ISO 16770:2019
(Main)Plastics — Determination of environmental stress cracking (ESC) of polyethylene — Full-notch creep test (FNCT)
Plastics — Determination of environmental stress cracking (ESC) of polyethylene — Full-notch creep test (FNCT)
This document specifies a method for determining the environmental stress cracking (ESC) resistance of polyethylene (PE) materials in a defined test environment. The test is carried out on notched test specimens machined from moulded sheets/specimens or from finished products. The test specimen is subjected to a static tensile load when immersed into an environment such as a surfactant solution held at a specified temperature. The time to failure is measured. The method has been specifically developed for polyethylene materials but can be used to evaluate PE products, such as pipes, fusion welds/fittings and blow-moulded PE containers to study the effect of aggressive environments, i.e. dangerous goods and chemicals. The method is suitable for use with test specimens moulded to chosen dimensions or machined from compression moulded sheets or injection moulded specimens, or from finished products, such as mouldings and pipes. When the test specimens are machined from extruded or moulded parts, the results can be affected not only by properties of the material, but also by stresses or orientation introduced during processing.
Plastiques — Détermination de la fissuration sous contrainte dans un environnement donné (ESC) du polyéthylène — Essai sur éprouvette entièrement entaillée (FNCT)
General Information
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 16770
Second edition
2019-09
Plastics — Determination of
environmental stress cracking (ESC)
of polyethylene — Full-notch creep
test (FNCT)
Plastiques — Détermination de la fissuration sous contrainte dans un
environnement donné (ESC) du polyéthylène — Essai sur éprouvette
entièrement entaillée (FNCT)
Reference number
ISO 16770:2019(E)
©
ISO 2019
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ISO 16770:2019(E)
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ISO 16770:2019(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 3
5 Apparatus . 3
6 Test specimens. 6
6.1 General . 6
6.2 Test specimens geometry . 6
6.3 Test specimen preparation . 8
6.3.1 General. 8
6.3.2 Test specimens machined from compression moulded sheets . 8
6.3.3 Test specimens machined from injection-moulded type 1A multipurpose
test specimen . 8
6.3.4 Test specimens machined from finished products . 8
6.4 Test specimen notching . 8
6.5 Conditioning of the test specimens . 9
7 Test environment . 9
7.1 General . 9
7.2 Recommended test environment . 9
7.2.1 Test environment No. 1 . 9
7.2.2 Test environment No. 2 .10
7.3 Other test environments .11
8 Test procedure .11
8.1 Selection of detergent, reference tensile stress and test temperature .11
8.2 Number of test specimens to be tested .11
8.3 Calculation of the test load .13
8.4 Application of the load to the test specimen .13
8.5 Removing individual specimens from a chamber .13
8.6 Measurement of the ligament area after failure .13
9 Expression of results .14
9.1 Calculation of the actual tensile stress .14
9.2 Calculation of time to failure at the reference tensile stress .14
10 Precision .15
11 Test report .15
Annex A (informative) Precision statement .17
Annex B (informative) Example of a recommended test report for FNCT determination .20
Annex C (informative) Practical examples of fracture surfaces and corresponding definition
of failure mode.22
Bibliography .25
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ISO 16770:2019(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 9,
Thermoplastics.
This second edition cancels and replaces the first edition (ISO 16770:2004), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the test conditions for bimodal PE materials with substantially increased crack-resistance have
been extended on an alternative more aggressive detergent;
— the compression moulding conditions for 10 mm × 10 mm test specimen (Type A) have been
modified;
— the specimen types have been updated, with injection moulded test specimens being included;
— precision data (repeatability) from interlaboratory testing have been included for Type A test
specimen.
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 16770:2019(E)
Introduction
Since mid-2000, the full-notch creep test (FNCT) has been widely used as one of the tests characterizing
the environmental stress cracking (ESC) of polyethylene (PE) materials used in demanding applications.
Among current ESC tests, the FNCT is relatively simple, easy to perform and still a sensitive laboratory
scale test method to characterize ESC resistance of PE materials. The advantage of FNCT is that it
enables the evaluation of the whole PE product range by variation of test conditions (test specimen
geometry, temperature, medium and load conditions).
The FNCT method was standardized in 2004 as a PE material property, with partial precision statement
based on the repeatability for a 6 mm × 6 mm-cross-sectional test specimen (Type B). Since then, several
interlaboratory tests have been performed, resulting in a repeatability statement for 10 mm × 10 mm-
cross-sectional test specimen (Type A). Type A specimen is prepared by using modified compression
moulding conditions, which give more consistent test results compared to the original moulding
conditions.
