Bend Test – Specimen Shapes
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Bend test – Specimen shapes
Test specimen geometry for three-point bending tests of thermoplastics
In the case of the three-point bend test on plastics, the test specimen with the dimensions 80 × 10 × 4 mm3 are usually used in accordance with the standard ISO 178 of the bend test of plastics [1–3]. This test specimen can produce directly by the injection moulding method or from the multipurpose test specimen type 1A by removing the shoulders of the test specimen. In this type of test specimen, a comparability of the internal state of the specimens with respect to the residual stress and orientations with other test methods is existing (see: Tensile test residual stresses orientations).
If the test specimens for the bending test are prepared from plates with different thicknesses (indirect preparation), the width and length of the test specimens must be adapted according to Table 1 in order to avoid impermissible influences (influences in the bend test) due to the shear stress occurring in the cross-section area. As a basic requirement, a length-to-thickness ratio l / h = 20 must be adhered to in the case of approximately isotropic and homogeneous materials. In addition, in this case, the manufacturing direction during the extrusion or calendering of the plates must be also observed.
Table 1: Test specimen geometry according to ISO 178
| Nominal thickness h (mm) | Width b (mm) | Length l (mm) |
|---|---|---|
| 1 < h ≤ 3 | 25.0 ± 0.5 | 20 – 60 |
| 3 < h ≤ 5 (4) | 10.0 ± 0.5 | 60 – 100 (64) |
| 5 < h ≤ 10 | 15.0 ± 0.5 | 100 – 200 |
| 10 < h ≤ 20 | 20.0 ± 0.5 | 200 – 400 |
| 20 < h ≤ 35 | 35.0 ± 0.5 | 400 – 700 |
| 35 < h ≤ 50 | 50.0 ± 0.5 | 700 – 1000 |
Test specimen geometry for bending tests on fiber-reinforced thermosets
When taking test specimens from long-fiber-reinforced, layered anisotropic plastics on a thermoset base (laminates), it must be taken into account that there is a risk of delamination (pulling side) or buckling (pressure side) in the case of short test specimen lengths. In order to avoid the interlaminar shear failure between the layers, the length-thickness ratio of the test specimens is increased to up to l / h = 50. Since the three-point bending test and the four-point bending test are permitted in the corresponding standard ISO 14125 [4], differences arise in this case. However, because of the comparatively low laminate thicknesses, test specimen lengths comparable to ISO 178 are used here (Table 2), which however are influenced by the type of reinforcement. In both standards, the test speed must be adapted to the type of test specimen in order to avoid impermissible influences on the deformation and damage mechanisms that occur.
Table 2: Test specimen geometry according to ISO 14125
| Material | h (mm) | b (mm) | l (mm) |
|---|---|---|---|
| Discontinuous reinforcement | 4 | 10.0 | 80 |
| Mat, fabric or mixed reinforcement | 4 | 15.0 | 80 |
| Traverse uni- or multidirectional reinforcement | 2 | 15.0 | 60 |
| Uni- or multi-directional reinforcement | 2 | 15.0 | 100 |
See also
- Bend test
- Bend test – Test influences
- Bend loading
- Bend test – Specimen preparation
- Bend test – Yield stress
References
| [1] | Bierögel, C.: Bend Test on Polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser Verlag, München (2022) 3. Edition, 133–143 (ISBN 978-1-56990-806-8; see under AMK-Library A 22) |
| [2] | ISO 178 (2019-04): Plastics – Determination of Flexural Properties |
| [3] | Bierögel, C., Grellmann, W.: Bend loading. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein, Volume VIII/6A3, Springer Verlag (2014) pp. 164–191 (ISBN 978-3-642-55166-6; see under AMK-Library A 16) |
| [4] | ISO 14125 (1998-03): Fibre-reinforced Plastic Composites – Determination of Flexural Properties; Technical Corrigendum Cor.1:2001-07 and Amd.1:2011-02 |