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  • 1. Bergwall, Mats
    et al.
    Hederberg, Hampus
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    Calmunger, Mattias
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    Murtagh, Timothy
    Rinn, Florence
    Loukil, Mohamed Sahbi
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
    Innovative Out-of-Autoclave Composite Manufacturing Method for Saving Cost2019Conference paper (Refereed)
  • 2.
    Loukil, Mohamed Sahbi
    Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå.
    Experimental and numerical studies of intralaminar cracking in high performance composites2013Doctoral 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).

    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
  • 3.
    Loukil, Mohamed Sahbi
    Division of Materials Science, Department of Engineering, Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden.
    Microcracking in fiber composites and degradation of thermo-elastic properties of laminates2011Licentiate 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 thermo-elastic 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 of damaged laminates 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.

  • 4.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, University of Lorraine, EEIGM, Nancy, France.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    ESPI analysis of crack face displacements in damaged laminates2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 94, p. 80-88Article in journal (Refereed)
    Abstract [en]

    Crack opening displacement (COD) and crack sliding displacement (CSD) are the two parameters that define slope of stiffness reduction with increasing crack density. The values of these parameters strongly depend on the crack geometry which is difficult to observe and quantify. In models we usually assume straight crack, no local delaminations (questionable for surface cracks) and use uniform spacing. In this paper COD of cracks in surface layers are analyzed using the displacement field on the edge and on the surface of a [90/0]s and [903/0]s carbon fiber/epoxy laminates subjected to tension. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). It is shown that including in the FEM model crack locations exactly as in the test, the COD of an individual crack is predicted accurately. The model with periodic and symmetric crack distribution significantly underestimates the COD whereas staggered crack distribution model in many cases renders very good results. Crack face sliding displacements, CSD were measured on the edge of [0/704/-704]s GF/EP laminate, showing that the sliding can not be neglected and that the COD versus CSD ratio for this laminate is similar to model predictions.

  • 5.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Farge, Laurent
    Institut Jean Lamour, Nancy Universite, France.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Universite.
    Characterization of damage evolution in thermo-echanical loading of composites materials2009In: Book of abstracts: 5th EEIGM International Conference on Advanced Materials Research, November 4-5, 2009, Nancy, France, 2009Conference paper (Refereed)
    Abstract [en]

    In the experimental part, the displacement field on the edges of a [0, 554, -554]s GF/EP laminate and [302,-602]s GF/EP laminate specimens with multiple transverse cracks is visualized and analyzed on dependence of the applied mechanical load by using ESPI(Electronic Speckle Pattern Interferometry).The different displacement profiles along the tensile-axis are drawn along the specimen edges at several distances from the mid-plane corresponding to the different plies. The measurements out-of-plane deformations are effected to determine and to compare the crack opening displacement (COD) with the crack face sliding displacement (CSD). In the numerical part, the strong dependence of the average normalized (COD) on the relative stiffness and thickness of the surrounding layers, based on FE results, is described by a simple empirical expression for non-interactive cracks. The FEM simulation results obtained by ABAQUS/CAE are compared with the general expressions for calculation of elastic constants of the damaged laminate developed by Lundmark and Varna [1]. The effect of crack interaction on the crack opening displacement (COD) is also discussed in this study. The effect on (COD) is described by the introduced “interaction function” which is determined using FE analysis.

    Download full text (pdf)
    fulltext
  • 6.
    Loukil, Mohamed Sahbi
    et al.
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Hussain, W.
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Kirti, A.
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Pupurs, Andrejs
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Thermoelastic constants of symmetric laminates with cracks in 90-layer: application of simple models2013In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 4, p. 157-166Article in journal (Refereed)
    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.

  • 7.
    Loukil, Mohamed Sahbi
    et al.
    Swerea SICOMP, Piteå, Sweden.
    Varna, Janis
    Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden.
    Characterization of damaged composite laminates using Electronic Speckle Pattern Interferometry (ESPI)2016In: 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 (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.

  • 8.
    Loukil, Mohamed Sahbi
    et al.
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering. RISE SICOMP, Linköping, Sweden.
    Varna, Janis
    Luleå University of Technology, Luleå, Sweden.
    Crack Face Sliding Displacement (CSD) as an Input in Exact GLOB-LOC Expressions for In-plane Elastic Constants of Symmetric Damaged Laminates2019In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, p. 1-23Article in journal (Refereed)
    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.

