The Definitive Guide to Three Point Bend Testing of Plastic to ASTM D790 & ISO 178

Learn more about ASTM D790
https://www.instron.us/en-us/testing-...

Learn more about ISO 178
https://www.instron.us/en-us/testing-...

This video shows how three different Instron systems and accessories are configured to perform the test according to the standards.

ASTM D790 is very similar to ISO 178, though it differs in several key points:
1. ISO 178 requires the use of either a deflectometer or compliance correction in order to determine modulus. In ASTM D790 this is only a recommendation, and modulus can be calculated by crosshead displacement alone.
2. Preferred specimen sizes are different, and because test speed is dependent on specimen depth, test speeds between the standards may vary. The preferred depth of ASTM D790 specimens is 3.2 mm. The preferred depth for ISO 178 specimens is 4 mm.
3. ASTM D790 allows only one test speed, whereas ISO 1780 allows a second (faster) test speed to be used after modulus is measured.

ASTM D790 and ISO 178 measures the flexural properties of a material while under a bending strain or deflection and determines the following mechanical properties:
• Tangent modulus- Also known as flexural modulus, this is the slope of the initial linear portion of the load deflection curve and is a measurement of the material's stiffness
• Secant modulus- The slope between the origin and a predefined point on the load deflection curve
• Chord modulus- The slope between two predefined points on the load deflection curve
• Flexural strength- The maximum flexural stress obtained during a bend test
• Flexural Stress at Break- The flexural stress at which a specimen breaks during a bend test. For some materials, the specimen breaks before a yield point, in which cases the flexural strength equals the flexural stress at break.

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