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  • 1.
    Almgren, Karin M
    et al.
    STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm.
    Gamstedt, Kristofer
    Department of Fiber and Polymer Technology, Royal Institute of Technology - KTH.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates2010In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, no 5, 762-771 p.Article in journal (Refereed)
    Abstract [en]

    One of the main drawbacks of wood fiber-based composite materials is their propensity to swell due to moisture uptake. Because the wood fibers are usually the main contributor to hygroexpansion, it is of interest to quantify the hygroexpansion coefficient of wood fibers, to compare and rank different types of fibers. This investigation outlines an inverse method to estimate the transverse hygroexpansion coefficient of wood fibers based on measurements of moisture induced thickness swelling of composite plates. The model is based on composite micromechanics and laminate theory. Thickness swelling has been measured on polylactide matrix composites with either bleached reference fibers or crosslinked fibers. The crosslinking modification reduced the transverse hygroexpansion of the composites and the transverse coefficient of hygroexpansion of the fibers was reduced from 0.28 strain per relative humidity for reference fibers to 0.12 for cross-linked fibers.

  • 2.
    Al-Ramahi, Nawres
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, RobertsLuleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.Varna, JanisLuleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Model for numerical simulation and parametric analysis of composite adhesivejoints under thermo-mechanical loading2017Conference proceedings (editor) (Refereed)
    Abstract [en]

    Abstract: The current investigation focuses on development and verification of a modelfor numerical simulation of performance of adhesive joints under tensile loading. Differentcombination of materials in joints is considered: metal-metal, composite-composite andcomposite-metal. The objective of this paper is to present simulation results of joints usingan accurate finite element model including non-linear behaviour and large deformation.Moreover, several loading scenarios are analysed, including simultaneous application oftemperature and mechanical load. Not only the effect of temperature on mechanicalperformance of materials (adhesive as well as adherents) is analysed but also built up ofresidual thermal stresses during the manufacturing of joints are taken into account. Thisapproach is demonstrated by simulation of tensile tests of joints at several temperatures.Two scenarios of application of temperature and mechanical load using large deformationtheory are considered: 1) the thermal and mechanical loads are applied simultaneously (theproperties of the materials are adjusted accordingly to their performance at differenttemperatures); 2) temperature is applied on specimen which is not macroscopicallyconstrained and the obtained stress distribution is used as initial state for the nextsimulation of mechanical loaded joint. The influence of edge effects (due to limited widthof the joint) on the stress distribution within the joint are studied. In order to eliminatethese effects the periodic boundary conditions (BC) are used in the numerical model.These BC are adjusted to optimize numerical model and obtain efficient calculation routinefor analysis of stresses within interior part of the structure. The validity of these BCs isevaluated and verified by analysing number of case studies. The comparison between full3D FEM model and simplified 2D model is carried out. The resulting stress distributions inthe overlap region of joints are presented for different joints (the parameters are: materialcombinations, material models, geometry of adhesive layer, constraints and BCs) withcomprehensive analysis and recommendations for optimal numerical model that can beused in joint design.

  • 3. Arab, A.
    et al.
    Stommel, M.
    Wallström, Lennart
    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.
    Stiffness prediction in green composites using homogenization techniques2013In: Proceedings of the 19th International Conference on Composite Materials: ICCM 2013, Montreal (Canada), 2013, 1214-1222 p.Conference paper (Refereed)
  • 4.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    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.
    Modelling stiffness and strength of non-crimp fabric composites: semi-laminar analysis2011In: Non-crimp fabrics composites: manufacturing, properties and applications, Cambridge: Woodhead Publishing Materials , 2011, 402-438 p.Chapter in book (Refereed)
  • 5.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    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.
    Olsson, Robin
    Swerea SICOMP AB, Mölndal.
    Multiscale modelling of non-crimp fabric composites2012In: Proceedings of the ASME International Mechanical Engineering Congress and Exposition--2012: presented at ASME 2012 International Mechanical Engineering Congress and Exposition, November 9-15, 2012 Houston, Texas USA, New York: American Society of Mechanical Engineers , 2012, Vol. 3, 581-590 p.Conference paper (Refereed)
    Abstract [en]