  • 9.
    Loukil, Mohamed Sahbi
    et al.
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering. RISE SICOMP, Sweden.
    Varna, Janis
    Lulea Univ Technol, Sweden.
    Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, article id UNSP 0021998319874369Article in journal (Refereed)
    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.

  • 10.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Z.
    Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Thermo-elastic properties degradation in damaged laminates with high density of transverse cracks2013Conference paper (Other academic)
    Abstract [en]

    During the service life multidirectional laminate with an arbitrary lay-up is subjected to varying thermo-mechanical loading and in result different number on intralaminar cracks can develop in different layers of the laminate. These cracks called also transverse cracks are reducing the thermo-elastic properties of laminates. A model called GLOB-LOC model was developed in [1,2] to calculate these properties for any given damage state. This analytical calculation scheme is exact if the input information is exact. However, the expressions contain two important approximate parameters (functions) which are obtained summarizing/fitting parametric analyzes results using FEM. These parameters are proportional to the average stress state change between two cracks with respect to the undamaged stress. The parameters govern the stiffness reduction of the laminate and have the physical meaning of average opening displacement (COD) and average sliding displacement (CSD) of a crack. The parameters COD and CSD depend mostly on neighboring layer thickness and stiffness which makes them robust parameters in the model. Unfortunately, the situation changes when the crack density (number of cracks per mm) is high and the distance between cracks is comparable with the crack size (cracked layer thickness). In this crack density range the cracks are interacting and the COD and CSD depend on the distance between them. In this paper the crack interaction effects on laminate stiffness are analyzed using FEM. Based on FEM results a simple interaction function which depends on the cracked and neighboring layer thickness, elastic properties and the distance between cracks is suggested. This interaction function multiplied with COD of a non-interactive crack is the input parameter in analytical model for thermo-elastic properties of damaged symmetric and balanced laminates. It is shown that these functions when used in the GLOB-LOC model give high accuracy for all thermo-elastic constants of damaged laminates. As an example, the predicted axial modulus and Poisson’s ratio of cross-ply laminates with cracks in inside 90-layer are shown for GF/EP composite in Fig.1. FEM data are shown with symbols, predictions according to the interaction function by solid curves and dotted lines represent the ply-discount model. It has to be reminded that the normalized crack density is introduced as inverse to the crack spacing normalized with ply thickness. Therefore, the values of the normalized density larger than 1 are extremely high and are with no practical interest. Data in this region are shown to demonstrate the asymptotic approaching to the ply-discount value.

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  • 11.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, France.
    Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 3, p. 287-301Article in journal (Refereed)
    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.

    Download full text (pdf)
    Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates
  • 12.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Application of the electronic speckle pattern interferometry (espi) technique for the characterization of damage in composite laminates2012Conference paper (Refereed)
    Abstract [en]

    Composite laminates under service loading undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode 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. In other words, the elastic modulus in the loading direction and the corresponding Poisson’s ratio decrease. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks [1]. The only correct way to validate these assumptions is through experiments. The main objective of this 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 edges of a [0/ 704/ -704]sand [0/554/ -554]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied and the (COD) dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry) [2,3]. 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. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the opening displacement along the cracks. The effect of crack interaction on the (COD) at high crack density is also investigated.

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    Application of the electronic speckle pattern interferometry (espi) technique for the characterization of damage in composite laminates
  • 13.
    Loukil, Mohamed Sahbi
    et al.
    Institut Jean Lamour, (SI2M), Nancy, France / Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, (SI2M), Nancy, France.
    Characterization of damaged composite laminates by an optical measurement of the displacement field2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: Institute of Physics (IOP), 2012, Vol. 31, p. 1-7Conference 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

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    Characterization of damaged composite laminates by an optical measurement of the displacement field
  • 14.
    Loukil, Mohamed Sahbi
    et al.
    Department of Engineering Sciences and Mathematics, Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Department of Engineering Sciences and Mathematics, Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Science et Ingénierie des Matériaux et Métallurgie (SI2M), Institut Jean Lamour, Nancy.
    Damage characterization in glass fiber/epoxy laminates using electronic speckle pattern interferometry2015In: Experimental techniques (Westport, Conn.), ISSN 0732-8818, E-ISSN 1747-1567, Vol. 39, no 2, p. 38-45Article in journal (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

