ELS-Specimen

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ELS-specimen

General remarks

The Anglo-Saxon abbreviation ELS stands for ‘End-Loaded Split’. The ELS-specimen is used to determine the interlaminar fracture toughness of Mode II loading [1].

Test specimen shape

Similar to the double cantilever beam (DCB) test (see also: DCB-specimen), a 50 mm long film is inserted. The dimensions are 170 x 20 mm². For carbon-fibre reinforced polymer (CFRP) laminates, B = 3 mm and glass-fibre reinforced polymers (GFRP) B = 5 mm are used to account for delamination growth in test specimens with different stiffnesses (if the test specimens are too thin, there is a risk of premature fracture in the crack initiation area). CCD cameras (CCD = charged coupled device) are used for visual assessment of crack growth.

Fig.:

End-Loaded Split (ELS) testing according ESIS TC 4

The force is applied uniformly via a load block and the test specimen is loaded at crosshead speeds of 1 to 5 mm/min. At the same time, the deflection is determined using a suitable measuring system and the compliance of the test specimen changes due to crack growth in the middle layer.

Equation for determining the energy release rate

The energy release rate is calculated by evaluating the recorded load–deflection diagrams [2].

$G_{IIc}={\frac {18\cdot F^{2}\cdot a^{2}}{B^{2}\cdot d^{3}\cdot E_{f}}}$

mit:

F		load (force)
B		specimen width
a		delaminations length
d		specimen thickness
Ef		flexural modulus

A prerequisite for applying the equation is that the loading and unloading curves match. Otherwise, the correction factors described in the ESIS TC 4 test specification [3] for this test must be determined (non-linearities are caused, for example, by friction effects between the opposing crack surfaces).

A comprehensive overview of suitable test specimens for fracture mechanics tests on plastics and composites can be found in fracture mechanics test specimens.

See also

• Fracture mechanical testing
• Specimen for fracture mechanics tests
• DCB-specimen
• FRMM-specimen
• Energy release rate

References

[1]	Altstädt, V.: Testing of Composite Materials. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 548/549 (ISBN 978-1-56990-806-89; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
[2]	Wang, H., Vu-Khanh, T.: Use of End-Loaded-Split (ELS) Test to Study Stable Fracture Behaviour of Composites under Mode II Loading. Composite Structure 36 (1996) 71–79; DOI: https://doi.org/10.1016/S0263-8223(96)00066-9
[3]	ESIS TC 4 (1995): Protocol for Interlaminar Fracture Testing of Composites (Mode I DCB-ISO 15024 and Mode II ELS-ESIS TC 4)