ENF-Specimen

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ENF specimen

General

The Anglo-Saxon abbreviation ENF stands for "End-Notched Flexure".

The ENF test specimen is used to determine the interlaminar fracture toughness of mode II loading. A critical energy release rate in plane strain is determined as a fracture mechanical parameter. Shear loading without much friction between the surfaces of a crack is assumed. Shear stresses and strains before the crack tip (see also crack opening) can have an influence on the calculation of the energy release rate.

Test specimen shape

Fig. 1:

Schematic illustration of ENF specimen

Determination equation

To calculate the energy release rate G_IIc for EDZ, the following equation applies

$G_{IIc}={\frac {9\cdot F^{2}\cdot a^{2}\cdot C_{b}}{2\cdot W\left(2\cdot L^{3}+3\cdot a^{3}\right)}}\left[{\frac {J}{m^{2}}}\right]$

width:

a		crack length
F		failure force (Fmax)
Cb		compliance of the test specimen
W		specimen width
L		distance between pressure fin and support

As a conservative estimate of the compliance, the simple beam theory is sufficient in many cases; the test is carried out with a three-point bending device (see bend test) and a defined initial crack of 25 mm.

Fig. 2:

ENF specimen in unloaded and loaded states according to JIS K 7086

The compliance can be determined experimentally or calculated according to

$C={\frac {2\cdot L^{3}+3\cdot a^{3}}{8\cdot E\cdot f\cdot d^{3}}}$

width:

E		flexural modulus in axial direction
d		half of elastic beam height
f		deflection
L		half of support distance

By substituting these equations, one obtains a calculated energy release rate

$G_{IIc}={\frac {9\cdot F^{2}\cdot a^{2}}{16\cdot W\cdot f\cdot d^{3}\cdot E}}$

A comprehensive compilation of suitable test specimens for fracture mechanics testing on plastics and composites is contained in test specimens for fracture mechanics tests.

Test system for performing of ENF tests

The test set-up shown in Fig. 3 was implemented at Polymer Service GmbH Merseburg to carry out fracture mechanics tests under Mode II loading on ENF specimens. The left partial image shows the installation of an ENF specimen in a universal testing machine Z 050 from Zwick Roell GmbH & Co. KG in unloaded condition. The right partial image shows a loaded condition with crack propagation in the test specimen.

Bild 3:

Test system for carrying out fracture mechanics tests on ENF specimens from the company Polymer Service GmbH Merseburg

See also

• Specimen for fracture mechanics tests
• Bend test

References

	Carlsson, L. A., Pipes, R. B.: Hochleistungsverbundwerkstoffe, B. G. Teubner, Stuttgart (1989) (ISBN 978-3-519-03250-2; e-Book ISBN 978-3-322-96703-9)
	Valisetly, R. R., Chamis, C. C.: ASTM STP 972 (1988) 41–72 (Composite Materials. Testing and Design / Eighth Conference)
	Russel, A. J., Street, K. N.: Moisture and Temperature Effects on the Mixed Mode Delamination. Fracture of Unidirectional Graphite / Epoxy. Delamination and Deponding of Materials, ASTM STP 876 (1985) 349
	Hodgkinson, J. M. (Ed.): Mechanical Testing of Advanced Fibre Composites, Woodhead Publishing, Cambridge (2000) (ISBN 978-1-8557-3891-1)
	Altstädt, V.: Testing of Composite Materials. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser Munich (2022) 3rd Edition, pp. 549–550 (ISBN 978-1-56990-806-8; see AMK-Library under A 22) Standards ASTM D 7905/D 7905M (2019): Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites JIS K 7086 (1993): Testing Methods for Interlaminar Fracture Toughness of Carbon Fiber Reinforced Plastics (JIS – Japan Industrial Standards)