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  • 101. Talreja, Ramesh
    Stiffness bases fatigue damage characterisation of fibrous composites1980In: Economic Computation and Economic Cybernetics Studies and Research, ISSN 0424-267x, Vol. 2, p. 1732-1739, article id 8Article in journal (Refereed)
    Abstract [en]

    The paper presents a simple procedure for measuring the changes in stiffness properties of a unidirectional fibrous composite caused by fatigue damage. Unlike previous attempts at stiffness based fatigue damage characterization, where one or two elastic constants have been investigated, changes in all four independent stiffness constants of an othotropic elastic lamina are considered. The results obtained for a unidirectional glass fiber reinforced polyester show that while small changes occur in the longitudinal elastic modulus and the Poisson's ratio of transverse to longitudinal strains, the shear modulus and the Poisson's ratio of longitudinal to transverse strains change significantly. The extension of this procedure to the fatiue damage characterization of laminates is discussed.

  • 102. Talreja, Ramesh
    Stiffness changes in an orthotropic lamina under fatigue1982In: Progress in science and engineering of composites: proceedings of the Fourth International Conference on Composite Materials, Oct. 25 - 28 1982, Tokyo, Japan / [ed] Tsuyoshi Hayashi, Tokyo: Japan Society for Composite Materials , 1982Conference paper (Refereed)
  • 103. Talreja, Ramesh
    Stiffness properties of composite laminates with matrix cracking and interior delamination1986In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 25, no 5-6, p. 751-762Article in journal (Refereed)
    Abstract [en]

    Two modes of damage in composite laminates are considered: the intralaminar damage (matrix cracking) and the interlaminar damage (interior delamination). Using a vectorial representation of damage as internal variables in a phenomenological theory, relationships between the overall stiffness properties and the intensity of damage in the individual modes are determined. These relationships show that the intralaminar damage reduces all elastic moduli for a general orientation of the damage entities (cracks) and changes the initial orthotropic symmetry of a laminate. The interlaminar damage, however, does not change the symmetry but reduces the moduli. Predictions of the elastic moduli changes are compared with experimental results, showing excellent agreement

  • 104. Talreja, Ramesh
    Stress-strain behavior of a Cr-mo-v steel under variable amplitude loads1976In: Proceedings of the Second International Conference on Mechanical Behavior of Materials, 16 - 20 August 1976, Sheraton-Boston Hotel, Boston, Massachusetts, Metals Park, Ohio: ASM International, 1976Conference paper (Refereed)
    Abstract [en]

    Recent viscoplastic theories have been successful in describing strain behavior under variable loads. However, convincing experimental evidence was lacking to elucidate the essentially viscoplastic nature of the inelastic behavior of metals. The paper discusses an investigation in which the inelastic behavior of a Cr-Mo-V steel was studied in uniaxial stress at room temperatures. At high stresses, the viscous strains are found to be significant and revived with each application following a prestrain. The laws of the dependence of viscous strains on stress and prestrain are determined experimentally.

  • 105.
    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

  • 106. Talreja, Ramesh
    Transverse cracking and stiffness reduction in composite laminates1985In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 19, no 4, p. 355-375Article in journal (Refereed)
    Abstract [en]

    A systematic classification of the effects of transverse cracking on the stress-strain response of composite laminates is presented. Stiffness reductions resulting from transverse cracking in glass/epoxy and graphite/epoxy laminates from crack initiation to crack saturation are predicted using the stiffness-damage relationships developed by the author in a previous work. Good agreement with the experimental results is found. An assessment of the ply discount method for predicting stiffness reductions at crack saturation is also made.

