ISO 12493:2023 ゴム、加硫または熱可塑性 — 非等温条件下での引張応力の測定 | ページ 2

Foreword

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This document was prepared by Technical Committee ISO/TC 45, Rubber and rubber products, Subcommittee SC 2, Testing and analysis.

This third edition cancels and replaces the second edition (ISO 12493:2017), which has been technically revised.

The main changes are as follows:

• another method (method B) for measuring tensile stress under non-isothermal conditions has been added. In this method, the change of stress is measured at a given strain and under variation of temperature at a given heating rate as a function of temperature.

Introduction

Usually, stress relaxation tests (see ISO 6914) are performed at constant temperature conditions because temperature has a strong impact on the relaxation time constants of rubber materials. Depending on the respective relaxation time constant or, more realistic, relaxation time spectrum, those measurements are more or less time consuming and can require testing times of several days, weeks or even months. Accelerated stress relaxation tests can be performed under non-isothermal conditions, if the temperature is increased at a certain constant heating rate, because relaxation processes are thermally activated and therefore occur faster at higher temperatures. Thus, the entire stress relaxation behaviour of the material can be scanned within a short period of time, typically a few hours. This method is designated as the temperature scanning stress relaxation (TSSR) test method and offers the ability to characterize rubber ─ vulcanized or thermoplastic ─ by short-term measurements. TSSR tests cannot replace conventional isothermal stress relaxation measurements, but are considered as a comparative test method, e.g. for the purpose of material pre-selection or in order to determine the state of crosslink density ─ which is an important reason for ageing phenomena ─ of a sample.

During non-isothermal testing, the material undergoes not only stress relaxation, but additional phenomena occur which need to be considered by adequate corrections. Most important in this case is an increase of retractive forces due to the Gough-Joule effect and thermal expansion of the sample. Whereas the latter can be numerically compensated by considering a proper value of the coefficient of thermal expansion (CTE), the increase of retractive forces offer the opportunity to calculate the crosslink density of the material, based on fundamental theory of rubber elasticity. Furthermore, an enhanced sensitivity of the test, with respect to specific relaxation processes is achieved by determination of the first derivative of the measured stress–temperature curve. Similar to stress relaxation measurements in the time domain, the latter can be used to calculate a corresponding relaxation temperature spectrum, instead of a relaxation time spectrum.

1 Scope

This document describes two methods for measuring stress in tension under non-isothermal conditions.

• Method A: The thermal stress is measured for various pre-strain and temperature conditions as a function of time.
• Method B: The change of stress is measured in a test piece at a given strain and under variation of temperature at a given heating rate as a function of temperature. In this way, the determination of the thermal-mechanical behaviour of a rubber can be accelerated, e.g. for the purpose of comparative testing of aging or estimating the upper limit of the operating temperature.

The measurement device, which is equipped with a suitable heating chamber, is used to record the stress as a function of time or temperature until the sample breaks or the stress has approached zero or for a certain time.

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.

3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

Bibliography