Bend Test – Shear Stress

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Bend test – Shear stress

Causes for the occurence of shear stresses

In the bend test on plastics, it is assumed in accordance with the preferred standards for the bending test of plastics [1, 2] that a pure normal stress state is present in the test specimen. In analogy to this, a pure shear stress condition should be present in the shear or punching test. In the presence of an exact adjustment and a sharp punching tool (Fig. 1a), a quality-compliant result is thus obtained. In the case of a blunt punching knife, rounding at the support span or a too large support distance, a deflection is additionally generated, which leads to a defective product (Fig. 1b).

Fig. 1:

Influence of bending in the punch process

Conversely, a too small support spacing or non-adjusted support radii can generate an impermissibly high shear stress in the cross-section. Although this stress component is not taken into account in the calculation, it usually leads to a reduction in the characteristic values of the bending test. The occurrence of shear stress in the bending test can be illustrated with the help of Fig. 2 [3, 4].

Formation and distribution of shear stresses in the flexure test specimen

Assuming a bending beam (Fig. 2 right) or four individual bending beams laid one on top of the other (Fig. 2 left) with identical geometry and made of the same material, i.e. comparable bending stiffness EI_y, an approximately identical deflection will occur under load. When comparing the edge contour, the bending beam will have a flat cross-section, while the bending beams will deform individually and thus cause a staircase-like contour of the cross-section.

If one creates a lamination structure of the bending beam with four bonded individual layers, then one can see that in order to maintain the flat edge contour, forces act between the layers which manifest themselves in a shear stress. If the load due to the shear stress becomes too great, then, as is known, buckling effects occur on the upper pressure side or delamination between the individual layers on the lower tension side.

Fig. 2:

Influence of bending in the punch process

The shear stress reaches its maximum in the area of the neutral fibre and has a parabolic distribution function (Fig. 2). To avoid inadmissible shear stress influences in the bending test, a length-to-thickness ratio of l/h = 20 or a support span-to-thickness ratio of L/h = 16 must be maintained according to ISO 178 [1]. Due to the shear sensitivity of laminates, the support span-to-thickness ratio in this case is L/h = 20 – 25. The Interlaminar shear strength (ILSS) can then be determined on laminates from the so-called "short-beam bend test", i.e. the bending test with a narrow support spacing [5, 6].

See also

• Bend test
• Bend test – Specimen preparation
• Bend test – Test influences
• Bend loading
• Bend test – Specimen shapes
• Bend Test – Yield Stress

References

[1]	ISO 178 (2019-04): Plastics – Determination of Flexural Properties
[2]	ISO 14125 (1998-03 Amd 1:2011-02): Fiber-Reinforced Plastic Composites – Determination of Flexural Properties
[3]	Bierögel, C.: Bend test on polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3. Edition, p. 133–143 (ISBN 978-1-56990-806-8; ePub ISBN 978-1-56990-802-2; see AMK-Library under A 22)
[4]	Bierögel, C., Grellmann, W.: Bend loading. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein, Volume VIII/6A3, Springer Verlag, Berlin (2014) 164–191, (ISBN 978-3-642-55165-9; see AMK-Library under A 16)
[5]	Altstädt, V.: Testing of composite materials. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3. Edition, pp. 537–538 (ISBN 978-1-56990-806-8; e-Book ISBN 978-1-56990-807-5; see AMK-Library under A 22
[6]	ISO 14130 (1997-12): Fibre-reinforced Plastic Composites – Determination of Apparent Interlaminar Shear Strength by Short-beam Method, Technical Corrigendum 1:2003-11