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  • 1.
    Advani, Suresh G.
    et al.
    University of Delaware, Department of Mechanic Engineering.
    Talreja, Ramesh
    A continuum approach to determination of elastic properties of short fibre composites1993In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 29, no 2, p. 127-137Article in journal (Refereed)
  • 2.
    Andersons, J.
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Sandmark, R.
    Materials division, SINTEF SI.
    Constrained fragmentation of composites under uniaxial loading1995In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 31, no 1, p. 26-33Article in journal (Refereed)
  • 3.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    The onset of mixed mode intralaminar cracking in a cross-ply composite laminate2008In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 44, no 6, p. 549-556Article in journal (Refereed)
    Abstract [en]

    The intralaminar fracture toughness of a unidirectionally reinforced glass/epoxy composite is determined experimentally at several mode I and mode II loading ratios. The crack propagation criterion, expressed as a quadratic form in terms of single-mode stress intensity factors (alternatively, linear in terms of energy release rates), approximates the test results reasonably well. The mixed-mode cracking criterion obtained is used to predict the intralaminar crack on set in a cross-ply glass/epoxy composite under off-axis tensile loading.

  • 4.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Rubenis, Ojars
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Estimation of laminate stiffness reduction due to cracking of a transverse ply by employing crack initiation- and propagation-based master curves2008In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 44, no 5, p. 441-450Article in journal (Refereed)
    Abstract [en]

    The applicability range of toughness-and strength-based criteria for progressive cracking of a transverse layer in a cross-ply composite laminate subjected to tensile loading is considered. Using a deterministic cracking model, approximate relations for the crack density as a function of stress are derived for initiation-and propagation-controlled types of cracking. The master-curve approach is applied to progressive cracking in glass/epoxy laminates. The accuracy of estimation of laminate stiffness reduction by using crack density master curves is evaluated.

  • 5. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Influence of flow-induced deformations of fabrics on the formation and transport of bubbles during liquid moulding processes2011In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 47, no 2, p. 221-232Article in journal (Refereed)
    Abstract [en]

    The viscous flow-induced deformation of non-crimp fabrics during liquid moulding processes, such as resin transfer moulding, and its influence on the creation of bubbles behind the liquid front are studied. A transverse flow with a low Reynolds number through random arrays of aligned cylinders is considered with account of changes in the transverse permeability of fibre bundles. A combined methodology of directly solving the two-dimensional Navier-Stokes equations for the flow in the vicinity of a single fibre and minimization of the dissipation rate in a system of fibres is employed. Sethian's level set method is used for transient calculations of the motion of the liquid-gas front, at which the capillary pressure is accounted for. The continuity is maintained, and local correlations between the dimensions of neighbouring gaps formed between bundles are used. The elastic deformations of the fibre bundles are calculated based on micromechanical analyses. The void fractions of inter-and intrabundle bubbles obtained differ for deformable and non-deformable fabrics, but both the cases compare well with those from real mouldings

  • 6. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jakovics, Andris
    University of Latvia.
    Permeability of clustered fibre networks: modelling of unit cell2003In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 39, no 3, p. 265-272Article in journal (Refereed)
    Abstract [en]

    The paper is focused on estimating the permeability of a clustered fiber network by variational methods. First, a laminar flow in ducts is considered by using polynomial trial functions. Then, a longitudinal flow through a square array is described by expanding the flow-rate field in trigonometric and Laurent series. Finally, a formal scheme for estimating the longitudinal permeability in a cluster with an irregular distribution of fibers is given. The irregular distribution is modeled by setting an individual effective radius for each fiber and then letting this fiber reach its minimum gravitational energy. The results obtained here form a basis for future predictions of the permeability of fibrous reinforcements.

