack
tip opening displacement δ and fracture toughness J parameters, or of a continuous δ- and
0,2BL 0,2BL
J-resistance curve. The values δ and J , regarded as specimen size insensitive, are engineering
0,2BL 0,2BL
estimates of the onset of stable crack extension, not to be confused with the actual initiation toughness
δ and J . Measurement of δ and J is described in Annex A.
i i i i
Two procedures are available for determining δ and J . The multiple specimen procedure
0,2BL 0,2BL
requires several nominally identical specimens to be monotonically loaded, each to different amounts
of displacement. Measurements of force and displacement are made and recorded. Specimen crack
fronts are marked (e.g. by heat tinting or post-test fatiguing) after testing, thus enabling measurement
of stable crack extension on the specimen halves after each specimen is broken open. Post-test cooling
of ferritic material specimens to ensure brittle behaviour can be helpful in preserving crack front
markings prior to breaking open the specimens.
A minimum of six specimens is required by the multiple-specimen method. When material availability
is limited, a single-specimen procedure based on either unloading compliance or the potential drop
technique may be used. There is no restriction on the single-specimen procedure providing sufficient
accuracy can be demonstrated. In all cases, certain criteria are to be met before δ or J values
0,2BL 0,2BL
and δ- or J-resistance curves are qualified by this standard method of test.
5.3 Fracture toughness symbols
Fracture toughness symbols identified in this document are given in Table 1.
Table 1 — Fracture toughness symbols
Size sensitive quantities Qualifying limits
Parameter Size insensitive quantities
(specific to thickness B tested) to R-curves
K
lc
K
K
J0,2BL
δ
c(B)
δ
i
δ δ δ , δ (Δa )
0,2BL(B) g g max
δ
0,2BL
δ , δ , δ
u(B) uc(B) m(B)
J
c(B)
J
i
J J J J (Δa )
0,2BL(B) g, g max
J
0,2BL
J , J , J
u(B) uc(B) m(B)
5.4 Test specimens
5.4.1 Specimen configuration and size
Dimensions and tolerances of specimens shall conform to Figures 3 to 5.
The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top
and bottom edges of the specimen to within 0,005 W.
NOTE 1 Integral or attachable knife edges for clip gauge attachment can be used (see Figures 8 and 9).
NOTE 2 For starter notch and fatigue crack configuration, see Figure 6.
NOTE 3 1,0 ≤ W/B ≤ 4,0 (W/B = 2 preferred).
NOTE 4 0,45 ≤ a/W ≤ 0,70. For K determination, 0,45 ≤ a/W ≤ 0,55.
lc
NOTE 5 Surface roughness Ra in micrometres.
a
See Figures 6 to 8 and 5.4.2.3.
Figure 3 — Proportional dimensions and tolerances for bend specimen
The intersection of the crack starter notch tips with the two specimen surfaces shall be equally distant from the top
and bottom edges of the specimen to within 0,005 W.
NOTE 1 Integral or attachable knife edges for clip gauge attachment can be used (see Figures 8 and 9).
NOTE 2 For starter notch and fatigue crack configuration, see Figure 6.
NOTE 3 0,8 ≤ W/B ≤ 4,0 (W/B = 2 preferred).
NOTE 4 0,45 ≤ a/W ≤ 0,70. For K determination, 0,45 ≤ a/W ≤ 0,55.
lc
+0,004W
NOTE 5
Alternative pin hole diameter, φ 0,188 W .
NOTE 6 Surface roughness Ra in micrometres.
a
See Figures 6 to 8 and 5.4.2.3.
Figure 4 — Proportional dimensions and tolerances for straight-notch compact specimen
The intersection of the crack starter notch tips with the t
...
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