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  • 51.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Varna, Janis
    COD-based simulation of transverse cracking and stiffness reduction in [S/90n]s laminates2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 5, p. 637-656Article in journal (Refereed)
    Abstract [en]

    Closed-form expressions for the thermo-elastic properties of [S/90n]s laminates with transverse cracks in the 90° layers are derived. Provided the normalised average crack-opening displacement (COD) is known, expressions contain only crack density, geometrical parameters and elastic constants of layers. The average COD dependence on the crack spacing and constraint effect of adjacent sub-laminates is analysed by using finite-element method in plane stress formulation. It is found that the out-of-plane elastic constants have an insignificant effect on COD. A simple power law relating average COD to elastic and geometrical parameters of constituents is derived. The obtained power law and the developed methodology are successfully used to predict the reduction of thermo-elastic properties and damage evolution of [±θ/904]s laminates. The crack-closure technique and Monte-Carlo simulations are used to model the damage development. The 90° layer is divided in to a large number of elements and Gc values are assigned to each element according to a Weibull distribution. Parameters in the Weibull distribution are determined by using experimental crack density versus strain curve for glass-fibre/epoxy [02/904]s cross-ply laminates. Damage development in [S/904]s laminates of the same material, containing sub-laminates with ±θ layers only, is modelled by using these Weibull parameters and the results are in good agreement with test data. The effect of the thickness of the 90° layer on damage development is discussed in strength and fracture mechanics formulation.

  • 52. Joffe, Roberts
    et al.
    Mattsson, David
    Modniks, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Compressive failure analysis of non-crimp fabric composites with large out-of-plane misalignment of fiber bundles2005In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, no 8, p. 1030-1046Article in journal (Refereed)
    Abstract [en]

    Failure initiation under compressive loading in non-crimpfabric composites containing bundles with out-of-plane orientation imperfections was analyzed using FEM in plane stress and linear elastic formulation. The bundle orientation imperfection in a composite unit was described by a sine function. Failure initiation strain was determined comparing failure functions corresponding to two alternative failure mechanisms: (a) plastic microbuckling in bundle due to mixed compressive and shear load; (b) plastic matrix yielding according to von Mises criterion. Parameters for compressive failure initiation analysis were bundle misalignment angle, fiber volume fraction inside the bundle and bundle volume fraction inside the composite unit. The support effect of the neighbouring material was analyzed varying boundary conditions and solving cases with particular configuration of surrounding material. Model prepreg tape GF/EP composite with different introduced levels of out-of-plane waviness of layers was used to validate the conclusions from parametric analysis.

  • 53.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, David
    Varna, Janis
    Damage tolerance of cross-ply non-crimp fabric composites2005In: The fifth Canadian international composites conference, CACSMA , 2005Conference paper (Other academic)
  • 54.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nystöm, Birgitha
    Swerea SICOMP AB.
    Rozite, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical performance of polymer composites reinforced with nonlinear cellulosic fibers2011Conference paper (Refereed)
  • 55.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pupure, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berthold, Fredrik
    Innventia AB.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micro-structure and Mechanical Properties in PLA Reinforced with Cellulosic Fiber Sheets Made by Wet Forming Method2017In: 8th International Conference on Composites Testing and Model Identification: CompTest2017 / [ed] S.V . Lomov, L. Gorbatikh, Y. Swolfs, 2017Conference paper (Refereed)
    Abstract [en]

    The current paper presents results of study of composites produced by compression moulding of sheets of comingled PLA and reinforcing fibers. The dynamic sheet former is employed to produce fiber mats (with the PLA in the form of fibers) which are consolidated into composite plates by using hot press. This processing route ensures that initial length of the fibers is preserved during the manufacturing and preferential fiber orientation is achieved. However, the internal structure of the composites in question is very complex and somewhat unpredictable, which complicates design of these materials. The main objectives of this paper are application and validation of micro-mechanical models on composites produced within this study as well as direct (experimental) and indirect (back-calculation) identification of input parameters to be used in the modelling. The main input parameters considered in this study are fiber orientation and porosity. The estimation of these parameters is done through micro–computed tomography but also by using micro-mechanical expressions in combination with experimental results (e.g. composite density, stiffness). The input parameters identified by different approaches are compared, then these parameters are used in the micro-mechanical models to predict stiffness of composites with different types of fibers and various fiber contents.