  • 7.
    Hajlane, Abdelghani
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Laboratoire CRISMAT UMR 6508 CNRS/ENSICAEN, Caen Cedex 4, France.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Identification of a Model of Transverse Viscoplastic Deformation for a UD Composite from Curvature Changes of Unsymmetric Cross-Ply Specimens2019In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 55, no 3, p. 363-384Article in journal (Refereed)
    Abstract [en]

    A novel testing methodology for the characterization of viscoplastic (VP) deformation of unidirectional (UD) composites under transverse tensile loading is described. The law of transverse VP deformation, which, due to specimen failure, is almost impossible to obtain using UD specimens, is identified from curvature changes of an unsymmetrical CF/EP [0/90] specimen. An initially thermally curved specimen is loaded in the axial direction to different high levels of tensile strain, with a specified holding time. After unloading and waiting for recovery of the viscoelastic strains, the curvature of the specimen had changed due to the irreversible transverse VP strain and microdamage had developed in the 90° ply. The damage effect is incorporated into the model by introducing the“effective stiffness” of the damaged layer, which is a function of the crack density and the local delamination length. A model based on the classical laminate theory is used to construct a relationship between the measured curvature after a loading step and the accumulated VP strain, temperature, and the effective stiffness. The back-calculated VP strains are analyzed, and their functional dependence on the stress and time in the viscoplasticity law is obtained. The model and the methodology are validated by comparing calculation results with test data for [902/0] laminates subjected to a similar loading. The methodology suggested could be very accurate on using a laminate with ultrathin plies in the test.

  • 8.
    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, p. 289-298Article 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.

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

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

  • 10.
    Katerelos, Dionysios
    et al.
    LTU.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Labou, D.
    FORTH/ICE-HT, Patras.
    Wallström, Lennart
    Alteration of the mechanical behaviour of polypropylene owing to successive introduction of multiwall carbon nanotubes and stretching2009In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 45, no 4, p. 423-434Article in journal (Refereed)
    Abstract [en]

    Stretching effects on the morphology of polypropylene/carbon nanotube composites is the focus of this work. The material under investigation was composed of isotactic polypropy lene (iPP) and multiwall car bon nanotubes (MWCNTs) in amount of 0.5 wt.%. The iPP and CNTs were mixed under sonication in a solvent, and the homogenized mixture was melted and pressed. The rectangular plates produced from the material were stretched by a constant load at a fixed temperature in order to obtain extended specimens. The scanning electron microscopy, a thermal gravimetric analysis, and the differential scanning calorimetry were employed to study variations in the structural morphology of the material. A dynamic mechanical analysis showed that the strain-induced crystallization of polypropylene and the possible Stone-Wales transformation of the carbon nanotubes due to stretching improved the mechanical performance of the nanocomposites considered.

  • 11. Katerelos, Dionysos
    et al.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Labou, D.
    FORTH/ICE-HT, Patras.
    Wallström, Lennart
    FORTH/ICE-HT, Patras.
    Erratum: Alteration of the mechanical behaviour of polypropylene owing to successive introduction of multiwall carbon nanotubes and stretching2009In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 45, no 5, p. 543-Article in journal (Other academic)
  • 12.
    Krasnikovs, A.
    et al.
    Riga Technical University, Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Transverse cracks in cross-ply laminates. 1. Stress analysis1997In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 33, no 6, p. 565-582Article in journal (Refereed)
    Abstract [en]

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

  • 13. Marjavaara, Daniel
    et al.
    Ebermark, Sofia
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Compression moulding simulations of SMC using a multi-objective surrogate-based inverse modeling approach2009In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 45, no 5, p. 503-515Article in journal (Refereed)
    Abstract [en]

    A multiobjective surrogate-based inverse modeling technique to predict the spatial and temporal pressure distribution numerically during the fabrication of sheet moulding compounds (SMCs) is introduced. Specifically, an isotropic temperature-dependent Newtonian viscosity model of a SMC charge is fitted to experimental measurements via numerical simulations in order to mimic the temporal pressure distribution at two spatial locations simultaneously. The simulations are performed by using the commercial computational fluid dynamics (CFD) code ANSYS CFX-10.0, and the multiobjective surrogate-based fitting procedure proposed is carried out with a hybrid formulation of the NSGA-IIa evolutionary algorithm and the response surface methodology in Matlab. The outcome of the analysis shows the ability of the optimization framework to efficiently reduce the total computational load of the problem. Furthermore, the viscosity model assumed seems to be able to re solve the temporal pressure distribution and the advancing flow front accurately, which can not be said of the spatial pressure distribution. Hence, it is recommended to improve the CFD model proposed in order to better capture the true behaviour of the mould flow.

