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  • 251.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Loukil, Mohamed Sahbi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Degradation of elastic properties of non-uniformly damaged composite laminates2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 12009Conference paper (Refereed)
    Abstract [en]

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

  • 252.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lundmark, P.
    Mattsson, David
    Megnis, M.
    Luleå tekniska universitet.
    Constitutive models for composite laminates with matrix cracks and fiber breaks2005In: Proceedings of the International Conference on Fracture and Damage Mechanics IV, 2005, p. 597-604Conference paper (Refereed)
  • 253.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lundmark, Peter
    Luleå tekniska universitet.
    Crack face sliding effect on stiffness of laminates with ply cracks2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 10, p. 1444-1454Article in journal (Refereed)
    Abstract [en]

    The rate of stiffness reduction in damaged laminates with increasing transverse crack density in plies depends on two micromechanical parameters: normalized crack face opening displacement (COD) and crack face sliding displacement (CSD). A FE-based parametric study shows that the only properties that affect the CSD are the thickness ratio and the in-plane shear stiffness ratio of the damaged and neighboring undamaged layers. The dependence is described by a power function with respect to the above mentioned properties. This relationship and the previously obtained power law for COD [Lundmark P, Varna J. Constitutive relationships for damaged laminate in in-plane loading. Int J Dam Mech 2005:14(3):235-59] are used in the damaged laminate constitutive relationships [Lundmark P, Varna J. Constitutive relationships for damaged laminate in in-plane loading. Int J Dam Mech 2005:14(3):235-59], which are closed form exact expressions for general symmetric laminates in in-plane loading. The model is validated analyzing reduction in shear modulus of [Sn,90m]s laminates and comparing with direct FE-calculations. The results are excellent in case of cracks in one layer only. For laminates with two orthogonal systems of cracks, the power law underestimates the CSD. To account for interaction between both systems of cracks, which is of importance for crack face sliding, the power law is modified using the effective shear modulus of the cracked neighboring layer.

  • 254.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oldenbo, Magnus
    Volvo, Göteborg.
    An incremental 2D constitutive model accounting for linear viscoelasticity and damage development in short fibre composites2005In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 64, no 11, p. 1509-1528Article in journal (Refereed)
    Abstract [en]

    A model accounting for linear viscoelasticity and microdamage evolution in short fibre composites is described. An incremental 2D formulation suitable for FE-simulation is derived and implemented in FE-solver ABAQUS. The implemented subroutine allows for simulation close to the final failure of the material. The formulation and subroutine is validated with analytical results and experimental data in a tensile test with constant strain rate using sheet moulding compound composites. FE-simulation of a four-point bending test is performed using shell elements. The result is compared with linear elastic solution and test data using a plot of maximum surface strain in compression and tension versus applied force. The model accounts for damage evolution due to tensile loading and neglects any damage evolution in compression, where the material has higher strength. Simulation and test results are in very good agreement regarding the slope of the load-strain curve and the slope change.

  • 255.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Paris, Frederico
    University of Seville.
    Cano, Juan Carlos del
    University of Valladolid.
    Effect of crack-face contact on fiber/matrix debonding in transverse tensile loading1997In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 57, no 5, p. 523-532Article in journal (Refereed)
    Abstract [en]

    Debond growth at the fiber/matrix interface caused by loading transverse to the fiber axis is analysed by the boundary-element method with the possibility of taking into account contact between fiber and matrix. Experimentally observed debonds that are typically much larger in the axial direction than in the arc direction are modeled as arc cracks under plane-strain conditions in a linear elasticity formulation. As a result of the geometrical configuration of the crack with respect to the load, the debonded zone is closed for large crack arc sizes, the interface failure occurring in a pure shear mode. Debond growth in the arc direction is analysed by an energy-balance criterion and the strain-energy release rate is calculated by using Irwin's crack-closure technique. Predicted debond size versus applied load relationships are compared with experimental data for two glass-fiber/epoxy systems and the critical strain-energy release rate of the interface in shear, Gc, is determined. The applicability of a simple analytical failure model, used for data reduction, is also discussed.

  • 256.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Persson, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Claudel, Florian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hajlane, Abdelghani
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Curvature of unsymmetric cross-ply laminates: Combined effect of thermal stresses, microcracking, viscoplastic and viscoelastic strains2017In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 36, no 4, p. 283-293Article in journal (Refereed)
    Abstract [en]

    Curvature of unsymmetrical [0/90] specimens caused by thermal stresses changes if the specimen is subjected to large axial strains introducing intralaminar cracks in the 90-layer. It is shown that the large curvature reduction can not be explained by cracking related stress release only. The large irreversible viscoplastic strains introduced during the axial tensile loading (with 5 min holding at high strain for crack counting) give the main contribution to the curvature change. The effect of transient viscoelasticity (VE) was found to be of minor significance. Simple approach based on effective damaged layer stiffness and constant irreversible strain is used in the framework of laminate theory to extract the viscoplastic and VE strains from experimental curvature data. The obtained fitting expressions for viscoplastic- and VE-strain development are successfully used to describe curvature change in [0/902] laminate subjected to the same test procedure. It is suggested that the used curved beam tests could be efficient to characterize the viscoplastic strain development in the thin 90-layers

  • 257.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Persson, Magnus
    Luleå tekniska univesitet.
    Hajlane, Abdelghani
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microdamage, viscoelasticity and viscoplasticity as main phenomena in thermal stress relaxation in laminated composites2016In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 713, p. 99-102Article in journal (Refereed)
    Abstract [en]

    Microdamage, viscoplastic and viscoelastic strain development in 90-layers of cross-ply laminates subjected to tensile loading is studied on unsymmetrical GF/EP laminates measuring the thermal curvature change. All three phenomena partially compensate for the effect of the thermal mismatch reducing the residual stress (specimen curvature). The viscoplastic strain contribution to curvature change is the largest whereas the effect of transient viscoelasticity is the smallest. Damage is included in the analysis through its effect on the effective transverse modulus of the 90- layer.