    Damage initiation and evolution in NCF composites leading to final failure includes a multitude of mechanisms and phenomena on several length scales. From an engineering point-of-view a computational scheme where all mechanisms would be explicitly addressed is too complex and time consuming. Hence, methods for macroscopic performance prediction of NCF composites, with limited input regarding micro- And mesoscale details, are requested. In this paper, multi-scale modelling approaches for in-plane transverse strength of NCF composites are outlined and discussed. In addition a simplistic method to predict transverse tensile and compressive strength for textile composites featuring low or no fibre waviness is presented

  • 6.
    Berglund, Lars A.
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Transverse cracking in laminated composites1995In: Advanced technology for design and fabrication of composite materials and structures: applications to the automotive, marine, aerospace, and construction industry / [ed] George C. Sih; A. Carpinteri; G. Surace, Kluwer Academic Publishers, 1995, 191-201 p.Conference paper (Refereed)
    Abstract [en]

    Models based on variational analysis are compared with respect to the elastic properties of cross-ply laminates with transverse cracks. All models use the principle of minimum complementary energy for calculation of the stress state in a laminate element between two cracks, as was the case with the original model by Hashin (1985). The models are based on assumptions of different degree of accuracy, where the most refined model was developed by the present authors. Experimental data for the longitudinal stiffness are compared with predictions for glass fiber/epoxy cross-ply laminates. The agreement between model predictions and data is good provided the initial assumptions of the stress state are sufficiently accurate

  • 7.
    Berglund, Lars A.
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Yuan, Jie
    Linköpings universitet.
    Transverse Cracking and Local Delamination in [04/90n]s and [90n/04]s Carbon Fiber/Toughened Epoxy Laminates1992In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 11, no 6, 643-660 p.Article in journal (Refereed)
    Abstract [en]

    [04/90n]s and [90n/04 ]s laminates were used to study 90°-layer failure mechanisms in HTA/6376 carbon fiber/toughened epoxy. The different stacking sequences were chosen to vary the stress states, since experimental results show differences in failure strains as well as in the local delamination behavior at the crack tip. Differences in the onset of transverse cracking and local delamination behavior between [04/90m]s and [90 n/04]s laminates was discussed using a model based on linear elastic fracture mechanics (LEFM). Although quantitative LEFM-predictions do not agree with data for the carbon fiber/toughened epoxy investigated, the general predictions for differences in and causes of local delamination behavior were confirmed by experimental data. The fact that the main features were correctly predicted, encourages the development of some modified LEFM-approach to describe toughened matrix composites.

  • 8.
    Berglund, Lars
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Yuan, Jie
    Effect of intralaminar toughness on the transverse cracking strain in cross-ply laminates1991In: Advanced Composite Materials, ISSN 0924-3046, E-ISSN 1568-5519, Vol. 1, no 3Article in journal (Refereed)
  • 9.
    Byström, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Jekabsons, Normunds
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An evaluation of different models for prediction of elastic properties of woven composites2000In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 31, no 1, 7-20 p.Article in journal (Refereed)
    Abstract [en]

    A critical analysis of two simple and convenient analytical models for calculation of elastic properties of woven fabric composites is performed. Predictions of these models are compared with results obtained using the method of reiterated homogenization and with experimental data for plain weave glass fiber and carbon fiber polyester composites. Three different scales are identified in the analysis. The first scale predictions, which are the tow properties (obtained by applying Hashin's concentric cylinder model, the Halpin-Tsai expressions or mathematical homogenization technique), are the most critical because they form the input information for woven composite modeling. It appears that the uncertainty in this information causes larger differences in predictions than the deviations between models of different degree of accuracy. This fact sets limits on the required accuracy of the models. Model comparisons reveal that the woven compoiste model based on isostrain assumption in the compoiste plane and isostress assumption for out-of-plane components is in very good agreement with both experimental data and the reiterated homogenization method, whereas the modified mosaic parallel model fails to describe composites with large interlaced regions.

