The ASTM D790 standard, like the ISO 178 standard, describes 3-point flexure tests on rigid and semi-rigid plastics as well as on fiber composites with long-fiber reinforcements.
Typical test results include:
- Flexural modulus
- Stresses and strains at the yield point, at maximum stress and at specimen break
The tests are performed up to a flexural strain of max. 5%.
- The specimens are manufactured by injection molding, or taken from sheets or plates via mechanical processing
- Normally, specimens with a cross-section of 3.2 mm x 12.7 mm are used for plastic molding compounds. The defined thickness/support ratio of 16 leads to a support span of 51 mm.
- For fiber composites, a thickness/support ratio of 16 can lead to undesired shear failure. If the ratio of tensile strength to shear strength is greater than 8, then larger support spans are used, with a ratio of 32, 40 or even 60 to the thickness of the specimen.
- The ASTM D638 standard allows for deflection measurement via the testing machine’s crosshead travel monitor. For more accurate measurements, the use of a directly measuring extensometer is recommended.
- Calculating the flexural stress and strain takes into account small deflections and does not take into account any friction at the supports. For this reason, the method is limited to flexural strain of 5 %.
- Extreme care is needed when measuring specimen dimensions for a flexure test. Since specimen thickness is calculated quadratically in the flexural stress, the measurement error result is also a quadratic function.A measurement error of only 0.1 mm with a specimen height of 3.2 mm (nominal) produces an error in flexural stress of more than 5%.
- The results of the flexure test show the material behavior, in particular, near the surface of the specimen. The deflections measured are approximately four times greater than the extensions in a tensile test.
- ZwickRoell testing machines can compensate for deformation of the load frame, force sensor, and flexure test tool using the testXpert III software.Measurement is sufficiently accurate via the testing machine's crosshead displacement transducer. Operating the machine is therefore made simple, especially for quality control purposes.
- If a high degree of reproducibility is required, we recommend using an extensometer that facilitates load-free deflection under the specimen. ZwickRoell offers sensor arms for flexure tests that can be used with the automatic extensometers makroXtens or multiXtens, instead of tensile sensor arms.
- Along with its dimensional accuracy, a key feature of our sophisticated flexure test kit is accurate alignment.The supports must be precisely aligned with one another and with the die. In the stress-strain diagram, angular errors can create a curve foot, which can significantly falsify determination of the modulus. Adjustable supports and well-designed gauges makes this task significantly easier. Centering stops on the supports make it easy to position the specimen with precision.