  • 15.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM, Nancy Cedex, France.
    Degradation of thermo-elastic properties of non-uniformly composite laminates2012Conference paper (Other academic)
    Abstract [en]

    During the service life composite laminates undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode is intralaminar cracking in layers. These cracks are reducing the thermo-elastic properties of laminates. A model called GLOB-LOC model was developed in to calculate these properties for any given damage state. This analytical calculation scheme is exact if the input information is exact. However, the expressions contain two important approximate parameters which are obtained summarizing/fitting parametric analyzes results using FEM. These parameters are proportional to the average stress state between cracks which governsthe stiffness reduction of the laminate and have the physical meaning of average opening displacement (COD) and average sliding displacement (CSD) of a crack. Using this relationship analytical shear lag and Hashin’s models, developed for axial modulus, can be applied for calculating both thermal expansion coefficients, in-plane moduli and Poisson’s ratios of damaged laminates. The approach is evaluated using FEM and it shown that the accuracy is rather similar as in axial modulus calculation. Most of the existing models are based on assumption that the crack distribution in the layer is uniform: the same distance between all cracks. Since optical observations show that it is not true, we present FEM simulation results to evaluate the effect of non-uniform crack distribution on elastic properties of RVE showing that the differences may be rather large. 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

  • 16.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks2013In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 48, no 1, p. 37-46Article in journal (Refereed)
    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.

  • 17.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Finite element analysis and experimental comparison of the crack opening displacement profile in the damaged cross-ply laminates2013In: Book of Abstracts: 7th EEIGM International Conference on Advanced Materials Research, 21–22 March 2013 in Luleå, Sweden, 2013Conference paper (Refereed)
    Abstract [en]

    During the service life composite laminates 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. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks. To validate these assumptions experimentally the displacement field on the surface of a [90/0/90], [90/0]s and [903/0]s carbon fiber/epoxy laminates specimens with multiple intralaminar cracks in the surface layer is studied. The profile of the COD along the width is investigated for each single crack. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). The effect of crack interaction as well as the effect of the thickness of 90° layers on the COD is given in this paper.

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    Finite element analysis and experimental comparison of the crack opening displacement profile in the damaged cross-ply laminates
  • 18.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, Nancy-Université, France.
    Parameter determination in damaged laminate model by optical full-field measurement of the displacement using ESPI2011Conference paper (Other academic)
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    The presentation at the conference
  • 19.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, Nancy-Université, France.
    Thermo-elastic properties of damaged laminates at high density of cracks in layers2011In: Aerospace - the global innovation driver: 6th International Conference "Supply on the Wings" ; November 02 - 04, 2011, Frankfurt, Germany ; conference proceedings ; in conjunction with AIRTEC, International Aerospace Supply Fair, Frankfurt: airtec GmbH , 2011Conference paper (Refereed)
  • 20.
    Loukil, Mohamed Sahbi
    et al.
    Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering. Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM, Nancy Cedex, France.
    Thermo-elastic properties of non-uniformly damaged laminates at high crack density2012Conference paper (Other academic)
    Download full text (pdf)
    Thermo-elastic properties of non-uniformly damaged laminates at high crack density
  • 21.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Zoubir
    Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Validation of FEM based damaged laminate model measuring crack opening displacement in cross-ply laminate using Electronic Speckle Pattern Interferometry (ESPI)2013Conference paper (Refereed)
    Abstract [en]

    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.