  • 10.
    Byström, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Persson, Lars-Erik
    Jekabsons, Normunds
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Using reiterated homogenization for stiffness computation of woven composites1998In: ICCE 5: Fifth International Conference on Composites Engineering / [ed] David Hui, 1998, 133-134 p.Conference paper (Refereed)
  • 11.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Mattsson, David
    Asp, Leif
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension2004In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 64, no 5, 675-692 p.Article in journal (Refereed)
    Abstract [en]

    Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of ‘novel' type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD

  • 12. Eitzenberger, Johannes
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Debond crack growth in fatigue along fiber in UD composite with broken fibers2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Refereed)
    Abstract [en]

    Assuming that Paris law is applicable for individual debond crack propagation along the fiber/matrix interface, the related strain energy release rate in a unidirectional composite is analysed using FEM and also using simple analytical considerations based on self-similar debond crack propagation. Model with axial symmetry consisting of three concentric cylinders is used: partially debonded broken fiber in the middle is surrounded by matrix cylinder which is embedded in a large block of effective composite with properties calculated using rule of mixtures and Halpin-Tsai expressions. It is shown for pure mechanical loading that the fiber elastic properties have a huge effect on the released energy, whereas fiber content in the composite in the considered realistic range has effect only for short debonds. The interaction between debonds approaching from both fiber fragment ends is investigated and related to material properties and geometrical parameters. It is shown that the self-similar debond propagation model gives slightly overestimated values of the strain energy release rate which may be related to interaction effects not included in the analytical model.

  • 13.
    Farge, L.
    et al.
    Laboratoire de Science et Génie des Surfaces, EEIGM.
    Ayadi, Z.
    Laboratoire de Science et Génie des Surfaces, EEIGM.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Optically measured full-field displacements on the edge of a cracked composite laminate2008In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 39, no 8, 1245-1252 p.Article in journal (Refereed)
    Abstract [en]

    The main objectives of this study are to visualize the displacement field on the edge of a [0, 554, -554]s GF/EP laminate specimen with multiple transverse cracks and to analyze the crack opening displacement dependence on the applied mechanical load. The specimen full-field displacement measurement was carried out using ESPI (Electronic Speckle Pattern Interferometry) and phase-shifting. ESPI is an optical technique that provides the displacement for every point on a surface. The measurement resolution is roughly 20 nm. The displacement measurement is along the tensile axis and takes place on the specimen edge. Using the displacement map, it is possible to obtain the displacement profiles along the tensile-axis. The different profiles were drawn along the specimen edge at several distances from the mid-plane corresponding to the different plies. Studies of the displacement discontinuities make it possible to carry out a measurement of the crack opening displacements corresponding to the cracks in the measurement field. The experimental results are in a good agreement with idealized straight crack model in low stress region and much larger in the high stress region, which is attributed to development of local inter-ply delaminations.

  • 14.
    Farge, L.
    et al.
    Nancy Université-INPL.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Nancy Université-INPL.
    Damage characterization of a cross-ply carbon fiber/epoxy laminate by an optical measurement of the displacement field2010In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, no 1, 94-101 p.Article in journal (Refereed)
    Abstract [en]

    Using Electronic Speckle Pattern Interferometry (ESPI), full-field displacement measurement was performed on the edge of a cracked cross-ply graphite/epoxy laminate subjected to a tensile loading. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the Crack Opening Displacement (COD) values along the cracks. The main objective of this study is to determine if the application of successive loads of increasing magnitude may have modified the existing cracks and thereby changed the COD dependence on the applied stress. Moreover, we have tested the applicability of the assumed linear elastic COD behavior in the presence of very high stress concentration at the crack tips. The profile of the opening along the crack was also studied.

  • 15. Farge, L.
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Study of the displacement field on the edge of cracked laminated composites2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Refereed)
    Abstract [en]

    Using Electronic Speckle Pattern Interferometry (ESPI), the full-field displacement measurement was obtained on the edge of a cracked laminate subjected to a tensile loading. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the Crack Opening Displacement (COD) values along the cracks. The main objective of this study is to determine if the application of high load may have modified the existing cracks and consequently changed the COD dependence on the applied stress. The profile of the opening was also studied.

  • 16.
    Farge, Laurent
    et al.
    Laboratoire d’Energètique et de Mécanique Théorique et Appliquee, Vandoeuvre Lés Nancy Cedex.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Cedex.
    Use of full-field measurements to evaluate analytical models for laminates with intralaminar cracks2012In: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 46, no 21, 2739-2752 p.Article in journal (Refereed)
    Abstract [en]

    Full-field displacement measurements between intralaminar cracks in cross-ply laminates were performed to evaluate the accuracy of shear lag models and Hashin’s model. The dependence of the average crack opening displacement on the crack density was measured and compared with model predictions. It was found that Hashin’s model overestimates the average COD by at least 25%. The value of the shear lag parameter was back-calculated by fitting. With the same goal the strain in the middle between two cracks was measured rendering shear lag parameter which was only 1% different. The found value does not agree with any of the used shear lag models. Measurement shows nonzero intralaminar shear zones in both layers covering a part of the ply thickness.

