Flexural properties of plastics are important to many applications. ASTM D790 describes the appropriate procedure in detail. Fixtures for three-point and four-point loading are used to evaluate rigid and semi-rigid materials. Crosshead position is not an acceptable means of measuring deflection when conducting four-point loading tests. Deflectometers or other strain gauge devices are required for direct flexure measurement when conducting a four-point bend test. Since the support span requirement is based on the specimen thickness, it is important to know the range of sample dimensions before selecting a fixture.
Loading and support anvil dimensions may differ from one standard to another, so we suggest reviewing ASTM D790 to fully understand the fixture requirements, as well as the test setup, procedures, and results requirements.