  • 107. Talreja, Ramesh
    et al.
    Akshantala, Nagendra V.
    School of Aerospace Engineering, Georgia Institute of Technology, Atlanta.
    An inadequacy in a common micromechanics approach to analysis of damage evolution in composites1998In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 7, no 3, p. 238-249Article in journal (Refereed)
    Abstract [en]

    This article examines a commonly used micromechanics approach to evolution of damage in composites. In this approach an initial crack pattern is assumed and on the basis of the local stress fields and a (brittle) fracture criterion, further evolution of the same crack pattern is determined. We question this assumed self-similarity in the damage mode by considering two alternative crack modes, one of which would result in deviation from the assumed crack pattern. Using a simple maximum stress criterion for formation of brittle cracks, it is found that the self-similarity breaks down before saturation of cracking in the assumed crack pattern. This seemingly inherent inadequacy in the damage evolution analysis is discussed.

  • 108. Talreja, Ramesh
    et al.
    Govada, Anil
    Virginia Polytechnic Institute and State University, Materials Response Group, Blacksburg, VA.
    II, Edmund G. Henneke
    Virginia Polytechnic Institute and State University, Materials Response Group, Blacksburg, VA.
    Quantitative assessment of damage growth in graphite epoxy laminates by acousto-ultrasonic measurements1984In: Review of progress in quantitative nondestructive evaluation: Proceedings of the Tenth Annual Review of Progress in Quantitative Nondestructive Evaluation, held August 7 - 12, 1983, at the University of California, Santa Cruz, California / [ed] Donald O. Thompson, New York: Plenum Publishing Co., N.Y. , 1984, Vol. 3 B, p. 1099-1106Conference paper (Refereed)
    Abstract [en]

    Composite materials fail by a complex development of many different damage modes, including matrix cracking, fiber-matrix debonds, delamination and fiber breakage. Nondestructive testing of composite materials must therefore concern itself with something other than the detection and geometrical location, sizing and orienting of single cracks. The acousto-ultrasonic techniques, developed by Vary, et al. , has the potential to respond to the development of damage states and to provide a quantitative parameter which monitors this damage development. This paper presents some preliminary data taken during quasi-static loading and fatigue of graphite epoxy laminates which indicate the plausibility of this statement. The ultrasonic signals received from transducers were amplified, digitized and stored in computer for later spectral analysis. A Fast Fourier Transform algorithm was used to perform spectral analysis. It is shown that the acousto-ultrasonic test is capable of sensitively monitoring the development of damage in graphite epoxy laminates

  • 109.
    Talreja, Ramesh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Joffe, Roberts
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    High temperature aging effects on damage and mechanical response of carbon/BMI composites1999Conference paper (Refereed)
  • 110. Talreja, Ramesh
    et al.
    Niu, Kangmin
    Optimal CAE, 39555 Orchard Hill Place, Ste. 200, Novi, MI.
    Modeling of wrinkling in sandwich panels under compression1999In: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 125, no 8, p. 875-883Article in journal (Refereed)
    Abstract [en]

    A wrinkling model for sandwich panels in compression is developed with the assumption of a continuous isotropic linear elastic core. Wrinkling stresses for the three modes, defined in a well-known book by H. G. Allen, are expressed by a unified, single expression. The expression depends linearly on a case parameter η, defined to specify the three cases of wrinkling: η = 0 for single-sided face wrinkling (case 1), η = 1 for in-phase wrinkling (case 2), and η = - 1 for out-of-phase wrinkling (case 3). It is shown that the stresses in all three cases are almost identical for short wavelength wrinkling and car be expressed by a single simplified analytical expression; however, they may differ significantly in moderate and long wavelength wrinkling, and for these cases they are given by simplified analytical expressions. It is proved that the in-phase wrinkling stress is the lowest among the three cases. Based on the analysis conducted, limitations of the commonly used Winkler and two-parameter models are discussed. Finally, engineering design procedures are recommended for the wrinkling effect in sandwich panels under compression.

