ASTM D6272 is a test method for the determinations of flexural properties for unreinforced and reinforced plastics. These materials include high-modulus composites and electrical insulating materials. This flexure test requires a 4 point fixture where the bottom span is set by the span-to-depth of 16:1 ratio, and the top is set at two points equally distant from the center line. The Instron® 4 point flexure fixture is fully compliant with the fixture requirements.
Two test procedures, Procedure A and B are used depending on the material properties. Procedure A is used for brittle materials that break at low deflections. Procedure B is used for flexible, more ductile materials that undergo large deflections. The specimen is deflected until break or until a strain of 5% is reached, whichever occurs first. To accurately measure the deflection, a calibrated device is required that meets the ASTM E83 Class C deflection measuring standard.
The challenges of testing to this standard are:
- Flexural strain measurement
- Repeatability of calculations
- Reporting calculations in compliance with the standard
- Flexural strain measurement – ASTM D6272 requires that the method used to measure strain is not influenced by the compliance of the machine. Advanced strain solutions include non-contacting extensometers, such as the Advanced Video Extensometer (AVE 2) and automatic contacting extensometers, such as the AutoX 750. These solutions require little influence from the operator, resulting in improved repeatability. Clip-on extensometers such as 2630-107 or 2630-110 attached to a deflectometer plunger also meet Class C deflection measuring requirements.
- Repeatability of calculations – When setting up the test, having an adequate preload is highly recommended to ensure accurate and consistent strain measurements. The amount of force applied to the specimen prior to starting the test directly impacts the repeatability of calculations, such as flexural modulus. Alternatively, slack correction 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 Bluehill® Universal testing software has advanced built-in methods available with pre-configured plastic-specific calculations helping to ensure compliance based on the ASTM D6272 standard.
It is important to review ASTM D6272 in order to fully understand the test setup, procedure, and results requirements.