Metallic materials — Measurement of fracture toughness at impact loading rates using precracked Charpy-type test pieces

ISO 26843:205 specifies requirements for performing and evaluating instrumented precracked Charpy impact tests on metallic materials using a fracture mechanics approach. Minimum requirements are given for measurement and recording equipment such that similar sensitivity and comparable measurements are achieved. Dynamic fracture mechanics properties determined using this International Standard are comparable with conventional large-scale fracture mechanics results when the corresponding validity criteria are met. Because of the small absolute size of the Charpy specimen, this is often not the case. Nevertheless, the values obtained can be used in research and development of materials, in quality control, and to establish the variation of properties with test temperature under impact loading rates. Fracture toughness properties determined through the use of this International Standard may differ from values measured at quasistatic loading rates. Indeed, an increase in loading rate causes a decrease in fracture toughness when tests are performed in the brittle or ductile-to-brittle regimes; the opposite is observed (i.e. increase in fracture toughness) in the fully ductile regime. More information on the dependence of fracture toughness on loading (or strain) rate is given in Reference [1]. In addition, it is generally acknowledged that fracture toughness also depends on test temperature. For these reasons, the user is required to report the actual test temperature and loading rate for each test performed. In case of cleavage fracture of ferritic steels in the ductile-to-brittle transition region, variability can be very large and cannot be adequately described by simple statistics. In this case, additional tests are required and the analysis is to be performed using a statistical procedure applicable to this type of test, see for example Reference [2]. NOTE Modifications to the analytical procedures prescribed in Reference [2] might be necessary to account for the effect of elevated (impact) loading rates.

Matériaux métalliques — Mesure de la ténacité d'éprouvettes type Charpy préfissurées soumises à un chargement d'impact

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INTERNATIONAL ISO
STANDARD 26843
First edition
2015-12-15
Metallic materials — Measurement
of fracture toughness at impact
loading rates using precracked
Charpy-type test pieces
Matériaux métalliques — Mesure de la ténacité d’éprouvettes type
Charpy préfissurées soumises à un chargement d’impact
Reference number
ISO 26843:2015(E)
ISO 2015
---------------------- Page: 1 ----------------------
ISO 26843:2015(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2015, Published in Switzerland

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ii © ISO 2015 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 26843:2015(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Symbols .......................................................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 3

5 Test specimens........................................................................................................................................................................................................ 5

6 Testing machines .................................................................................................................................................................................................. 6

7 Test procedures and measurements................................................................................................................................................ 6

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

7.2 Impact velocity ........................................................................................................................................................................................ 7

7.3 Time to fracture ...................................................................................................................................................................................... 7

7.4 Multiple specimen tests................................................................................................................................................................... 7

7.5 Single-specimen tests ........................................................................................................................................................................ 7

7.6 Post-test crack length measurements ................................................................................................................................. 8

8 Evaluation of fracture mechanics parameters ...................................................................................................................... 8

9 Test report ................................................................................................................................................................................................................... 9

9.1 Organization .............................................................................................................................................................................................. 9

9.2 Specimen, material, and test environment ..................................................................................................................... 9

9.2.1 Specimen description .................................................................................................................................................. 9

9.2.2 Specimen dimensions .................................................................................................................................................. 9

9.2.3 Material description ........................................................................................................................................... ........... 9

9.2.4 Test environment .........................................................................................................................................................10

9.3 Fatigue precracking conditions .............................................................................................................................................10

9.4 Test data qualification ...................................................................................................................................................................10

9.4.1 Limitations .........................................................................................................................................................................10

9.4.2 Crack length measurements ...............................................................................................................................10

9.4.3 Fracture surface appearance ..............................................................................................................................10

9.4.4 Resistance curves .........................................................................................................................................................10

9.4.5 Checklist for data qualification .........................................................................................................................10

9.5 Test results...............................................................................................................................................................................................11

Annex A (normative) Test machines suitable for each test procedure ..........................................................................12

Annex B (informative) Estimation of strain rate ...................................................................................................................................13

Annex C (normative) Dynamic evaluation of fracture toughness ........................................................................................14

Annex D (normative) Determination of resistance curves at impact loading rates by

multiple specimen methods ..................................................................................................................................................................19

Annex E (normative) Estimation of J-Δa R-curves using the normalization method ...................................21

Annex F (normative) Determination of characteristic fracture toughness value J ...............................24

