Bend Test – Test Influences

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Bend test − Test influences

Bend test − Test influences

So-called self-adjusting supports are still sometimes used in the practical testing bend tests (Fig. 1a). These test systems work very well with metallic materials, since the forces generated are much greater than with plastics. Since experience has shown that the maintenance condition of such devices has a considerable influence, and an exactly plane-parallel bearing of the test specimens is problematic, additional stress components (Fig. 1b) can arise which are not taken into account in the calculation.

Fig. 1:

Influence of incline bending and torsional moments on stress distribution

The so-called inline (skew) bending, i.e. bending around two axes of the specimen, can occur as a result of the resulting misalignment of the bending device and thus leads to increased material stress. As can be seen from Fig. 1b, a torsional component will usually occur at the same time, which is not negligible. Considering the superposition of the different stress components, e.g. in the shape change hypothesis, it becomes clear that the comparative stress adequately describes the material effort. If additional stresses now occur, this value remains identical, but the weighting of the individual stress components shifts. In the worst case, therefore, the flexural strength calculated is always too low, since it is based only on the normal stress arising in the cross-section and does not include the other stress components [1].

See also

• Bend test
• Bend test – Specimen preparation
• Bend test – Shear stress
• Bend loading
• Bend test – Specimen shapes
• Bend test – Yield stress

Reference

[1]	Bierögel, C.: Bend Test on Polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser Munich (2022) 3. Edition, 133–143 (ISBN 978-1-56990-806-8; see under AMK-Library A 22)