ISO 23524:2022
(Main)Plastics — Determination of fracture toughness of films and thin sheets — Essential work of fracture (EWF) method
Plastics — Determination of fracture toughness of films and thin sheets — Essential work of fracture (EWF) method
1.1 This document specifies the principles and the method for determining the fracture toughness of polymeric films and thin sheets in the crack opening mode (mode I) under plane stress conditions. The essential work of fracture (EWF) method is based on the use of double edge notched tensile (DENT) specimens. 1.2 The method is suitable for use with films or thin sheets, of thickness not greater than 1 mm, made of ductile polymeric materials, in which fracture propagation is stable (crack growth is always driven by the external applied force). If, at any time during the test, brittle fracture occurs, with fast crack propagation driven by the elastic energy stored in the specimen, the sample is not suitable for the application of the present test method.
Plastiques — Détermination de la ténacité à la rupture des films et feuilles minces — Méthode du travail essentiel de rupture (EWF)
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 23524
First edition
2022-10
Plastics — Determination of fracture
toughness of films and thin sheets
— Essential work of fracture (EWF)
method
Plastiques — Détermination de la ténacité à la rupture des films et
feuilles minces — Méthode du travail essentiel de rupture (EWF)
Reference number
ISO 23524:2022(E)
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ISO 23524:2022(E)
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ISO 23524:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 3
4 Principle . 3
5 Apparatus . 4
6 Test specimens . 6
6.1 Specimen geometry . 6
6.2 Preparation of test specimens . 6
6.2.1 General . 6
6.2.2 Width and length . 7
6.2.3 Ligament length . 7
6.2.4 Number of specimens . 7
6.2.5 Specimen notching. 7
6.2.6 Conditioning . 8
7 Procedure for the determination of EWF . 8
7.1 Testing speed . 8
7.2 Force-displacement curves . 8
7.3 Calculation of the overall fracture energy W . 8
f
7.4 Stress criterion . 9
7.5 Linear regression . 9
7.6 Outlying data criterion . 10
7.7 Results table . 11
8 Precision .11
9 Test report .12
Annex A (informative) Example .13
Annex B (informative) Interlaboratory test results .18
Bibliography .21
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ISO 23524:2022(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
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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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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).
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 2,
Mechanical behavior.
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.
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ISO 23524:2022(E)
Introduction
Fracture occurs under plane stress displaying gross ductility in many practical applications of
polymeric materials in which they are used as thin sheets or films (e.g. packaging and coatings). It is
inappropriate to adopt thicker test specimens, which are generally used in fracture tests, to measure
the fracture toughness in such cases. Thicker test specimens suppress crack tip ductility and bring
about a change in stress state which does not occur in practice. The essential work of fracture (EWF)
method, described in this document, provides toughness measurement under plane stress. The method
[1]
which is relatively simple is based on a suggestion by Broberg , further developed first by Cotterell,
[2],[3] [4]-[9]
Reddel and Mai for metals and then by a series of workers for ductile polymers. More recent
reviews on this method are given in References [10], [11], [12].
The method assumes that the overall energy associated with fracture can be partitioned into two
components: the essential work necessary to create new surfaces in the so-called fracture process
zone, and the non-essential work dissipated for the plastic deformation in the surrounding volume, the
process zone.
The essential work of fracture has been shown to be a material property, i.e. independent of the
[13],[14]
specimen geometry, for a given sheet thickness , when the condition of full yielding of the
specimen ligament before the onset of crack propagation is fulfilled. In this case, the essential work
of fracture is a parameter that gives an intrinsic material property dependent only on thickness and
therefore useful in product design. However, the condition of full yielding of the ligament is usually
difficult to verify without specific instrumentation, not commonly available in every laboratory.
Even if this condition is not fulfilled, the EWF test method can still be applied to determine the essential
work and non-essential work of the fracture energy, which are repeatable and reproducible parameters
useful in the development of new materials, in quality control and interlaboratory comparisons.
This document describes the EWF method independently of the verification of the full ligament yielding
condition.
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INTERNATIONAL STANDARD ISO 23524:2022(E)
Plastics — Determination of fracture toughness of films
and thin sheets — Essential work of fracture (EWF)
method
1 Scope
1.1 This document specifies the principles and the method for determining the fracture toughness of
polymeric films and thin sheets in the crack opening mode (mode I) under plane stress conditions. The
essential work of fracture (EWF) method is based on the use of double edge notched tensile (DENT)
specimens.
1.2 The method is suitable for use with films or thin sheets, of thickness not greater than 1 mm,
made of ductile polymeric materials, in which fracture propagation is stable (crack growth is always
driven by the external applied force). If, at any time during the test, brittle fracture occurs, with fast
crack propagation driven by the elastic energy stored in the specimen, the sample is not suitable for the
application of the present test method.