0,2Bd

Annex G (normative) Validity criteria ..............................................................................................................................................................25

Annex H (normative) Determination of fracture toughness in terms of J-integral ...........................................27

Annex I (informative) Example test reports ..............................................................................................................................................29

Bibliography .............................................................................................................................................................................................................................34

© ISO 2015 – All rights reserved iii
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ISO 26843:2015(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on the meaning of ISO specific terms and expressions related to conformity

assessment, as well as information about ISO’s adherence to the WTO principles in the Technical

Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information

The committee responsible for this document is ISO/TC 164, Mechanical testing of metals, Subcommittee

SC 4, Toughness testing — Fracture (F), Pendulum (P), Tear (T).
iv © ISO 2015 – All rights reserved
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ISO 26843:2015(E)
Introduction

This International Standard is closely related to ISO 14556 and was derived from a draft procedure

prepared by the Working Party “European Standards on Instrumented Precracked Charpy Testing”

of the European Structural Integrity Society (ESIS) Technical Subcommittee on Dynamic Testing at

Intermediate Strain Rates (TC5).
© ISO 2015 – All rights reserved v
---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 26843:2015(E)
Metallic materials — Measurement of fracture toughness
at impact loading rates using precracked Charpy-type test
pieces
1 Scope

This International Standard specifies requirements for performing and evaluating instrumented

precracked Charpy impact tests on metallic materials using a fracture mechanics approach. Minimum

requirements are given for measurement and recording equipment such that similar sensitivity and

comparable measurements are achieved.

Dynamic fracture mechanics properties determined using this International Standard are comparable

with conventional large-scale fracture mechanics results when the corresponding validity criteria are

met. Because of the small absolute size of the Charpy specimen, this is often not the case. Nevertheless,

the values obtained can be used in research and development of materials, in quality control, and to

establish the variation of properties with test temperature under impact loading rates.

Fracture toughness properties determined through the use of this International Standard may differ

from values measured at quasistatic loading rates. Indeed, an increase in loading rate causes a decrease

in fracture toughness when tests are performed in the brittle or ductile-to-brittle regimes; the opposite

is observed (i.e. increase in fracture toughness) in the fully ductile regime. More information on the

dependence of fracture toughness on loading (or strain) rate is given in Reference [1]. In addition, it is

generally acknowledged that fracture toughness also depends on test temperature. For these reasons,

the user is required to report the actual test temperature and loading rate for each test performed.

In case of cleavage fracture of ferritic steels in the ductile-to-brittle transition region, variability can

be very large and cannot be adequately described by simple statistics. In this case, additional tests are

required and the analysis is to be performed using a statistical procedure applicable to this type of test,

see for example Reference [2].

NOTE Modifications to the analytical procedures prescribed in Reference [2] might be necessary to account

for the effect of elevated (impact) loading rates.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method

ISO 148-2, Metallic materials — Charpy pendulum impact test — Part 2: Verification of testing machines

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

ISO 14556, Steel — Charpy V-notch pendulum impact test — Instrumented test method

ISO 26203-2, Metallic materials — Tensile testing at high strain rates — Part 2: Servo-hydraulic and

other test systems
3 Symbols

For the purposes of this International Standard, the following symbols given in Table 1 apply.

© ISO 2015 – All rights reserved 1
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ISO 26843:2015(E)
Table 1 — Symbols and definitions used in this International Standard
Symbol Definition Unit

Nominal crack length (for the purposes of fatigue precracking, an assigned value less mm

than a )
a Final crack length (a + Δa) mm
f 0
a Length of machined notch mm
a Initial crack length mm
Δa Crack extension (a – a ) mm
Δa Crack extension limit for J-controlled crack extension mm
max
Δa Crack extension corresponding to displacement s mm
B Specimen thickness mm
B Specimen effective thickness as defined in Formula (E.7) mm
B Specimen net thickness after side-grooving mm
C Compliance of the test machine m/N
C Specimen elastic compliance m/N
C Specimen theoretical compliance m/N
E Young’s modulus of elasticity GPa
dε/dt Strain rate s
f Output frequency limit Hz
F Applied force N
F Applied force at onset of unstable crack extension in Figure 1 N
F Maximum fatigue precracking force during the final precracking stage N
F Applied force at onset of general yielding as defined in ISO 14556 N
F Maximum applied force as defined in ISO 14556 N
F Applied force corresponding to a displacement s N
J Dynamic J-integral MJ/m
J Dynamic equivalent of J in ISO 12135 (with B = 10 mm) MJ/m
cd c(B)
J J at upper limit of J-controlled crack extension MJ/m
J Limit of J -R material behaviour defined by this test method MJ/m
d,max d
J Dynamic equivalent of J in ISO 12135 (with B = 10 mm) MJ/m
ud u(B)
J Dynamic equivalent of J in ISO 12135 (with B = 10 mm) MJ/m
0,2Bd 0,2BL(B)
2 −1
dJ /dt Rate of change of dynamic J-integral MJ/m s
0,5
K Dynamic stress intensity factor MPa m
0,5
K Dynamic stress intensity factor calculated from J-integral MPa m
dyn 0,5