  • 17.
    Giannadakis, Konstantinos
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mannberg, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    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.
    The sources of inelastic behavior of glass fibre/vinylester non-crimp fabric [±45]s laminates2011In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 30, no 12, 1015-1028 p.Article in journal (Refereed)
    Abstract [en]

    The non-linear and time-dependent stress–strain response of NCF [+-45]s laminates in tension is studied. Testing methodology is suggested to separate and quantify the effect of damage development, non-linear viscoelastic effects, and viscoplasticity on the inelastic response. This is achieved by decomposition of viscoelastic and viscoplastic response, both of them being affected by microdamage accumulated during the service life. Material model based on Schapery’s work on viscoelasticity and Zapas viscoplastic function with added damage terms is presented and used. Simulation is performed and validated with constant stress rate tensile tests, identifying the non-linear viscoelasticity and viscoplasticity as the major sources of the non-linear response.

  • 18.
    Giannadakis, Konstantinos
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Szpieg, Magdalena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical performance of a recycled carbon fibre/PP composite2011In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 51, no 5, 767-777 p.Article in journal (Refereed)
    Abstract [en]

    A composite made of recycled carbon fibres in recycled polypropylene matrix is studied experimentally to describe the features of the elastic and time dependent nonlinear mechanical behaviour. The properties of the developed material have a large variability to be addressed and understood. It was found that the stress-strain curves in tension are rather nonlinear at low strain rate and the strength is sensitive to strain rate. The elastic properties' reduction for this composite after loading to high strains is rather limited. More important is that even in the "elastic region" due to viscoelastic effects the slope of loading-unloading curve is not the same and that at higher stress large viscoplastic strains develop and creep rupture is typical. The time and stress dependence of viscoplastic strains was analysed and described theoretically. The viscoelastic response of the composite was analysed using creep compliance, which was found to be slightly nonlinear

  • 19.
    Giannadakis, Konstantinos
    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.
    Analysis of non-linear shear stress-strain response of unidirectional GF/EP composite2014In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 62, 67-76 p.Article in journal (Refereed)
    Abstract [en]

    Reasons for nonlinear stress-strain curves in shear of unidirectional glass fibre composite are analysed. Laminate with stacking sequence [45/-45]s is used in tensile quasi-static as well as in tensile creep and strain recovery tests to study the development of viscoelastic and viscoplastic shear strains in local coordinates of the ply. It is shown that Zapa’s integral representation of viscoplasticity is applicable for this material and methodology for parameter determination is given. Schapery’s nonlinear viscoelastic material model was used for shear response characterization and the nonlinearity parameters’ dependence on the shear stress was determined and described by fitting functions. Microdamage development is quantified by measuring axial modulus and Poisson’s ratio of the laminate. The obtained nonlinear viscoelastic, viscoplastic model with included effect of microdamage was successfully used to predict the nonlinear shear stress-strain curve in strain controlled tensile loading.

  • 20. Giannadakis, Konstantinos
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of Curing Conditions on Mechanical Properties and Chemical Shrinkage in Polymers2013In: 9th International Conference on Composite Science and Technology: 2020 - Scientific and Technical Challenges / [ed] Michelle Meo, Lancaster: PA Destech Publications Inc. , 2013, 989-990 p.Conference paper (Refereed)
    Abstract [en]

    The effect that different curing time/temperature conditions bring to the final properties of a polymeric resin, along with measurements of their chemical shrinkage have been investigated in the present study.

  • 21.
    Giannadakis, Konstantinos
    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.
    Effect of thermal aging and fatigue on failure resistance of aerospace composite materials2009In: 5th International EEIGM/AMASE/FORGEMAT Conference on Advanced Materials Research, Bristol: IOP Publishing Ltd , 2009Conference paper (Refereed)
    Abstract [en]

    Exposing a laminate structure to thermal cycles and to temperatures close to the curing temperature, followed by mechanical fatigue, cause interlaminar and intralaminar cracks leading to degradation of the mechanical properties. The effect of thermal and mechanical fatigue, as well as thermal aging, on carbon fiber composite laminate structures is under study in the present paper.