  • 111. Talreja, Ramesh
    et al.
    Singh, Chandra Veer
    Department of Aerospace Engineering, Texas A&M University.
    Continuum damage mechanics: A modeling approach for comprehensive assessment of subcritical damage in composites2007Conference paper (Refereed)
    Abstract [en]

    The concept of "failure" has served the engineering practice well for designing structures to perform safely in service environments. Historically, this concept has its roots in experience with metals where the onset of undesirable material condition is yielding followed ultimately by unstable behavior characterized by maximum load carrying capacity. This classical concept of material failure was subsequently broadened to include fracture, which deals with onset and instability of growth of a preexisting flaw. In heterogeneous materials, particularly in composites, where multiple materials exist, the failure and fracture concepts are not as straightforward. The concept of damage is instead more useful, with its onset and criticality as two end conditions.

  • 112.
    Talreja, Ramesh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Effect of aging on damage and stiffness changes in IM7/K3B laminates1998In: Mechanical behavior of advanced materials, American Society of Mechanical Engineers , 1998Conference paper (Refereed)
  • 113. Talreja, Ramesh
    et al.
    Weibull, Waloddi
    Probability of fatigue failure based on residual strength1977Report (Refereed)
  • 114. Talreja, Ramesh
    et al.
    Yalvaç, Selim
    Georgia Institute of Technology, Atlanta.
    Yats, Larry
    Georgia Institute of Technology, Atlanta.
    Wetters, Derrick
    Georgia Institute of Technology, Atlanta.
    Transverse cracking and stiffness reduction in cross ply laminates of different matrix toughness1992In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 26, no 11, p. 1644-1663Article in journal (Refereed)
    Abstract [en]

    Cross ply laminates of (02,902)s configuration having AS4 graphite fibers in three epoxy resins of different toughness: 3501-6, Tactix 556 and Tactix 695, have been tested to determine their transverse cracking behavior and the associated mechanical response under longitudinal tensile loading. The test data are analyzed using Talreja's continuum damage model [1-3]. The material constants needed in the model to predict the inplane stiffness changes are determined. The measured Poisson's ratio, which shows significant change, is compared with the prediction of the model. The constants for the three materials are found to increase with the increase in their fracture toughness.

  • 115.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Akshantala, Nagendra
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Stiffness changes in laminates caused by intralaminar cracks and shear deformation1997In: Composites and functionally graded materials, American Society of Mechanical Engineers , 1997, p. 165-168Conference paper (Refereed)
    Abstract [en]

    An experimental study has been conducted to study stiffness changes in laminates containing inclined (off-axis) plies. The class of laminates studied is glass-epoxy (0, ±θ4, 01/2)s with θ = 25, 40, 55 and 70. It is found that for 0>40 multiple cracks occur in the θ-plies while for θ<40 no cracks are found. However, for all θ angles the laminate moduli change. A continuum damage mechanics approach is used to calculate stiffness changes due to cracks. In the absence of cracks the shear modulus reduction in the θ-plies, determined from independent tests on a (±45)s laminate, is found to provide good prediction of the observed stiffness changes.

  • 116.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Akshantala, Nagendra V.
    School of Aerospace Engineering, Georgia Institute of Technology, Atlanta.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Crack opening displacement and the associated response of laminates with varying constraints1999In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 8, no 2, p. 174-193Article in journal (Refereed)
    Abstract [en]

    This paper reports an experimental study of the opening displacement behavior of 90-ply cracks in a class of composite laminates where the constraint to cracking is varied systematically. The study is motivated by a synergistic approach to damage mechanics proposed by Talreja (1996) where the crack opening displacement forms a key micromechanically determined parameter in a continuum damage formulation of the composite response. It is found that although the opening displacement shows scatter from point to point along a crack length, its average over the crack length forms a robust parameter. This parameter, when introduced as the damage influence coefficient in a continuum description of damage, allows satisfactory predictions of the elastic moduli changes due to transverse cracks in the considered class of laminates with varying constraints.