K (t) Stress intensity factor – time history from the impact response curve method MPa m

0,5
K Dynamic plane strain fracture toughness MPa m
0,5

K Dynamic stress intensity factor calculated from J-integral at the onset of cleavage MPa m

Jcd
0,5 −1
dK /dt Rate of change of dynamic stress intensity factor MPa m s
KV Absorbed energy as defined in ISO 148-1 J

KV Available potential energy corresponding to a reduced pendulum impact velocity v J

0 0
M Total mass of the moving striker of the pendulum kg
n Strain hardening exponent of the Ramberg-Osgood material law —
N Number of available test specimens —
2 © ISO 2015 – All rights reserved
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ISO 26843:2015(E)
Table 1 (continued)
Symbol Definition Unit

Dynamic flow stress, defined as the average of dynamic yield strength and dynamic

R MPa
tensile strength

R Dynamic tensile strength determined at the strain rate of the fracture toughness test MPa

Dynamic yield (proof) strength determined at the strain rate of the fracture tough-

R MPa
ness test
R Yield (proof) strength measured at quasistatic strain rate MPa
s Specimen displacement (calculated according to ISO 14556) mm
s Plastic component of specimen displacement mm
S Span between outer loading points mm
T Temperature °C
t Time s
t Time to fracture s
t Time at the onset of crack propagation s
t Signal rise time s
t Time at striker impact s
τ Period of force oscillation s
v Initial striker impact velocity m s

v Striker impact velocity corresponding to the maximum available energy of the pendu-

m s
lum
W Specimen effective width mm
W Energy at maximum force as defined in ISO 14556 J

W Plastic component of the area under the force-displacement test record up to displace-

ment s

W Total fracture energy under the force-displacement test record up to displacement s J

W Calculated energy from area under complete force-displacement test record up to

F = 0,02 F as defined in ISO 14556
W Available impact energy J
ν Poisson’s ratio —
4 Principle

This International Standard prescribes impact bend tests which may be performed on fatigue precracked

Charpy-type specimens to obtain dynamic fracture mechanics properties of metallic materials. This

International Standard extends the procedure for V-notch impact bend tests in accordance with

ISO 148-1, and may be used for the evaluation of the master curve reference temperature in accordance

with Reference [2] provided that the corresponding validity requirements are met. Instrumented

testing machines are required together with ancillary instrumentation and recording equipment in

accordance with ISO 14556.

Fracture toughness properties depend on material response reflected in the force-time diagrams

described in Table 2 and Figure 1. The logical structure for fracture property determination and

validation is shown in the flow chart of Figure 2.
© ISO 2015 – All rights reserved 3
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ISO 26843:2015(E)
Table 2 — Fracture toughness properties to be determined

Material response/fracture behaviour Corresponding diagram type R-curve Characteristic pa-