  • 22.
    Giannadakis, Konstantinos
    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.
    Minimization of complementary energy to predict shear modulus of laminates with intralaminar cracks2012In: 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 most common damage mode and the one examined in this work is the formation of intralaminar cracks in layers of laminates. These cracks can occur when the composite structure is subjected to mechanical and/or thermal loading and eventually lead to degradation of thermo-elastic properties. In the present work, the shear modulus reduction due to cracking is studied. Mathematical models exist in literature for the simple case of cross-ply laminates. The in-plane shear modulus of a damaged laminate is only considered in a few studies. In the current work, the shear modulus reduction in cross-plies will be analysed based on the principle of minimization of complementary energy. Hashin investigated the in-plane shear modulus reduction of cross-ply laminates with cracks in inside 90-layer using this variational approach and assuming that the in-plane shear stress in layers does not depend on the thickness coordinate. In the present study, a more detailed and accurate approach for stress estimation is followed using shape functions for this dependence with parameters obtained by minimization. The results for complementary energy are then compared with the respective from literature and finally an expression for shear modulus degradation is derived.

  • 23.
    Giannadakis, Konstantinos
    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.
    Potential of a simple variational analysis in predicting shear modulus of laminates with cracks in 90-layers2014In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 48, no 15, 1843-1856 p.Article in journal (Refereed)
    Abstract [en]

    The potential of accurate modelling of the shear modulus reduction of laminates with cracked 90-layers using models based on the minimization of the complementary energy with improved stress description in the constraint layers is evaluated. This group of models refine Hashin’s model by introducing shape functions with unknown parameters to represent the out-of-plane shear stress distribution across the constraint layer thickness. The Hashin’s model becomes a particular case of the presented when the shape parameter approaches to zero. The most accurate shape parameters are found in the result of minimization. Three models are compared: the present variational model, Hashin’s model and the shear lag model introduced by Soutis which assumes linear out-of-plane shear stress distribution over an unknown part of the layer. It is shown in this paper that the size of this part may be determined by minimization of the complementary energy. The present model is the most accurate amongst the three, whereas Soutis’ model is more accurate than the Hashin’s model for laminates with constraint layer, thicker than the cracked layer. The comparison with finite element method results shows reasonable agreement. Agreement can be improved developing models with better description of the stress state in the cracked layer.

  • 24.
    Giannadakis, Konstantinos
    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.
    Potential of simple analysis based on minimization of complementary energy to predict shear modulus in laminates with transverse cracks2011In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, 2011Conference paper (Refereed)
  • 25.
    Graciani, E.
    et al.
    University of Seville.
    Mantic, V.
    University of Seville.
    Paris, Federico
    Escuela Superior de Ingenieros, Universidad de Sevilla, University of Seville.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fiber–matrix debonding in composite materials: Axial loading2016In: Modeling damage, fatigue and failure of composite materials, Cambridge: Woodhead Publishing Limited, 2016, 117-141 p.Chapter in book (Refereed)
    Abstract [en]

    The propagation of fiber–matrix interfacial debonding under axial loading is analyzed, using the single-fiber fragmentation test as a reference, in order to determine fiber–matrix failure properties. A data reduction technique is presented in which the fiber–matrix Mode II interfacial fracture toughness is obtained from the measurements of the average debond crack growth. A set of boundary element models are employed to evaluate the energy release rate associated with interfacial crack propagation. The interfacial friction coefficient is parametrically varied until a constant value of the energy release rate (which is then equal to the fiber–matrix Mode II interfacial fracture toughness) is obtained. The applicability of the properties evaluated is demonstrated using a set of finite element models with cohesive elements.

  • 26.
    Graciani, Enrique
    et al.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Mantič, Vladislav
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Patris, Frederico
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A BEM analysis of the single fibre fragmentation test: influence of curing temperature in crack propagation2004In: 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, Rhodos, 2004Conference paper (Refereed)
  • 27.
    Graciani, Enrique
    et al.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Mantič, V.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Paris, F.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Single fiber fragmentation test: a BEM analysis2003In: A collection of technical papers: A collection of technical papers, Reston, Va: American Institute of Aeronautics and Astronautics, AIAA , 2003, Vol. Vol. 2, 988-997 p.Conference paper (Refereed)
    Abstract [en]

