ISO/FDIS 26843

Metallic materials — Measurement of fracture toughness of steels at impact loading rates using precracked Charpy specimens

ISO 26845:2009 specifies requirements for performing and evaluating instrumented precracked Charpy impact tests on steels using a fracture mechanics approach. Minimum requirements are given for measurement and recording equipment, such that similar sensitivity and comparable measurements are achieved. ISO 26845:2009 can be applied to other metallic materials by agreement. Dynamic fracture mechanics properties determined using ISO 26845:2009 are comparable to 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 service evaluation and to establish the variation of properties with test temperature under impact loading rates.

Matériaux métalliques — Mesure de la ténacité à la rupture d'éprouvettes Charpy préfissurées en acier soumises à des charges dynamiques

General Information

• Status: Withdrawn
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: ISO/TC 164/SC 4 - Fatigue, fracture and toughness testing
• Drafting Committee: ISO/TC 164/SC 4 - Fatigue, fracture and toughness testing
• Current Stage: 5098 - Project deleted
• Completion Date: 19-Sep-2011

Standards Content (Sample)

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 26843
ISO/TC 164/SC 4
Metallic materials — Measurement of
Secretariat: ANSI
fracture toughness of steels at impact
Voting begins on:
loading rates using precracked Charpy
2009-07-01
specimens
Voting terminates on:
2009-09-01
Matériaux métalliques — Mesure de la ténacité à la rupture

d'éprouvettes Charpy préfissurées en acier soumises à des charges
dynamiques


Please see the administrative notes on page iii

RECIPIENTS OF THIS DRAFT ARE INVITED TO
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BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 26843:2009(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
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DARDS TO WHICH REFERENCE MAY BE MADE IN
©
NATIONAL REGULATIONS. ISO 2009

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ISO/FDIS 26843:2009(E)
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ii © ISO 2009 – All rights reserved

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ISO/FDIS 26843:2009(E)
In accordance with the provisions of Council Resolution 15/1993, this document is circulated in the
English language only.

© ISO 2009 – All rights reserved iii

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ISO/FDIS 26843:2009(E)
Contents Page
Foreword. v
Introduction . vi
1 Scope. 1
2 Normative references . 1
3 Symbols and definitions. 1
4 Principle . 3
5 Test specimens . 5
6 Testing machines. 6
7 Test procedures and measurements . 7
7.1 General . 7
7.2 Key data . 7
7.3 Impact velocity . 7
7.4 Time to fracture . 7
7.5 Multiple specimen tests. 7
7.6 Single-specimen tests . 7
7.7 Crack length measurements after testing . 8
8 Evaluation of fracture mechanics parameters. 10
9 Test report. 11
Annex A (informative) Test machines suitable for each test procedure. 13
Annex B (informative) Estimation of strain rate . 14
Annex C (normative) Dynamic evaluation of fracture toughness . 15
Annex D (normative) Determination of resistance curves at impact loading rates by multiple
specimen methods. 21
Annex E (normative) Estimation of J-∆a R-curves by single-specimen methods. 23
Annex F (normative) Determination of characteristic fracture toughness values J or δ . 27
0,2Bd 0,2Bd
Annex G (normative) Validity criteria .29
Annex H (normative) Fracture mechanics parameters. 30
Bibliography . 32

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ISO/FDIS 26843:2009(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 26843 was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals, Subcommittee
SC 4, Toughness testing — Fracture (F), Pendulum (P), Tear (T).