  • 14.
    Modniks, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Porike, E.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, J.
    Institute of Polymer Mechanics, University of Latvia.
    Evaluation of the apparent interfacial shear strength in short-flax-fiber/PP composites2012In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 48, no 5, p. 571-578Article in journal (Refereed)
    Abstract [en]

    An important factor affecting the mechanical characteristics of composite materials is the adhesion of constituents which can be characterized by the interfacial shear strength (IFSS). Most of the experimental methods for the evaluation of IFSS involve model single-fiber composites. By contrast, the method proposed by Bowyer and Bader is based on an analysis of the stress-strain curve of a short-fiber-reinforced composite in tension. In the present study, the method is applied to the experimental tension curves of short-flax-fiberreinforced polypropylene to evaluate the IFSS and the fiber orientation factor of the composite. The IFSS was found to depend on the volume fraction of fibers and the treatment to improve adhesion.

  • 15.
    Parfeev, V.M.
    et al.
    Institute of the Mechanics of Polymers.
    Kurzemnieks, A. Kh.
    Institute of the Mechanics of Polymers.
    Joffe, Roberts
    Destruction of polyethylene-oxide films during storage1995In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 30, no 4, p. 335-339Article in journal (Refereed)
    Abstract [en]

    In lengthy storage of PEO films the amorphous part of the material is subjected most to destruction under the effect of environmental factors; consequently, films with a higher degree of crystallinity are more suitable for storage. The resistance of individual sections of film to environmental factors is a random variable, and the nature of the encountered patterns of destruction can therefore be reproduced with the aid probabilistic models.

  • 16.
    Pupure, Liva
    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.
    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.
    An analysis of the nonlinear behavior of lignin-based flax composites2013In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 49, no 2, p. 139-154Article in journal (Refereed)
    Abstract [en]

    A lignin composite reinforced with 30% flax fibers at two levels of relative humidity, 34 and 66%, was used in this study. The nonlinearity of the composite was analyzed by studying the degradation of its modulus and the development of viscoelastic and viscoplastic strains. The reduction in the modulus of lignin-based composites in tension starts before the maximum in the stress-strain curve is reached and can be as large as 50%. With increasing relative humidity, these effects are slightly magnified. The time-dependent phenomena in tension were examined in short-term creep and strain recovery tests, demonstrating a rather high viscoplastic strain in lignin composites. Both viscoelastic and viscoplastic strains are larger at a higher relative humidity

  • 17.
    Pupurs, Andrejs
    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.
    Fracture mechanics analysis of debond growth in a single-fiber composite under cyclic loading2011In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 47, no 1, p. 109-124Article in journal (Refereed)
    Abstract [en]

    A model is developed to analyze the growth of a fiber/matrix debond along a broken fiber interface in a single-fiber composite subjected to tension-tension fatigue. The Paris law expressed in terms of debond growth and strain energy release rates is used. An analytical solution for the Mode II energy release rate G II is obtained for long debonds, where the interface crack growth is self-similar. For short debonds, the interface crack interacts with the fiber break, and therefore a FEM modeling in combination with the virtual crack closure technique was performed to calculate the increase in G II . Finally, the calculated G II dependences are summarized in simple expressions that are used to simulate the debond growth in fatigue. A parametric study of the effect of Paris law parameters on the debond growth is performed.