  • 56.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Analytical modeling of elastic properties reduction in symmetric and balanced lamintes due to cracks in 90 layers1998In: Book of abstracts: Tenth International Conference Mechanics of Composite Materials : April 20 - 23, 1998, Riga, Latvia / [ed] V. Tamuzs, 1998Conference paper (Refereed)
  • 57.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Analytical modeling of stiffness reduction in symmetric and balanced laminates due to cracks in 90° layers1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 11, p. 1641-1652Article in journal (Refereed)
    Abstract [en]

    Stiffness reduction in [S,90n]s symmetric laminates, containing orthotropic sub-laminates (S) and cracked 90° layer, is analyzed. Closed-form expressions relating stiffness changes to the transverse crack density are derived. They contain only material properties, laminate geometry and a stress-perturbation function that is proportional to the normalized average crack-opening displacement. Stress-distribution models [shear lag, based on variational approach, and finite-element analysis (FEA)] are adopted for the [S,90n]s configurations and used to calculate the stress-perturbation function. Predictions are compared with experimental data for [θ, 904]s θ=0, 15, 30, 40 glass-fiber/epoxy-resin laminates. Generally the FEA model slightly underestimates stiffness reduction whereas both of the variational models used lead to similar results, slightly lower than experimental. Even the shear-lag model may be successfully used if the shear-lag parameter is first obtained from fitting test results for cross-ply laminate of the same material.

  • 58.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Compressive strength of unidirectional polymer composites obtained from test of multidirectional laminates2001In: 2001 Mechanics and Materials Summer Conference: June 27-29, 2001 Sheraton San Diego Hotel and Marina ; technical program and book of abstracts ; MMC2001, 2001Conference paper (Refereed)
  • 59.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Damage evolution modeling in multidiractional laminates and the resulting nonlinear response1999In: ICCM-12 Europe: 12th International Conference on Composite Materials ; Palais des Congrès, Paris [July 5th - 9th] 1999 / [ed] Thierry Massard, Tours: ICCM , 1999Conference paper (Refereed)
  • 60.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Effect of bundle waviness on compressive strength and notch sensitivity of non-crimp fabric composites2002Conference paper (Refereed)
    Abstract [en]

    Non-Crimp Fabrics (NCF) and vinylester resins seem to give lower compression strength data than similar prepreg-based composites. However, the lower data turned out to be not only material issue, but a problem associated with testing methodology, namely design of test fixtures and specimen configuration. Specimen geometry optimization study led to research concerning notch sensitivity of NCF. Investigations were performed testing specimens with circular center holes and symmetric sharp edge notches. In all cases reduction of compressive strength, corresponding to the net cross-section, due to introduced notches was not observed. Although NCF composites showed rather low compressive strength if compared with similar prepreg materials, they also exhibit almost complete notch insensitivity. This can be explained by irregular and uneven internal structure of NCF composite. Due to complex internal geometry NCF composite fails close to an internal defect, prior to the failure in vicinity of the notch, where stress concentration takes place.

  • 61.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Failure analysis of fiber bundles with waviness in composites under compression2004In: MACM 2004 conference proceedings: Proceedings of the Tenth International Conference on Marine Applications of Composite Materials, Composites education association , 2004Conference paper (Refereed)
  • 62.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Fiber/matrix interface fracture toughness identification in single fiber fragmentation test1997In: Material identification using mixed numerical experimental methods: proceedings of the EUROMECH Colloquium held in Kerkrade, The Netherlands, 7-9 April 1997 / [ed] Hugo Sol, Encyclopedia of Global Archaeology/Springer Verlag, 1997Conference paper (Refereed)
  • 63.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Inelastic response of laminated due to damage accumulation2000In: Composites from fundamentals to exploitation: [the premier composites conference in Europe ; proceedings] / ECCM9, 4 - 7 June 2000, Brighton Conference Centre, UK, London: Institute of Materials , 2000Conference paper (Refereed)
  • 64.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Manufacturing and characterization of polypropylene reinforced with carbon nano-tubes2004In: From nano-scale interactions to engineering structures: ECCM 11, 11th European Conference on Composite Materials ; May 31 - June 3, 2004, Rhodes, Greece / [ed] Costas Galiotis, 2004Conference paper (Other academic)
  • 65.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Mechanics of non-crimp fabric reinforced composites in ship structures2002In: MACM 2002 conference proceedings: proceedings of the Ninth International Conference on Marine Applications of Composite Materials ; March 19 - 21, 2002, Radisson Oceanfront Resort, Melbourne, Florida, USA, Composites education association , 2002Conference paper (Refereed)
  • 66.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Berglund, Lars
    Luleå tekniska universitet.
    Acoustic emission from composite laminates with simple damage characteristics1995In: Fifth International Symposium on Acoustic Emission from Composite Materials, July 10-14, 1995, Sundsvall's KonferensCenter, Hotel Sundsvall, Sundsvall Sweden, Columbus, Ohio: American Society for Nondestructive Testing , 1995, p. 179-186Conference paper (Refereed)
    Abstract [en]