    Numerical analysis was performed with the aid of the Boundary Element Method (BEM). Contact conditions were taken into account using a weak formulation of the equilibrium compatibility equations along the interface of the solids. Comparison of the different kinds of failure was made in terms of the energy release rate

  • 28.
    Graciani, Enrique
    et al.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Mantič, Vladislav
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Paris, Federico
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Numerical analysis of debond propagation in the single fibre fragmentation test2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 15-16, 2514-2520 p.Article in journal (Refereed)
    Abstract [en]

    A numerical analysis, using the Boundary Element Method, of the stress state within the specimen in the Single Fibre Fragmentation Test is presented first. Thermal residual stresses and fibre-matrix interfacial friction along the debonding crack faces have been considered in the study. Special attention has been paid to the axial stresses along the fibre and the interfacial tractions and relative displacements in the neighbourhood closest to the debonding crack tips. In order to analyze the debond propagation, the associated Energy Release Rate has been evaluated from the near-tip elastic solution. Numerical results show that both the effects of thermal residual stresses and of fibre-matrix interfacial friction are opposed to the debond propagation. Additionally, the effect of the debond propagation on the load transfer through the interface has been studied, showing that fibre-matrix interfacial friction has a weak influence on the distance needed to re-establish the nominal axial load within the fragment.

  • 29.
    Graciani, Enrique
    et al.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Mantič, Vladislav
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Paris, Frederico
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An axisymmetric boundary element analysis of interface cracks in fiber reinforced composites2005In: Advances in boundary element techniques VI / [ed] A.P.S. Selvadura; C.L. Tan; M.H. Aliabad, Eastleigh: Intellect Ltd., 2005Conference paper (Refereed)
  • 30.
    Graciani, Enrique
    et al.
    University of Seville, Escuela Superior de Ingenieros, Camino de los Descubrimientos.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mantič, V.
    University of Seville, Escuela Superior de Ingenieros, Camino de los Descubrimientos.
    Blázquez, A.
    University of Seville, Escuela Superior de Ingenieros, Camino de los Descubrimientos.
    Paris, F.
    University of Seville, Escuela Superior de Ingenieros, Camino de los Descubrimientos.
    Evaluation of interfacial fracture toughness and friction coefficient in the single fiber fragmentation test2011In: 11th International Conference on the Mechanical Behavior of Materials (ICM11) / [ed] Mario Guagliano; Laura Vergani, Elsevier, 2011, 2478-2483 p.Conference paper (Refereed)
    Abstract [en]

    Single fiber fragmentation test is extensively employed to characterize the fiber-matrix interface in composites. A novel technique for evaluating fracture toughness and friction coefficient at the fiber-matrix interface in an epoxy sample containing a single glass fiber is proposed. Using experimental measurements of the average fragment and debond lengths, Boundary Element (BE) models of the portion of sample corresponding to the average fiber fragment are created for increasing values of the applied strain. From the solution of the BE models, energy release rate (ERR) during crack propagation is evaluated using a Fracture Mechanics based approach which accounts for fiber-matrix interfacial friction. The calculated evolution of the ERR has a reasonably linear dependency on the applied strain, with a decreasing slope for increasing values of the interfacial friction coefficient. Since debond growth is stable, ERR should equal the interfacial fracture toughness during debond propagation. Consequently, interfacial friction coefficient and fracture toughness can be simultaneously determined by parametrically varying the friction coefficient until a null slope is obtained in the linear fit of the numerical solution of the ERR as a function of the applied strain. The applicability of the proposed technique is demonstrated with experimental results taken from the literature.

  • 31.
    Grasiani, Enrique
    et al.
    Grupo de Elasticidad y Resistencia de Materiales, Escuela Técnica Superior de Ingenieros de Sevilla.
    Mantič, Vladsilav
    Grupo de Elasticidad y Resistencia de Materiales, Escuela Técnica Superior de Ingenieros de Sevilla.
    Paris, Fernando
    Grupo de Elasticidad y Resistencia de Materiales, Escuela Técnica Superior de Ingenieros de Sevilla.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Numerical analys of the single fibre fragmentation test using cohesive elements2010In: ECCM 14: 14th European Conference on Composite Materials ; June 7 - 10, 2010, Budapest, Hungary, Budapest: BUTE Dep. of Polymer Engineering , 2010Conference paper (Refereed)
  • 32.
    Hajikazemi, Mohammad
    et al.
    Aerospace Engineering Department and Centre of Excellence in Computational Aerospace Engineering, Amirkabir University of Technology, Iran.
    Sadr, Mohammad Homayoun
    Aerospace Engineering Department and Centre of Excellence in Computational Aerospace Engineering, Amirkabir University of Technology, Iran.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Analysis of cracked general cross-ply laminates under general bending loads: A variational approach2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 22, 3089-3109 p.Article in journal (Refereed)
    Abstract [en]