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    Validation of FEM based damaged laminate model measuring crack opening displacement in cross-ply laminate using Electronic Speckle Pattern Interferometry (ESPI)
  • 22.
    Loukil, Mohamed Sahbi
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Varna, Janis
    Division of Materials Science, Luleå University of Technology, Sweden.
    Ayadi, Zoubrir
    Institut Jean Lamour, SI2M, Université de Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France.
    Caractérisation de l’endommagement des matériaux composites pour l’aéronautique à partir des mesures plein champ des déplacements et modélisation: [Measurement of Crack Opening Displacement in Damaged Composite Aerospace Laminates Using ESPI]2012Conference paper (Refereed)
    Abstract [fr]

    La rupture finale d'un composite est le résultat de l'accumulation de divers mécanismes élémentaires coexistants qui dépendent notamment de la nature des matériaux constitutifs et des sollicitations mécaniques imposées. Ces mécanismes s'accompagnent généralement du phénomène de dé-cohésion de l'interface fibre-matrice, de la rupture des fibres, de la rupture transverse ou longitudinale de la matrice. Le mécanisme d’endommagement le plus facilement observable lors d’un essai de traction est la micro-fissuration des plis. Ces fissures sont parallèles à la direction des fibres et s’étendent sur toute l’épaisseur du pli. L’apparition et la croissance du nombre de ces fissures engendrent une réduction progressive de la rigidité globale du composite. L’approche GLOB-LOC est l’une des approches quipermet d’obtenir l’ensemble des constantes thermo-élastiques du composite à partir de deux paramètres: l’ouverture et le glissement moyens des lèvres des fissures, normalisés par rapport à la contrainte appliquée. Dans cette approche, ces deux paramètres sont évalués à partir d’expressions analytiques simples obtenues enanalysant les résultats issus de nombreuses modélisations par des éléments finis[1,2]. Naturellement, ce type de modélisation n’est applicable qu’à des fissures «idéales»: fissures rectilignes, isolées, perpendiculaires au plan moyen avec une pointe de fissure située à l’interface entre les plis. En pratique, les fissures réelles peuvent différer sensiblement de ce modèle idéal, notamment du fait des modes d’endommagement supplémentaires engendrés par les fissures elles-mêmes. Lorsque le composite est sollicité mécaniquement, les concentrations de contraintes en pointe de fissures peuvent favoriser la création d’une zone où le pli fissuré et le pli adjacent sont décollés (phénomène de délamination). On pourra également observer des fissures secondaires dans la matrice du pli adjacent. Il est évident que l’apparition de ces nouvelles formes d’endommagement modifiera la dépendance de l’ouverture et du glissement des lèvres des fissures avec le chargement appliqué. Il est donc nécessaire de trouver un moyen de mesure permettant l’estimation expérimentale des valeurs de l’ouverture moyenne et du glissement moyen des lèvres des fissures. En principe, ces grandeurs ne peuvent pas être déterminées à partir d’une mesure ponctuelle, la mesure du profil du déplacement le long des fissures est en effet nécessaire. Cette exigence conduit naturellement l’expérimentateur à envisager l’emploi d’une méthode optique «plein champ» pour les mesures des déplacements. L’objectif principalde ce papierest de caractériser l’endommagement des matériaux composites (vert/époxy) utilisés dans le domaine aéronautique. En utilisant l’interférométrie de speckle (ESPI), des mesures de champs de déplacements aux bords des échantillons et dans différentes couches du stratifié [3] ainsique des études par élément finis ont été effectuées dans le but de calculer l’ouverture et le glissement des lèvres des fissures. L’effet des propriétés élastiques des matériaux sur l’endommagement aussi bien quel’effet d’interaction entre les fissures ont été déterminés. Une discussion essais/calculs est enfin réalisée afin de juger la validité des hypothèses retenues.

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    Article in French
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    Article in English
  • 23.
    Pupurs, Andrejs
    et al.
    Composites Centre Sweden, Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Luleå tekniska universitet, Materialvetenskap.
    Loukil, Mohamed Sahbi
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Kahla, Hiba Ben
    Composites Centre Sweden, Luleå University of Technology, Luleå, Sweden.
    Mattsson, D.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers2016In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 82, p. 244-252Article in journal (Refereed)
    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.

