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
Current Stage
9093 - International Standard confirmed
Start Date
15-Jul-2010
Completion Date
16-Oct-2020
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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 18872:2007(E)
ISO 2007
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ISO 18872:2007(E)
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© ISO 2007

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ii © ISO 2007 – All rights reserved
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ISO 18872:2007(E)
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

© ISO 2007 – All rights reserved iii
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ISO 18872:2007(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.

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
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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.

© ISO 2007 – All rights reserved 1
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ISO 18872:2007(E)
4.1
true stress

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

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

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
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ISO 18872:2007(E)

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

...

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