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 mid span (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
Both three-point and four-point loading flexure fixtures are used to evaluate plastics. Deflectometers or other strain measuring devices, such as a video extensometer and LVDT have historically been required for direct flexure measurement when conducting four point loading tests. Direct strain measurement devices were not a requirement for three-point loading tests until the 2010 release of ISO 178. Prior to this, crosshead position was an acceptable means of measuring flexural strain.
Note. ISO typically allows a grace period of 5 years in which the new requirements are not mandatory.
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 3 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.