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 dates references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 291, Plastics — Standard atmospheres for conditioning and testing
ISO 2818, Plastics — Preparation of test specimens by machining
ISO 4593, Plastics — Film and sheeting — Determination of thickness by mechanical scanning
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
ISO 16012, Plastics — Determination of linear dimensions of test specimens
3 Terms, definitions and symbols
3.1 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 https:// www .electropedia .org/
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ISO 23524:2022(E)
3.1.1
initial distance between the grips
L
g
distance between the grips before the beginning of the test
Note 1 to entry: It is expressed in millimetres (mm).
Note 2 to entry: See Figure 1.
3.1.2
gauge length
L
0
initial distance between the grips L (3.1.1) when the displacement is measured by the change in the
g
distance between the grips during the test
Note 1 to entry: It is expressed in millimetres (mm).
3.1.3
extensometer gauge length
L
0e
gauge length (3.1.2), set equal to the ligament length b, when the displacement is measured by an
extensometer
Note 1 to entry: It is expressed in millimetres (mm).
3.1.4
displacement
x
increase in the gauge length L (3.1.2), or in the extensometer gauge length L (3.1.3), occurring from
0 0e
the beginning of the test
Note 1 to entry: It is expressed in millimetres (mm).
3.1.5
test speed
v
rate of separation of the gripping jaws
Note 1 to entry: It is expressed in millimetres per minute (mm/min).
3.1.6
overall fracture energy
W
f
energy measured by the area under the force-displacement curves
Note 1 to entry: It is expressed in millijoules (mJ).
3.1.7
specific fracture energy
w
sf
ratio of the overall fracture energy, W , (3.1.6) to the minimum cross-section area of the specimen
f
Note 1 to entry: The minimum cross-section area of the specimen is given by the ligament length, b, times the
thickness, h.
w = W / hb
sf f
2
Note 2 to entry: It is expressed in kilojoules per square metre (kJ/m ).
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ISO 23524:2022(E)
3.1.8
shape factor
β
dimensionless geometrical factor accounting for the shape of the plastically deformed zone around the
2
fracture zone, the volume of which is proportional to hb
3.1.9
essential work of fracture
w
e
specific energy to create new surface
2
Note 1 to entry: It is expressed in kilojoules per square metre (kJ/m ).
3.1.10
non-essential work of fracture
w
p
energy per unit volume dissipated for plastic deformation in the volume around the fracture zone
3
Note 1 to entry: It is expressed in megajoules per cubic metre (MJ/m ).
3.1.11
maximum stress
σ
max
maximum force, F divided by the minimum cross-section area of the specimen given by the ligament
max
length, b, times the thickness, h
σ = F /hb
max max
Note 1 to entry: It is expressed in megapascals (MPa).
3.1.12
average maximum stress
σ
m
average of the maximum stress, σ (3.1.11) values obtained on the 25 specimens used for the essential
max
work of fracture, w (3.1.9) determination
e
Note 1 to entry: It is expressed in megapascals (MPa).
3.2 Symbols
h specimen thickness, expressed in millimetres (mm) See Figure 1.
B specimen width, expressed in millimetres (mm) See Figure 1.
b un-cracked ligament length, expressed in millimetres (mm) See Figure 1.
L specimen length, expressed in millimetres (mm) See Figure 1.
W energy necessary to create new surfaces, expressed in millijoules
s
(mJ)
W energy dissipated for plastic deformation in the volume around the
pl
fracture zone, expressed in millijoules (mJ)
F force
F maximum force, expressed in Newton (N)
max
x displacement at maximum force, expressed in millimetres (mm)
Fmax
4 Principle
The principle of the experimental technique is to prepare a series of double edge-notched tensile
specimens (see Figure 1) having the same thickness (h), width (B) and length (L) and varying ligament
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ISO 23524:2022(E)
length (b). Specimens are extended along their major longitudinal axis at constant displacement rate up
to fracture. The overall fracture energy W is measured from the relevant force-displacement traces.
f
This energy is supposed to be made of two additive components: W = W + W . The first component
f s pl
(W ) is the energy necessary to create new surfaces and thus proportional to the fractured area (hb).
s
It can therefore be expressed as (W = w hb), where w is the essential work of fracture. The second
s e e
component (W ) is the energy dissipated for plastic deformation in the volume around the fracture
pl
2
zone. This volume can be expressed as β hb , β being a shape factor. Thus, W can be expressed
pl
2
as (W = w β hb ) where w is the non-essential work of fracture (i.e. the energy per unit volume
pl p p
dissipated for plastic deformation). Accordingly, the specific energy to fracture (w = W /hb) can be
sf f
written as shown in Formula (1):
ww=+wbβ (1)
sf ep
where the essential work of fracture, w , and the product βw shall then be determined from a least
e p
squares linear regression to w versus b experimental data.
sf
To obtain a valid value of w , some limitations shall be taken into consideration:
e
— plane stress conditions shall prevail: this limits the minimum acceptable ligament length;
In this document, a fixed minimum ligament of 5 mm is specified for all considered specimen
thicknesses.
