ASTM D638-14 Poisson's Ratio for Plastics

Plastics tensile standards, such as ASTM D638, cover a range calculations including Poisson's Ratio - a calculation that has proven to be challenging to obtain repeatable and accurate results.

Poisson’s Ratio is measured anywhere in the linear region as long as the linear region extends past 0.25% strain. When the linear region does not extend to this strain value, the calculation is taken between 0.25% to 0.5% strain.

The challenges of testing to this standard are:

  • Repeatable strain measurement
  • Reducing knife edge effects
  • Minimizing operator influence
  • Reporting calculations in compliance with the standard
Poisson Ration Graph

Instron's Solution:

  • Repeatable strain measurement – An extensometer capable of measuring both axial and transverse strain with an accuracy of 1% is required by the standard.  Many labs may be using extensometers which are not in compliance with these requirements.  Poisson’s Ratio is measured in the elastic portion of the test where very little strain has occurred, it is important to consider this when selecting an appropriate extensometer.  Instron’s biaxial clip-on extensometers, such as 2650-563, provide the accuracy needed to obtain Poisson’s ratio.
  • Reducing knife edge effects – During testing, knife edge penetration or knife edge slipping can cause inaccurate results.  The 2650 series biaxial extensometers provide interchangeable knife edges to ensure that the extensometer is suitable for both rigid and softer plastics.  Options include conical point for extremely rigid materials, line contact for thermoplastics, and vee profiles for thin section test pieces.
  • Minimizing operator influence - Because the measurement of Poisson's Ratio is so sensitive, operators can influence results through factors such as specimen alignment and extensometer attachment.  Consistent specimen alignment can be achieved through the use of specimen centering devices.  Care should be taken when attaching the extensometer to ensure that it is centered on the specimen for each test.  
  • Reporting calculations in compliance with the standard – Instron Bluehill® Universal testing software has advanced pre-built methods available with pre-configured plastic specific calculations helping to ensure compliance.  Using features such as an appropriate preload is critical for achieving accurate, repeatable result. A preload eliminates the compressive forces on a specimen that often occur when initially placed into grips.

It is important to review ASTM D638-14 (A3) in order to fully understand the test setup, procedure, and results requirements.


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