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ISO/FDIS 26843:2009(E)
Introduction
This International Standard is closely related to ISO 14556 and was derived from a draft procedure prepared
by the “European Standards on Instrumented Precracked Charpy Testing” Working Party of the European
Structural Integrity Society (ESIS) Technical Subcommittee on Dynamic Testing at Intermediate Strain Rates
(TC 5).

vi © ISO 2009 – All rights reserved

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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 26843:2009(E)

Metallic materials — Measurement of fracture toughness of
steels at impact loading rates using precracked Charpy
specimens
1 Scope
This International Standard specifies requirements for performing and evaluating instrumented precracked
Charpy impact tests on steels using a fracture mechanics approach. Minimum requirements are given for
measurement and recording equipment, such that similar sensitivity and comparable measurements are
achieved.
This International Standard can be applied to other metallic materials by agreement. Dynamic fracture
mechanics properties determined using this International Standard are comparable to 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 service evaluation 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 can 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. Additional information on the dependence of
[1]
fracture toughness on loading (or strain) rate is given in Anderson . In addition, it is generally acknowledged
that fracture toughness also depends on test temperature. For these reasons, the user reports the actual test
temperature and loading rate for each test performed.
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 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 3785, Metallic materials — Designation of test specimen axes in relation to product texture
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
3 Symbols and definitions
For the purposes of this document, the symbols given in Table 1 apply.
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ISO/FDIS 26843:2009(E)
Table 1 — Symbols used in this International Standard
Symbol Definition Unit
a crack length mm
a initial crack length mm
o
∆a crack extension [a – a ] mm
o
∆a crack extension corresponding to displacement s mm
s
a length of machined notch mm
n
a fatigue crack length mm

f
B specimen thickness mm
B specimen effective thickness as defined in Equation (E.7) mm
e
B specimen net thickness after side-grooving mm
N
C compliance of the test machine m/N
m
δ crack-tip opening displacement (CTOD) mm
δ dynamic equivalent of δ in ISO 12135 mm
0,2Bd 0,2BL
−1
dδ/dt rate of crack-tip opening displacement mm s
E Young's modulus of elasticity GPa
−1
dε/dt strain rate s
f output frequency limit Hz
g
F force N
F applied force at onset of unstable crack extension in Figure 1 – Type I N
cd
F maximum fatigue precracking force during the final precracking stage N
f
F applied force at onset of yielding as defined in ISO 14556 N
gy
F maximum applied force as defined in ISO 14556 N
m
2
J experimental equivalent of the J-integral MJ/m
2
J dynamic equivalent of J in ISO 12135 MJ/m
cd c
2
J dynamic equivalent of J in ISO 12135 MJ/m
ud u
2
J dynamic equivalent of J in ISO 12135 MJ/m
0,2Bd 0,2BL
2 −1
dJ/dt rate of change of J-integral MJ/m s
dyn
0,5
K dynamic stress intensity factor MPa m
I
0,5
K dynamic plane strain fracture toughness MPa m
Id
0,5 −1
dK/dt rate of change of stress intensity factor MPa m s
KV absorbed energy as defined in ISO 148-2 J
M total mass of moving striker kg
n strain hardening exponent of the Ramberg-Osgood material law —
N number of available test specimens —
dynamic flow stress, defined as the average of dynamic yield strength and dynamic tensile
R MPa
fd
strength
R dynamic tensile strength determined at the strain rate of the fracture toughness test MPa
md
R dynamic yield (proof) strength determined at the strain rate of the fracture toughness test MPa
pd
R yield (proof) strength measured at quasistatic strain rate MPa
p
s displacement (calculated in accordance with ISO 14556) mm
s plastic component of displacement mm
pl
S span between outer loading points mm
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ISO/FDIS 26843:2009(E)
Table 1 (continued)
Symbol Definition Unit
T temperature °C
t time s
t time to fracture s
f
t time at the onset of crack propagation s
i
t signal rise time s
r
t time of striker impact s
o
τ period of force oscillation s
−1
v striker impact velocity m s
o
W specimen effective width mm
W energy at maximum force defined in ISO 14556 J
m
W actual total fracture energy (area under the force-displacement diagram up to displacement s) J
s
W non-recoverable fracture energy corresponding to force F and displacement s J
sp s
calculated energy from area under complete force-displacement curve to F = 0,02 F as
m
W J
t
defined in ISO 14556
W available impact energy J
o
initial distance of the notch opening gauge measurement position from the notched edge of
z mm
the specimen [see ISO 12135:2002, Figure 8b)]
v Poisson's ratio —
4 Principle
This International Standard prescribes impact bend tests which may be performed on fatigue precracked
Charpy notch specimens to obtain dynamic fracture mechanics properties of materials. This International
Standard extends the procedure for V-notch impact bend tests in accordance with ISO 148, and may be used
[2]
for evaluation of the Master Curve in accordance with ASTM E 1921 . 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 is shown in the flow chart of
Figure 2.
Table 2 — Fracture toughness properties to be determined
Corresponding
Characteristic
Material response/fracture behaviour diagram type R-curve
parameters
(see Figure 1)
Essentially linear-elastic I — K (dK /dt)
Id I
Elastic-plastic, unstable fracture without significant
II — J (B,dJ/dt)
cd
stable crack extension (∆a < 0,2 mm)
Elastic-plastic, unstable fracture after significant
II — J (B,∆a,dJ/dt)
ud
stable crack extension [0,2 mm u ∆a u 0,15 (W−a )]
o
Elastic-plastic, unstable fracture after substantial J −∆a J (dJ/dt)
d 0,2Bd
III
stable crack extension [∆a > 0,15 (W−a )] δ −∆a δ (dδ/dt)
o d 0,2Bd
J −∆a J (dJ/dt)
d 0,2Bd
Elastic-plastic; no unstable fracture IV
δ −∆a δ (dδ/dt)
d 0,2Bd

