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Plastics tensile standards, such as ASTM D638 and ISO 527-2, cover a range of plastics materials and calculations, including Poisson's Ratio - a calculation that has proven to be one of the more challenging calculations to obtain repeatable and accurate results.
Reproducibility studies have shown great variance in the measurement of Poisson’s Ratio, revealing inconsistent results between labs, as well as within individual labs. Difficulty in measuring the ratio relates directly to the measurement of transverse and axial strain at very small strain ranges. Poisson’s Ratio is defined by the division of transverse strain by axial strain, describing 2-dimensional deformation while the material is subjected to a uniform stress. This ratio is ideally constant within the linear elastic region for isentropic, homogenous materials.
Poisson’s Ratio is typically measured at strain values between 0.05% and 0.25%. An extensometer with the appropriate accuracy is critical to achieving acceptable results. Depending on the standard being followed, an extensometer with specific accuracy specifications may be required. For ASTM D638, the extensometer must be capable of measuring strain with an accuracy of 1% of the relevant value or better.
For the majority of plastics, we recommend bi-axial extensometers - high-resolution extensometers that measures both axial and transverse strain simultaneously. Selecting appropriate grips is also important for obtaining consistent results. We typically recommend pneumatic side acting grips since they are self-aligning with adjustable clamping pressures that allow for consistent clamping pressures from one specimen to the next.
In addition to equipment selection, setting up an appropriate test method is critical for achieving accurate, repeatable results. It was discovered that a “preload” improves the repeatability of results by eliminating compressive forces on specimens that often result when initially placed into grips. Compressive forces may cause specimens to bend, so without a proper a preload, the axial and transverse strain measurement may be taken while the specimen is bent, producing inaccurate results. The preload value should be high enough that it will straighten a specimen without stretching it.
Software packages, such as Bluehill®, include automatic preload as a standard feature. However, not all software packages have this feature. In these cases, operators may apply preloads manually by adjusting the position of the cross-head before starting each test. It is important to zero balance the axial and transverse extensometers after the preload is reached. In addition to preload, a suitable sampling rate is important. For very stiff materials, sampling rates as high as 500 Hz is recommended. For other materials, lower sampling rates are sufficient.
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