  • 258.
    Varna, Janis
    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.
    Characterization of viscoelasticity, viscoplasticity, and damage in composites2019In: Creep and Fatigue in Polymer Matrix Composites / [ed] Rui Miranda Guedes, Elsevier, 2019, 2, p. 497-530Chapter in book (Refereed)
    Abstract [en]

    Empirical inelastic constitutive material models for composites and testing methodology for parameter determination in these models are analyzed. Short fiber as well as long unidirectional fiber reinforced composites are analyzed in situations when the main sources of inelastic behavior are a combination of (a) nonlinear viscoelasticity; (b) nonlinear viscoplasticity; and (c) microdamage-induced reduction of thermoelastic properties, all three evolving with time and stress. These phenomena are included in a common material model. The necessary tests for model identification are tensile quasistatic loading-unloading tests and creep tests at different stress levels with recorded strain recovery after load removal. The methodology is demonstrated presenting models for (a) shear in layers of [45/−45]s laminates; (b) response of short fiber composites (SMC with glass fiber bundles; composites with natural or man-made cellulosic fibers in bio-based resins).

  • 259.
    Varna, Janis
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Incremental forms of Schapery’s model: convergence and inversion to simulate strain controlled ramps2016In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 20, no 4, p. 535-552Article in journal (Refereed)
    Abstract [en]

    Schapery’s nonlinear viscoelastic model is written in incremental form, and three different approximations of nonlinearity functions in the time increment are systematically analysed with respect to the convergence rate. It is shown that secant slope is the best approximation of the time shift factor, leading to significantly higher convergence rate. This incremental form of the viscoelastic model, Zapas’ model for viscoplasticity, supplemented with terms accounting for damage effect is used to predict inelastic behaviour of material in stress controlled tests. Then the incremental formulation is inverted to simulate stress development in ramps where strain is the input parameter. A comparison with tests shows good ability of the model in inverted form to predict stress–strain response as long as the applied strain is increasing. However, in strain controlled ramps with unloading, the inverted model shows unrealistic hysteresis loops. This is believed to be a proof of the theoretically known incompatibility of the stress and strain controlled formulations for nonlinear materials. It also shows limitations of material models identified in stress controlled tests for use in strain controlled tests.

  • 260.
    Varna, Janis
    et al.
    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.
    Steady-State Debond Growth Along Broken Fibers in Unidirectional Composites During Cyclic Loading2013In: 9th International Conference on Composite Science and Technology: 2020 - Scientific and Technical Challenges / [ed] Michelle Meo, Lancaster: PA Destech Publications Inc. , 2013, p. 981-983Conference paper (Refereed)
    Abstract [en]

    Several regions with different governing failure mechanisms may be distinguished analyzing axial tension-tension cyclic loading of unidirectional (UD) composite. At high loads multiple fiber fracture takes place during the first loading cycle. The relative number of broken fibers depends on the maximum stress and can be estimated assuming that the fiber strength is a statistical property following Weibull distribution. Multiple fiber fracture (fragmentation) is possible because the load is transferred into the broken fiber through the interface. In cyclic loading with constant amplitude we usually assume that fibers do not experience fatigue and all fiber breaks occur during the first cycle. During the following cyclic loading debonds start to grow along the fiber/matrix interface and this is the main mode of damage progression analyzed in this paper. The extensive debonding reduces the load borne by the broken fiber causing new fiber breaks and connecting the multiple fiber breaks, finally leading to catastrophic rupture of the UD composite. Hence, understanding of the debond growth process in fatigue is crucial for fatigue life analysis in this load region.

  • 261.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rozite, Liva
    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.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Non-linear behaviour of PLA based flax composites2012In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 41, no 2, p. 49-60Article in journal (Refereed)
    Abstract [en]

    The mechanical behaviour of polylactic acid/flax fibre composite in tension was investigated by analysing elastic properties and loading curves. The observed non-linearity was attributed to microdamage, viscoelastic and viscoplastic response, suggesting Schapery's type of model for viscoelasticilty and Zapas' model for viscoplasticity. It was found that after loading at stress levels below the maximum possible, the elastic modulus is not affected, and therefore, damage does not need to be included in the material model. Viscoplastic and viscoelastic strain development was analysed in creep and strain recovery tests at several high stress levels. The identified and validated material model is non-linear viscoelastic and viscoplastic with slight non-linearity even in the elastic strain term. It appears that there is no region of linear viscoelasticity for this material. Non-linear elasticity, viscoelasticity and viscoplasticity are equally responsible for the observed non-linearity in the tensile tests

  • 262.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nättinen, Kalle
    VTT.
    Lampinen, Johanna
    VTT.
    Time dependent behavior of flax/starch composites2012In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 16, no 1, p. 47-70Article in journal (Refereed)
    Abstract [en]

    The time dependent mechanical response of flax fiber reinforced thermoplastic starch matrix composite and neat starch is analyzed. It is demonstrated that the response is highly sensitive with respect to the relative humidity (with specific saturation moisture content in the composite) and special effort has to be made to keep it constant. It was found that micro-damage accumulation and resulting elastic modulus reduction in this type of composites is limited. The highly nonlinear behavior of composites is related to nonlinear viscoelasticity and viscoplasticity. These phenomena are accounted for by simple material models suggested in this study. The stress dependent nonlinearity descriptors in these models are determined in creep and strain recovery tests at low as well as high stresses

  • 263.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Erratum to: Integration of Macro- and MicroDamage Mechanics for the Performance Evaluation of Composite Materials2012In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 48, no 3, p. 353-Article in journal (Refereed)
  • 264.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Integration of macro- and microdamage mechanics for the performance evaluation of composite materials2012In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 48, no 2, p. 145-160Article in journal (Refereed)
    Abstract [en]

    When subjected to mechanical loads, composite materials can generate complex configurations of multiple cracks that collectively change the average material response and thereby the performance, e.g., the fatigue life. Over many years, various approaches have been developed to predict the properties of composite materials with damage, but none of them is capable of treating other than a few special cases. In this paper, we address the two main approaches, described as macro- and microdamage mechanics, and demonstrate that, by integrating these in a combined approach, an effective methodology for performance evaluation can be achieved to treat a broad range of composite materials