NOTE The requirement of plane stress conditions generally limits the minimum acceptable ligament
length to at least 5 times the thickness (see Reference [12]). In this document, for the maximum considered
thickness of 1 mm (see 1.2), the requirement gives a minimum ligament length of 5 mm. For thinner films
smaller ligament lengths can be made, however this gives rise to difficulties in specimen preparation and
handling. Therefore, a fixed value of 5 mm for the minimum ligament length b is adopted in the present
min
document.
— no edge effects: this condition limits the minimum notch length. This limits in turn the maximum
ligament length;
— full yielding of the specimen ligament before crack onset: this last requirement ensures that the
fracture mechanism is the same irrespective of ligament length and that w is a material property,
e
i.e. independent of the specimen geometry, for a given sheet thickness. However, as already stated in
the Introduction, this document will not consider the verification of this condition. If the condition
is not verified, w is not a geometry independent material property but, nevertheless, the method
e
provides a useful, repeatable and reproducible characterization of fracture toughness. This last
limitation, therefore, will not be taken into consideration in this document.
5 Apparatus
The testing machine shall be in accordance with ISO 7500-1 and ISO 9513, and shall meet the
specifications given in 5.1 and in 5.2.
5.1 Force measurement system, in accordance with class 1 as defined in ISO 7500-1 in the relevant
range of forces.
5.2 Extensometer, in accordance with ISO 9513, class 1. The accuracy of this class shall be attained
in the strain range over which measurements are being made.
For the measurement of the displacement, the use of an extensometer is preferred. Non-contact
extensometers or low drag-force contact extensometers can be used. If low drag-force contact
extensometers are used, ensure that the force applied to the specimen by the extensometer in the
test direction does not exceed 2 % of the maximum force F measured on the specimen having the
max
smallest ligament length b (see 5.4 and 6.2.3).
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ISO 23524:2022(E)
When using an extensometer, the gauge length L shall be set equal, for each specimen, to the relevant
0e
nominal value of the ligament length b (see 6.2.3) and shall be perpendicular to the ligament plane; its
centre shall correspond with the centre of the specimen.
If suitable extensometers are not available, the displacement shall be measured by the change in the
distance between the grips during the test (grip separation). The initial distance between the grips L
g
shall correspond, for all the specimens, to the gauge length L (see 3.1.2 and 3.1.3).
0
Extension measurements using the crosshead displacement shall be corrected for the compliance of
the machine. If the machine is equipped with built in routines for compliance correction, these shall be
applied.
When the displacement is measured by the change in distance between the grips, the overall fracture
energy W includes both the plastic energy dissipated in the region surrounding the fracture zone and
f
the viscoelastic energy dissipated far from the fracture zone. Instead, when using an extensometer
and a gauge length L equal to the ligament length b, only the plastic energy involved in the fracture
0e
[15]
process zone is considered in the evaluation of the overall fracture energy, W .
f
The value of the essential work of fracture, w , is not influenced by the displacement measurement
e
method (extensometer or grip separation), but the value of the product βw , (the shape factor times the
p
non-essential work of fracture) will be overestimated when the displacement is measured by the
change in distance between the grips.
5.3 Tensile testing machine, capable of maintaining the test speed, v, required by the present
procedure (see 7.1), i.e. 10 mm/min, with the tolerance of ±20 %.
5.4 Vernier caliper and thickness gauge
All dimensions shall be measured in accordance with ISO 16012.
Width and length of the specimens shall be measured with an accuracy of 0,05 mm.
The thickness, h, shall be measured by a dead weight thickness gauge according to ISO 4593.
The thickness, h, of each specimen shall be measured (after notching) along the ligament with an
accuracy of 1 % of the nominal thickness or 0,001 mm, whichever is greater. Readings every 5 mm of
ligament length shall be made and the average value shall be used as the value of the specimen thickness
h.
The ligament length shall be measured by means of a vernier caliper, by placing the tips of the caliper
jaws as close as possible to the two notch tips. The ligament length shall be measured with an accuracy
of ±0,05 mm.
