Flexure Test

ISO 178 and ASTM D790

Die 3-Punkt-Biegeprüfungen, die in ISO 178 und in ASTM D 790 beschrieben sind, stellen klassische Charakterisierungsmethoden für steife und halbsteife Kunststoffe dar. Typische Prüfergebnisse sind der Biegemodul, die Spannung bei 3,5 % Dehnung sowie Spannungen und Dehnungen am Steckpunkt und bei Probenbruch.
In the characterization of molding materials, specimens are taken from the mid-section of the specimen as described 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 method is limited to a flexural strain of 3.5% by the standards, which corresponds to a deflection of 6 mm for ISO specimens with a height of 6 mm. The calculation error accepted by the standards in this range still lies under 1% of the measured value. 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 4.0 mm (nominal) produces an error in flexural stress of about 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. Zwick testing machines can compensate for deformation of the load frame, force sensor, and flexure test tool using the testXpert software. Measurement is sufficiently accurate via the testing machine's crosshead displacement transducer. Operating the machine is 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. Zwick offers sensor arms for flexure tests that can be used with the automatic extensometers, makroXtens and multiXtens, instead of tensile sensor arms.

Along with its dimensional accuracy, a key feature of our sophisticated flexure test kit is its ability to be accurately aligned. The supports 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 gages makes this task significantly easier. Centering stops on the supports make it easy to position the specimen with precision.

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3-point flexure test on plastics

3-point flexure test on plastics with makroXtens to ISO 178

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