(see Figure 1) rameters
J , K , K (B, dK /
cd Jcd Id d
Linear-elastic I —
dt, dJ /dt)
Elastic-plastic, unstable fracture with
II — J , K (B,dJ /dt)
cd Jcd d
Δa < 0,2 mm
Elastic-plastic, unstable fracture with
II — J (B,Δa,dJ /dt)
ud d
0,2 mm ≤ Δa ≤ 0,15 (W − a )
Elastic-plastic, unstable fracture with
III J -Δa J (dJ /dt)
d 0,2Bd d
Δa > 0,15 (W − a )
Elastic-plastic; no unstable fracture IV J -Δa J (dJ /dt)
d 0,2Bd d
Force
Force
Type I
Type II
t t
Force Time Time
f f
Force
Type III
m F
Type IV
Test
termination
Figure 1 — Typical force-time diagrams (schematic)
4 © ISO 2015 – All rights reserved
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ISO 26843:2015(E)
Fracture toughness test
at impact loading rates
Unstable Fracture
Stable
Figure 1 types I or II behaviour
Figure 1 types III or IV
Figure 1
type
Test method
No Yes
Yes No
t < 3τ
t < 3τ
Use lower
Quasistatic Quasistatic
Use lower Multi-specimen
Single-specimen
impact
impact elastic-plastic
elastic-plastic
evaluation
normalization
velocity
velocity
fracture fracture
method
method
mechanics mechanics
dynamic eval.
(Annex E)
(Annex H) (Annex H)
(Annex D)
methods
(Annex C)
repeat test with
different ∆a
J , K J , K , J
Id cd Jcd cd Jcd ud
J-R curve
Yes
0,2 ≤ ∆a ≤ 0,15
∆a < 0,2 mm
AnneAnnex Fx F
(W−a )
Yes
J , K J
JJ , , KK
Validity (Annex G) Validity (Annex G)
cd Jcd ud
cdcd JcJcdd
Figure 2 — Flow chart for the application of the test method
5 Test specimens

5.1 Specimens shall be prepared in accordance with the standard specimens of ISO 148-1, with or

without the 2,0 mm V-notch, followed by fatigue precracking.

5.2 Specimens shall be fatigue precracked in bending to produce an initial crack length, a , in the range

0,30 ≤ a /W ≤ 0,70.

If the results in terms of J are to be directly comparable with full-size standard fracture toughness

values such as J (as defined in ISO 12135), then a /W shall be in the range 0,45 < a /W < 0,70.

0,2BL 0 0

Shorter crack lengths may be more advantageous, as a stiffer test piece increases the probability of a

successful test.

5.3 To initiate fatigue precracking, machine or spark erode a slot into the specimen. For specimens

with an existing V-notch, fatigue precracking may initiate at the bottom of the notch. The length of the

machined notch, a , shall be at least 1,0 mm shorter than the desired initial crack length, a .

m 0

5.4 During the final 1,3 mm or 50 % of precrack extension, whichever is less, the maximum fatigue

precracking force shall be the lower of:
© ISO 2015 – All rights reserved 5
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ISO 26843:2015(E)
0,8BW−a
F= R (1)
f p
 
 
WBB
W 
 
F=ξ×E (2)
f  
 a   S
 
   
 
 
 
−4 1/2
where ξ = 1,6 × 10 m and the function f is given in Formula (H.2).
 
 

The ratio of minimum-to-maximum fatigue precracking force shall be in the range 0 to 0,1 except that

to expedite crack initiation one or more cycles of −1,0 may be first applied.
NOTE For plain-sided specimens, B = B.

5.5 When fatigue precracking is performed at temperature T and testing is performed at temperature

T , F in Formula (2) shall be factored by the ratio R [T ] / R [T ], where R [T ] is the quasistatic yield

2 f p 1 pd 2 p 1

strength at temperature T and R [T ] is the dynamic yield strength at temperature T . In addition, F

1 pd 2 2 f

determined from Formula (1) shall be evaluated using the smaller value of R [T ] and R [T ].

p 1 pd 2

5.6 Specimens may be side grooved, preferably after fatigue precracking, using a V-notch cutter in

accordance with ISO 148-1 to a depth of 1,0 mm on each side. Side grooving is recommended for all J -Δa

R-curve tests. For details of crack length measurement, see 9.4.2.
6 Testing machines

6.1 The tests may be carried out using testing machines of the general types specified in Annex A. Not

all machines can perform all types of test (see Annex A for more details). In all cases, the striker and anvil

dimensions shall conform to ISO 148-2.