  • 265.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Objedinenie mekhaniki makro- i mikropovrezdenia dlja ocenki funkcionalnykh kharakhteristik kompozitnykh materialov2012In: Mechanika kompozitnych materialov, ISSN 0203-1272, Vol. 48, no 2, p. 211-234Article in journal (Refereed)
    Abstract [en]

    When subjected to mechanical loads, composite materials can generate complex configurations of multiple cracks that collectively change the average material response and thereby the performance, e.g., the fatigue life. Over many years, various approaches have been developed to predict the properties of composite materials with damage, but none of them is capable of treating other than a few special cases. In this paper, we address the two main approaches, described as macro- and microdamage mechanics, and demonstrate that, by integrating these in a combined approach, an effective methodology for performance evaluation can be achieved to treat a broad range of composite materials

  • 266.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Zhuang, Linqi
    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.
    Ayadi, Zoubir
    Universite de Lorraine, Nancy.
    Growth and interaction of debonds in local clusters of fibers in unidirectional composites during transverse loading2017In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 734, p. 63-66Article in journal (Refereed)
    Abstract [en]

    Fiber/matrix debonding in transverse tensile loading of a unidirectional composite is analyzed calculating energy release rate (ERR) for interface crack propagation. Non-uniform fiber distribution (local hexagonal fiber clustering) is assumed in the model. The matrix region containing the central fiber with the debond and the 6 surrounding fibers is embedded in a large block of homogenized composite which has the same fiber content as the region analyzed explicitly. Some of the fibers surrounding the central fiber may also have a debond. The effect of the local clustering and of the presence of other debonds on magnification of the ERR is analyzed

  • 267.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Zrida, Hana
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Analysis of Microdamage in Thermally Aged CF/Polyimide Laminates2017In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 53, no 1, p. 45-58Article in journal (Refereed)
    Abstract [en]

    Microdamage in the layers of CF Thornel® T650 8-harness satin-weave composites with a thermosetting NEXIMID® MHT-R polyimide resin, designed for high service temperatures, is analyzed. After cooling down to room temperature (RT), a multiple intrabundle cracking due to tensile transverse thermal stresses was observed in the [(+45/–45)/(90/0)]2s laminates studied. Then, the composite was subjected to two ramps of thermal cycling quantifying the increase in crack density in its layers. A comparison of two ramps with the same lowest temperature showed that the highest temperature in the cycle where thermal stresses were low had a significant detrimental effect on the thermal fatigue resistance of the composite. The effect of holding it at 288°C for 40 days was also studied: many new cracks formed in it after cooling down to RT. During the time at the high temperature, the mechanical properties degraded with time, and the crack density versus aging time was measured at RT. Then, both aged and nonaged specimens were tested in uniaxial quasi-static tension quantifying the damage development in layers of different orientation. Cracking in the layers was analyzed using fracture mechanics arguments and probabilistic approaches: a) a simple one, not considering crack interaction; b) Monte Carlo simulations. It is shown that cracking in the off-axis layers which are not in contact with the damaged 90°-layer can be predicted based on the Weibull analysis of the 90°-layer, whereas in the off-axis layer contacting the 90°-layer, the crack density is much higher due to the local stress concentrations caused by cracks in the 90°-layer. The thermal treatment degraded the cracking resistance in the surface and adjacent layer, whereas the composite close to the midplane was not changed.

  • 268.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Zrida, Hana
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fernberg, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microdamage analysis in thermally aged CF/polyimide laminates2016In: IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981, E-ISSN 1757-899X, Vol. 139, article id 012050Article in journal (Refereed)
    Abstract [en]

    Microdamage in layers of CF Thornel® T650 8-harness satin woven composite with thermosetting polyimide NEXIMID® MHT-R resin was analysed. After cooling to room temperature multiple intra-bundle cracking due to tensile transverse thermal stresses was observed in the studied [(+45/-45)/(90/0)]2s composite. The composite was subjected to thermal cycling quantifying the increase of crack density in layers. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant detrimental effect. Exposure for 40 days to 288°C caused many new cracks after cooling down to room temperature. Both aged and not aged specimens were tested in uniaxial quasi-static tension. Cracking was analysed using fracture mechanics and probabilistic approaches. Cracking in off-axis layers was predicted based on Weibull analysis of the 90- layer. The thermal treatment degraded the cracking resistance of the surface layer and of the next layer.

  • 269.
    Wu, W.
    et al.
    Katholieke Universiteit Leuven.
    Desaeger, M.
    Katholieke Universiteit Leuven.
    Verpoest, I.
    Katholieke Universiteit Leuven.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An improved analysis of the stresses in a single-fibre fragmentation test: I. Two-phase model1997In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 57, no 7, p. 809-819Article in journal (Refereed)
    Abstract [en]

    In order to improve the analysis of the single-fibre fragmentation test, a modified variational analysis, based on the principle of minimum complementary energy, has been derived to describe the stress states around breaks in a single fibre embedded in an infinite matrix. The variational analysis includes the non-uniform stress distribution in the radial direction of the matrix cylinder, and allows for crack interaction. The stress profiles along the interface between the fibre and matrix are presented and compared with the prediction by the variational approach presented by Nairn. The predictions are also compared with the results from finite element analysis and good agreement is obtained. While the present analysis is more accurate than existing analytical models, it leads to very simple final expressions. Thus, it is convenient to be used for data reduction of the single-fibre fragmentation test results. In particular, it can be easily generalized to a three-phase model, including an interphase between fibre and matrix. The extensive applications of this model are also presented.