    Interpretation of acoustic emission (AE) signals in terms of damage mechanisms in composite materials and structures is often difficult. This is because of the multitude of damage mechanisms. In the present study, glass fiber/epoxy (GF/EP) (0m/90n)s cross-ply laminates were studied in unixial tensile loading. Since the damage mechanisms in such laminates are primarily matrix cracking, acoustic emission interpretation was facilitated. Real-time optical microscopy and AE studies were conducted in order to identify details of the damage development. It was found that a multitude of secondary cracking events occurred at higher strains. These events may explain why stress analysis models based on straight and regularly spaced transverse cracks are not successful. In addition, it was clearly demonstrated that in order to interpret damage development with some confidence from AE-data, direct observations of damage events must also be performed.

  • 67.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Berglund, Lars
    Luleå tekniska universitet.
    Analysis of single fiber fragmentation data1995In: Deformation and fracture of composites : 3rd International conference: Papers and programme, London: Institute of metals , 1995, p. 126-131Conference paper (Refereed)
  • 68. Joffe, Roberts
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Interfacial debonding in model composites1996Conference paper (Other academic)
  • 69.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Berglund, Lars
    Luleå tekniska universitet.
    Interfacial fracture toughness characterization in single fibre fragmentation test1995In: Ninth International Conference Mechanics of Composite Materials: book of abstracts ; Riga, Latvia, October 17-20, 1995, 1995Conference paper (Refereed)
  • 70.
    Kahla, Hiba Ben
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Institut Jean Lamour, SI2M, Université de Lorraine.
    Ayadi, Z.
    Edgren, F.
    GKN Aerospace.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Statistical model for initiation governed intralaminar cracking in composite laminates during tensile quasi-static and cyclic tests2018In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 116, p. 1-12Article in journal (Refereed)
    Abstract [en]

    A simple model for predicting intralaminar cracking in laminates under cyclic loads is proposed and validated. The model is limited to low stresses and low crack density and is based on the assumption that the non-uniformity of the fiber distribution is the main reason for the observed large variation of cracking resistance along the transverse direction of the layer. Hence, the resistance variation in quasi-static and in cyclic loading can be described by the same parameter. At low crack density the failure resistance variation is more significant than the variation of the stress state in the specimen, the latter becoming dominant at high crack density. At low crack density the Weibull distribution for probability of intralaminar cracking is used for crack density growth simulation during cyclic loading. Assuming the non-uniformity of the fiber distribution as the cause for variation of cracking resistance, the Weibull shape parameter in cyclic loading is the same as in quasi-static loading case while the scale parameter is assumed to degrade with the applied number of cycles and this dependence is described by a power function. Thus, the determination of parameters is partially done using quasi-static tests and partially using cyclic tests, significantly reducing the necessary testing time. The predictions of dependency of the cracking on the stress and number of cycles are validated against experimental observations of cracking in the 90-plies of quasi-isotropic non-crimp fabric (NCF) laminates as well as in tape based cross-ply laminates.

  • 71.
    Kahla, Hiba Ben
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Characterization and modelling of multiple intralaminar cracking initiation under tensile quasi-static and fatigue loading2018Conference paper (Refereed)
    Abstract [en]

    The first failure mode in tensile quasi-static and in tension-tension fatigue (cyclic) loading of composite laminates is intralaminar cracking in layers with off-axis fiber orientation. These tunnel-building cracks are result of combined action of in-plane transverse and shear stresses. We assume that due to non-uniform fiber distribution (clustering) which leads to local stress concentrations, different positions in the layer have different resistance to crack initiation (initiation strength). If so, the weakest position in quasi-static loading is also the weakest in fatigue and some of the distribution parameters for fatigue behavior can be obtained in quasi-static tests, thus significantly reducing the number of required fatigue tests. Methodology is suggested and validated for cases when the cracking is initiation governed- initiated crack almost instantly propagates along fibers. Distribution parameters are identified using data in low crack density region where stress perturbations from cracks do not interact. Monte- Carlo simulations are performed for cracking in layers under quasi-static and cyclic loading using novel approach for computationally efficient stress state calculation between existing cracks. 