  • 117.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Jakovics, A.
    Department of Physics & Mathematics Latvia University.
    A study of the opening displacement of transverse cracks in cross-ply laminates1993In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 2, no 3, p. 272-289Article in journal (Refereed)
    Abstract [en]

    Transverse cracks in cross-ply laminates are investigated experimentally to reveal the essential characteristics of their opening displacement under tensile loads. The average crack opening displacement is studied as a function of the longitudinal overall strain and the effects of matrix toughness and transverse ply thickness on this parameter are examined. The interactive effects between closely spaced transverse cracks are also ex amined and found to be significant. Implications of the experimentally observed features on the micromechanics and continuum damage type models are discussed.

  • 118.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Akshantala, N.V.
    Georgia Institute of Technology.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Damage in composite laminates with off-axis plies1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 14, p. 2139-2147Article in journal (Refereed)
    Abstract [en]

    Damage in off-axis plies of composite laminates is studied by examining the configuration [0/±θ4/01/2]s with θ=25, 40, 55, 70 and 90 subjected to tensile loading in the axial direction. It is found that for the values of θ, where the stress in the off-axis plies normal to the fibers is tensile, ply cracks lying along fibers initiate and increase in number, while for other θ values the plies do not undergo this damage, as expected. However, the overall laminate elastic moduli are also found to change for the θ values where no ply cracks exist. It is postulated that a shear-induced degradation of the off-axis plies is responsible for the observed laminate moduli changes. The prediction of changes in these moduli by using the ply shear modulus measured on [±θ4]s appears to support this postulate. For the case of moduli changes caused by ply cracks the recently proposed synergistic damage-mechanics approach [1] is applied. The implications of the findings of this work on a class of continuum damage-mechanics formulations proposed in the literature are discussed.

  • 119.
    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.
    A synergistic damage-mechanics analysis of transverse cracking in [±θ/904]s laminates2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 5, p. 657-665Article in journal (Refereed)
    Abstract [en]

    The deformational response of [±θ/904]s laminates under longitudinal tensile loading is treated by a synergistic approach that combines continuum damage mechanics (CDM) and micromechanics. The constraint of θ° plies on transverse cracks in 90° plies is represented by a crack-opening displacement (COD) parameter in the CDM model and is expressed in terms of the ply properties and ply thickness ratios on the basis of a finite-element calculation. A methodology is proposed for predicting the laminate stiffness variations with crack density and applied strain (or stress) for various values of ply orientation θ and is found to give good results.

  • 120. Varna, Janis
    et al.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Intralaminar cracking evolution in symmetric laminates1999Conference paper (Refereed)
  • 121.
    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.

  • 122.
    Varna, Janis
    et al.
    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.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mixed micromechanics and continuum damage mechanics approach to transverse cracking in [S,90°n]s laminates2001In: Mechanika kompozitnych materialov, ISSN 0203-1272, Vol. 37, no 2, p. 193-209Article in journal (Refereed)
  • 123.
    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,90)s laminates2000Conference paper (Refereed)
  • 124.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Crack opening displacement as a basic parameter for stiffness reduction modeling in laminated composites1999In: ICCM-12 Europe: 12th International Conference on Composite Materials ; Palais des Congrès, Paris, Tours: ICCM , 1999Conference paper (Refereed)
  • 125.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, Andrejs
    Riga Technical University, Latvia.
    Kumar, Rajesh A.
    Georgia Institute of Technology.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    A synergistic damage mechanics approach to viscoelastic response of cracked cross-ply laminates2004In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 13, no 4, p. 301-334Article in journal (Refereed)
    Abstract [en]

    A synergistic damage mechanics approach, i.e., a combined continuum damage mechanics (CDM) and micromechanics approach, is developed to characterize the viscoelastic response of cross-ply laminates with transverse cracks. The approach relies upon a second-order tensor-based description of damage wherein the crack opening displacement appears as an explicit function. The time variation of this function is calculated by micromechanics and is inserted into the continuum damage formulation. The relaxation moduli calculated for fixed states of damage (transverse crack density) by this approach agree well with independently calculated values by a CDM approach and finite element model. (Example materials: carbon-epoxy and glass-epoxy composites.)

