Flexure tests on composites are performed using the 3-point or 4-point test method. The measured flexural moduli and flexural strengths are strongly influenced by the laminate structure and therefore do not correlate with the measured tensile properties.
Flexure tests are performed to determine important mechanical characteristics. The laminate of the specimen is loaded on the bottom with a tensile force and on the top with a compressive force. No force is applied to the laminate planes in the middle. Furthermore, in this test setup, in-plane shear stresses arise, which can be minimized by selecting a wide span in proportion to the thickness of the specimen.
In the case of multi-directional laminates, a flexure test can be used to identify layup errors, which is useful for quality control and goods inwards checks.
In flexure tests, precise alignment of the tool's supports and dies is also mandatory to achieve accurate and reproducible test results. ZwickRoell has developed special flexure tools for these types of tests, in which all elements can be adjusted to one another with precision.
3-point flexure tests on composites are common and easy to carry out. The deflection can be measured with the testing machine's crosshead travel monitor if the machine deformation is compensated.
- The flexure modulus is determined between 10% and 50% Fmax (EN 2562), or 10% and 25% Fmax (EN 2746), or between two strain limits (ISO and ASTM).
- The span to thickness ratio is defined as 32.1 by ASTM. ISO uses 20:1 for GFRP and 40:1 for CFRP, EN standards use 16:1 for GFRP and 40:1 for CFRP, thus subjecting the specimen to only low shear forces.
The advantage of the 4-point flexure test on composites is the shear-force-free area between the force application points of the double die.
- The flexural modulus is determined between 0.05% and 0.25% strain (ISO 14125) or 0.1% and 0.3% strain (ASTM D7264).
- The central support span can be 1/3 (ISO 14125) or 1/2 (ASTM D7264) of the lower support span. The span to thickness ratio is 32:1 in ASTM. ISO uses 22.5:1 for GFRP and 40.5:1 for CFRP.
- In accordance with standards, flexure can be directly measured in the middle between the supports.