In addition, due to recent development of new PE pipe materials, known as PE 100 RC, which exhibit
substantially improved crack resistance, it became necessary to define test conditions for these
materials. Testing of these materials using the conditions specified in the previous edition of this
document (i.e. ISO 16770:2004) resulted in failure times of one year or longer. As a consequence, this
document includes extended test conditions to cover PE materials with substantially increased crack
resistance. This accelerated test procedure using extended test conditions allows failure times to be
reduced substantially compared to conventional test conditions given in ISO 16770:2004. In addition, a
good correlation between accelerated and conventional test methods was derived; see References [4],
[7] and [9].
The FNCT is a material characterization and production monitoring test, which, by strictly maintaining
the test conditions defined, enables relevant and reliable comparison among similar PE materials or
group of PE materials.
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INTERNATIONAL STANDARD ISO 16770:2019(E)
Plastics — Determination of environmental stress cracking
(ESC) of polyethylene — Full-notch creep test (FNCT)
1 Scope
This document specifies a method for determining the environmental stress cracking (ESC) resistance
of polyethylene (PE) materials in a defined test environment. The test is carried out on notched test
specimens machined from moulded sheets/specimens or from finished products. The test specimen is
subjected to a static tensile load when immersed into an environment such as a surfactant solution held
at a specified temperature. The time to failure is measured.
The method has been specifically developed for polyethylene materials but can be used to evaluate PE
products, such as pipes, fusion welds/fittings and blow-moulded PE containers to study the effect of
aggressive environments, i.e. dangerous goods and chemicals.
The method is suitable for use with test specimens moulded to chosen dimensions or machined
from compression moulded sheets or injection moulded specimens, or from finished products, such
as mouldings and pipes. When the test specimens are machined from extruded or moulded parts,
the results can be affected not only by properties of the material, but also by stresses or orientation
introduced during processing.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 294-1, Plastics — Injection moulding of test specimens of thermoplastic materials — Part 1: General
principles, and moulding of multipurpose and bar test specimens
ISO 2818, Plastics — Preparation of test specimens by machining
ISO 7500-2, Metallic materials — Verification of static uniaxial testing machines — Part 2: Tension creep
testing machines — Verification of the applied force
ISO 17855-2, Plastics — Polyethylene (PE) moulding and extrusion materials — Part 2: Preparation of test
specimens and determination of properties
ISO 20753, Plastics — Test specimens
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
failure
complete separation of the two halves of the test specimen
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ISO 16770:2019(E)
3.2
brittle failure
failure (3.1), in which the fracture surface exhibits no permanent material deformation to the naked
eye, e.g. stretching, elongation or necking down
Note 1 to entry: If the brittle area is less than 20 % of the total ligament, the failure is ranked as ductile.
Note 2 to entry: See Annex C for examples.
Note 3 to entry: The beginning of the transition to ductile failure (3.3) behaviour may be indicated by an extended
ligament, which may form in the centre (see Annex C for examples).
3.3
ductile failure
failure (3.1), in which the fracture surface clearly exhibits permanent material deformation with
stretching, elongation and necking down
Note 1 to entry: See Annex C for examples.
3.4
nominal ligament area
A
n
cross-sectional area of the test specimen remaining after notching
2
Note 1 to entry: It is expressed in square millimetres (mm ).
3.5
measured ligament area
A
L
actual cross-sectional area of the test specimen remaining after notching determined after testing
2
Note 1 to entry: It is expressed in square millimetres (mm ).
3.6
nominal tensile stress
σ
n
normal force per unit area of the nominal ligament area (3.4) of the test specimen
Note 1 to entry: It is expressed in megapascals (MPa).
3.7
actual tensile stress
σ
L
normal force per unit area of the measured ligament area (3.5) of the test specimen
Note 1 to entry: It is expressed in megapascals (MPa).
3.8
reference tensile stress
σ
L,ref
selected normal force per unit area of the measured ligament area (3.5) of the test specimen used for
determination of comparable time to failure (3.1)
Note 1 to entry: It is expressed in megapascals (MPa).
3.9
time to failure at the reference tensile stress
t
f,ref
time to failure corresponding to the reference tensile stress (3.8), calculated by interpolating in the
measured dependence of time to failure vs. actual tensile stress (3.7) for individual tested specimens
Note 1 to entry: It is expressed in hours (h).
