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  • 1.
    Di Stasio, Luca
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. IJL, EEIGM, Université de Lorraine.
    Influence of microstructure on debonding at the fiber/matrix interface in fiber-reinforced polymers under tensile loading2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    At the end of the second decade of the XXI century, the transportation industry at large faces several challenges that will shape its evolution in the next decade and beyond. The first such challenge is the increasing public awareness and governmental action on climate change, which are increasing the pressure on the industrial sectors responsible for the greatest share of emissions, the transportation industry being one of them, to reduce their environmental footprint. The second big challenge lies instead in the renewed push towards price reduction, due to increased competition (as for example, in the market for low-Earth orbit launchers, the entry of private entities) and innovative business models (like ride-sharing and ride-hailing in the automotive sector or low-cost carriers in civil aviation). A viable and effective technical solution strategy to these challenges is the reduction of vehicles’ structural mass, while keeping the payload mass constant. By reducing consumption, a reduced weight leads to reduced emissions in fossil-fuels powered vehicles and to increased autonomy in electrical ones. By reducing the quantity of materials required in structures, a weight reduction strategy favors in general a reduction of production costs and thus lower prices. Transportation is however a sector where safety is a paramount concern, and structures must satisfy strict requirements and validation procedures to guarantee their integrity and reliability during service life. This represents a significant constraint which limits the scope of the weight reduction approach. In the last twenty years, the development of a novel type of Fiber-Reinforced Polymer Composite (FRPC) laminates, called thin-ply laminates, proposes a solution to these competing requirements (weight with respect to structural integrity) by providing at the same time weight reduction and increased strength. Several experimental investigations have shown, in fact, that thin-ply laminates are capable of delaying, and even suppress, the onset of transverse cracking. Transverse cracks are a kind of sub-critical damage in FRPC laminates and occur early in the failure process, causing the degradation of elastic properties and favoring other, often more critical, modes of damage (delaminations, fiber breaks). Delay and suppression of transverse cracks were already linked, at the end of the 1970’s, to the use of thinner plies inside a laminate. However, thin-plies available today on the market are at least 10 times thinner than those studied in the 1970’s. This characteristic changes the length scale of the problem, from millimeters to micrometers. At the microscale, transverse cracks are formed by several fiber/matrix interface cracks (or debonds) coalescing together. Understanding the mechanisms of transverse cracking delay and suppression in thin-ply laminates requires detailed knowledge regarding onset of transverse cracking at the microscale, and thus the study of the mechanisms that favor or prevent debond initiation and growth. The main objective of the present work is to investigate the influence of the microstructure on debond growth along the fiber arc direction. To this end, models of 2-dimensional Representative Volume Elements (RVEs) of Uni-Directional (UD) composites and crossply laminates are developed. The Representative Volume Elements are characterized by different configurations of fibers and different damage states. Debond initiation is studied through the analysis of the distribution of stresses at the fiber/matrix interface in the absence of damage. Debond growth on the other hand is characterized using the approach of Linear Elastic Fracture Mechanics (LEFM), specifically through the evaluation of the Mode I, Mode II and total Energy Release Rate (ERR). Displacement and stress fields are evaluated by means of the Finite Element Method (FEM) using the commercial solver Abaqus. The components of the Energy Release Rate are then evaluated using the Virtual Crack Closure Technique (VCCT), implemented in a custom Python routine. The elastic solution of the debonding problem presents two different regimes: the open crack and the closed crack behaviour. In the latter, debond faces are in contact in a region of finite size at the debond tip; in the latter, the debond is everywhere open and no contact exists between the faces. In the open crack regime, it is known that stress and displacement fields at the debond tip present an oscillating singularity. A convergence analysis of the VCCT in the context of the FEM solution is thus required to guarantee the validity of results and represents the first step of the work presented in this thesis. It is found that the total ERR does not depend on the size of elements at the debond tip, while the values of Mode I and Mode II ERR depend on element size in the open crack or mixed mode case. It is furthermore shown that Mode I and Mode II ERR do not converge, i.e. their asymptotic behavior for decreasing element size is not bounded. Thus, error reduction between successive iterations cannot be used to validate the solution and comparison with another method is required. Results obtained with the Boundary Element Method (BEM), available in the literature, are selected to this end. Debond growth under remote tensile loading is then studied in Representative Volume Elements of: UD composites of varying thickness, measured in terms of number of rows of fibers, from extremely thin (one fiber row) to thick ones; cross-ply laminates with a central 90◦ ply of varying thickness, measured as well in terms of number of rows of fibers, from extremely thin (one fiber row) to thick ones; thick UD composites (modelled as infinite along the through-the-thickness direction). Different damage configurations are also considered, corresponding to different stages of transverse crack onset: non-interacting isolated debonds; interacting debonds distributed along the loading direction; debonds on consecutive fibers along the through-the-thickness direction. Among the most relevant results, it is found that neither the 90◦ ply thickness nor the 0◦ ply thickness influences debond ERR in cross-ply laminates, differently from what is observed for transverse cracks with the so-called ply-thickness and ply-block effects. On the other hand, debond interaction along the loading direction is shown to influence significantly the Energy Release Rate, but this interaction possesses a characteristic distance (in terms of number of undamaged fibers) that defines the region of influence between debonds. Finally, an estimation of debond size at initiation and of debond maximum size is proposed based on arguments from stress analysis (for initiation) and on Griffith’s criterion from LEFM (for propagation). For a debond in a cross-ply laminate, its maximum size is estimated to lie in the range 40◦ − 60◦ , which is in strong agreement with previous results from microscopic observations available in the literature.

