ISO 18488:2025

International
Standard
ISO 18488
Second edition
Polyethylene (PE) materials for
2025-10
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 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 2
6 Test specimens . 3
6.1 Test specimen geometry and dimensions .3
6.2 Test specimen preparation .4
7 Test procedure . 4
8 Data analysis . 5
9 Test report . 6
Annex A (informative) Strain hardening behaviour and the Neo-Hookean constitutive model . 7
Bibliography .10

iii
Foreword
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This document was prepared by Technical Committee 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.
This second edition cancels and replaces the first edition (ISO 18488:2015), which has been technically
revised.
The main changes are as follows:
— the definition for strain hardening modulus, , has been improved;
p
— the definitions that are also in ISO 527-1 have been removed from this document;
— an improved depiction of L in Figure 1;
— the gripping distance in Table 1 has been clarified;
— the tolerance for the gauge length, l has been increased;
0,
— regrind from PE products has been added in 6.2 to align with the product standards (e.g. ISO 4437-2 and
ISO 4437-3);
— reference to ISO 293 for compression moulding has been added;
— the description of the measurement of the thickness, h, and width, b , have been clarified;
— the explanations of the data analysis (Clause 8) and the strain hardening behaviour and the Neo-Hookean
constitutive model (Annex A) have been revised.
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.

iv
Introduction
Resistance to slow crack growth of polyethylene materials is related in general to the lifetime of the material
and thus, the lifetime of polyethylene products, e.g. pipes and fittings. The slow crack growth behaviour of a
polyethylene material can be regarded as a combination of resistance to deformation of the crystalline phase
(manifested as yield stress) and the amorphous phase (entangled chains and tie molecules) as reported by
[2],[3],[5],[6]
Kramer and Brown. The resistance to disentanglement of polymer chains in the amorphous phase of
a polymer structure upon application of constant load will determine its resistance against slow crack growth.
The strain hardening modulus of polyethylene material is a measure of the resistance to disentanglement
of the entangled chains and tie molecules of this polymer and is an intrinsic property. The strain hardening
modulus of polyethylene is obtained from a “true stress vs. draw ratio” curve above the natural draw
ratio. The “true stress vs. draw ratio” curve of a compression moulded sheet is relatively easily obtained
using a tensile test apparatus equipped with an appropriate extensometer. The test time for measuring 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 differentiation between polyethylene materials. It has been
[9],[10]
demonstrated that the strain hardening modulus is sensitive to structural parameters of polyethylene
and corresponds very well with several environmental stress cracking test methods for polyethylene, such
[7] [8]
as environmental stress cracking resistance (ESCR), Pennsylvania notch test (PENT), full-notch creep
[4] [11]
test (FNCT), cracked round bar test (CRB) and notched pipe test (NPT) .

v
International Standard ISO 18488:2025(en)
Polyethylene (PE) materials for piping systems —
Determination of strain hardening modulus in relation to
slow crack growth — Test method
1 Scope
This document 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.
This document specifies how to obtain a strain hardening modulus measurement from “true stress vs. draw
ratio” curves on compression moulded samples. Details of the necessary equipment, precision and sample
preparation for the generation of data are given.
This document provides a method that is valid for all types of polyethylene, independent from the
manufacturing technology, comonomer or catalyst type used for pipes and fittings applications.
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 293, Plastics — Compression moulding of test specimens of thermoplastic materials
ISO 527-1, Plastics — Determination of tensile properties — Part 1: General principles
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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 527-1 and the following apply.
ISO and IEC maintain terminology 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/
3.1
length
l
distance between the gauge marks on the central part of the test specimen at any given moment
Note 1 to entry: Length is expressed in millimetres (mm).

3.2
true stress
σ
true
draw ratio (3.3) multiplied with the normal force per unit area of the original cross-section within the
gauge length
Note 1 to entry: True stress is expressed in megapascals (MPa).
3.3
draw ratio
λ
ratio of the length (3.1) to the gauge length
Note 1 to entry: This is demonstrated in the following formula:
l Δl
λ==1+
l l
Note 2 to entry: Draw ratio is expressed as a dimensionless ratio, or as a percentage (%).
3.4
natural draw ratio
final draw ratio (3.3) where further neck propagation in the narrow parallel sided portion of the test
specimen stops and the strain hardening region starts
Note 1 to entry: Natural draw ratio is expressed as a dimensionless ratio, or as a percentage (%).
3.5
strain hardening modulus

p
slope of the Neo-Hookean constitutive model fitted to data beyond the natural draw ratio (3.4) between a
draw ratio of 8 and up to the point of maximum stress, but not above a draw ratio of 12
Note 1 to entry: Strain hardening modulus is expressed in megapascals (MPa).
4 Principle
Test specimens cut from a compression moulded sheet are subjected to a tensile test at 80 °C. The “true
stress vs. draw ratio” 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.
p
5 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.
5.1 Tensile-testing machine, which shall conform to ISO 527-1 and shall be capable of maintaining a test
speed, v, of (20 ± 2) mm/min. The tensile-testing machine shall include:
5.1.1 Load cell, which shall conform to 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 test specimens and 120 N for 1,0 mm thick
test specimens.
5.1.2 Extensometer, which shall conform to Class 1 as defined in ISO 9513 usable over the full elongation
of the test specimen up to and including λ = 12. The traverse displacement shall not be used as a measure of
strain, ε. For a 0,30 mm thick test specimen, a non-contact extensometer is preferred.
5.1.3 Temperature chamber, to control the temperature at (80 ± 1) °C.

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