Type of Strain Being Measured
Materials exhibit a wide range of plastic and elastic properties, and hence there are many different types of extensometers to suit different testing needs and applications. Axial extensometers measure axial strain, typically up and down the specimen length. Transverse extensometers measure transverse strain, which is measured across the specimen width. Biaxial extensometers measure both axial and transverse strain, which generally manifests in an increase in axial length and a reduction in specimen width and thickness. Circumferential extensometers measure circumferential strain as diameters change. Some applications require the use of averaging extensometers, which take and report multiple strain measurements during a test when the material being tested cannot be adequately assessed with a single strain measurement (for example, composites with multiple fiber layers where specimen misalignment can have a significant effect on test results). While most extensometers are static, meaning that they measure strain during a single test, some extensometers are dynamic, and designed to be used through repeat cycling during fatigue testing.
Clip on extensometers are highly accurate, economical, and easy to use. For these reasons they are probably the most common type of extensometer used in the materials testing industry. They are suitable for low-elongation materials and must be applied and removed by hand before the specimen fails, making them less efficient than automatic or video extensometers.
Long Travel extensometers are used on materials that are expected to stretch significantly during testing. They are useful for testing high-elongation materials such as elastomers and polymers with elongations of up to 7,000%.
Capacitive extensometers are typically used for high temperature applications up to 600°C where some strain gauges are not suitable.
Automatic extensometers are typically designed with automation in mind and have features to reduce operator interaction. They do not have fixed travel restrictions like clip-on extensometers and are suitable for a variety of tests, gauge lengths, and specimen geometries.
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Non-Contacting Video extensometers measure strain without physically contacting the specimen. These are ideal for delicate or notch-sensitive materials where a 'knife edge' from a contact extensometer may cause premature failure. Non-contacting video extensometers can be safely used through failure without risk of damaging the extensometer. They are also an excellent choice when testing with environmental chambers, as the test operator does not need to open the chamber in order to acquire an accurate strain measurement.
Crack Opening Displacement (COD) Gauges can be used for two purposes. First, to measure crack growth under dynamic conditions, and second to help determine specimen toughness once the crack growth stage has been completed.Deflectometers, or linear variable deflection transformers, are often used to determine displacement during compression or bend tests.
Other Ways to Measure Strain
Extensometers are not the only way to measure strain, and some applications and test standards allow or require other methods of strain measurement. Bonded strain gauges are often prescribed by standards in the composites industry, and are often used in pairs to compensate for misalignment of the specimen. Some standards imply that crosshead displacement is a reliable source for strain measurement. If you plan to use crosshead displacement as a measurement, be sure to use Instron's Bluehill® Universal compliance correction feature to ensure the most accurate readings possible. If you are still uncertain about which extensometer or strain measuring device best fits your needs, please contact your local Instron representative for assistance.