6th International Conference on Composites Testing and Model Identification O.T.Thomsen, Bent F. Sørensen and Christian Berggreen (Editors) Aalborg, 2013
VALIDATION OF FEM BASED DAMAGED LAMINATE MODEL MEASURING CRACK OPENING DISPLACEMENT IN CROSS-PLY LAMINATE USING ELECTRONIC
SPECKLE PATTERN INTERFEROMETRY (ESPI)
M.S. Loukil1,2, J. Varna1 and Z. Ayadi2
1
Department of Engineering Sciences and Mathematics, Luleå University of Technology SE-97187 Luleå, Sweden
Email: janis.varna@ltu.se, web page: http://www.ltu.se
2
Institut Jean Lamour, Université de Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy Cedex, France
web page: http://www.ijl.nancy-universite.fr
Keywords: Damaged Laminate, Crack opening displacement (COD), ESPI, FEM ABSTRACT
Composite laminates during service undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined in this work is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness.
Finite element method (FEM) studies were performed in [1,2] to understand which material and geometry parameters affect the COD and CSD most and simple empirical relationships (power law) were suggested.
All these studies and analysis assume a linear elastic material with idealized geometry of cracks. The only correct way to validate these assumptions is through experiments.
The effect of material properties on COD was measured experimentally using optical microscopy of loaded damaged specimens in [3,4]. It was shown that the measured average values of COD are affected by the constraining layer orientation and stiffness.
The experimental determination of the average COD and CSD needs the measurement of the displacement for all points of the crack surfaces, which justifies the use of full-field measurement technique, Electronic Speckle Pattern Interferometry (ESPI). ESPI is an optical technique that provides the displacement for every point on a surface and offers the possibility to measure both, the in-plane and out-of-plane displacement without surface preparation.
This technique was used in [5,6] to measure the COD for inside cracks on the specimen’s edge. It was shown that the profile of the crack on the edge is elliptical. The main objective of this paper is to study cracks in surface layers by measuring the COD along the crack path. For this reason the cross-ply laminate with surface cracks was selected.
In particular, the displacement field on the surface of a [903/0]S carbon fiber/epoxy laminate specimen
with multiple intralaminar cracks is studied and the relative displacement dependence on the applied mechanical load is measured.
By looking to the displacement field the cracks appear as singularities and the corresponding displacement jumps are directly related to COD and CSD. The transverse cracks are parallel to the fiber orientation in the layer, which in our case corresponds to a 90 direction with respect to the tensile axis. Consequently, there is no relative sliding of the crack faces and the only displacement of these crack faces corresponds to COD. In other words, the crack displacement discontinuity measured on the surface is directly the COD.
M.S. Loukil, J. Varnaand Z. Ayadi 0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.05 0.10 0.15 0.20 N o rm a li ze d T h ic k n e ss
Nomalized COD (µm/Mpa)
Experimental results FEM results 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0 0.2 0.4 0.6 0.8 1 1.2 N o ma li ze d C O D (µ m/ M p a) Normalized Width Experimental results FEM results
Figure 1. The COD as a function of the normalized thickness
Figure 2 . The COD along the normalized width of the sample
In this work, the crack opening displacement profile along the thickness of the damaged 90° layers is investigated and compared with FEM. A very good agreement is shown in Fig1.
To study edge effects on the opening displacement, the COD is measured along the crack path and the results are presented in Fig 2.
REFERENCES
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loading, International Journal of Damage Mechanics, 14 (3), 2005, pp. 235-261
[2] P. Lundmark and J. Varna, Crack face sliding effect on stiffness of laminates with ply cracks,
Composite Science and Technology, 66, 2006, pp. 1444–54
[3] J. Varna, L. A. Berglund, R. Talreja and A. Jakovics, A study of the crack opening displacement
of transverse cracks in cross ply laminates, International Journal of Damage Mechanics, 2, 1993, pp. 272–89
[4] J. Varna, R. Joffe, N. V. Akshantala and R. Talreja, Damage in composite laminates with
off-axis plies, Composite Science and Technology, 59, 1999, pp. 2139-2147
[5] L. Farge, Z. Ayadi and J.Varna,Optically measured full-field displacements on the edge of a
cracked composite laminate, Composite. Part A, 39, 2008, pp.1245-1252
[6] L. Farge, J. Varna and Z. Ayadi, Damage characterization of a cross-ply carbon fiber/epoxy
laminate by an optical measurement of the displacement field, Composite Science and
