ISO 18872:2007

Plastics - Determination of tensile properties at high strain rates

ISO 18872:2007 specifies procedures for determining the tensile properties of moulding and extrusion plastics over a wide range of strain rate, including high rates appropriate to impact-loading situations. Properties are determined through a combination of measurements at low and moderate strain rates, the use of mathematical functions to model these results, the rate-dependence of parameters and the determination of parameters at high strain rates by extrapolation. Tensile properties at high strain rates are then derived by calculation. In this way, the experimental problems and associated errors with the measurement of properties at high rates are avoided.

Plastiques — Détermination des propriétés en traction à hautes vitesses de déformation

General Information

• Status: Published
• Publication Date: 11-Feb-2007
• ICS: 83.080.10 - Thermosetting materials
• Technical Committee: ISO/TC 61/SC 2 - Mechanical behavior
• Drafting Committee: ISO/TC 61/SC 2/WG 1 - Static behavior
• Current Stage: 9020 - International Standard under periodical review
• Start Date: 15-Oct-2025
• Completion Date: 15-Oct-2025

Overview

ISO 18872:2007 - Plastics - Determination of tensile properties at high strain rates describes a standardized approach to obtain reliable tensile properties of moulding and extrusion plastics across a wide range of strain rates, including the high rates relevant to impact-loading situations. Instead of attempting direct high-speed tests (which can introduce large experimental errors), the standard combines low/moderate-rate tensile measurements with mathematical modelling and extrapolation to calculate tensile response at high strain rates.

Key topics and requirements

• Scope and principle
  • Uses measured stress–strain data at low and moderate speeds, models the stress vs true plastic strain curves, and extrapolates model parameters to high strain rates.
• Test speeds
  • Mandatory speeds: 0.1, 1, 10 and 100 mm/s. Optional intermediate speeds: 0.3, 3 and 30 mm/s if needed for confidence in extrapolation.
• Specimen geometry
  • Use standard ISO 527-2 specimen types for low-strain (yield-region) measurements.
  • For post-yield/high-strain work, a reduced-width specimen with a circular waist and a small gauge length (~4 mm) is required to localize strain and improve accuracy.
• Apparatus and instrumentation
  • Servo-hydraulic testing machines are recommended for speeds above ~10 mm/s.
  • Use high-stiffness, low-mass force transducers (e.g., piezoelectric) and lightweight or non-contact extensometers to reduce resonance and measurement error.
  • Data-acquisition rates must be sufficient to capture force and extensometer signals at the chosen test speeds.
• Measured and calculated quantities
  • Engineering stress/strain, Poisson’s ratio, true stress, true strain, true plastic strain, plastic strain rate, and tensile modulus.
  • Separation of elastic and plastic components and modeling of hardening curves to enable extrapolation.
• Referenced documents
  • Normative references include ISO 527-1, ISO 527-2 and ISO 2818 (specimen preparation).

Applications and who uses it

ISO 18872:2007 is used by:

• Materials and polymer engineers characterizing rate-dependent mechanical properties.
• Test laboratories performing plastics qualification for impact-prone applications.
• Product designers and finite element analysts who need rate-dependent hardening curves for simulation of dynamic events (impact, crash, drop tests).
• Failure analysts and regulatory bodies assessing performance under high strain-rate loading.

Practical benefits:

• Avoids unreliable direct high-speed testing by providing a reproducible extrapolation methodology.
• Produces inputs suitable for dynamic finite element modelling and design validation.

Related standards

• ISO 527-1: General principles for tensile testing of plastics
• ISO 527-2: Test conditions for moulding and extrusion plastics
• ISO 2818: Preparation of test specimens by machining

Keywords: ISO 18872:2007, tensile properties, high strain rates, plastics testing, true stress, true plastic strain, impact loading, tensile modulus, ISO 527.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18872
First edition
2007-02-15
Plastics — Determination of tensile
properties at high strain rates
Plastiques — Détermination des propriétés en traction à hautes
vitesses de déformation
Reference number
©
ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
1 Scope . 1
2 Normative references . 1
3 Principle. 1
4 Terms and definitions. 1
5 Apparatus . 2
5.1 Test assembly . 2
5.2 Extensometers . 2
5.3 Data-recording equipment . 2
6 Test specimens . 2
6.1 Low-strain measurements . 2
6.2 High-strain measurements . 2
7 Conditioning. 3
8 Test procedure . 3
8.1 General. 3
8.2 Test speeds . 3
8.3 Recording of data . 3
9 Calculation and expression of results. 3
9.1 Low-strain measurements . 3
9.2 High-strain measurements . 4
10 Modelling stress versus plastic strain curves.6
10.1 Low-strain measurements . 6
10.2 High-strain measurements . 6
11 Determination of properties over a wide range of strain rates . 6
11.1 Tensile modulus. 6
11.2 Stress/plastic strain curves . 6
12 Precision. 7
Annex A (informative) Modelling true stress against true plastic strain curves: Low-strain
measurements. 8
Annex B (informative) Modelling true stress against true plastic strain curves:
High-strain measurements . 11

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 18872 was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 2, Mechanical
properties.
iv © ISO 2007 – All rights reserved

INTERNATIONAL STANDARD ISO 18872:2007(E)