  • 72.
    Kahla, Hiba Ben
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Similarities and differences in micro-damage mechanisms during tensile quasi-static and cyclic loading of NCF composites2018Conference paper (Refereed)
    Abstract [en]

    A comparative study of damage mechanisms governing the mechanical behavior of a [-45/90/45/0]s HTS40/RTM6 NCF laminate under quasi-static and tension-tension cyclic loading was performed. Intralaminar cracking in 90˚-layers is described assuming the Weibull distribution of transverse cracking initiation strength. Based on the assumption that the non-uniform fiber distribution is the reason for strength variation, the fatigue model parameters are determined using the crack density in a quasi-static tensile test and in a cyclic test with just one stress level. A triggering mechanism was observed when the cracking which starts in the 90˚-layer leads to immediate cracking in the neighboring off-axis layers even if the average stress there is low. In the quasi-static test, delaminations are small even at high strain levels. At low fatigue strain level, a similar behavior was observed, however in high strain cyclic tests, the delaminations are growing faster at the edges and inside the composite. The delamination length is the largest at the edge and it decreases towards the middle of the laminate. The rate of the decrease depends on the interface and the fatigue strain applied. Delaminations which have propagated inside the specimen increase the opening and the sliding displacements of the intralaminar cracks thus causing much larger stiffness reduction than cracks without delaminations.

  • 73.
    Kahla, Hiba Ben
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå.
    Microcracking in layers of composite laminates in cyclic loading with tensile transverse stress component in layers2015In: 20th International Conference on Composite Materials: Copenhagen, 19-24th July 2015, ICCM , 2015, article id 2204-1Conference paper (Refereed)
    Abstract [en]

    Intralaminar cracking in layers of a quasi-isotropic carbon fiber NCF laminate in tension-tension cyclic loading is studied experimentally. Methodology based on modified Weibull analysis is suggested to combine quasi-static and fatigue testing to identify parameters in the crack density growth model. The validity of the assumptions for the given material is experimentally confirmed. The suggested methodology can lead to significant time and material savings in composites fatigue behaviour characterization.

  • 74. Katerelos, Dionisios
    et al.
    Lundmark, Peter
    SICOMP AB, Swedish Institute of Composites.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Galiotis, Costas
    University of Patras.
    Raman spectroscopy investigation of stiffness change and residual strains due to matrix cracking2006In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 42, no 6, p. 535-546Article in journal (Refereed)
    Abstract [en]

    The matrix cracking models developed for cross-ply composite laminates have been poorly extended in the past to more complex geometries used in practice, and they are still under development. In this paper, a new detailed analysis of the effect of matrix cracking on the behaviour of cross-ply and [0/45]s laminates under uniaxial tension is attempted. The model used in this work is applicable both to cross-ply laminates and unbalanced systems. It gives exact closed-form expressions for all thermomechanical properties of a general symmetric laminate with cracks in arbitrary layers. The theoretical approach is backed by experimental data obtained by microscopic strain-state variation measurements within a specimen, with using the technique of laser Raman spectroscopy. Glass-fibre-reinforced epoxy systems were investigated. Embedded aramid fibres-sensors within the 0° ply and near the 0°/θ ° interface were necessary due to the poor Raman signal of glass. Using experimental Raman data, the residual strain and the stiffness reduction are determined as functions of increase in crack density. The stiffness reduction is predicted with a high accuracy, whereas the measured residual strains are larger than predicted. The good results for the reduction in the elastic modulus show that the basic assumption of the model is accurate. The difference is explained by the viscoelastic-viscoplastic behaviour of the off-axis layer in shear, which in creases the "apparent" residual strain.

