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ISO 178 investigates the flexural behavior of plastics that are important to many applications. Used to determine the flexural strength, flexural modulus, and other aspects of the flexural stress/strain relationship under the conditions defined, it applies to a freely supported beam, loaded at midspan (three-point loading test).
This ISO 178 test method is suitable for use with the following range of materials: - Thermoplastic moulding and extrusion materials, including filled and reinforced compounds in addition to unfilled types
- Rigid thermoplastic sheets
- Thermosetting moulding materials, including filled and reinforced compounds
- Thermosetting sheets
We use a three-point and four point loading flexure fixture to evaluate the materials. For three-point loading, we found that crosshead position is an acceptable means of measuring deflection. However for four point loading the crosshead position is not an acceptable means of measuring deflection. Deflectometers or other strain gauge devices are required for direct flexure measurement when conducting a four-point bend test. The support span requirement is based on a ratio of the specimen thickness. It is important to know the range of sample dimensions before selecting a fixture. Loading and support anvil dimensions may differ on specimen thickness. We use the preload feature of Bluehill® Software to determine the start of loading. Strain measurement devices can automatically re-balance at this point. The load sustained by the specimen is measured during this procedure. To record the test it is important to have a high-speed intelligent test data logging rate. This is the transfer rate to the PC of 100Hz or more to capture and record all data. We use a three-point and four-point loading flexure fixture fitted to a tensile testing machine. This is usually of single-column or dual-column table top design. Use testing software, such as Bluehill 2 test method, to input specimen details, set the desired test control, automatically calculate the desired results and statistics, and produce a test report all in accordance with the standard. We suggest reviewing ISO 178 to fully understand the test fixture and results requirements.
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