  • 24.
    Pupurs, Andrejs
    et al.
    Swerea SICOMP AB / Luleå University of Technology, Luleå, Sweden.
    Varna, Janis
    Luleå University of Technology, Luleå, Sweden.
    Loukil, Mohamed Sahbi
    RISE - Research Institutes of Sweden, Swerea SICOMP AB.
    Mattsson, David
    RISE - Research Institutes of Sweden, Swerea SICOMP AB.
    Damage development and stiffness reduction in laminates in out-of-plane loading2014In: 16th European Conference on Composite Materials, ECCM 2014, European Conference on Composite Materials, ECCM , 2014Conference paper (Refereed)
    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. The effective stiffness of layer with cracks as a function of crack density is back-calculated comparing in-plane stiffness of laminates with and without damage. The accuracy of the CLT and effective stiffness approach is demonstrated comparing with bending stiffness results from FEM simulated 4-point bending test on laminate with damage. Analytical model for damaged laminate stiffness is presented which gives similar values for effective stiffness as FEM calculations for unit cell. Effect of local delaminations initiated from transverse cracks is analyzed.

  • 25.
    Strömberg, Karl-Otto
    et al.
    Flexprop AB, Sweden.
    Borgenvall, Stefan
    SAAB Aeronautics, Sweden.
    Loukil, Mohamed Sahbi
    Swerea SICOMP AB.
    Noharet, Bertrand
    Swedish ICT ACREO, Sweden.
    Sterner, Carola
    Swedish ICT ACREO, Sweden.
    Lindblom, Magnus
    Swedish ICT ACREO, Sweden.
    Festin, Örjan
    RISE - Research Institutes of Sweden, Swerea IVF AB.
    Next Generation Manufacturing Fixtures: CFRP Structures Using "in Situ" Health Monitoring2015In: SAE Technical Papers, ISSN 0148-7191, article id 2015-01-2619Article in journal (Refereed)
    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.

  • 26.
    Varna, Janis
    et al.
    Luleå tekniska universitet, Materialvetenskap.
    Joffe, Roberts
    Luleå tekniska universitet, Materialvetenskap.
    Loukil, Mohamed Sahbi
    Luleå tekniska universitet, Materialvetenskap.
    Microdamage initiation and growth in laminated composites2009Conference paper (Refereed)
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    fulltext
  • 27.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Loukil, Mohamed Sahbi
    Swerea SICOMP AB.
    Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 14, p. 1945-1959Article in journal (Refereed)
    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.

  • 28.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Loukil, Mohamed Sahbi
    Swerea SICOMP, Sweden.
    Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 14, p. 1945-1959Article in journal (Refereed)
    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

  • 29.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Sweden.
    Loukil, Mohamed Sahbi
    Swerea SICOMP, Sweden.
    Stiffness prediction of damaged laminates based on the concept of laminate effective stiffness2016In: 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 (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.

  • 30.
    Varna, Janis
    et al.
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Loukil, Mohamed Sahbi
    Division of Materials Science, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Z.
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy Cedex, France.
    Degradation of elastic properties of non-uniformly damaged composite laminates2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, Institute of Physics Publishing (IOPP), 2013, Vol. 48, article id 12009Conference paper (Refereed)
    Abstract [en]

    Stiffness reduction model for laminates with non-uniformly distributed intralaminar cracks is presented and used to analyze the effect of non-uniformity and the accuracy of predictions based on uniform spacing. The values of the normalized crack opening displacement (COD) as affected by the presence of other cracks are used to calculate the axial modulus of cross-ply laminates with cracks. The COD is calculated using finite element method (FEM), considering two closest neighbors of the crack and using the smallest versus the average crack spacing ratio as non-uniformity parameter. Assuming uniform spacing the axial modulus reduction is overestimated. A "double-periodic" approach is presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems.

  • 31.
    Zrida, Hana
    et al.
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France / Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden .
    Loukil, Mohamed Sahbi
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, France / Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden .
    Varna, Janis
    Division of Polymer Engineering, Luleå University of Technology, Luleå, Sweden.
    Ayadi, Z.
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Crack opening displacement determination in damaged cross-ply laminate using electronic speckle pattern interferometry (ESPI)2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: Institute of Physics (IOP), 2012, Vol. 31, article id 012002Conference paper (Refereed)
    Abstract [en]

    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. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks. To validate these assumptions experimentally the displacement field on the surface of a [90/0/90] carbon fiber/epoxy laminate specimens with multiple intralaminar cracks in the surface layer is studied and the COD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). The displacement jumps corresponding to cracks are clearly visible and can be used to determine the opening displacement along the cracks. The effect of crack interaction on the COD at high crack density is also investigated.

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