The ISO 178 standard specifies the determination of the flexural properties of plastics. In a 3-point flexure test, the specimen is placed on two parallel-positioned anvils and bent via a compression die. The specimen is not allowed to break during this process.
A further description of the 3-point flexure test on plastics can be found in the ASTM D790 standard.
The 3-point flexure tests specified in ISO 178 and ASTM D790, describe classic characterization methods for rigid and semi-rigid plastics.
Typical test results include:
- Flexural modulus
- Stress at 3.5 % elongation
- Stresses and elongations at the yield point and at specimen break
In the characterization of molding materials, specimens are taken from the mid-section of the specimen as specified in ISO 20753.This ensures that the polymer has the same processing state for the flexure test as it does for the tensile test.
Calculating the flexural stress and strain typically takes into account small deflections and does not take into account any friction at the supports. For this reason, the standards limit this method to a flexural strain of 3.5%, which corresponds to a deflection of 6 mm for ISO specimens with a height of 4 mm. The calculation error accepted by the standards in this range still lies under 1% of the measured value.
Exercising extreme care is a requirement 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 4.0 mm (nominal) produces an error in flexural stress of about 5%.
The deflection measurement is carried out according to the particular objective of the testing laboratory.
- In the simplest case, only the maximum force or stress needs to be determined and no deflection measurement is required.
- If only deflection values are to be determined that result in an outer fiber strain greater than 1%, that is, the 3.5% flexural yield strength for example, then a simple measurement via the crosshead travel monitor of the testing machine is sufficient.
- Type III defined in the standard describes the case of a quality assurance or R&D task with flexural modulus measurement that only serves for internal comparison or tolerance monitoring purposes, and therefore, it is not so much the correctness but good repeatability of the measurement that matters. In this case, to simplify the test arrangement, the crosshead travel monitor of the testing machine and a compliance correction can be used.
- If the objective is to achieve accurate and correct test results that also hold up in comparison with other testing laboratories, then use of a direct deflection measurement in accuracy class 1 is mandatory.
- 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.