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ISO 16770:2019(E)
4 Principle
A test specimen in the form of a square-section bar with coplanar notches in each face at the centre,
is subjected to a static load in a temperature-controlled environment containing a surface-active
detergent solution. The geometry of the test specimen is such that plane strain conditions are obtained,
and brittle failure occurs under appropriate tensile load and temperature conditions. The time for this
brittle failure to occur after loading is recorded.
NOTE 1 Distilled water is also a suitable environment for carrying out this test.
NOTE 2 For testing of PE-based container materials, other environments can also be used, such as dangerous
goods and chemicals or suitable liquid test media.
5 Apparatus
5.1 Loading device, constructed and maintained in accordance with ISO 7500-2. The class of the
apparatus shall be stated and reported.
A suitable device for applying the load is a lever-arm loading machine with an arm ratio between 4:1
and 10:1. A typical example of such device is shown in Figure 1. The lever-arm ratio R is equal to L /L .
1 2
When the lever-arm is fitted with the top specimen grip and the weight carrier, it shall be horizontal,
i.e. balanced.
Other suitable loading mechanisms are permitted.
The specimen grips shall be designed to prevent slippage of the test specimen and to ensure that the
load is transmitted axially through the test specimen, for example via a low-friction coupling, to prevent
bending and torsion of the test specimen during the test. An example of test specimen grip assembly is
shown in Figure 2.
In addition to the above example, the tensile load may be applied directly using deadweight
pneumatically actuated loading or any other means of producing a constant load. The loading device
shall be capable of applying the load to an accuracy of ±1 %. The balanced loading apparatus as
[3]
described in ISO 22088-2 can also be used.
As the applied load is a critical parameter, the operation and calibration of the equipment shall be
checked on a regular basis. Loading rate shall be kept as steady as possible to avoid shock-loading of
the notched test specimen. Motor-driven loading system with speed control should be preferably used.
The calibration of a lever-arm machine can be checked by hanging a series of known weights on the
specimen side of the lever-arm and counterbalancing these in turn with weights on the weight hanger.
The ratio of the former to the latter provides a direct measure of the arm ratio and hence a check on the
operation of the equipment.
In case of multiple specimens testing, care shall be taken to ensure that when one or more specimens
fail, the remaining test specimens remain unaffected.
NOTE Measurement of the extension of the test specimen or movement of the lever-arm can provide useful
information. The rate of extension of the test specimen increases when the initiation of the crack from the notch
has occurred and increases rapidly when failure is imminent.
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ISO 16770:2019(E)
Dimensions in millimetres
Key
1 counterweight
2 low-friction roller on hinge
3 balance-lever arm
4 example of environmental chamber
5 environment
6 weights
7 weight carrier
8 grip
9 test specimen
NOTE The distance between the grips is 50 mm corresponding with the specimen length of 100 mm.
Figure 1 — Loading device
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ISO 16770:2019(E)
Key
1 small environmental chamber (example)
2 coupling pin
3 grub screw to prevent slipping
4 clamp bolt
5 glass tube
6 notch
7 heat-shrink tube
8 test specimen
Figure 2 — Specimen grip assembly
5.2 Thermostatically controlled chamber, designed to contain the test environment and ensure
immersion of at least the notched area of the test specimen(s). The chamber shall be constructed of
material(s) which do not affect the environment, and which are not affected by it. The temperature of
the environment shall be controlled to maintain the test specimens within ±1,0 K of the specified test
temperature throughout the duration of the test.
The homogeneity and uniform dispersal of the environment shall be ensured. If the cloud point of the
environment solution is lower than the test temperature, phase separation occurs and so moderate
laminar flow or adequate stirring equipment is required. It shall also be ensured that the results
achieved at each location in the immersion bath are the same.
5.3 Temperature-measuring device. A calibrated thermometer, thermocouple or thermistor with an
accuracy of ±0,2 K is suitable.
5.4 Timing device, with an accuracy of the timing device shall be ±1 min. The timing device shall
automatically indicate or record the point when the test specimen fails by excessive displacement of
the grips.
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ISO 16770:2019(E)
5.5 Notching device, designed so that the notches in the specimen are coplanar and the plane of the
notching is perpendicular to the tensile axis of the test specimen. The machine shall have a device to
ensure that the notches are placed in the centre of the test specimen. The notch tip radius shall be less
than 10 µm.
Indentation using a razor blade of appropriate thickness is a preferred method for notching the tests
specimens. Recommended razor blade thickness is 0,15 mm to 0,25 mm.
Alternatively, a cutting machine with a tool as broaching device can also be used to machine the notches,
provided that the notch tip radius is less than 10 µm.
