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Experimental and numerical studies of intralaminar cracking in high performance composites
Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå.
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: urn:nbn:se:liu:diva-150431ISBN: 978-91-7439-718-5 (print)ISBN: 978-91-7439-719-2 (electronic)OAI: oai:DiVA.org:liu-150431DiVA, id: diva2:1241339
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
List of papers
1. Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks
Open this publication in new window or tab >>Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks
2013 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 48, no 1, p. 37-46Article in journal (Refereed) Published
Abstract [en]

Thermo-elastic constants of symmetric and balanced laminates with intralaminar cracks in 90-layers depend on the opening displacement (COD) of the crack. The COD dependence on the interaction between cracks in the same layer is studied using FEM. The COD dependence on crack density is described by interaction function in form of tanh(). This interaction function multiplied with COD of non-interactive crack is the input parameter in analytical model for thermo-elastic properties of damaged symmetric and balanced laminates. Predictions performed for cross-ply laminates with cracks in inside and in surface layers and for quasi-isotropic laminates with different position of the 90-layer are in a very good agreement with direct FEM calculations.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150448 (URN)10.1016/j.compositesa.2012.12.012 (DOI)000317880200005 ()2-s2.0-84873630424 (Scopus ID)
Note

Validerad; 2013; 20130125 (andbra)

Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
2. Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates
Open this publication in new window or tab >>Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates
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
Keywords
Laminates, transverse cracking, crack-opening displacement, elastic properties, damage mechanics
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150444 (URN)10.1177/0021998312440126 (DOI)000313902100002 ()2-s2.0-84872914468 (Scopus ID)
Note

Validerad; 2013; 20120323 (andbra)

Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
3. Thermoelastic constants of symmetric laminates with cracks in 90-layer: application of simple models
Open this publication in new window or tab >>Thermoelastic constants of symmetric laminates with cracks in 90-layer: application of simple models
Show others...
2013 (English)In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 4, p. 157-166Article in journal (Refereed) Published
Abstract [en]

The change of thermoelastic properties of cross-ply and quasi-isotropic laminates with intralaminar cracks in layers is analysed. Predictions are performed using previously derived general expressions for stiffness of symmetric damaged laminates as dependent on crack density and crack face opening and sliding. It is shown that the average crack opening displacement can be linked with the average value of axial stress perturbation between two cracks. Using this relationship, analytical shear lag and Hashin’s models, developed for axial modulus, can be applied for calculating thermal expansion coefficients, in-plane moduli and Poisson’s ratios of damaged laminates. The approach is evaluated using finite element method and it is shown that the accuracy is rather similar to that in axial modulus calculation.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150434 (URN)10.1179/1743289811Y.0000000064 (DOI)000318218300005 ()2-s2.0-84876901876 (Scopus ID)
Note

Validerad; 2013; 20130308 (andbra)

Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved
4. Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry
Open this publication in new window or tab >>Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry
2015 (English)In: Experimental techniques (Westport, Conn.), ISSN 0732-8818, E-ISSN 1747-1567, Vol. 39, no 2, p. 38-45Article in journal (Refereed) Published
Abstract [en]

The degradation of the elastic properties of composite laminates with intralaminar cracks is caused by reduced stress in the damaged layer which is mainly due to two parameters: the crack opening displacement (COD) and the crack sliding displacement (CSD). In this paper these parameters are measured experimentally, providing laminate stiffness reduction models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ +704/ −704]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied, and the COD and CSD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (electronic speckle pattern interferometry). By studying the displacement discontinuities, the crack face displacements were measured. A comparison between the COD and the CSD (for the same crack) is performed

Place, publisher, year, edition, pages
John Wiley & Sons, 2015
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150440 (URN)10.1111/ext.12013 (DOI)000351440000006 ()2-s2.0-84924981891 (Scopus ID)
Note

Validerad; 2015; Nivå 2; 20130111 (andbra)

Available from: 2016-09-29 Created: 2018-08-23 Last updated: 2018-08-27Bibliographically approved

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Loukil, Mohamed Sahbi

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