Support Distance

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Support distance or Support span

Definition

The support distance, also known as the support span, is the geometric distance between the abutments supporting the test specimen in the bending test, measured at the centre of the abutments.

Support distance in the bending test

In the quasi-static bending test, a shear stress occurs in the test specimen in addition to the normal stress, which can influence the determination of the characteristic values in the bending test. To minimise this effect, the ratio between the span L and the test specimen height h (designations according to ISO 178) must be observed when testing plastics:

$L=(16\pm 1)h$

When using three-point bending test specimens ( SENB-specimens), often still referred to as ISO standard bars in testing practice, with the dimensions 80 x 10 x 4 mm³, this results in a support span of 64 mm.

For very thick test specimens or plastic composites that contain coarse-grained fillers, it may be necessary to select a larger ratio L/h to avoid delamination due to shearing. This applies in particular to laminates or other composites with a layered structure if the interlaminar shear strength (short beam test) is not to be determined. In this case, the ratio L/h = 20 to 25 should be used to eliminate the shear stress component. In the case of very soft plastics such as polyethylene ( abbreviation: PE) etc., a larger support span or a modified support radius can also be used to reduce the compression of the abutments into the test specimen.

Charpy impact test

The Charpy impact bending test is carried out on notched and unnotched test specimens in three-point support and is used to assess the toughness behaviour of plastics under impact loading. It is standardised in ISO 179. The prismatic test specimens can be produced directly by injection moulding or by machining from pressed or cast plates. Type 1 test specimens, which can be taken from multipurpose test specimens in accordance with ISO 3167 Type A, are primarily used for thermoplastics. They have a length of 80 mm, a width of 10 mm, a thickness of 4 mm and a support span of 62 mm.

The instrumented Charpy impact test (ICIT)

For dynamic loading in the instrumented Charpy impact test (ICIT), the support span (designated s here in ISO 179) is an important setting parameter with regard to an optimum signal shape. For a fracture mechanics evaluation, the requirement of s/W = 4 (W - test specimen width) is derived from investigations into the influence of the support width. In addition, the a/W ratio and the pendulum hammer speed must also be optimised.

For a large number of applications in the plastics sector, the following is obtained with

s/W = 4

a/W = 0.2 and

vH = 1 ms-1

are generally applicable stress conditions.

See also

• Instrumented Charpy impact test
• Bend test – Influences
• ICIT – Limits of fracture mechanics evaluation
• ICIT – Support span method

References

• ISO 178 (2019-04): Plastics – Determination of Flexural Properties
• ISO/DIS 179-1 (2025-05): Plastics – Determination of Charpy Impact Properties – Part 1: Non-instrumented Impact Test (Draft)
• ISO 179-2 (2020-05): Plastics – Determination of Charpy Impact Properties – Part 2: Instrumented Impact Test
• Bierögel, C.: Bend Test on Polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3. Edition, S. 133–139, ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
• Grellmann, W.: Zähigkeitsbewertung mit bruchmechanischen Methoden. In: Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4. Edition, pp. 255–259 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)