  • 126.
    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)
  • 127.
    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

  • 128.
    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

  • 129.
    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.

  • 130.
    Zhuang, Linqi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Analysis of Formation of the Critical State in Tensile Failure of Unidirectional Composites2016In: ASME 2015 International Mechanical Engineering Congress and Exposition, New York: American Society of Mechanical Engineers , 2016, Vol. 9 : Mechanics of Solids, Structures and Fluids, article id UNSP V009T12A031Conference paper (Refereed)
    Abstract [en]

    Unidirectional (UD) composites are building blocks in most load bearing structural components for lightweight applications in aerospace, automotive and wind energy industries. The loss of the structural load bearing capacity is governed by the instability of the fiber breakage process in the UD composites. When subjected to increasing or repeated tensile loading along fiber direction, the first failure event within these composites occurs as discrete fibers break at weak points followed by fiber/matrix debonding due to high stress concentration caused by fiber breaks. Upon further loading, or on repeated loading, more fiber breaks occur along with other accumulated damage events such as debond growth and matrix cracking. Final failure of a UD composite occurs when a critical fracture plane is formed by interconnecting individual broken fibers and associated debonding through matrix cracking. This failure process has emerged from numerous experimental studies, which also suggest that the critical fracture plane contains only a small number of broken fibers for commonly used composites such as glass/epoxy and carbon/epoxy. However, the mechanisms underlying the critical fracture plane formation are not clear. As the first step to clarify the creation of a critical fracture plane, the conditions for connectivity of a broken fiber end with neighboring broken fibers is studied in this work. In order to investigate the local stress field surrounding the broken fiber, a finite element (FE) model is constructed in which six neighboring fibers 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. To account for the consequence of the stress enhancement at the broken fiber end, a debond crack at the fiber/matrix interface extending a short distance from the fiber end is included in the analysis. Realizing that the debond crack by itself would not connect with other fiber failures, focus of the stress and failure analysis is placed on deviation of the debond crack laterally into the matrix. For this purpose, matrix cracking in two possible modes ductile and brittle is considered, Energy based criteria are used to study the competition between the cracking modes and the crack path into the matrix from the end of debond to the neighboring fibers is determined. Next the failure of the neighboring fibers caused by the intense stress field accompanying the matrix cracks is studied. The conditions for generating a plane connecting the initially broken fiber end to subsequent fiber failures are finally determined. Further ongoing studies are aimed at clarifying the limiting conditions for avoiding the fiber failure criticality, and thereby improving the load bearing capacity of UD composites. The statistical considerations regarding fiber failure will also be incorporated in these studies.

  • 131.
    Zhuang, Linqi
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effects of Voids on Postbuckling Delamination Growth in Unidirectional Composites2014In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 51, no 5, p. 936-944Article in journal (Refereed)
    Abstract [en]

    This work examines the effects of manufacturing induced voids on the postbuckling behavior of delaminated unidirectional composites. In the finite element model developed, a through-width delamination is introduced close to one surface of a flat panel, and a void is placed in the delamination plane ahead of each delamination front. The panel is subjected to compression in the fiber direction. The postbuckling delamination growth is studied by calculating the strain energy release rate (SERR) using the virtual crack closure technique. Local stress analyses of the region near the delamination front are also performed to further investigate the void effects. It is found that although the presence of void does not significantly alter the postbuckling transverse displacement of the delaminated panel, the induced stress perturbation by the void affects the SERR. The Mode II SERR as well as the total SERR increase depending on the size of the void and its distance from the delamination front. Since the Mode I SERR shows non-monotonic behavior with the applied load, the effects of voids are studied on its maximum value.

  • 132.
    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. 

  • 133.
    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.

  • 134.
    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.

  • 135.
    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

123 101 - 135 of 135
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