  • 75. Katerelos, Dionisos
    et al.
    Lundmark, P.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    McCartney, L.N.
    Galiotis, C.
    A meso-scopic experimental determination of matrix damage effects in composite laminates2006In: From the science of composites to engineering applications: the dawning future of composites: ECCM 12, 12th European Conference on Composite Materials, Biarritz, 29th August - 1st September 2006 ; [conference proceedings] / [ed] Jacques Lamon, Biarritz, 2006Conference paper (Refereed)
  • 76.
    Katerelos, Dionysos T.G.
    et al.
    Luleå tekniska universitet, FORTH/ICE-HT, Patras, Institute of Chemical Engineering and High Temperature Processes, Ionian Islands Technological Educational Institute, Faculty of Music Technology, Department of Sound and Musical Instruments Technology.
    Krasnikovs, Andrejs
    Department of Strength of Materials, Riga Technical University, Riga Technical University, Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Variational models for shear modulus of symmetric and balanced laminates with cracks in 90-layer2015In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 71, p. 169-179Article in journal (Refereed)
    Abstract [en]

    Three analytical models with increasing complexity, all based on minimization of complementary energy, are compared in their ability to predict shear modulus reduction of laminates with intralaminar cracks in 90-layer. The very elegant and simple (and the less accurate) model by Hashin assumes linear out-of-plane shear stress distribution across the ply in all layers. The second model assumes exponential shape of these stresses in the constraint layer keeping linear assumption in the cracked layer. The model developed in the present paper accounts for nonlinear out-of-plane shear stress thickness distribution in all layers described by shape functions determined in the procedure of minimization. Increasing the complexity of the model the predicted shear modulus of the damaged laminate increases approaching to value obtained using finite elements (FE). Results show that for laminates with relatively thick cracked layers the stress state description in the cracked layer should be refined whereas for laminates with constraint layer thicker than the cracked layer more accurate stress description in the constraint layer is necessary. More accurate solutions could be derived using the described methodology, but the involved complexity and the numerical routines required for their application diminish their value comparing with direct FE solution

  • 77.
    Katerelos, Dionysos T.G.
    et al.
    Technological Educational Institute of Ionian Islands, Lixouri, Greece.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Secondary damage effect on stress redistribution in laminated composites2013In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 22, no 5, p. 752-769Article in journal (Refereed)
    Abstract [en]

    Intralaminar matrix cracking is recognized as the primary mode of damage appearing in laminates. Local Raman strain measurements in neighboring layers show that the distribution shape in the vicinity of the crack tip is smoothened with increasing magnitude of the applied strain. This behavior is attributed to secondary damage development in the neighboring supporting plies close to the transverse crack tip. In this article, an approximate analytical variational model, based on the principle of minimum complementary energy, is proposed for the investigation of the stress state in a glass/ epoxy cross-ply laminate with cracks in 90°layer and with homogenized damage (reduced elastic properties) in part of the 0° layer. The extent of damage (fiber breaks) is estimated combining modeling with measured local strain distributions. The model is applied in order to examine the effect of ply thickness on the magnitude of the secondary damage at the transverse crack tip

  • 78. Katerelos, Dionysos T.G.
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Secondary damage modes in laminates initiated by intralaminar cracks2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Refereed)
    Abstract [en]

    Damage developing within laminated composites is a result of complex combinations of thermo-mechanical and environmental loads. Intralaminar (transverse) cracking is the first mode of damage appearing. The cracks act as stress concentration locations resulting in the development of additional damage modes. These modes include fibres breakage, matrix yielding, longitudinal splitting and local delaminations. The mode of damage that will grow depends on the laminate lay-up and local statistical strength of the layers and interfaces. In the present paper the stress state in the proximity of the transverse crack tip is investigated in a Glass Fibre/Epoxy composite system. An analytical model is proposed, in which the intact layers are divided into appropriate sub-layers for the stress components calculation. Knowing the SCF value at the tip of the crack, the possibility of fibres breakage in the adjacent layer is calculated using a statistical model, while the matrix failure is examined through a matrix failure criterion. SCF is recalculated taking into account the possible local stiffness reduction due to new modes of damage and the theoretical results are correlated with experimental findings.