The lengths of the two notches, measured by means of a vernier caliper as the distance between each
notch tip and the nearest specimen border, shall be equal within 0,5 mm
The two notch tips shall lie on a plane perpendicular to the longitudinal axis of the specimen. Maximum
permissible deviation from perpendicularity, measured as the maximum distance between the two
planes, perpendicular to the longitudinal specimen axis, each containing one of the notch tips, shall be
2 % of the ligament length b.
5.5 Optical microscope
Notch tip radius is required to be smaller than 10 μm (see 6.2.5). This requirement shall be checked by
means of an optical microscope using a magnification of 200X or higher. Annex B (see Figure B1) gives
some examples on how to perform this verification.
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ISO 23524:2022(E)
6 Test specimens
6.1 Specimen geometry
The double edge notched tension specimen (DENT) shall be used for this test method. Figure 1 defines
the relevant dimensions and their nomenclature.
The specimen is a rectangular strip with two notches, cut on the centre of the long sides and with the
notch plane perpendicular (see 5.4) to the long axis of the specimen. The two notches have equal length,
within the tolerance given in 5.4. The notches shall have a sharp tip as detailed in 5.5 and 6.2.5.
Key
L specimen length
h specimen thickness
B specimen width
b ligament length
L initial distance between grips
g
Figure 1 — DENT specimen and nomenclature
6.2 Preparation of test specimens
6.2.1 General
Preparation of the test specimen shall be made in accordance with ISO 2818. In many cases, an
appropriate cutting press or a strip cutter can be used.
For a single test run, 25 specimens shall be prepared. All specimens shall have the same width, B, and
length, L (see 6.2.2), but with different ligament lengths, b (see 6.2.3 and 6.2.4).
The following specifications and guidelines shall be observed.
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ISO 23524:2022(E)
6.2.2 Width and length
The initial step is to cut 25 rectangular specimens of width, B, and length, L, from the test material. If
films or sheets from which specimens are cut out are anisotropic, all the specimens shall have the same
orientation, parallel or perpendicular, with respect to the anisotropy axes of the sample (in a film, for
example, they are usually identified as machine direction, MD and transverse direction, TD). This shall
be recorded, by specifying the direction in which force is applied during the test.
The specimens shall have the following dimensions:
— thickness, h: the thickness of the film or thin sheet sample to be tested;
— width, B: 35 mm ± 0,2 mm;
— specimen length, L: ≥ 100 mm;
— initial distance between grips, L : 60 mm ± 0,5 mm;
g
— gauge length:
— when the displacement is measured by the change in the distance between the grips during
the test (see 5.2), gauge length shall be equal to the initial distance between the grips L (see
g
Figure 1). Gauge length shall be indicated by L ;
0
— when the displacement is measured by an extensometer gauge length shall be the extensometer
gauge length, set equal to the ligament length, b as specified in 5.2. Gauge length shall be
indicated by L ;
0e
— ligament length, b: see 6.2.3 and 6.2.4;
— position and dimensions of the notches: see 6.1 and 6.2.4.
6.2.3 Ligament length
To satisfy the validity conditions for EWF some limits on ligament length shall be considered.
— plane stress conditions shall prevail: this limits the minimum acceptable ligament length. In this
document a fixed minimum ligament of 5 mm is specified for all considered specimen thicknesses
(see Clause 4).
— no edge effects: this condition limits the minimum notch length. This limits in turn the maximum
ligament length b , which shall be smaller than half the specimen width B. A maximum ligament
max
length value: b = 15 mm shall be adopted.
max
6.2.4 Number of specimens
Twenty five (25) specimens are required, with the ligament length varying between b and b
min max
(see 6.2.3). Interlaboratory testing comparisons have shown that the best results are obtained when
most specimens are concentrated near the minimum and maximum ligament lengths and some of them
(about 20 % of the total number) have ligament length close to the mean value.
Therefore, specimen notching will be performed, according to 6.2.5, aiming to obtain 10 specimens
having b = 5 mm; 5 specimens having b = 10 mm; and 10 specimens having b = 15 mm. These are
nominal values, and the actual b value shall be measured for each specimen in accordance with 5.4.
6.2.5 Specimen notching
The sharpness of the notches is a critical factor for the repeatability and the reproducibility of
the essential work of fracture w . Annex B reports RR results showing the importance of the notch
e
sharpness. Annex B also gives some examples on the verification of notch tip sharpness by means of an
optical microscope.
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ISO 23524:2022(E)
In this document, the value of 10 μm is specified as the maximum limit to the notch tip radius of
curvature, to be verified as described in 5.5.
The use of common razor blades and a check of the notch tip sharpness by means of an optical microscope
make it feasible to satisfy the requirement of a notch tip radius smaller than 10 μm.
...
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