    A novel variational model is developed that can predict the stress transfer between plies in cracked general cross-ply laminates subject to general in-plane (Nxx, Nyy, Nxy) and out-of-plane bending (Mxx, Myy, Mxy) loading. The effects of thermal residual stresses are taken into account. Admissible stress systems, which satisfy equilibrium and all boundary and interface conditions, are constructed and the principle of minimum complementary energy is employed to find the optimal solution. The approach yields rigorous lower bounds for stiffness matrices. A methodology based on Levin's theorem is developed to evaluate the effective thermal expansion coefficients of non-symmetric cracked laminates. A ply-refinement technique is used in order that through-thickness variations of the stress components can be precisely taken into account. It is found that the developed method, when used in conjunction with ply refinement technique, results in stress fields and thermo-mechanical properties comparable in accuracy to refined finite and boundary element solutions.

  • 33.
    Huang, Y.
    et al.
    Texas A&M University, College Station.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A statistical approach to evaluate the effect of manufacturing quality on transverse cracking in cross ply laminates2011In: 18th International Conference on Composites Materials, ICCM 2011: Jeju; South Korea; 21 August 2011 through 26 August 2011, 2011Conference paper (Refereed)
  • 34.
    Huang, Yongxin
    et al.
    Wind R&D Center, Siemens Energy.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Statistical Methodology for Assessing Manufacturing Quality Related to Transverse Cracking in Cross Ply Laminates2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 95, 100-106 p.Article in journal (Refereed)
    Abstract [en]

    We present a statistical analysis based methodology for making assessment of the manufacturing quality of cross ply composite laminates as it relates to its effect on transverse cracking evolution. Assuming a two-parameter Weibull distribution of tensile strength of the transverse plies to represent randomly distributed manufacturing defects, multiple crack formation in the plies is simulated in the non-interactive and interactive regimes of cracking using the local stress fields calculated by a variational analysis. The statistical methodology is demonstrated on crack density evolution in cross ply laminates manufactured by four different processing routes and loaded in monotonic tension in the axial direction. The differences in the crack density evolution, supposedly due to different defect population induced by the four manufacturing conditions, could be described by the proposed statistical simulation method.

  • 35.
    Huang, Yongxin
    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.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The effect of manufacturing quality on transverse cracking in cross ply laminates2011In: Processing and fabrication of advanced materials. XlX / [ed] D. Bhattacharyya; R.J.T. Lin; T.S. Srivatsan, Auckland: Centre for Advanced Composite Materials, University of Auckland , 2011, 552-559 p.Conference paper (Refereed)
  • 36. Jacobs, Edgard
    et al.
    Verpoest, Ignaas
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    On the use and limitations of concentric cylinder models for polymer matrix composites1999In: 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)
  • 37.
    Jekabsons, Normunds
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micromechanics of damage accumulation in a 2.5D woven C-fiber/SiC ceramic composite2001In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 37, no 4, 289-298 p.Article in journal (Refereed)
    Abstract [en]

    The morphology of a C/PyC/SiC ceramic composite is studied and its microdamage mechanisms are revealed. Some models based on the micromechanical fracture mechanics are proposed to describe the possible damage processes. The influence of the observed microdamage on the stiffness reduction, is studied. It is shown that the delamination between the layers and the cracks in the coating alone cannot explain the stiffness reduction observed in tests. Debonding between the SiC coating and pyrocarbon is suggested as another mechanism of the stiffness reduction.

  • 38.
    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, 637-656 p.Article 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.

  • 39. 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, 1030-1046 p.Article 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.

  • 40.
    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)
  • 41.
    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)
  • 42.
    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)
  • 43.
    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, 1641-1652 p.Article 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.

  • 44.
    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)
  • 45.
    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)
  • 46.
    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.

  • 47.
    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)
  • 48.
    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)
  • 49.
    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)
  • 50.
    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)
123456 1 - 50 of 261
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