6.2 Details of machine instrumentation and calibration procedures are specified in ISO 14556.

6.3 For every test in which the entire force signal has been recorded (i.e. the force returns to the

baseline), the difference between KV and W shall be within ±15 % of KV or ±1 J, whichever is larger. If

this requirement is not met but the difference does not exceed ±25 % of KV or ±2 J, whichever is larger,

[3]

force values may be adjusted until KV = W . If the difference exceeds ±25 % of KV or ±2 J, whichever is

larger, the test shall be discarded and the calibration of the instrumented striker user shall be checked

and if necessary repeated. If recording of the entire force signal is not possible (for example due to the

specimen being ejected from the machine without being fully broken), conformance to the requirements

stated earlier shall be demonstrated by testing, using the same experimental apparatus, at least five

Charpy specimens (precracked, non-precracked, or a mix of precracked and non-precracked) of similar

absorbed energy level, for which the entire force signal is recorded. In all cases, the difference between

KV and W shall be within ±15 % of KV or ±1 J, whichever is larger.
7 Test procedures and measurements
7.1 General

Tests are performed in general accordance with the standard Charpy impact test of ISO 148-1, with

allowance for other types of machines, as specified in Annex A.
6 © ISO 2015 – All rights reserved
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ISO 26843:2015(E)

The force-displacement diagram is recorded according to ISO 14556, from which the key data values

F , F , W , and W are determined. In addition to the procedures of ISO 14556, specific procedures for

m cd m t

determining striking velocity, available energy, and crack lengths are given below. These data form the

basis for evaluation of toughness parameters according to Annexes D to F.

NOTE The force F in this International Standard corresponds to the force F (crack initiation force) in

cd iu
ISO 14556.
7.2 Impact velocity

This International Standard applies to any impact velocity, v , in excess of those corresponding to

the testing rates prescribed by ISO 12135. Commonly used impact velocities are in the range from

−1 −1
1 ms to 5,5 ms .

NOTE 1 Impact velocities for pendulum or falling weight testing machines can vary by adjusting the striker

release height.

NOTE 2 The reduced impact velocity, v , can be determined as follows: release the pendulum from the

appropriately reduced height, without a specimen on the supports. Read the energy KV (in J) indicated by the

pointer on the analogue scale. From this, the reduced impact velocity is calculated for a 300 J pendulum as:

300−KV
vv= (3)
00s
300

where v is the impact velocity corresponding to the maximum potential energy of the pendulum

(machine capacity), in this case 300 J. If the pendulum maximum available energy is different from

300 J, replace 300 in Formula (3) with the actual maximum available energy. A reduced velocity (1 m/s

to 2 m/s) can be advantageous, particularly in case of brittle behaviour, as it reduces the effect of

oscillations by lowering their relative amplitude and by increasing their number within the time to

fracture t (see 8.2).
7.3 Time to fracture

When the time t to initiate unstable fracture is less than 3τ, with τ being the period of force oscillation,

fracture occurs after less than three oscillations in the force-time or force-displacement record. In this

case, the instant of crack initiation is not detectable in the force signal with adequate accuracy due to

[4][5][6]

the force oscillations (see Figure 1, type I) and the test cannot be evaluated in accordance with

this International Standard. Reducing the test impact velocity is recommended for further testing in

order to increase the number of oscillations preceding fracture.

NOTE Dynamic evaluation methods have been proposed for determining t independently of force

measurements, when time to fracture t < 3τ. Examples are the impact response curve method and the crack tip

strain gauge method described in Annex C.
7.4 Multiple specimen tests

To determine dynamic J -R curves by multi-specimen techniques, the fracture process is interrupted at

a certain stable crack extension Δa and the process is repeated until an adequate number of data points

are available to define the J -R curve. This procedure is described in Annex D.
7.5 Single-specimen tests

Several single-specimen techniques have been proposed in the literature to estimate dynamic

J -R curves. However, only the normalization method described in Annex E is supported by this

International Standard.
© ISO 2015 – All rights reserved 7
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ISO 26843:2015(E)
7.6 Post-test crack length measurements

After a test has been performed, the specimen shall be broken open, if necessary, and the fracture

surfaces shall be examined to determine the initial crack length a and the amount of stable crack

extension Δa (if applicable). The measurement of initial crack length and stable or unstable crack

extension (if applicable) shall be performed in accordance with ISO 12135 (nine-point average method).

NOTE 1 For some tests, it may be necessary to mark the extent of stable crack extension before opening the

specimen. Stable crack extension may be marked by heat tinting or by post-test fatiguing. Care is to be taken

to minimize post-test deformation. Cooling materials which exhibit a ductile-to-brittle transition may help to

ensure brittle behaviour during specimen opening.

NOTE 2 In the case of poor contrast between fatigue crack, stable crack, and brittle crack after heat tinting,

when using a microscope for crack length measurement, the use of dark field illumination and/or filters may

be beneficial. Digitizing the fracture surface and subsequently evaluating the digital image by image analysis

software may be
...

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