ISO 18488:2015

INTERNATIONAL ISO
STANDARD 18488
First edition
2015-09-01
Polyethylene (PE) materials for piping
systems — Determination of Strain
Hardening Modulus in relation to slow
crack growth — Test method
Matériaux polyéthylène (PE) pour systèmes de canalisations —
Détermination du module d’écrouissage en relation avec la
propagation lente de fissures — Méthode d’essai
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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ii © ISO 2015 – All rights reserved

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. 4
6.1 Test specimen geometry and dimensions . 4
6.2 Test specimen preparation . 4
7 Test procedure . 5
8 Data analysis . 6
9 Test report . 6
Annex A (informative) Neo-Hookean constitutive model . 8
Bibliography .10
Foreword
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The committee responsible for this document is ISO/TC 138, Plastics pipes, fittings and valves for the
transport of fluids, Subcommittee SC 5, General properties of pipes, fittings and valves of plastic materials
and their accessories — Test methods and basic specifications.
iv © ISO 2015 – All rights reserved

Introduction
Resistance to slow crack growth is related in general to the lifetime of polyethylene and thus, the
lifetime of polyethylene products, e.g. pipes and fittings. The slow crack growth behaviour can be
regarded as a combination of yield stress and the capability of disentanglement of tie molecules as
[3] [6] [7]
reported by Kramer and Brown. , , The disentanglement capability of a polymer will determine
its resistance against slow crack growth.
The strain hardening modulus of a polymer is a measure of the disentanglement capability of the tie
molecules of this polymer and is an intrinsic property. The strain hardening modulus of polyethylene
is obtained from a stress-strain curve above the natural draw ratio. The stress-strain curve of a
compression moulded sample is relatively easily obtained using a tensile test apparatus equipped with
an optical extensometer. The test time of the strain hardening modulus is a consequence of the speed
of tensile testing and is therefore constant for all measurements and independent of the slow crack
growth property of the tested material itself.
The strain hardening modulus value allows discrimination between materials. It has been demonstrated
that the strain hardening modulus corresponds very well with several environmental stress cracking
[4] [5] [8]
test methods for high density polyethylene. , ,
INTERNATIONAL STANDARD ISO 18488:2015(E)
Polyethylene (PE) materials for piping systems —
Determination of Strain Hardening Modulus in relation to
slow crack growth — Test method
1 Scope
This International Standard specifies a method for the determination of the strain hardening modulus
which is used as a measure for the resistance to slow crack growth of polyethylene.
The strain hardening modulus is obtained from stress-strain curves on compression moulded samples.
This International Standard describes how such measurement is performed and how the strain
hardening modulus shall be determined from such a curve. Details of the required equipment, precision,
and sample preparation for the generation of meaningful data are given.
This International Standard provides a method that is valid for all types of polyethylene, independent
from the manufacturing technology, comonomer, catalyst type, that are used for pipes and fittings
applications.
NOTE This method could be developed for materials for other applications.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 527-1, Plastics — Determination of tensile properties — Part 1: General principles
ISO 7500-1, Metallic materials — Verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Verification and calibration of the force-measuring system
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE The symbols and their terms and definitions, as given below, are in line with ISO 527-1 and/or ISO 16241.
3.1
gauge length
l
initial distance between the gauge marks on the central part of the test specimen
Note 1 to entry: It is expressed in millimetres (mm).
3.2
thickness
h
smaller initial dimension of the rectangular cross-section in the central part of a test specimen
Note 1 to entry: It is expressed in millimetres (mm).
3.3
width
b
larger initial dimension of the rectangular cross-section in the central part of a test specimen
Note 1 to entry: It is expressed in millimetres (mm).
3.4
test speed
v
rate of separation of the gripping jaws
Note 1 to entry: It is expressed in millimetres per minute (mm/min).
3.5
length
l
distance between the gauge marks on the central part of the test specimen at any given moment
Note 1 to entry: It is expressed in millimetres (mm).
3.6
stress
σ
normal force per unit area of the original cross-section within the gauge length (3.1)
Note 1 to entry: It is expressed in megapascals (MPa).
3.7
stress at yield
σ
y
stress at the strain at yield (3.10)
Note 1 to entry: It is expressed in megapascals (MPa).
3.8
true stress
σ
true
draw ratio (3.11) multiplied with the normal force per unit area of the original cross-section within the
gauge length (3.1)
Note 1 to entry: It is expressed in megapascals (MPa).
3.9
strain
ε
increase in length (3.5) per unit original length of the gauge
Note 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).
3.10
strain at yield
yield strain
ε
y
first occurrence in a tensile test of strain increase without a stress increase
Note 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).
2 © ISO 2015 – All rights reserved

3.11
draw ratio
λ
length (3.5) per unit original length of the gauge
Note 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).
3.12
strain hardening modulus

p
slope of the Neo-Hookean constitutive model between a true strain (3.9) of 8 and up to the point of
maximum stress (3.6), but not above 12
Note 1 to entry: It is expressed in megapascals (MPa).
4 Principle
Test specimens cut from compression moulded sheet are subjected to a tensile test at 80 °C. The stress-
strain curve is obtained sufficiently beyond the natural draw ratio. The strain hardening modulus is
determined from the slope of this curve in the area after the natural draw ratio.
5 Apparatus
5.1 Tensile-testing machine, complying with ISO 527-1 and capable of maintaining a test speed of
(20 ± 2) mm/min.
5.1.1 Load cell, which shall comply with Class 1, as defined in ISO 7500-1. The load cell shall be able
to accurately measure forces in the range of 40 N for 0,30 mm thick samples and 120 N for 1,0 mm
thick samples.
5.1.2 Extensometer, which shall comply with Class 1, as defined in ISO 9513. The traverse
displacement shall not be used as a measure of strain. For a 0,30 mm thick specimen, a non-contact
extensometer is preferred.
5.1.3 Temperature chamber, to control the temperature at (80 ± 1) °C.
5.1.4 Clamps, remote operation to enable closing and opening without the need to open the
temperature chamber is recommended.
5.2 Devices for measuring the thickness and width of the test specimens.
5.2.1 Thickness shall be measured with a device with an accuracy of 0,005 mm and a device with a
contact dimension of less than the width of the parallel specimen section (4,0 mm).
5.2.2 Width shall be measured with a device with an accuracy of 0,01 mm. Care shall be taken not to
change the width of the test specimen by deformation. Therefore, it is recommended to use a microscope
to measure width to avoid deforming the test specimen.
5.3 Punch knife, in accordance with 6.1 shall be used.
6 Test specimens
6.1 Test specimen geometry and dimensions
The test specimen geometry as shown in Figure 1 shall be used. A large clamping area of the specimen
to avoid slippage in the clamps is required to accurately measure the strain hardening regime.
Figure 1 — Test specimen
Table 1 — Dimensions of test specimens
Dimension Size
mm
L Start length between clamps 30,0 ± 0,5
l Gauge length 12,5 ± 0,1
l Length of narrow parallel 16,0 ± 1,0
sided portion
l Length between the parallel 46 ± 1,0
portions of the clamp area
a
l Minimum overall length 70
R Radius 10,0 ± 0,5
R Radius 8,0 ± 0,5
b Width at narrow parallel 4,0 ± 0,1
sided portion
b Width at ends 20,0 ± 1,0
h Thickness 0,30 + 0,05/-0,03
or
1,0 ± 0,1
a
A greater overall length may be necessary to ensure that only
...