  • 2.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. IJL, EEIGM, Université de Lorraine.
    Ayadi, Zoubir
    IJL, EEIGM, Université de Lorraine.
    Finite Element solution of the fiber/matrix interface crack problem: Convergence properties and mode mixity of the Virtual Crack Closure Technique2019In: Finite elements in analysis and design (Print), ISSN 0168-874X, E-ISSN 1872-6925, Vol. 167, article id 103332Article in journal (Refereed)
    Abstract [en]

    The bi-material interface arc crack has been the focus of interest in the composite community, where it is usually referred to as the fiber-matrix interface crack. In this work, we investigate the convergence properties of the Virtual Crack Closure Technique (VCCT) when applied to the evaluation of the Mode I, Mode II and total Energy Release Rate of the fiber-matrix interface crack in the context of the Finite Element Method (FEM). We first propose a synthetic vectorial formulation of the VCCT. Thanks to this formulation, we study the convergence properties of the method, both analytically and numerically. It is found that Mode I and Mode II Energy Release Rate (ERR) possess a logarithmic dependency with respect to the size of the elements in the crack tip neighborhood, while the total ERR is independent of element size.

  • 3.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Université de Lorraine, Nancy, France.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Université de Lorraine, Nancy, France.
    Effect of boundary conditions on microdamage initiation in thin ply composite laminates2018Conference paper (Refereed)
    Abstract [en]

    The Energy Release Rate (ERR) and the contact zone size for a fiber/matrix interface debond are studied for a thin-ply glass fiber/epoxy laminate. The main objective is to analyze the effect on the debonding process of the presence of a traction-free specimen surface or an adjacent material, in the form of a stiffer UD ply or by considering it as part of a thick 90° layer, at different levels of fiber content. To this end, a model of Representative Volume Element (RVE) subjected to different combinations of boundary conditions is proposed. It is found that the constraining effect of the adjacent ply favors at high fiber volume fractions the opening of small debonds (10 − 40°) for the same level of strain. The results agree well and provide a mechanical explanation to previous microscopic observations available in the literature [4].

  • 4.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. IJL, EEIGM, Université de Lorraine.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    IJL, EEIGM, Université de Lorraine.
    Effect of the proximity to the 0°/90° interface on Energy Release Rate of fiber/matrix interface crack growth in the 90°-ply of a cross-ply laminate under tensile loading2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed)
    Abstract [en]

    Models of Representative Volume Elements (RVEs) of cross-ply laminates with different geometric configurations and damage states are studied. Debond growth is characterized by the estimation of the Mode I and Mode II Energy Release Rate (ERR) using the Virtual Crack Closure Technique (VCCT). It is found that the presence of the 0°/90° interface and the thickness of the 0° layer have no effect, apart from laminates with ultra-thin 90° plies where it is however modest. The present analysis support the claim that debond growth is not affected by the plythickness effect.

