CLS-Specimen

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CTS-specimen

The Anglo-Saxon abbreviation CLS stands for ‘Crack-Lap Shear’.

General information

The CLS-specimen was originally designed for investigating shear-dominated failure in adhesive joints (see also: adhesive joints – determination of characteristic values).

Test specimen shape [1, 2]

Specimen shape

Fig. 1:

Crack-Lap Shear (CLS) specimen

This test specimen shape with a ‘free shear layer’ was introduced in the literature by Wilkins [1] and Valisetty [2] (see Fig. 1). The CLS test specimen does not have a pure mode II load at the crack tip. The unbalanced configuration of the CLS test specimen results in a normal stress (mode I). As a result, this test specimen is referred to as a mixed-mode test specimen.

A geometrically non-linear finite element analysis by Law and Wilkins [3] showed that the mode II component varies with the load. However, the change in the Mode II component is small compared to the expected fluctuations in the critical force. For a typical carbon fibre/epoxy resin system with a layered structure, for example, the mode II component is approximately 70 %.

Equation for determining the energy release rate

Altstädt describes a CLS-specimen in [4] in which only the clamping position is subjected to stress (Fig. 2).

Fig. 2:

CLS-specimen according to ESIS TC 4

Since there is no Mode II stress alone at the crack tip, this is a mixed-mode stress. In order to obtain a natural initial crack (see also: initial crack length), the crack is first opened and extended to a certain length. The crosshead speed is preferably 0.5 mm/min, whereby only the clamped position is stressed; the free shear lag is not stressed. Between loading and unloading, the crack length is recorded to determine the compliance. Based on a strength analysis, the compliance C and the energy release rate G can be determined for the plane strain state:

C={\frac {L}{B\cdot E\cdot d_{1}}}+{\frac {a\left(d_{1}-d_{2}\right)}{B\cdot E\cdot d_{1}\cdot d_{2}}}

G_{I/IIc}={\frac {F^{2}\cdot \left(d_{1}-d_{2}\right)}{2\cdot B^{2}\cdot E\cdot d_{1}\cdot d_{2}}}

with

a		initial crack length
d₁		thickness of the clamping layer
d₂		thickness of the free shear layer

An extensive summary of suitable test specimens for fracture mechanics investigations on plastics and composite materials is included in test specimens for fracture mechanics tests.

References

[1]	Wilkins, D. J., Eisenmann, R. A., Camin, R. A., Margolis, W. S., Benson, R. A.: ASTM STP 775 (1982), Short Fiber Reinforced Composite Materials
[2]	Valisetty, R. R., Chamis, C. C.: ASTM STP 972 (1988) 41–72, Composite Materials: Testing and Design. Editor: Whitcomb, J. D. (ISBN 0-8031-0980-6; ISBN 978-0-8031-0980-3)
[3]	Law G. E., Wilkinson, D. J.: Delamination Failure Criteria for Composite Structures, Final Report NAV-GD-0053 (Mai 15, 1984)
[4]	Altstädt, V.: Testing of Composite Materials. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, p. 553 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)

General information

Test specimen shape [1, 2]

Equation for determining the energy release rate

See also

References