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ISO/FDIS 26843:2009(E)


a)  Type I b)  Type II


c)  Type III d)  Type IV
Figure 1 — Typical force-time diagrams — Schematic
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ISO/FDIS 26843:2009(E)

Figure 2 — Flow chart for selection 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. By agreement, alternative specimen dimensions may be
used.
5.2 To initiate fatigue precracking, machine or spark erode a slot into the specimen to a depth of at least
1,0 mm less than the desired initial crack length, a . For specimens with an existing V-notch, fatigue
o
precracking may initiate at the bottom of the notch.
5.3 During the final 1,3 mm or 50 % of precrack extension, whichever is less, the maximum fatigue
precracking force shall be the lower of the value found using either Equation (1) or (2):
2
0,8BW()−a
o
F = (1)
f
S
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ISO/FDIS 26843:2009(E)
⎡⎤
⎢⎥
WBB
W
⎛⎞
N
⎢⎥
FE= ξ (2)
f ⎜⎟
⎢⎥
⎛⎞aS
⎝⎠
o
f
⎢⎥
⎜⎟
W
⎝⎠
⎣⎦
⎛⎞a
o
−4 1/2
where ξ = 1,6 × 10 m and the function f is given in Equation (H.6).
⎜⎟
W
⎝⎠
The ratio of minimum to maximum fatigue pre-cracking 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.
N
5.4 When fatigue precracking is performed at temperature T and testing is done at temperature T , F in
1 2 f
Equation (2) shall be factored by the ratio R [T ]/R [T ], where R [T ] is the yield strength at temperature T
p 1 p 2 p 1 1
and R is the yield strength at temperature T . In addition, F determined from Equation (1) shall be evaluated
p 2 f
using the lowest value between R [T ] and R [T ].
p 1 p 2
NOTE Experience has shown for a wide variety of steels that a fatigue precrack can be initiated in a Charpy
specimen with an initial mean force of 2 kN and a range of ±1 kN at a/W of 0,3 which are both progressively reduced by
equal amounts to a level of 0,7 kN over the final 0,5 mm of crack extension. Small 10 % progressive reductions in the
force levels as crack extension progresses are made in order to avoid retardation effects during crack extension.
5.5 Specimens are fatigue precracked in three-point or pure bending to produce an initial crack length, a ,
o
normally in the range of 0,3 < a /W < 0,55.
o
If the results in terms of J or δ are to be directly comparable to full-size standard fracture toughness values
such as J or δ (as defined in ISO 12135), a /W shall be in the range 0,45 < a /W < 0,55. Otherwise,
0,2BL 0,2BL o o
shorter crack lengths may be more advantageous.
[3][5]
NOTE An impact response curve for 0,28 < a /W < 0,32 can be established .
o
5.6 Specimens may be side grooved 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-∆a R-curve tests. For details of crack length
measurement, see 7.7.
6 Testing machines
6.1 The tests may be carried out using testing machines of the general types specified in Annex A. Other
machines which comply with the calibration and other requirements are not excluded. Not all machines can
perform all types of test (see Annex A). 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. until the force returns to the
baseline), the difference between KV and W shall be within ±15 % of KV or ±1 J, whichever is the greater. If
t
this requirement is not met, but the difference does not exceed ±25 % of KV or ±2 J, whichever is the greater,
[6]
force values may be adjusted until KV = W . If the difference exceeds ±25 % of KV or ±2 J, whichever is
t
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 in this
subclause shall be demonstrated by testing at least five non-precracked Charpy specimens of similar
absorbed energy level.
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ISO/FDIS 26843:2009(E)
7 Test procedures and measurements
7.1 General
Tests are performed in a manner similar to the standard Charpy impact test of ISO 148-1, especially with
regard to the pendulum hammer and the handling of pre-cooled or pre-heated specimens.
7.2 Key data
The force-displacement diagram is recorded in accordance with ISO 14556, from which the key data values
F , F , W and W are determined. Additional to the procedures of ISO 14556 are the procedures for striking
m cd m t
velocity, available energy and measurement of crack lengths, which are specified in this clause. These data
form the basis for evaluation of toughness parameters according to Annexes C to G.
7.3 Impact velocity
−1 −1
This International Standard applies to any impact velocity, v , typically in the range from 1 ms to 5,5 ms .
o
NOTE 1 Impact velocities for pendulum or falling weight testing machines can be varied by adjusting striker release
height.