  • 18.
    Sas, Gabriel
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Carolin, Anders
    Täljsten, Björn
    A model for predicting the shear bearing capacity of FRP-strengthened beams2008In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 44, no 3, p. 245-256Article in journal (Refereed)
    Abstract [en]

    The shear failure of reinforced concrete beams needs more attention than the bending failure since no or only small warning precedes the failure. For this reason, it is of utmost importance to understand the shear bearing capacity and also to be able to undertake significant rehabilitation work if necessary. In this paper, a design model for the shear strengthening of concrete beams by using fiber-reinforced polymers (FRP) is presented, and the limitations of the truss model analogy are highlighted. The fracture mechanics approach is used in analyzing the bond behavior between the FRP composites and concrete. The fracture energy of concrete and the axial rigidity of the FRP are considered to be the most important parameters. The effective strain in the FRP when the debonding occurs is determined. The limitations of the anchorage length over the cross section are analyzed. A simple iterative design method for the shear debonding is finally proposed.

  • 19.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Aerospace Engineering, Texas A&M University, Texas, USA.
    Studies on the failure analysis of composite materials with manufacturing defects2013In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 49, no 1, p. 35-44Article in journal (Refereed)
    Abstract [en]

    A strategy is discussed for the cost reduction of composite structures by allowing manufacturing defects while satisfying the performance requirements. To implement the strategy, cost/performance trade-offs must be made. These trade-offs require a proper assessment of the effect of defects on the performance-limiting conditions, such as the loss of required stiffness and/or strength. For illustration, two failure modes in composite laminates are considered: the delamination fracture and the progressive ply cracking under quasi-static loading. In the first case, the manufacturing-induced defects are treated directly by examining their effects on the energy release rate, while in the second case, the defects are taken into account indirectly via their influence on the statistics of local failure

  • 20.
    Tamuzs, V.
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Beilin, V.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Valdmanis, V.
    Institute of Polymer Mechanics, University of Latvia.
    Multicracking of brittle laminates1995In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 30, no 6, p. 529-539Article in journal (Refereed)
    Abstract [en]

    1. A short review of unidirectional multiply constrained cracking models is presented. 2. Differential equations for two-dimensional constrained cracking are derived based on the shear lag model and asymptotic analysis of long and short cracks is presented. Numerical results for straight and kinked cracks are presented, too. 3. Two methods of unidirectional multicracking analysis are generalized to a two-dimensional case in order to obtain the constrained crack patterns. A deterministic energetical approach is used to estimate the size of regular cracking cells. Stochastic crack patterns are obtained by multiple cracking process simulation

  • 21.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Mixed micromechanics and continuum damage mechanics approach to transverse cracking in [S, 90n ]s , laminates2001In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 37, no 2, p. 115-126Article in journal (Refereed)
    Abstract [en]

    The stiffness reduction in [S, 90 n ] slaminates due to transverse cracking in 90-layers is analyzed using the synergistic continuum damage mechanics (SCDM) and a micromechanics approach. The material constants involved in the SCDM model are determined using the stiffness reduction data for a reference cross-ply laminate. The constraint efficiency factor, which depends on the stiffness and geometry of neighboring layers, is assumed to be proportional to the average crack opening displacement (COD). The COD as a function of the constraint effect of adjacent layers and crack spacing is described by a simple power law. The crack closure technique and Monte Carlo simulations are used to model the damage evolution: the 90-layer is divided into a large number of elements and the critical strain energy rate G c having the Weibull distribution is randomly assigned to each element. The crack density data for a [02/904] s cross-ply laminate are used to determine the Weibull parameters. The simulated crack density curves are combined with the CDM stiffness reduction predictions to obtain the stiffness versus strain. The methodology developed is successfully used to predict the stiffness reduction as a function of crack density in [±Θ/904] s laminates.

  • 22.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Transverse cracks in cross-ply laminates 2. Stiffness degradation1998In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 34, no 2, p. 153-170Article in journal (Refereed)
  • 23.
    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)
  • 24.
    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

  • 25.
    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.

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