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This part of ASTM D638 specifies the test conditions for determining the tensile properties of unreinforced and reinforced plastics and applies to dumbbells (Types I – V), rods, and tubular specimen types. These properties include ultimate strength, strain at failure, and modulus. The mechanical properties of plastics can change dramatically when certain additives are incorporated into the formulation. Some of these properties are strength, ductility, and toughness.
Similar for many materials, extensometers are required to accurately measure strain. A plastic stress curve versus strain curve most often contains a linear elastic region, as well as a non-linear plastic region. Very little strain occurs within the linear region. Because modulus is measured within this region, extensometers with a high resolution are necessary. Extensometers with high resolution typically have limited travel and must be removed from the specimen during a test. Non-contacting extensometers, such as the Advanced Video Extensometer (AVE) or Standard Video Extensometer (SVE), have the resolution and flexibility to measure modulus and strain to failure for most plastics. In addition to extensometers, we recommend mechanical wedge grips or pneumatic side-acting grips for testing plastics.
It is important to read and review the standard to which you are testing in order to fully understand the test setup, procedures, and results requirements.
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