  • 270.
    Wu, W.
    et al.
    Katholieke Universiteit Leuven.
    Jacobs, E.
    Katholieke Universiteit Leuven.
    Verpoest, I.
    Katholieke Universiteit Leuven.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Variational approach to the stress-transfer problem through partially debonded interfaces in a three-phase composite1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 4, p. 519-535Article in journal (Refereed)
    Abstract [en]

    In our previous study (Wu W, Verpoest I, Varna J. Compos Sci Technol 1998;58(12):1863-77) of the stress-transfer problem around a single fibre, we presented a variational approach based on the principle of minimum complementary energy, not only in the perfectly bonded zone but also in the zone with a discontinuous interface of a two-phase composite. This approach is adapted in this paper to derive an accurate axisymmetric analytical model for the description of the stress state around the fibre breaks and partially debonded interfaces of a three-phase composite with a fibre, coating and matrix. The debonded fibre/coating interface is treated as a special external boundary on which a presumed interfacial shear stress with some free parameters is specified. Once the parameters are given, minimisation of complementary energy can be applied for both debonded- and bonded-interface zones together, to extract the most accurate closed-form solution. The shear stress at the debonded interface (the right values of free parameters) is finally found by substituting the calculated radial stresses in the Coulomb friction law and minimising the discrepancy by a simple numerical iteration. As the minimisation procedure is applied for the both debonded and bonded zones simultaneously, the strong interaction of the two zones is correctly described. This model also includes the matrix axial stress non-uniformity in the radial direction. The stress profiles along both axial and radial directions are presented and closely compared with the results from a finite element model and both agree quite well. A number of applications of this model are also discussed.

  • 271. Wu, W.
    et al.
    Verpoest, I.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A model for the stress distribution around the two breaks in a single fiber fragmentation test1996In: Realising their commercial potential: ECCM-7, Seventh European Conference on Composite Materials, 14 - 16 May 1996, London, UK. / [ed] Michael G. Bader, Cambridge: Woodhead Publishing Materials , 1996Conference paper (Refereed)
  • 272.
    Wu, W.
    et al.
    Katholieke Universiteit Leuven.
    Verpoest, I.
    Katholieke Universiteit Leuven.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A novel axisymmetric variational analysis of stress transfer into fibreS through a partially debonded interface1998In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 58, no 12, p. 1863-1877Article in journal (Refereed)
    Abstract [en]

    Most existing models for the problem of fibre/matrix stress-transfer through a partially debonded interface roughly solve the stress distribution in the debonded zone, neglecting the presence of the perfectly bonded zone. However the stress interactions between two zones is what makes the problem essentially different from the stress-transfer problem for a perfectly bonded interface. This paper suggests a variational approach based on the principle of minimum complementary energy not only in a perfectly bonded zone but also in a zone with a discontinuous interface. The debonded interface is treated as an external boundary on which a presumed interfacial shear stress is specified. A new analytical model, including stress non-uniformity in the radial direction and crack interaction, is derived to describe the stress state around fibre breaks and debonding tips in a single fibre embedded in an infinite matrix. For the presumed shear stress at the debonded interface the minimisation procedure renders the most accurate closed-form solution (under used assumptions) for both interactive zones. Finally, the ‘best' shear stress distribution at the debonded interface is found by using Coulomb's friction law and simple numerical iterations. The stress profiles along both axial and radial directions are presented and compared with results from a numerical model[1] available in the literature and also from finite-element analysis. Good agreements are achieved. Extensive applications of this approach and the derived model are also discussed

  • 273.
    Wu, W.
    et al.
    Katholieke Universiteit Leuven.
    Verpoest, I.
    Katholieke Universiteit Leuven.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An improved analysis of the stresses in a single-fibre fragmentation test-II. 3-phase model1998In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 58, no 1, p. 41-50Article in journal (Refereed)
    Abstract [en]

    A new model has been developed for the analyses of the stress state in a single-fibre specimen containing a coated fibre. This model is a generalization of the 2-phase model developed in Part I[1] for the case of 3 phases: fibre/coating/matrix. The stress distributions in radial and axial directions are obtained by using the principle of minimum complementary energy. Assumptions made to simplify the stress analysis are: (a) the axial stress distribution is uniform across the fibre cross-section; (b) the axial stress distribution is uniform across the coating thickness; (c) the non-uniform axial stress distribution in the matrix is given by a decreasing function which is obtained by a minimization procedure. The model developed has been used in a parametric analysis to determine the effect of the coating properties and geometry on the rate of stress transfer and shear stress concentration at the fibre end.

  • 274. Wu, W.
    et al.
    Verpoest, I.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Determination of interfacial fracture toughness from the fragmentation test data by variational models1999In: ICCM-12 Europe: 12th International Conference on Composite Materials ; Palais des Congrès, Paris / [ed] Thierry Massard; Alain Vautrin, Tours: ICCM , 1999Conference paper (Refereed)
  • 275.
    Wu, W.
    et al.
    Katholieke Universiteit Leuven.
    Verpoest, I.
    Katholieke Universiteit Leuven.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Prediction of energy release rate due to the growth of an interface crack by variational analysis2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 3, p. 351-360Article in journal (Refereed)
    Abstract [en]

    In our previous studies (Wu W, Verpoest I, Varna J. A novel axisymmetric variational analysis of the stress transfer into fibre through a partially debonded interface. Composites Science and Technology 1998;58:1863-77 and Wu W, Jacobs E, Verpoest I, Varna J. Variational approach to the stress transfer problem through partially debonded interfaces in a three phase composite. Composites Science and Technology 1999;59:519-35) on the stress transfer problem for a single fragment of a fibre or a fibre with an additional interphase, embedded in an infinite matrix with a partially debonded interface, we presented two axisymmetric models, based on the principle of minimum complementary energy. In this paper, some parts of models, useful in the application of fracture mechanics, are first summarised. An expression is then given for the strain energy release rate due to a crack extension in an arbitrary composite system subjected to mixed traction-displacement boundary conditions and with thermal residual stresses included. This expression is suitable for a stress-based variational model. An important advantage is that it can include the friction work at the crack surfaces, in a rather simple but exact way, without using both the displacement and stress distributions at the crack surfaces as inputs. As an application of the expression, we illustrate how to calculate the energy release rate due to the growth of an interface crack in a single fibre fragment with a partially debonded interface under thermomechanical load, by using models in the above references. It is found that the energy release rate can be calculated by using only the rate of change of the strain energy related to the perturbation stress components. A second application is the energy change which is due to fibre breakage. The numerical results indicate that the energy release rate due to the growth of an interface debond or a similar problem can now be determined in a reliable way for both 2- and 3-phase composites.