  • 5.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Université de Lorraine, EEIGM, IJL, Nancy, France.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Université de Lorraine, EEIGM, IJL, Nancy, France.
    Energy release rate of the fiber/matrix interface crack in UD composites under transverse loading: Effect of the fiber volume fraction and of the distance to the free surface and to non-adjacent debonds2019In: Theoretical and applied fracture mechanics (Print), ISSN 0167-8442, E-ISSN 1872-7638, Vol. 103, article id 102251Article in journal (Refereed)
    Abstract [en]

    The effects of crack shielding, finite thickness of the composite and fiber content on fiber/matrix debond growth in thin unidirectional composites are investigated analyzing Representative Volume Elements (RVEs) of different ordered microstructures. Debond growth is characterized by estimation of the Energy Release Rates (ERRs) in Mode I and Mode II using the Virtual Crack Closure Technique (VCCT) and the J-integral. It is found that increasing fiber content, a larger distance between debonds in the loading direction and the presence of a free surface close to the debond have all a strong enhancing effect on the ERR. The presence of fully bonded fibers in the composite thickness direction has instead a constraining effect, and it is shown to be very localized. An explanation of these observations is proposed based on mechanical considerations.

  • 6.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. IJL, EEIGM, Université de Lorraine.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Estimating the average size of fiber/matrix interface cracks in UD and cross-ply laminates2019In: / [ed] A. Turon, P. Maimi, M. Fagerström, 2019Conference paper (Refereed)
    Abstract [en]

    Initiation and propagation of fiber/matrix interface cracks are analyzed in Representative Volume Elements (RVEs) of UD and cross-ply laminates. By studying the distribution of stresses at the fiber/matrix interface in the undamaged case, an estimate of the initial flaw size is derived. By adopting a 2-parameters energy-based criterion for propagation [1], we then proceed to the estimation of the expected debond size in different microstructural arrangements. Finally, the results are compared with microscopic observations available in the literature [2].

  • 7.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. IJL, EEIGM, Université de Lorraine.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    IJL, EEIGM, Université de Lorraine.
    Growth of interface cracks on consecutive fibers: on the same or on the opposite sides?2019Conference paper (Refereed)
    Abstract [en]

    The growth of fiber/matrix interface cracks (debonds) locatedon consecutive fibers along the through-the-thickness (vertical)direction is studied in glass fiber-epoxy UD composites. Debonds couldappear, along the vertical direction, on the same or on opposite sides oftheir respective fibers. Determining which configuration is the mostenergetically favorable to debond growth is the objective of this paper.To this end, two different families of Representative Volume Elements(RVEs) are developed: the first implements the classic condition ofcoupling of the vertical displacements to model a unit cell repeating symmetrically along the vertical direction; the second uses a novel setof boundary conditions, proposed here by the authors, to represent a unitcell repeating anti-symmetrically along the vertical direction. The modelis analyzed in the context of Linear Elastic Fracture Mechanics (LEFM)and the Mode I and Mode II Energy Release Rate are evaluated toinvestigate crack growth. The calculation is performed using the VirtualCrack Closure Technique (VCCT) in the framework of the Finite ElementMethod (FEM). It is found that Mode I dominated propagation is favoredwhen debonds are located on the same sides of their respective fibers;while for larger (Mode II-dominated) debonds, Mode II ERR is higher whenthey lie on the opposite sides. No interaction effect is present when atleast two fully bonded fibers are located between the partially debonded ones.

  • 8.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Université de Lorraine, Nancy, France.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Université de Lorraine, Nancy, France.
    Investigation of Scaling Laws of the Fiber/Matrix Interface Crack in Polymer Composites through Finite Element-based Micromechanical Modeling2019In: Book of abstracts of the 10th EEIGM International Conference on Advanced Materials Research, Moscow, Russia, 2019Conference paper (Refereed)
  • 9.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Université de Lorraine, Nancy, France.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Université de Lorraine, Nancy, France.
    Micromechanical modeling of thin ply effects on microdamage in Fiber Reinforced Composite laminates2017In: Book of abstracts, Saarbrücken, Germany, 2017Conference paper (Refereed)
  • 10.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Université de Lorraine, Nancy, France.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Université de Lorraine, Nancy, France.
    Micromechanical models of transverse cracking in ultra-thin Fiber-Reinforced Composite laminates2017Conference paper (Other academic)
1 - 10 of 10
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