Plastics — Determination of tensile properties at high strain
rates
1 Scope
This International Standard specifies procedures for determining the tensile properties of moulding and
extrusion plastics over a wide range of strain rates, including high rates appropriate to impact-loading
situations. Properties are determined through a combination of measurements at low and moderate strain
rates, the use of mathematical functions to model these results, the rate-dependence of parameters and the
determination of parameters at high strain rates by extrapolation. Tensile properties at high strain rates are
then derived by calculation. In this way, the experimental problems and associated errors with the
measurement of properties at high rates are avoided.
The measurement of properties at low and moderate strain rates is based on ISO 527-2, which identifies the
types of plastics materials to which this International Standard is applicable.
2 Normative references
The following referenced documents are indispensable for the application 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 527-1:1993, Plastics — Determination of tensile properties — Part 1: General principles
ISO 527-2, Plastics — Determination of tensile properties — Part 2: Test conditions for moulding and
extrusion plastics
ISO 2818, Plastics — Preparation of test specimens by machining
3 Principle
Tensile stress versus strain curves are measured in accordance with ISO 527-2 at selected speeds in the
range 0,1 mm/s to 100 mm/s. In order to maximize the accuracy of these results at the higher speeds, it is
necessary to pay attention to certain features of the design of the test assembly as described in Clause 5.
Measurements are also made of the variation of Poisson's ratio with strain. From these results, values of true
stress and true plastic strain are calculated at each strain rate. A mathematical function is used to accurately
model the shape of each stress/plastic strain curve. The variation of parameters in this function with strain rate
is also modelled to enable the values of parameters at higher strain rates to be determined by extrapolation.
Stress/strain curves at these higher strain rates can then be derived by calculation.
4 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 527-1 and the following apply.
4.1
true stress
σ
T
force divided by the cross-sectional area of the specimen within the gauge length at the same time as the
force is measured
4.2
true strain
ε
T
incremental increase in the gauge length divided by the gauge length at the same time as the increase is
measured
4.3
true plastic strain
ε
Tp
true strain at any true stress σ minus the elastic component of true strain ε at that stress
T Te
5 Apparatus
5.1 Test assembly
See ISO 527-1:1993, 5.1, for general guidance on apparatus. Servo-hydraulic testing machines usually need
to be employed to achieve test speeds above 10 mm/s. At test speeds above around 10 mm/s, errors may
arise in the measurement of force. These are associated with the presence of resonance modes in the force
transducer, the test specimen and components in the test assembly. To maximize the speed range over which
measurements of satisfactory accuracy can be made, attention should be paid to the design of the test
assembly so that it incorporates a high stiffness (e.g. piezoelectric) force transducer and components of low
mass and high rigidity.
5.2 Extensometers
To maximize the upper limit for the test speed at which accurate measurements are possible, light-weight
extensometers or non-contacting devices should be employed. For the measurement of large strains in the
specimen, devices capable of defining a small gauge length (typically 4 mm) should be used (see 6.2).
5.3 Data-recording equipment
The data-acquisition rate of the equipment used to record force and extensometer signals shall be high
enough to accurately record the shape of the force/extension curve at all test speeds.
6 Test specimens
6.1 Low-strain measurements
For the measurement of properties at strains below the yield strain (see ISO 527-1:1993, 4.7), ISO specimen
geometries 1A, 1B or 1BA shall be used. Where specimens are cut from sheet or mouldings, the machining
shall be carried out in accordance with ISO 2818.
6.2 High-strain measurements
6.2.1 At strains above the yield strain, where the stress reaches a maximum or increases only slowly with
strain, the strain distribution in the gauge region in standard specimens becomes non-uniform. In extreme
situations, this is visible as a neck, and is the reason that International Standards refer to recording the
nominal strain (see ISO 527-1:1993, 4.10) through measurements of changes in the grip separation. These
strain values have an unknown error which, for some materials, can be very large. Where higher accuracy is
2 © ISO 2007 – All rights reserved

required, an alternative specimen geometry shall be used as shown in Figure 1. This specimen has a uniform
thickness but the width is reduced from 10 mm to 8 mm by a circular waist cut at the centre of the specimen
length. The specimen thickness is not critical, so it can be machined from the central region of type 1A or 1B
specimens or from sheet or mouldings (see ISO 2818). The region of strain localization is now in the centre of
the specimen, which is where axial and transverse strains are measured. A gauge length of 4 mm ± 1 mm
shall be used for the measurement of axial strain (see 6.2.2). Transverse strains are needed for the
determination of true stresses and these can be measured using transverse extensometers applied to the
specimen width or thickness.
6.2.2 Because of strain localization at strains beyond yield, a small gauge length shall be used in order to
achieve a fairly uniform strain distribution in the gauge region. This small gauge length will give rise to a
significant reduction in the accuracy of strain measurements at low strains. For this reason, standard
specimens are used for the determination of properties at strains below the yield strain (see 6.1). At strains
above this, use of the small gauge length with the new specimen in Figure 1 will give strain measurements of
satisfactory accuracy.
7 Conditioning
See ISO 527-1:1993, Clause 8.
8 Test procedure
8.1 General
See ISO 527-1:1993, 9.1 to 9.5.
8.2 Test speeds
Specimens shall be tested at speeds of 0,1 mm/s, 1 mm/s, 10 mm/s and 100 mm/s. If results at the highest
speed are unreliable, or if greater confidence is required in the analysis of results (see 11.2), additional
speeds may be used which should be selected from the values 0,3 mm/s, 3 mm/s and 30 mm/s.
8.3 Recording of data
Record the force and the changes in the gauge length and width or thickness of the specimen at suitable
intervals of time throughout the test.
9 Calculation and expression of results
9.1 Low-strain measurements
9.1.1 Determination of engineering stresses σ, engineering strains ε, tensile moduli E and Poisson's
ratio µ
Using results from type 1A, 1B or 1BA specimens, determine the stress and strain values up to the yield strain
for each test speed (see ISO 527-1:1993, 10.1 and 10.2). From these results, calculate the tensile modulus
values at each strain rate using the method of ISO 527-1:1993, 10.3. Calculate also an average valu
...