NOTE 2 The reduced impact velocity, v , can be determined:
o
⎯ releasing the pendulum from the appropriately reduced height, without a specimen on the specimen's supports;
⎯ reading the energy KV (in J) indicated by the pointer on the analogue scale;
o
⎯ from this, the reduced impact velocity is calculated for a 300 J pendulum using Equation (3):
300 − KV
0
vv= (3)
00s
300
If the pendulum capacity is different than 300 J, replace 300 in Equation (3) with the actual pendulum capacity.
A reduced velocity (1 m/s to 2 m/s) can be advantageous, especially for brittle materials, since it reduces the
effect of oscillations by lowering their relative amplitude and by increasing their number within the fracture time,
t (see 8.2).
f
7.4 Time to fracture
[7][8][9]
When the time, t, to initiate unstable fracture is less than 3τ , the instant of crack initiation is not
f
detectable in the force signal with adequate accuracy because of oscillations (see Type I of Figure 1) and an
independent measurement of t is required as described in Annex C.
f
7.5 Multiple specimen tests
To determine dynamic R-curves by multi-specimen techniques, the fracture process is interrupted at a certain
stable crack extension ∆a. This procedure is described in Annex D.
7.6 Single-specimen tests
It is possible to estimate dynamic R-curves by single-specimen techniques, as described in Annex E.
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ISO/FDIS 26843:2009(E)
7.7 Crack length measurements after testing
7.7.1 General
After a test has been performed, the fracture surfaces of the specimens are examined to determine the initial
crack length, a , and the amount of stable crack extension, ∆a (if applicable). The procedure is based on
o
measurements of average crack length. Although an area average method might provide a more reliable
estimate, the five-point average method produces acceptable results.
Difficulties may arise in measuring highly irregular crack fronts with spikes or regions of disconnected crack
extension. For such situations, it may only be practicable to estimate the crack length by ignoring the spikes or
subjectively averaging the crack extension regions. Care shall be exercised when the results derived from
highly irregular crack fronts are used to assess structural integrity. For ductile steels, it is recommended that
such assessments be based solely on crack initiation parameters and no advantage be taken of the crack
extension fracture resistance behaviour.
The method used to measure crack length and the occurrence of irregular crack fronts shall, in all cases, be
reported.
7.7.2 Five-point average method
7.7.2.1 Initial crack length is to be measured to an accuracy of 0,25 % or 0,05 mm, whichever is the
greater, between two reference lines defined at the minimum thickness positions as shown in Figure 3. The
measurements are to be made at five equally spaced points for all tests.
7.7.2.2 For K tests, the initial crack length, a , shall be obtained as a simple average. For J and CTOD
o
tests, measurements are made at five equally spaced points, where the outer points are located at 0,01B
inward from the reference lines. The value of a is obtained by first averaging the two measurements at 0,01B
o
inward from the surfaces, and then averaging this value with the three inner points. If the difference between
a and any of the individual measurement points contributing to a exceeds ±10 %, then report non-uniform
o o
crack extension, as required in the test report (see Clause 9).
7.7.2.3 Measure to an accuracy of 0,25 % or 0,01 mm, whichever is the greater, the minimum difference
between the end of the machined notch and the end of the fatigue precracked zone. If the difference does not
exceed 0,05 a , report insufficient fatigue precrack. The plane of the fatigue precrack shall be within 10° of the
o
central plane of the machined notch.
7.7.2.4 Measure the total crack extension, ∆a, between the initial and final crack fronts to an accuracy of
0,05 mm using the appropriate averaging procedure described in 7.7.2.1 and 7.7.2.2.
7.7.2.5 Measure the maximum and minimum crack extension between measurement positions 2 to 4 in
Figure 3. If the difference is greater than 20 % of the average crack extension, ∆a or 0,15 mm, whichever is
the greater, then report non-uniform crack extension as required.
7.7.2.6 Examine the fracture surfaces for evidence of arrested unstable crack extension regions ahead of
the fatigue precrack and/or any other unusual features. Record the number of regions and associated fracture
appearance as required.
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ISO/FDIS 26843:2009(E)

