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Tensile test according to ISO 527
The methods of the standard
The mechanical testing of plastics is an essential part of modern materials technology. The ISO 527 standard is a central standard that regulates the determination of the mechanical properties of plastics. In this article, you will learn which aspects are particularly important and how you can apply the standard optimally.
The general principles are described in the test standard ISO 527-1, while the tests are described in parts 2 to 5.
- Part 1: General principles
- Part 2: Test conditions for moulding and extrusion compounds
- Part 3: Test conditions for films and sheets
- Part 4: Test conditions for isotropic and anisotropic fibre-reinforced plastic composites
- Part 5: Test conditions for unidirectional fibre-reinforced plastic composites
The corresponding US standard is ASTM D638.
High demands are placed on the testing machine. On the one hand, this applies to force measurement, but also to test speed and strain measurement. An extensometer is essential for sufficiently accurate strain measurement to determine the modulus of elasticity. Most specimens have a measuring length of 50 or 75 mm. And since the modulus has to be determined at a very low strain of 0.05 to 0.25%, the required accuracy class for the extensometer is 0.5. Clip-on, video and special long-path extensometers are suitable for this purpose.
ISO527 describes the determination of the characteristic values E (modulus of elasticity), Rp (yield stress), Rm (tensile strength), A (elongation at break), μ (Poisson’s ratio) using a tensile test.
- Method A (without extensometer)
- Method B (with extensometer)
Sample preparation: the key to precise results
Correct sample preparation is a decisive factor for reproducible results. ISO 527 specifies the exact geometry of the test specimens. For plastic films, for example, the specifications of ISO 527-3 are relevant, while more rigid materials are tested according to ISO 527-2.
The samples should be free of surface defects and cleanly processed. They must also be conditioned before testing in order to minimise influences such as humidity or temperature.
How long and under what conditions must samples be conditioned? The conditioning time and temperature depend on the material properties and the environment. Conditioning for 24 hours at 23 °C and 50 % relative humidity is often recommended.
Carrying out the tensile test
The tensile test according to ISO 527 requires precise adjustment of the testing machine. Here are some key points:
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Test speed: The standard defines different speeds depending on the type of material. Typically, a speed of 1 mm/min to 50 mm/min is used, depending on the specimen and material. A slow speed of 1 mm/min is preferred for determining the modulus of elasticity.
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Ambient temperature: The tests should be carried out in a controlled temperature range. Standard conditions are 23 °C, unless other requirements apply.
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Calibration: The testing machine must be calibrated regularly to avoid measurement errors.
How is the modulus of elasticity (modulus of elasticity) measured? The modulus of elasticity is determined from the linear area of the stress-strain curve. This is usually done at low loads to ensure that the material is in the elastic range. The standard recommends a linear regression in the range between 0.05 % and 0.25 % elongation.
Testing machine equipment: the right equipment for every method
A powerful testing machine is essential to fulfil the requirements of ISO 527. The following equipment is required for the different methods:
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Tensile testing device: A precise and stable tensile testing device that offers high accuracy in force and displacement measurement is essential.
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Force transducers: For measuring tensile forces in various ranges. Select a force transducer whose measuring range is optimised for the material to be tested.
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Strain measurement: Optical or mechanical extensometers are required to record precise strain values. Mechanical extensometers are suitable for rigid materials, while non-contact systems such as video or laser extensometers are more suitable for ductile materials.
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Climatic chamber: An integrated climatic chamber is required for tests at different temperatures.
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Software: Modern testing software enables the test parameters to be controlled and the results to be analysed in accordance with the specifications of the standard.
What additional equipment is required for testing films or composite materials? Special clamping devices are required for films to prevent the samples from slipping. Composite materials often require additional strain measurement systems that work precisely even with irregular break points.
Which properties are determined?
The most important mechanical parameters that are determined in a tensile test in accordance with ISO 527 are
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Tensile strength: Maximum stress that a material can withstand.
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Modulus of elasticity: A measure of the stiffness of the material.
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Elongation at break: The elongation of the material at the time of breakage.
What to do if the material does not show a clear breaking point? In such cases, the maximum elongation at a defined force (e.g. 90 % of the maximum stress) is specified.
ISO 527 and other standards: What are the differences?
A frequent comparison is made between ISO 527 and the American standard ASTM D638. Although both standards pursue similar test objectives, there are differences in the geometry of the specimens and in the precision of the strain measurement. These differences should be taken into account to ensure international comparability.
Is it possible to directly compare results between ISO 527 and ASTM D638? No, the differences in sample geometry and test conditions can lead to deviations. A conversion or adaptation of the conditions is necessary.
Downloads and links:
Testing machines
Universal testing machines AGX-V
Shimadzu EZ-X materials testing machine
Tensile testing machine Compression testing machine AGS-X
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