  • 79.
    Katerelos, D.T.G.
    et al.
    Institute of Chemical Engineering and High Temperature Processes.
    Lundmark, P.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Galiotisa, C.
    University of Patras.
    Analysis of matrix cracking in GFRP laminates using Raman spectroscopy2007In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 67, no 9, p. 1946-1954Article in journal (Refereed)
    Abstract [en]

    A Raman spectroscopic technique was used to investigate the elastic modulus degradation and the residual strain development in cross-ply and [0/452/0]T glass fibre/epoxy laminates. Glass has a poor Raman signal and, therefore, embedded aramid fibres were used as strain sensors for mapping of the internal strain development within the 0° ply due to matrix cracking in the off-axis ply. The elastic modulus and the residual strain were derived from the changes of the distance between well identified strain "peaks" on the aramid fibres. The elastic modulus reduction and the residual strain development are described by a closed form model. The modulus reduction predictions are accurate, whereas the experimental residual strain is higher than that predicted. Since both phenomena are governed by the same elastic parameters, the discrepancy cannot be explained by purely elastic analysis. Development of inelastic strains in the off-axis layer is suggested as a possible cause

  • 80. Katerelos, D.T.G.
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage development in composites initiated after matrix cracking2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    Stress state in the vicinity of transverse cracks tip is investigated. An analytical model is used for the stress components calculation. Knowing the stress concentration factors values at the tip of the crack, the possibility of new damage modes initiation is examined. SCF is recalculated taking into account the possible local stiffness reduction due to new modes of damage.

  • 81.
    Katerelos, D.T.G.
    et al.
    FORTH/ICE-HT, Patras.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Galiotis, C.
    FORTH/ICE-HT, Patras.
    Energy criterion for modelling damage evolution in cross-ply composite laminates2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 12, p. 2318-2324Article in journal (Refereed)
    Abstract [en]

    The energy dissipated in cross-ply laminates during loading-unloading loops is obtained from stress-strain curves for cross-ply laminates and used in an energy based approach to predict the development of matrix cracking. The dissipated energy is correlated to the crack density growth data recorded for a reference laminate. The critical strain energy release rate, Gc obtained in this way is increasing with the applied strain. This phenomenon reflects the statistical nature of Gc distribution in the 90-layer: the first cracks (lower strain) develop in positions with lower fracture toughness. The obtained Gc data are in a good agreement with fracture toughness data obtained using LEFM based "compliance calibration" model in which the stiffness change with increasing strain is used. Finally, the matrix cracking development is successfully simulated using in the LEFM model, the data for critical strain energy release rate and an earlier derived stiffness-crack density relationship. It has been demonstrated that knowing the laminates geometry and measuring the laminate stiffness reduction with strain or (alternatively measuring the dissipated energy) the damage evolution may be simulated, thus reducing the necessity for optical observations to validation only.

  • 82.
    Krasnikovs, A.
    et al.
    Institute of Mechanics, Concrete Mechanics Laboratory, Riga Technical University.
    Kononova, O.
    Institute of Mechanics, Riga Technical University.
    Khabbaz, A.
    Majmaah University.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Engineering Sciences and Mathematics, Luleå University of Technology.
    Elasto-plastic single fiber pulling out of matrix with friction2014In: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22- 26 June 2014, European Conference on Composite Materials, ECCM , 2014Conference paper (Refereed)
    Abstract [en]

    Fibers are bridging cracks in short fiber brittle matrix composites. Post cracking composite material load - bearing capacity is dependent on each single fiber pull-out process. Numerical modeling results for single elasto-plastic fiber, is pulling out of elastic matrix, are presenting in the paper. Numerical results were compared with the data for single steel fiber pull-out of concrete matrix. 2D elastic and 3D elasto-plastic modeling, using the finite element method (FEM) approach, taking into account friction between fiber and matrix is presenting. Numerical modeling was performed for straight shape fiber was embedded into elastic matrix under variable angles to direction of applied pulling force. Fibers were embedded into matrix at variable depth. Friction between fiber and matrix and matrix shrinkage as well as elastic and plastic deformations in fiber are under investigation.

  • 83.
    Krasnikovs, A.
    et al.
    Riga Technical University, Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Transverse cracks in cross-ply laminates. 1. Stress analysis1997In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 33, no 6, p. 565-582Article in journal (Refereed)
    Abstract [en]