  • 276. Wu, W.
    et al.
    Verpoest, I.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stress analysis in a three phase material with fibre, coating and matrix1997In: Damage and failure of interfaces: proceedings of the First International Conference on Damage and Failure of Interfaces, Vienna, Austria, 22 - 24 September 1997 / [ed] Hans-Peter Rossmanith, Rotterdam: Balkema Publishers, A.A. / Taylor & Francis The Netherlands , 1997Conference paper (Refereed)
  • 277.
    Xu, Johanna
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindbergh, Göran
    School of Chemical Science and Engineering, KTH Royal Institute of Technology.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Carbon fiber composites with battery function: Stresses and dimensional changes due to Li-ion diffusion2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 20, p. 2729-2742Article in journal (Refereed)
    Abstract [en]

    Structural composite materials that simultaneously carry mechanical loads, while storing electrical energy offers the potential of significantly reduced total component weight owing to the multifunctionality. In the suggested micro-battery, the carbon fiber is employed as a negative electrode of the battery and also as a composite reinforcement material. It is coated with a solid polymer electrolyte working as an ion conductor and separator while transferring mechanical loads. The coated fiber is surrounded by a conductive positive electrode material matrix. This paper demonstrates a computational methodology for addressing mechanical stresses arising in a conceptualized micro-battery and dimensional changes of the cell during electrochemical cycling, caused by time-dependent gradients in lithium ion concentration distribution.

  • 278.
    Xu, Johanna
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindbergh, Göran
    School of Chemical Science and Engineering, KTH Royal Institute of Technology.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Multiphysics modeling of mechanical and electrochemical phenomena in structural composites for energy storage: Single carbon fiber micro-battery2018In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 37, no 10, p. 701-715Article in journal (Refereed)
    Abstract [en]

    This paper presents a framework for multiphysics modeling of a novel type of multifunctional composite material, structured on microscale, with ability to function as battery cell in addition to carry mechanical load. The micro-battery consists of a single carbon fiber surrounded by very thin solid electrolyte coating and embedded in a matrix which is a porous material containing active particles able to intercalate lithium. During battery operation (discharging and charging) the structural battery constituents undergo volume changes, caused by lithium-ion movement. The presented mathematical model is solved numerically using COMSOL software and results are used to analyze the physical phenomena occurring in the structural battery material. Parametric analysis is performed to reveal the significance of geometrical parameters like fiber volume fraction in the battery and the porosity content in the matrix on the multifunctional performance of the composite unit including its swelling/shrinking during charging/discharging.

  • 279.
    Xu, Johanna
    et al.
    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.
    Lindbergh, Göran
    KTH Royal Institute of Technology, School of Chemical Science and Engineering.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Multifunctional composites: Modeling intercalation induced stresses in constituents of micro-battery2016In: ECCM 2016: Proceeding of the 17th European Conference on Composite Materials, European Conference on Composite Materials , 2016Conference paper (Refereed)
    Abstract [en]

    A structural battery that simultaneously carries mechanical loads while storing electrical energy offers the potential of significantly reduced total vehicle weight owing to the multifunctionality. Carbon fiber is employed as negative electrode of the battery and also as a composite reinforcement material. It is coated with a solid polymer electrolyte working as an ion conductor and separator whilst transferring mechanical loads. The coated fiber is surrounded by a conductive positive electrode material. This paper demonstrates a methodology for addressing mechanical stresses arising in a conceptualized micro battery cell during electrochemical cycling, caused by time dependent gradients in lithium ion concentration distribution in the carbon fiber

  • 280.
    Xu, Johanna
    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.
    Matrix and interface cracking in cross-ply composite structural battery under combined electrochemical and mechanical loading2020In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 186, article id 107891Article in journal (Refereed)
    Abstract [en]

    In this paper propagation of matrix cracks and debonds at the coating/matrix interface in the 90°-layer of a cross-ply structural composite battery are studied numerically. The structural composite battery consists of micro-battery units, made of a solid electrolyte coated carbon fiber embedded in an electrochemically active polymer matrix. During charging the fiber swells and the matrix shrinks leading to high stresses on the fiber/matrix scale and to anisotropic free expansion of the composite ply. Two load cases are considered, pure electrochemical load (intercalation) and combined electrochemical and thermomechanical load. Energy release rates (ERR) of radial matrix cracks along two potential propagation paths are calculated using 2-D finite element models of the transverse plane in a cross-ply laminate with a square packing of fibers in the 90°-ply and using homogenized 0°-ply. Results show that the matrix crack growth towards the nearest fiber is unstable, and that the debond crack growth is in mixed mode. For a cross-ply structural battery composite the sequence of macro-scale crack forming events differs from a conventional cross-ply composite, as well as for a UD composite battery laminate. The most likely course of failure events in a cross-ply laminate are: 1) vertical radial matrix crack initiation and unstable growth; 2) debond is initiated at certain length of the matrix crack.

  • 281.
    Xu, Johanna
    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.
    Matrix and interface microcracking in carbon fiber/polymer structural micro-battery2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 53, no 25, p. 3615-3628Article in journal (Refereed)
    Abstract [en]

    In this paper, the propagation of radial matrix cracks and debond cracks at the coating/matrix interface in unidirectional carbon fiber structural micro-battery composite are studied numerically. The micro battery consists of a solid electrolyte-coated carbon fiber embedded in an electrochemically active polymer matrix. Stress analysis shows that high hoop stress in the matrix during charging may initiate radial matrix cracks at the coating/matrix interface. Several 2-D finite element models of the transverse plane with different arrangements of fibers and other matrix cracks were used to analyze the radial matrix crack growth from the coating/matrix interface of the central fiber in a composite with a square packing of fibers. Energy release rates of radial cracks along two potential propagation paths are calculated under pure electrochemical loading. The presence of a radial matrix crack imposes changes in the stress distribution along the coating/matrix interface, making debonding relevant for consideration. Results for energy release rates show that the debond crack growth is governed by mode II.