5.6 Optical device. A suitable optical device, such as a measuring microscope, comparator or similar
device, is required to allow accurate measurement of the actual ligament area (area between the tips of
2
the notches) after failure. The accuracy of the measurement shall be better than ±0,01 mm .
6 Test specimens
6.1 General
The method specifies preferred dimensions of the test specimens. Tests which are carried out on
specimens of different dimensions or with different notches, or specimens which are prepared under
different conditions, may give results which are not comparable. Other factors, such as conditioning of
the test specimens, can also influence the test results. Consequently, all these factors shall be controlled
and recorded.
6.2 Test specimens geometry
The test specimen geometry, showing notch and the ligament area is given in Figure 3.
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ISO 16770:2019(E)
Key
1 ligament area
2 notches
w width
b thickness
l length
d notch depth
Figure 3 — Test specimen showing the notch and the ligament area
The dimensions of the test specimens are given in Table 1. The appropriate test specimen type will be
selected according to polymer end-use application.
Table 1 — Dimensions of the test specimens
Length Width Thickness Notch depth
Test specimen
type
mm mm mm mm
A 100 ± 2 10,0 ± 0,1 10,0 ± 0,1 1,6
B 90 ± 2 6,0 ± 0,1 6,0 ± 0,1 1,0
C 100 ± 2 10,0 ± 0,1 4,0 ± 0,1 1,6/0,6
Type A test specimen is intended for pipe or sheet materials.
Type B test specimen is intended for extrusion or blow moulding materials.
Type C Test specimen is intended for injection moulding materials.
If other test specimens are used, these shall be made such that the ligament area is approximately 50 %
of the total cross-sectional area of the specimen. This is to make sure that test specimen failure will
occur under the prescribed conditions of brittle mode.
NOTE 1 The use of different test specimen geometries will give different test results with the same PE
material.
NOTE 2 Comparisons between materials are valid only if the same test specimen geometry, preparation
conditions and test conditions are used.
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ISO 16770:2019(E)
6.3 Test specimen preparation
6.3.1 General
The test specimens shall be prepared from compression moulded sheets, from injection moulded
multipurpose test specimen type 1A according to ISO 20753, or from finished products (pipes,
fittings, etc.).
6.3.2 Test specimens machined from compression moulded sheets
[1]
For compression moulding, the moulding conditions given in Table 2 shall be used. ISO 293 ,
[2]
ISO 17855-2 and ISO 11542-2 give additional general guidelines for moulding and cooling conditions.
Table 2 — Conditions for compression moulding of the test specimens
Moulding Average cooling Preheating time Full pressure
Thickness Full pressure
temperature rate time
mm °C °C/min min MPa min
+05,
180 15 ± 2 20 5 10
6
−01,
+05,
200 3,5 ± 1,5 25 10 40
10
−01,
NOTE 1 Higher value of upper tolerance of the thickness enables adopting the sheet thickness to its nominal value.
NOTE 2 Preheating pressure is 0,5 MPa.
Use of a positive mould is necessary.
The demoulding temperature shall be ≤ 40 °C.
The control of moulding parameters, especially the cooling rate is critical during test specimen preparation. Inconsistent
cooling rates can lead to significant deviations in test results due to the effect on crystallinity. It is critical that the moulding
machine is capable of maintaining a constant cooling rate.
After compression moulding, annealing of the test specimens shall be carried out by conditioning the
test specimens for 3 h in an oven at a temperature of 100 °C and then slowly cooled down to room
temperature.
If the tested PE material is a powder, it may be necessary to calendar or compound the material prior to
the compression moulding step. It is essential to make sure that the powder is heat-stabilized when this
is done. The use of different moulding conditions will affect the test results.
The test specimens shall be machined to size from the moulded sheet or test specimen in accordance
with ISO 2818 at least 24 h after moulding.
6.3.3 Test specimens machined from injection-moulded type 1A multipurpose test specimen
The test specimens shall be prepared by machining from multipurpose test specimen type 1A, prepared
in accordance with ISO 294-1 and ISO 17855-2.
6.3.4 Test specimens machined from finished products
When testing finished products, the test specimens shall be cut from extruded or moulded products in
accordance with ISO 2818.
6.4 Test specimen notching
For notching the test specimens, a suitable notching device (5.5) shall be used. The test specimens shall
be notched at room temperature. Notching shall be performed at least 24 h after the test specimen
preparation to allow for stress relaxation. Care shall be taken to avoid blunting the notch during this
operation, e.g. to avoid use of excessive speed and/or force, as this will invalidate the test results.
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ISO 16770:2019(E)
When a razor blade is used, the speed of razor indentation shall not
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