    During service loading of cross-ply laminates, transverse cracks occur in plies. The cracks parallel to the fiber direction are extended over the full thickness of transverse plies and often cross the entire test specimen width. It is widely recognized that the changes of laminate thermomechanical constants, caused by the transverse cracking of composite laminates, can be significant. Theoretical stress analysis in the cross-ply laminates in the vicinity of cracks is performed using numerical (FE) and analytical methods. The effect of transverse cracks on the degradation of elastic properties will be discussed in Part 2 [1]. Approximate analytical micromechanical models based on shear lag predictions, variational analysis, and numerical 2D finite element calculations were verified in their predictive abilities. The three variational models used are based on the principle of minimum complementary energy and have different degrees of accuracy with respect to the stress assumptions used (Hashin's, 2D 0° and 2D 0°/90° models). Using FEM, the plane stress and strain state were analyzed. The effect of material properties and layer thickness on the stress distribution in a 90° layer was evaluated by varying the crack spacing. The crack opening displacement (COD), normalized with respect to the far field strain, is proposed as a measure of reduction of the mechanical properties. Since the CODs are rather insensitive to the crack spacing (crack density) in a wide region, they will be used in modeling the stiffness reduction in these laminates [1].

  • 84.
    Lindström, Mikael
    et al.
    STFI-Packforsk AB.
    Gamstedt, Kristofer
    STFI-Packforsk AB.
    Barthold, Fredrik
    STFI-Packforsk AB.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wickholm, Kristina
    STFI-Packforsk AB.
    Hierarchical design as a tool in development of wood-based composite applications2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Refereed)
    Abstract [en]

    Traditionally new materials have been developed in order to solve technological problems within the aerospace, military or medicine fields. Renewable materials are developed from another perspective; they are developed in order to meet the demands from a sustainable society. If they are developed to become replacement materials, they have to be better or cheaper in order to take market shares or there has to be a political willingness to promote them. Economical subsidies are never sound and a better strategy to introduce these new materials is to identify their unique properties and possibilities and to market them.One such strategy, materials design, will be discussed in this paper. Renewable materials open for new strategies in design. Traditionally, in the life-cycle analysis (LCA) perspective materials should be recycled as materials and preferably as the original material quality. This implies that there has to be a limited number of materials and they need to be identifiable in order to be sorted. Renewable, natural fibre reinforced materials will degrade during recycling. The consequence of this is that these materials should better be recycled as energy. An integrated materials and product design will therefore be a new sustainable strategy. Researchers involved in R&D, i.e. research and development, of new materials have been involved in discussions and collaborations with the R&D department of companies producing and marketing materials. In parallel, industrial designers and engineers involved in product development and design have had on-going discussion with the same companies, but through their marketing department. This means that there is generally no communication between R&D and end-product designers. This paper suggests an R&D&D concept, Research and Development and Design, a hierarchical, integrated materials and product design strategy.

  • 85.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, University of Lorraine.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    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.

  • 86.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Farge, Laurent
    Institut Jean Lamour, Nancy Universite.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    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, 2009, p. 32-Conference paper (Refereed)
  • 87.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hussain, W.
    Kirti, A.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    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.

  • 88.
    Loukil, Mohamed Sahbi
    et al.
    Swerea SICOMP.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Characterization of damaged composite laminates using Electronic Speckle Pattern Interferometry (ESPI)2016In: ECCM 2016: Proceeding of the 17th European Conference on Composite Materials, 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.

  • 89.
    Loukil, Mohamed Sahbi
    et al.
    RISE SICOMP, Sweden. Department of Management and Engineering, Linköping University, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle 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 Poisson’s 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.

  • 90.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    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

  • 91.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Finite element analysis and experimental comparison of the crack opening displacement profile in the damaged cross-ply laminates2013Conference 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.

  • 92.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Thermo-elastic properties degradation in damaged laminates with high density of transverse cracks2013Conference paper (Refereed)
  • 93.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Validation of FEM based damaged laminate model measuring crack opening displacement in cross-ply laminate using Electronic Speckle Pattern Interferometry (ESPI)2013Conference paper (Refereed)
  • 94.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour.
    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.

  • 95.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Application of the electronic speckle pattern interferometry (espi) technique for the characterization of damage in composite laminates2012Conference paper (Refereed)
  • 96.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Characterization of damaged composite laminates by an optical measurement of the displacement field2011In: Book of abstracts: 6th EEIGM International Conference, Advanced Materials Research7th and 8th November, 2011 EEIGM, Nancy, France, 2011, p. 37-Conference paper (Refereed)
  • 97.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, (SI2M).
    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: IOP Publishing Ltd , 2012Conference 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

  • 98.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    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.

  • 99.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, Nancy-Université.
    Parameter determination in damaged laminate model by optical full-field measurement of the displacement using ESPI2011Conference paper (Other academic)
  • 100.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, Nancy-Université.
    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)
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