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

Status
Published
Publication Date
09-Oct-2022
Current Stage
6060 - International Standard published
Due Date
29-Jul-2022
Completion Date
10-Oct-2022
Ref Project

Buy Standard

Standard
ISO 23524:2022 - Plastics — Determination of fracture toughness of films and thin sheets — Essential work of fracture (EWF) method Released:10. 10. 2022
English language
21 pages
sale 15% off
Preview
sale 15% off
Preview

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)
© ISO 2022
---------------------- Page: 1 ----------------------
ISO 23524:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on

the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below

or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
© ISO 2022 – All rights reserved
---------------------- Page: 2 ----------------------
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

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

iii
© ISO 2022 – All rights reserved
---------------------- Page: 3 ----------------------
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

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 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 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.
© ISO 2022 – All rights reserved
---------------------- Page: 4 ----------------------
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.
© ISO 2022 – All rights reserved
---------------------- Page: 5 ----------------------
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/
© ISO 2022 – All rights reserved
---------------------- Page: 6 ----------------------
ISO 23524:2022(E)
3.1.1
initial distance between the grips
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

initial distance between the grips L (3.1.1) when the displacement is measured by the change in the

distance between the grips during the test
Note 1 to entry: It is expressed in millimetres (mm).
3.1.3
extensometer gauge length

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

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
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
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

ratio of the overall fracture energy, W , (3.1.6) to the minimum cross-section area of the specimen

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
Note 2 to entry: It is expressed in kilojoules per square metre (kJ/m ).
© ISO 2022 – All rights reserved
---------------------- Page: 7 ----------------------
ISO 23524:2022(E)
3.1.8
shape factor

dimensionless geometrical factor accounting for the shape of the plastically deformed zone around the

fracture zone, the volume of which is proportional to hb
3.1.9
essential work of fracture
specific energy to create new surface
Note 1 to entry: It is expressed in kilojoules per square metre (kJ/m ).
3.1.10
non-essential work of fracture

energy per unit volume dissipated for plastic deformation in the volume around the fracture zone

Note 1 to entry: It is expressed in megajoules per cubic metre (MJ/m ).
3.1.11
maximum stress
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

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
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
(mJ)
W energy dissipated for plastic deformation in the volume around the
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

© ISO 2022 – All rights reserved
---------------------- Page: 8 ----------------------
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.

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).

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

zone. This volume can be expressed as β hb , β being a shape factor. Thus, W can be expressed

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.

To obtain a valid value of w , some limitations shall be taken into consideration:

— 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,

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

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).
© ISO 2022 – All rights reserved
---------------------- Page: 9 ----------------------
ISO 23524:2022(E)

When using an extensometer, the gauge length L shall be set equal, for each specimen, to the relevant

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

shall correspond, for all the specimens, to the gauge length L (see 3.1.2 and 3.1.3).

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

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

[15]
process zone is considered in the evaluation of the overall fracture energy, W .

The value of the essential work of fracture, w , is not influenced by the displacement measurement

method (extensometer or grip separation), but the value of the product βw , (the shape factor times the

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

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.
© ISO 2022 – All rights reserved
---------------------- Page: 10 ----------------------
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
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.
© ISO 2022 – All rights reserved
---------------------- Page: 11 ----------------------
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;
— 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

Figure 1). Gauge length shall be indicated by L ;

— 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 ;
— 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

sharpness. Annex B also gives some examples on the verification of notch tip sharpness by means of an

optical microscope.
© ISO 2022 – All rights reserved
---------------------- Page: 12 ----------------------
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.

...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.