a)  Plain-sided specimen b)  Side-grooved specimen

Key
1 machined notch 5 crack growth
2 fatigue precrack 6 final crack front
3 initial crack front 7 side groove
4 stretch zone
a
Measure initial and final crack lengths at positions 1 to 5.
Figure 3 — Measurement of crack length on precracked Charpy specimens
7.7.3 Area average method
7.7.3.1 This method provides the most reliable result for crack length measurement. Both the accuracy
and precision of measurement of initial crack length, a , and stable crack extension, ∆a, by this method is
o
superior to the five-point average method, especially in the case of highly irregular crack fronts.
7.7.3.2 The area average method is intended to determine the crack length based on the real contour of
the crack front. This is done by use of a measuring microscope (reflected light microscope) mounted on an
X-Y translation stage with position sensors and a computer interface.
The displacement measurement chain shall be calibrated.
NOTE For measurement microscopes with a X-Y translation stage and a position sensor, an overall sensitivity of the
displacement measurement chain of 0,02 mm can easily be achieved for both axes and is considered sufficient.
7.7.3.3 As shown in Figure 4, the contour of the relevant area of the fracture surface is defined step by
step, first positioning the X-Y translation stage, then manually marking each step and storing the data in a
computer. The edge of the specimen is always taken as the reference line, therefore the machined notch area
is included in the measurement area. For efficiency, markings are only required when there are significant
changes in the direction of the contour.
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ISO/FDIS 26843:2009(E)
For materials with inhomogeneous microstructure, resulting in fracture surfaces with high roughness, or in the
case of poor contrast among fatigue crack, stable crack and brittle crack after heat tinting, the use of dark field
illumination and/or filters may be beneficial. Care shall always be taken that the specimen is properly aligned
with respect to the axes of the X-Y translation stage.
7.7.3.4 After digitizing a contour, the corresponding area value is calculated using the trapezium formula.
The average value of initial crack length, a , may then be calculated by dividing the fatigue crack area value
o
by the specimen thickness, B. For determining the average value of stable crack extension, ∆a, the value of
stable crack area reduced by the fatigue crack area is divided by the thickness, B, of the specimen. For side-
grooved specimens, B is replaced by the net section thickness, B
...