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Loukil, Mohamed Sahbi
Alternative names
Publications (10 of 31) Show all publications
Loukil, M. S. & Varna, J. (2019). Crack Face Sliding Displacement (CSD) as an Input in Exact GLOB-LOC Expressions for In-plane Elastic Constants of Symmetric Damaged Laminates. International journal of damage mechanics, 1-23
Open this publication in new window or tab >>Crack Face Sliding Displacement (CSD) as an Input in Exact GLOB-LOC Expressions for In-plane Elastic Constants of Symmetric Damaged Laminates
2019 (English)In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, p. 1-23Article in journal (Refereed) Epub ahead of print
Abstract [en]

The crack opening and crack sliding displacements of both faces of an intralaminar crack are the main parameters defining the significance of each crack in laminate stiffness degradation, according to the previously published GLOB-LOC approach for symmetric laminates with an arbitrary number of cracks in all plies. In the exact stiffness expressions of this approach, the crack density is always multiplied by crack opening displacement and crack sliding displacement. The dependence of crack opening displacement on geometrical and elastic parameters of adjacent plies was studied previously and described by simple fitting functions. The crack sliding displacement has been analyzed for low-crack densities only and the proposed finite element method-based fitting expressions are oversimplified not including the out-of-plane ply stiffness effects. Based on finite element method analysis, more accurate expressions for so-called non-interactive cracks are suggested in the presented article. For the first time the shear stress perturbations are analyzed and interaction functions are presented with the feature that they always lead to slightly conservative predictions. The presented simple fitting functions, when used in the GLOB-LOC model, give predictions that are in a good agreement with finite element method results and with experimental data for laminates with damaged off-axis plies in cases when crack face sliding is of importance. The significance of including crack sliding displacement in stiffness predictions is demonstrated.

Place, publisher, year, edition, pages
Sage Publications, 2019
Keywords
Composite laminates, crack sliding displacement, damage, finite element analysis, transverse cracking
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-161739 (URN)10.1177/1056789519866000 (DOI)000479429300001 ()2-s2.0-85070320457 (Scopus ID)
Available from: 2019-11-08 Created: 2019-11-08 Last updated: 2019-11-13Bibliographically approved
Loukil, M. S. & Varna, J. (2019). Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation. Journal of composite materials, Article ID UNSP 0021998319874369.
Open this publication in new window or tab >>Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation
2019 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, article id UNSP 0021998319874369Article in journal (Refereed) Epub ahead of print
Abstract [en]

The concept of the "effective stiffness" for plies in laminates containing intralaminar cracks is revisited presenting rather accurate fitting expressions for the effective stiffness dependence on crack density in the ply. In this article, the effective stiffness at certain crack density is back-calculated from the stiffness difference between the undamaged and damaged laminate. Earlier finite element method analysis of laminates with cracked 90-plies showed that the effective longitudinal modulus and Poissons ratio of the ply do not change during cracking, whereas the transverse modulus reduction can be described by a simple crack density dependent function. In this article, focus is on the remaining effective constant: in-plane shear modulus. Finite element method parametric analysis shows that the dependence on crack density is exponential and the fitting function is almost independent of geometrical and elastic parameters of the surrounding plies. The above independence justifies using the effective ply stiffness in expressions of the classical laminate theory to predict the intralaminar cracking caused stiffness reduction in laminates with off-axis plies. Results are in a very good agreement with (a) finite element method calculations; (b) experimental data, and (c) with the GLOB-LOC model, which gives a very accurate solution in cases where the crack face opening and sliding displacements are accurately described.

Place, publisher, year, edition, pages
SAGE PUBLICATIONS LTD, 2019
Keywords
Effective shear modulus; effective stiffness; stiffness prediction; transverse cracking
National Category
Applied Mechanics
Identifiers
urn:nbn:se:liu:diva-161862 (URN)10.1177/0021998319874369 (DOI)000491802900001 ()
Available from: 2019-11-11 Created: 2019-11-11 Last updated: 2019-11-11
Bergwall, M., Hederberg, H., Calmunger, M., Murtagh, T., Rinn, F. & Loukil, M. S. (2019). Innovative Out-of-Autoclave Composite Manufacturing Method for Saving Cost. In: : . Paper presented at EUROMAT19, Stockholm, 2-5 September 2019.
Open this publication in new window or tab >>Innovative Out-of-Autoclave Composite Manufacturing Method for Saving Cost
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2019 (English)Conference paper, Oral presentation only (Refereed)
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-162818 (URN)
Conference
EUROMAT19, Stockholm, 2-5 September 2019
Available from: 2019-12-19 Created: 2019-12-19 Last updated: 2020-02-06
Varna, J. & Loukil, M. S. (2017). Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation. Journal of composite materials, 51(14), 1945-1959
Open this publication in new window or tab >>Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation
2017 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 14, p. 1945-1959Article in journal (Refereed) Published
Abstract [en]