  • 282.
    Zhang, H.
    et al.
    Luleå tekniska universitet.
    Ericson, M.L.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Transverse single-fibre test for interfacial debonding in composites: 1. Experimental observations1997In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 28, no 4, p. 309-315Article in journal (Refereed)
    Abstract [en]

    Micromechanical methods for studies of interfacial debonding in fibre composites provide the possibility of comparing different materials. Since most previous composite studies focus on shear loading of the fibre matrix interface, the debonding process in single-fibre glass fibre/epoxy composites was studied in situ by optical microscopy during transverse tensile loading. Specimens had cylindrical debond cracks of known dimensions created in fragmentation tests. Mechanisms for debond growth were described for two materials where the difference was in glass fibre surface treatment. As the debonds reached a critical size in the arc (circumferential) direction, unstable debond growth occurred in the fibre direction. The debond angle at instability was fairly similar for both materials, although the material based on glass fibre treated with a coupling agent reached twice as high stress before instability.

  • 283. Zhang, H.
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ericson, M.
    Berglund, Lars
    Luleå tekniska universitet.
    Interfacial debonding of single fibers in transverse tension1994In: Composite materials, mechanics and processing : September 20 - 22, 1994, John M. Clayton Conference Center, University of Delaware, Newark, Delaware, Society of Plastics Engineers, 1994, p. 595-602Conference paper (Refereed)
    Abstract [en]

    A micromechanical method to measure and analyze interfacial debonding of single fibers in transverse tension was developed. The damage process was observed in-situ by optical microscopy. Two glass fiber/epoxy (GF/EP) systems were studied with different fiber surface treatment. Results from calculations of the interfacial fracture toughness for Mode I and Mode II failure are presented. The theoretical analysis is based on a modification of Toya's stress analysis of a debonded circular inclusion, calculation of the work required to close the crack and a mixed-mode failure criterion

  • 284.
    Zhuang, Linqi
    et al.
    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.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Universite, Science et Ingénierie des Matériaux et Métallurgie (SI2M), Institut Jean Lamour, Nancy, Laboratoire de Science et Génie des Surfaces, EEIGM, Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Effect of fiber clustering on debond growth energy release rate in UD composites with hexagonal packing2016In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 161, p. 76-88Article in journal (Refereed)
    Abstract [en]

    Steady-state energy release rate (ERR) for fiber/matrix interface debond growth originated from fiber break in unidirectional composite is calculated using 3-D FEM models with hexagonal fiber arrangement. In the model the debonded fiber is central in the hexagonal unit which is surrounded by effective composite. The effect of neighboring fibers focusing on local fiber clustering on the ERR is analyzed by varying the distance between fibers in the unit. The steady-state ERR is calculated from potential energy difference between a unit in the bonded region far away from the debond front and a unit in the debonded region far behind the debond front. The ERR for different modes of crack propagation is obtained from a FEM model containing a long debond by analyzing the stress at the debond front.Results show that in mechanical axial tensile loading fracture Mode II is dominating, it has strong angular dependence (effect of closest fibers) but the average ERR is not sensitive to the local fiber clustering. In thermal loading the Mode III is dominating and the average ERR is highly dependent on the distance to neighboring fibers. However, for realistic loads the thermal ERR is much smaller than the mechanical.

  • 285.
    Zhuang, Linqi
    et al.
    Department of Aerospace Engineering, Texas A&M University, Texas A&M University, College Station.
    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.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Universite, Science et Ingénierie des Matériaux et Métallurgie (SI2M), Institut Jean Lamour, Nancy, Laboratoire de Science et Génie des Surfaces, EEIGM, Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, University of Lorraine, F-54010, Nancy.
    Fiber/matrix debond growth from fiber break in unidirectional composite with local hexagonal fiber clustering2016In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 101, p. 124-131Article in journal (Refereed)
    Abstract [en]

    Energy release rate (ERR) for fiber/matrix debonding in composite with local fiber clustering, subjected to axial tension, has been investigated numerically by a 3-D finite element (FE) model. In the model, broken fiber is central in a hexagonal unit which is embedded in an effective composite. Fiber/matrix debond with circular front is assumed to be originated from the fiber break. The effect of the local fiber clustering on ERR is studied by varying distance between the broken fiber and the neighboring fibers. For very short debonds as well as for long debonds (almost steady-state growth) the ERR was calculated by both the J integral and the Virtual crack closure technique (VCCT). Results show that the debond growth is Mode II dominated and that the ERR strongly depends on the angular coordinate. The local fiber clustering has larger effect on the angular variation for shorter debonds and the effect increases with larger local fiber volume fraction. The results obtained from the 3-D hexagonal model are compared with those obtained previously using 5-cylinder axisymmetric model developed by the same authors. The ERR values from 5-cylinder axisymmetric model could be considered as upper bound for the 3-D hexagonal model.

  • 286.
    Zhuang, Linqi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Texas A&M, College Station; University of Lorraine, France.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. SICOMP Swerea.
    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. Texas A&M University, College Station, TX .
    Ayadi, Zoubir
    University of Lorraine.
    Effects of Inter-Fiber Spacing on Fiber-matrix Debond Crack Growth in Unidirectional Composites under Transverse Loading2018In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 109, p. 463-471Article in journal (Refereed)
    Abstract [en]

    The energy release rate (ERR) of a fiber-matrix debond crack in a unidirectional composite subjected to transverse tension is studied numerically. The focus of the study is the effect of the proximity of the neighboring fibers on the ERR. For this, a hexagonal pattern of fibers in the composite cross-section is considered. Assuming one fiber to be debonded at certain initial debond arc-length, the effect of the closeness of the surrounding six fibers on the ERR of the crack is studied with the inter-fiber distance as a parameter. Using an embedded cell consisting of discrete fibers in a matrix surrounded by the homogenized composite, a finite element model and the virtual crack closure technique are used to calculate the ERR. Results show that the presence of the local fiber cluster accelerates the crack growth up to a certain initial crack angle, beyond which the opposite effect occurs. It is also found that the residual stress due to thermal cooldown reduces the ERR. However, the thermal cooldown is found to enhance the debond growth in plies within a cross-ply laminate.

