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Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates
Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
Division of Materials Science, Luleå University of Technology, Luleå, Sweden.ORCID iD: 0000-0001-9649-8621
Institut Jean Lamour, France.
2013 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 3, p. 287-301Article in journal (Refereed) Published
Abstract [en]

Stiffness reduction simulation in laminates with intralaminar cracks is usually performed assuming that cracks are equidistant and crack density is the only parameter needed. However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In this article, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model, in which the normalized average crack-opening and crack-sliding displacements are the main characteristics of the crack, is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis, the crack-opening displacement and crack-sliding displacement are calculated using finite element method, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution, we obtain lower bond to elastic modulus. A ‘double-periodic’ approach presented to calculate the crack-opening displacement of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers of cross-ply laminates, whereas for high crack density in surface layers, it underestimates the modulus reduction.

Place, publisher, year, edition, pages
Sage Publications, 2013. Vol. 47, no 3, p. 287-301
Keywords [en]
Laminates, transverse cracking, crack-opening displacement, elastic properties, damage mechanics
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
URN: urn:nbn:se:liu:diva-150444DOI: 10.1177/0021998312440126ISI: 000313902100002Scopus ID: 2-s2.0-84872914468OAI: oai:DiVA.org:liu-150444DiVA, id: diva2:1241269
Note

Validerad; 2013; 20120323 (andbra)

Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
In thesis
1. Experimental and numerical studies of intralaminar cracking in high performance composites
Open this publication in new window or tab >>Experimental and numerical studies of intralaminar cracking in high performance composites
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The macroscopic failure of composite laminates subjected to tensile increasing load is preceded by initiation and evolution of several microdamage modes. The most common damage mode and the one examined in this thesis is intralaminar cracking in layers. Due to this kind of microdamage the laminate undergoes stiffness reduction when loaded in tension. For example, the elastic modulus in the loading direction and the corresponding Poisson’s ratio will decrease.

The degradation of the elastic properties of these materials is caused by reduced stress in the damaged layer which is mainly due to two parameters: crack opening displacement (COD) and crack sliding displacement (CSD). At fixed applied load these parameters depend on the properties of the damaged and surrounding layers, on layer orientation and on thickness. When the number of cracks per unit length is high (high crack density in the layer) the COD and CSD are reduced because of to crack interaction.

The main objective of the first paper is to investigate the effect of crack interaction on COD using FEM and to describe the identified dependence on crack density in a simple and accurate form by introducing an interaction function dependent on crack density. This interaction function together with COD of non-interactive crack gives accurate predictions of the damaged laminate properties. The application of this function to more complex laminate lay-ups is demonstrated. All these calculations are performed assuming that cracks are equidistant.

However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In the second paper, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis the COD and CSD are calculated using FEM, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution we obtain lower bond to elastic modulus. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction.

In the third paper, the thermo-elastic constants were calculated using shear lag models and variational models in a general calculation approach (GLOB-LOC) for symmetric laminates with transverse cracks in 90° layer. The comparison of these two models with FEM was presented for cross-ply and quasi-isotropic laminates.

Using FEM, we assume linear elastic material with ideal crack geometry. Fiber bridging over the crack surface is possible which can affect COD and CSD. The only correct way to validate these assumptions is through experiments.

The main objective of the fourth and the fifth paper is to measure these parameters for different laminate lay-ups in this way providing models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edge of a [90/0]s and [903/0]s carbon fiber/epoxy laminates specimens with multiple intralaminar cracks in the surface layer is studied. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry).

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2013. p. 194
Series
Doctoral thesis, ISSN 1402-1544
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150431 (URN)978-91-7439-718-5 (ISBN)978-91-7439-719-2 (ISBN)
Public defence
2013-10-04, E231, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2018-08-27 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved

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