The old concept of the effective stiffness of a 90-layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates. It is shown, focusing on the effective transverse modulus, that the expected dependence of this property on composite elastic properties and laminate lay-up is very weak and follows very simple rules. Calculations show that the effective longitudinal modulus and Poisson’s ratio of the layer are not affected at all by intralaminar cracking. Simple fitting curve for effective transverse modulus change with normalized crack density is obtained from analysis of GF/EP cross-ply laminate. It is shown, comparing with FEM results and experimental data, that this expression can be used as a ‘master curve’ in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different unidirectional composites.

Place, publisher, year, edition, pages
Sage Publications, 2017
Keywords
Cracks; Curve fitting; Elasticity; Finite element method; Laminates; Stiffness, Composite elastic properties; Crack opening displacements; Intralaminar crackings; Longitudinal modulus; Parametric -analysis; Transverse cracking; Transverse modulus; Unidirectional composites, Laminated composites
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-150456 (URN)10.1177/0021998316658965 (DOI)2-s2.0-85020108151 (Scopus ID)
Available from: 2017-07-04 Created: 2018-08-22 Last updated: 2018-08-23Bibliographically approved
Varna, J. & Loukil, M. S. (2017). Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation. Journal of composite materials, 51(14), 1945-1959
Open this publication in new window or tab >>Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation
2017 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 14, p. 1945-1959Article in journal (Refereed) Published
Abstract [en]

The old concept of the effective stiffness of a 90-layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates. It is shown, focusing on the effective transverse modulus, that the expected dependence of this property on composite elastic properties and laminate lay-up is very weak and follows very simple rules. Calculations show that the effective longitudinal modulus and Poisson’s ratio of the layer are not affected at all by intralaminar cracking. Simple fitting curve for effective transverse modulus change with normalized crack density is obtained from analysis of GF/EP cross-ply laminate. It is shown, comparing with FEM results and experimental data, that this expression can be used as a ‘master curve’ in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different unidirectional composites

Place, publisher, year, edition, pages
Sage Publications, 2017
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150457 (URN)10.1177/0021998316658965 (DOI)000402456700001 ()2-s2.0-85020108151 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-06-08 (andbra)

Available from: 2017-06-08 Created: 2018-08-22 Last updated: 2018-08-23Bibliographically approved
Loukil, M. S. & Varna, J. (2016). Characterization of damaged composite laminates using Electronic Speckle Pattern Interferometry (ESPI). In: Proceedings of the 17th European Conference on Composite Materials ECCM17 - 17th European Conference on Composite Materials, 26-30th June 2016, Munich, Germany: . Paper presented at 17th European Conference on Composite Materials, Munich, Germany, 26-30th June 2016. European Conference on Composite Materials
Open this publication in new window or tab >>Characterization of damaged composite laminates using Electronic Speckle Pattern Interferometry (ESPI)
2016 (English)In: Proceedings of the 17th European Conference on Composite Materials ECCM17 - 17th European Conference on Composite Materials, 26-30th June 2016, Munich, Germany, European Conference on Composite Materials , 2016Conference paper, Published paper (Refereed)
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
European Conference on Composite Materials, 2016
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150439 (URN)2-s2.0-85017686439 (Scopus ID)9783000533877 (ISBN)
Conference
17th European Conference on Composite Materials, Munich, Germany, 26-30th June 2016
Note

Publicerad CD-Rom

Available from: 2017-05-05 Created: 2018-08-23 Last updated: 2018-08-23Bibliographically approved
Pupurs, A., Varna, J., Loukil, M. S., Kahla, H. B. & Mattsson, D. (2016). Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers. Composites. Part A, Applied science and manufacturing, 82, 244-252
Open this publication in new window or tab >>Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers
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2016 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 82, p. 244-252Article in journal (Refereed) Published
Abstract [en]

Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of damaged layer is suggested for bending stiffness determination of laminate with intralaminar cracks in surface 90-layers and delaminations initiated from intralaminar cracks. The effective stiffness of a layer with damage is back-calculated comparing the in-plane stiffness of a symmetric reference cross-ply laminate with and without damage. The in-plane stiffness of the damaged reference cross-ply laminate was calculated in two ways: 1) using FEM model of representative volume element (RVE) and 2) using the analytical GLOB-LOC model. The obtained effective stiffness of a layer at varying crack density and delamination length was used to calculate the A, B and D matrices in the unsymmetrically damaged laminate. The applicability of the effective stiffness in CLT to solve bending problems was validated analyzing bending of the damaged laminate in 4-point bending test which was also simulated by 3-D FEM.

Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150453 (URN)10.1016/j.compositesa.2015.11.012 (DOI)000370997000026 ()2-s2.0-84956666458 (Scopus ID)
Note

Validerad; 2016; Nivå 2; 20151117 (andbra)

Available from: 2016-09-29 Created: 2018-08-23Bibliographically approved
Varna, J. & Loukil, M. S. (2016). Stiffness prediction of damaged laminates based on the concept of laminate effective stiffness. In: Proceedings of the 17th European Conference on Composite Materials ECCM17: 17th European Conference on Composite Materials, 26-30th June 2016, Munich, Germany. Paper presented at 17th European Conference on Composite Materials, Munich, Germany, 26-30th June 2016. European Conference on Composite Materials
Open this publication in new window or tab >>Stiffness prediction of damaged laminates based on the concept of laminate effective stiffness
2016 (English)In: Proceedings of the 17th European Conference on Composite Materials ECCM17: 17th European Conference on Composite Materials, 26-30th June 2016, Munich, Germany, European Conference on Composite Materials , 2016Conference paper, Published paper (Refereed)
Abstract [en]

IThe concept of the effective stiffness of a unidirectional layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates with damaged 90-layer. The effective stiffness of the damaged layer is obtained from the difference between damaged and undamaged laminate stiffness. The effective longitudinal modulus and Poisson's ratio of the layer are equal to their initial values. A very simple expression for the effective transverse modulus change with normalized crack density has sufficient accuracy and generality to be used in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different UD composites.

Place, publisher, year, edition, pages
European Conference on Composite Materials, 2016
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:liu:diva-150458 (URN)2-s2.0-85018536461 (Scopus ID)9783000533877 (ISBN)
Conference
17th European Conference on Composite Materials, Munich, Germany, 26-30th June 2016
Available from: 2017-05-05 Created: 2018-08-22 Last updated: 2018-08-23Bibliographically approved
Loukil, M. S., Varna, J. & Ayadi, Z. (2015). Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry. Experimental techniques (Westport, Conn.), 39(2), 38-45
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
Strömberg, K.-O., Borgenvall, S., Loukil, M. S., Noharet, B., Sterner, C., Lindblom, M. & Festin, Ö. (2015). Next Generation Manufacturing Fixtures: CFRP Structures Using "in Situ" Health Monitoring. Paper presented at SAE AeroTech Congress and Exhibition, AEROTECH 2015; Washington State Convention and Trade CenterSeattle; United States; 22 September 2015 through 24 September 2015; Code 115952. SAE Technical Papers, Article ID 2015-01-2619.
Open this publication in new window or tab >>Next Generation Manufacturing Fixtures: CFRP Structures Using "in Situ" Health Monitoring
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2015 (English)In: SAE Technical Papers, ISSN 0148-7191, article id 2015-01-2619Article in journal (Refereed) Published
Abstract [en]

Lightweight Production Technology (LWPT) is today a well-established technology in the automotive industry. By introducing light weight fixtures manufactured from Carbon Fiber Reinforced Plastics (CFRP) in aeronautical applications, new challenges as well as possibilities of in-situ health monitoring emerges. The present paper present results from experimental investigations using optical fibers with multiplex Bragg gratings (FBG) as strain gauges in an industrial CFRP fixture. Fixtures were manufactured of laminates made from CFRP. Measurements have been performed on a single CFRP beam with dimensions (8000 × 500 × 500 mm), used as a structural part in a larger assembly (9000 × 4000mm). The optical fibers were placed in between two laminates on two sides of the beam. The measurement data from the FBGs were compared and correlated to the measured displacements of the beam and the applied loads. The results show that for an eight meter long CFRP beam, it is possible to measure and detect displacements of 0.12 mm (load = 36kg), and resolve a displacement of 0.02 mm (Δload = 4.75kg). The experimental results were also compared to FEM calculations on the displacement of the beams when subject to the same load. Here measurements and calculations showed very good agreement. Overall the results imply that FBGs show a high potential as a suitable in-situ health monitoring of lightweight fixtures for the aeronautic industry as well as other applications. Copyright © 2015 SAE International.

Place, publisher, year, edition, pages
SAE International, 2015
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-150454 (URN)10.4271/2015-01-2619 (DOI)2-s2.0-84959440985 (Scopus ID)
Conference
SAE AeroTech Congress and Exhibition, AEROTECH 2015; Washington State Convention and Trade CenterSeattle; United States; 22 September 2015 through 24 September 2015; Code 115952
Available from: 2017-03-30 Created: 2018-08-23Bibliographically approved
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