  • 287.
    Zhuang, Linqi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Texas A and M University, College Station, TX, United States; University of Lorraine, SI2M, Nance, France.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Texas A and M University, College Station, TX, United States.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of cooldown induced fiber/matrix interfacial2017In: 32nd Technical Conference of the American Society for Composites 2017 / [ed] Yu W.,Pipes R.B.,Goodsell J., DEStech Publications Inc , 2017, Vol. 2, p. 1095-1102Conference paper (Refereed)
    Abstract [en]

    Despite efforts to improve the overall quality of composite materials, the presence of fiber/matrix interfacial defects seems inevitable. In composites with high fiber volume fraction, the small inter-fiber spacing can lead to development of high tensile radial stress at the fiber/matrix interfaces on cooldown from a high cure temperature. This stress can cause failure from defects at the interfaces earlier before any mechanical loads are applied. In the present paper, we study further progression of cracking from a preexisting disbond (debonding crack) that has been formed by thermal cooldown on remotely applying transverse tension to the composite. In the finite element model, a local region of hexagonally packed fibers embedded in a homogenized composite is analyzed. The cooldown induced disbond is assumed to initiate at the location where tensile radial stress resulting from cooldown is the highest. Energy release rate of the debonding crack is calculated by the Virtual Crack Closure Technique (VCCT). Upon loading, it is found that the debonding crack tends to grow towards the symmetry plane normal to the loading direction. Furthermore, this crack is found not to kink out of the interface until it has fully propagated past the symmetry plane. As a result, further growth of the cooldown induced disbond as well as the potential kinking process are found to be the same as when the disbond initiates due to applied transverse tension. 

  • 288.
    Zhuang, Linqi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of manufacturing induced fiber break on local tensile failure in composites2016In: Proceedings of the American Society for Composites: 31st Technical Conference, ASC 2016 / [ed] Davidson B.D.,Czabaj M.W.,Ratcliffe J.G, DEStech Publications Inc , 2016Conference paper (Refereed)
    Abstract [en]

    Fiber breakages are commonly found during composites manufacturing process. In the current study, the effect of manufacturing induced fiber break on local tensile failure in unidirectional (UD) composites is investigated numerically. In the finite element (FE) model, a broken fiber is placed centrally in a hexagonally packed UD composite and is assumed to be perfectly bonded to the matrix. Since the stress perturbation caused by a single fiber breakage is local, only the six most affected nearest-neighbor fibers are modeled and are placed as a ring of concentric axisymmetric cylinder embedded in the matrix. The discrete fiber region is surrounded by a concentric outer cylinder ring of homogenized composite. The entire FE model is subjected to axial tensile loading. Upon loading, it is found that matrix crack would most likely initiate perpendicular to fiber axis by cavitation due to tri-axial stress state near fiber break, and the thermal residual stress is found to promote the cavitation process. Once the matrix crack initiates from fiber break, fracture mechanics methodology is adopted by using extended finite element method (XFEM) to simulate the matrix crack propagation. The stress concentration factors (SCF) along the neighboring fibers are calculated during matrix crack propagation and obtained results show that the maximum SCF is the highest when matrix crack reaches a neighboring fiber. Finally, the statistical consideration regarding neighboring fiber failure is incorporated and it is found that the initial fiber breakage, together with the matrix cracking that follows, greatly enhance the probability of neighboring fiber failing at the local region close to the original fiber-break plane, which indicates that a planar fracture plane is expected if final tensile failure of UD composite starts from a manufacturing induced fiber break.

  • 289.
    Zhuang, Linqi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tensile failure of unidirectional composites from a local fracture plane2016In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 133, p. 119-127Article in journal (Refereed)
    Abstract [en]

    Final failure in composite structures often occurs from fiber failures in a local region. This paper aims to study the formation of a critical fracture plane that simulates the local failure region in a unidirectional composite. The nucleation site for the critical plane is assumed to be a broken fiber. An axisymmetric finite element model is constructed to study the progression of failure from the broken fiber placed at the center and surrounded by intact fibers. Two scenarios are considered: one, where the broken fiber results from a manufacturing process, and two, where a fiber fails at a weak point under loading. In the first case, a matrix crack is found to initiate from the broken fiber end and grow normal to the fiber axis, while in the second case, a matrix crack kinks out of an (assumed) short fiber/matrix debond crack and grows out towards the neighboring fibers. The consequent stress enhancement in the neighboring fibers is analyzed to determine their probability of failure. The influence of the initial debond length on the formation of the critical fracture plane is also studied.

  • 290.
    Zhuang, Linqi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Texas A&M University, College Station, TX, USA.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA. Department of Aerospace Engineering, Texas A&M University, College Station, TX, USA.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Transverse crack formation in unidirectional composites by linking of fibre/matrix debond cracks2018In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 107, p. 294-303Article in journal (Refereed)
    Abstract [en]

    Plausible mechanisms of transverse crack formation in unidirectional (UD) composites under applied tension normal to fibres are investigated numerically using a finite element model. Two initial scenarios are considered: Scenario 1 where a pre-existing single fibre/matrix debond crack kinks out into the matrix and induces fibre/matrix debonding at neighbouring fibres, and Scenario 2 where multiple pre-existing debond cracks link up by the debond growth and crack kink-out process. The 2-D finite element model consists of a circular region of matrix with a central fibre surrounded by six fibres in a hexagonal pattern. The region is embedded in a homogenized UD composite of rectangular outer boundary. Energy release rates (ERRs) of interface cracks and kinked-out cracks are calculated under applied tension normal to fibres. Results show that Scenario 2 is more likely to lead to formation of a transverse crack than Scenario 1. These results provide understanding of the roles of fibre clustering and fibre volume fraction on transverse crack formation in composites

  • 291.
    Zrida, Hana
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fernberg, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, Ecole Européenne d’Ingénieurs en Génie des Matériaux, Université de Lorraine.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microcracking in thermally cycled and aged Carbon fibre/polyimide laminates2017In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 94, no 1, p. 121-130Article in journal (Refereed)
    Abstract [en]

