ASTM D790 is a test method to determine flexural properties of unreinforced and reinforced plastics and electrical insulating materials. This three-point bend test requires that the loading nose and fixed supports have a diameter of 10mm and the span of the supporting anvils be 16 times the depth (thickness) of the specimen. Instron’s 2810-400 bend fixture is fully compliant with these requirements.
The challenges of testing to this standard are:
- Flexural strain measurement
- Repeatability of calculations
- Reporting calculations in compliance with the standard
- Flexural strain measurement – There are two methods for reporting strain that are compliant with ASTM D790. Type I tests use crosshead position for deflection measurement. Compliance correction is recommended but not required. The second method, Type II, uses a deflectometer or extensometer. A clip-on extensometer attached to a deflectometer plunger or Instron’s AutoX have the accuracy to measure flexural modulus.
- Repeatability of calculations – When setting up the test, having an adequate preload is highly recommended to ensure accurate and consistent strain measurements. How much force is applied to the specimen prior to starting the test directly impacts the repeatability of calculations such as flexural modulus. Alternatively, a slack correction, also referred to as a tow compensation, can be used to ensure that the calculations are performed on the appropriate section of the testing data.
- Reporting calculations in compliance with the standard – Instron's Bluehill® Universal testing software has advanced pre-built methods available with pre-configured plastic specific calculations helping to ensure compliance.
It is important to review ASTM D790-15e2 in order to fully understand the test setup, procedure, and results requirements.
For more information view the Definitive Guide to ASTM D790 Flexure Testing of Plastics.