    Carbon fibre T650 8-harness satin weave fabric composites with thermosetting polyimide resin designed for high service temperatures are solidified at 340 °C. High thermal stresses develop after cooling down to room temperature, which lead to multiple cracking in bundles of the studied quasi-isotropic composite. The composites are subjected to two thermal cycling ramps and the increase of crack density in each bundle is quantified. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant effect on thermal fatigue resistance. During thermal aging tests at 288 °C the mechanical properties are degrading with time and the crack density after certain aging time is measured. Aging and fatigue effects are separately analysed showing that part of the cracking in thermal cycling tests is related to material aging during the high temperature part of the cycle. Numerical edge stress analysis and fracture mechanics are used to explain observations. The 3-D finite element edge stress analysis reveals that there is large edge effect that induces a large difference in the damage state between the different layers on the edge. The linear elastic fracture mechanics explains the higher initiated and propagated crack density in the surface layers comparing to the inner layers.

  • 292.
    Zrida, Hana
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fernberg, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP AB, Piteå.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of extreme temperatures on micro-damage development in CF/polyimide laminates2015In: 20th International Conference on Composite Materials: Copenhagen, 19-24th July 2015, ICCM , 2015, article id 2204-2Conference paper (Refereed)
    Abstract [en]

    CF Thornel® T650 8-harness satin weave fabric composite with thermosetting polyimide NEXIMID® MHT-R resin designed for high service temperatures is produced at around 390°C and therefore high thermal stresses develop after cooling down to room temperature. Thermal transverse stresses in bundles/layers are tensile and lead to multiple intra-bundle /intra-laminar cracking. When the composite plate is subjected to large and repeated temperature variations, new cracks can appear due to thermally induced fatigue stress. Experimental results show that the highest temperature inthe cycle, where thermal stresses are low, has a significant detrimental effect on thermal fatigue resistance. Another observed phenomenon is thermal aging: at high temperature the mechanical properties are degrading with time. Aging and fatigue effects were separately analyzed for quasi-isotropic laminates with lay-up [(+45/-45)/(90/0)]2s.

  • 293.
    Zrida, Hana
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Giannadakis, Konstantinos
    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, Nancy Universite.
    The effect of mesostructure heterogenity on the cracks initiation and the displacement distribution in NCF composites2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, IOP Publishing Ltd , 2012, article id 012023Conference paper (Refereed)
    Abstract [en]

    Non Crimp Fabrics (NCF) are promising new generation composite materials. They are now being used in some sections of composite industry, for example in wind turbine blades and boat hulls. The aerospace industry also shows an increasing interest in this material, thanks to the low cost of its manufacturing process. NCFs are special types of textile composites, made of layers of parallel fiber bundles oriented in different directions and separated by resin. Due to the manufacturing process the fiber bundles are not perfectly straight. They show a certain degree of waviness which decreases the stiffness and the strength of the material. The heterogeneous mesostructure affects the mechanical properties of the material and the failure mechanisms. This was studied using both numerical and experimental methods. In our experimental approach, a carbon fiber/epoxy resin laminate with uniform fiber distribution was manufactured by voluntarily introducing waviness to simulate the NCF composites. The displacement map was studied against the thickness of a sample loaded in tension, using ESPI (Electronic Speckle Pattern Interferometry). This can give us a primary idea of the micro damage initiation and the cracks' shapes.

  • 294.
    Zrida, Hana
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Loukil, Mohamed Sahbi
    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, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Crack opening displacement determination in damaged cross-ply laminate using electronic speckle pattern interferometry (ESPI)2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: IOP Publishing Ltd , 2012Conference paper (Refereed)
    Abstract [en]

    Composite laminates during service undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined in this work is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks. To validate these assumptions experimentally the displacement field on the surface of a [90/0/90] carbon fiber/epoxy laminate specimens with multiple intralaminar cracks in the surface layer is studied and the COD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). The displacement jumps corresponding to cracks are clearly visible and can be used to determine the opening displacement along the cracks. The effect of crack interaction on the COD at high crack density is also investigated.

  • 295.
    Zrida, Hana
    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.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Universite, Science et Ingénierie des Matériaux et Métallurgie (SI2M), Institut Jean Lamour, Nancy, Laboratoire de Science et Génie des Surfaces, EEIGM, Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effective stiffness of curved 0°-layers for stiffness determination of cross-ply non-crimp fabric composites2014In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 33, no 14, p. 1339-1352Article in journal (Refereed)
    Abstract [en]

    The effect of the 0°-tow waviness on axial stiffness of cross-ply non-crimp fabric composites is analysed using multiscale approach. The curved 0°- and 90°-layers are represented by flat layers with effective stiffness properties and classical laminate theory is used to calculate the macroscopic stiffness. The effective 0°-layer stiffness is calculated analysing isolated curved 0°-layers subjected not only to end loading, but also to surface loads. The surface loads are identified in a detailed finite element analysis and approximated by a sinus shaped function with amplitude depending on the waves parameters. The sinus shaped surface loads are then applied to an isolated curved 0°-layer finite element model together with end loading to calculate the effective stiffness of the layer. Finally, the effective 0°-layer stiffness was successfully used to calculate the macroscopic stiffness of the composite proving validity of the approach being used and showing that, without losing accuracy, elastic properties in the 90°-layers with bundle structure can be replaced by the transverse stiffness of the homogenised 90°-layer material.

  • 296.
    Zrida, Hana
    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.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Master curve approach to axial stiffness calculation for non-crimp fabric biaxial composites with out-of-plane waviness2014In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 64, p. 214-221Article in journal (Refereed)
    Abstract [en]

    The effect of 0∘-tow out-of-plane waviness on the biaxial Non-Crimp-Fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0∘-layer and the 90∘-layer with varying thickness. It is shown that the NCF composite knock-down factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the knock-down factor for the curved 0∘-layer. Numerical analysis showed that 90∘-layer knock-down factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for knock-down factors for cases with different wavelength and amplitudes based on two FE calculations only.

3456 251 - 296 of 296
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