Browsing by Author "Sockalingam, Subramani"

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    Dynamic effects of single fiber break in unidirectional glass fiber-reinforced composites
    (Sage Publications, 2016-09-15) Ganesh, Raja; Sockalingam, Subramani; Haque, Bazle Z. (Gama); Gillespie, John W. Jr.; Raja Ganesh, Subramani Sockalingam, Bazle Z. (Gama) Haque and John W. Gillespie, Jr.; Ganesh, Raja; Sockalingam, Subramani; Haque, Bazle Z. (Gama); Gillespie, John W. Jr.
    In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process which leads to stress concentrations in the interface, matrix and neighboring fibers that can propagate at high speed over long distances. To gain better understanding of this event, a fiber-level finite element model of a 2-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Moreover, dynamic interfacial failure is predicted where debonding initiates, propagates and arrests at longer distances than predicted by models that assume quasi-static fiber breakage. In the case of higher strength fibers breaks, unstable debond growth is predicted. A stability criterion to define the threshold fiber break strength is derived based on an energy balance between the release of fiber elastic energy and energy absorption associated with interfacial debonding. A contour map of peak dynamic stress concentrations is generated at various break stresses to quantify the zone-of-influence of dynamic failure. The dynamic results are shown to envelop a much larger volume of the microstructure than the quasi-static results. The implications of dynamic fiber fracture on damage evolution in the composite are discussed.
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    Experimental characterization of tensile properties of epoxy resin by using micro-fiber specimens
    (Sage Publications, 2016-09-21) Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr.; Jun Misumi, Raja Ganesh, Subramani Sockalingam, John W Gillespie; Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr
    In unidirectional carbon fiber-reinforced plastic laminates, the distance between fibers can varies from submicron to micron length scales. The mechanical properties of the matrix at this length scale are not well understood. In this study, processing methods have been developed to produce high quality epoxy micro-fibers with diameters ranging from 100 to 150 µm that are used for tensile testing. Five types of epoxy resin systems ranging from standard DGEBA to high-crosslink TGDDM and TGMAP epoxy systems have been characterized. Epoxy macroscopic specimens with film thickness of 3300 µm exhibited brittle behavior (1.7 to 4.9% average failure strain) with DGEBA resin having the highest failure strain level. The epoxy micro-fiber specimens exhibited significant ductile behavior (20 to 42% average failure strain) with a distinct yield point being observed in all five resin systems. In addition, the ultimate stress of the highly cross-linked TGDDM epoxy fiber exceeded the bulk film properties by a factor of two and the energy absorption was over 50 times greater on average. The mechanism explaining the dramatic difference in properties is discussed and is based on size effects (the film volume is about 2000 times greater than the fiber volume within the gage sections) and surface defects. Based on the findings presented in this paper, the microscale fiber test specimens are recommended and provide more realistic stress–strain response for describing the role of the matrix in composites at smaller length scales.
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    Experimental characterization of tensile properties of epoxy resin by using micro-fiber specimens
    (SAGE Publications, 2016) Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr.; Jun Misumi, Raja Ganesh, Subramani Sockalingam and John W Gillespie Jr.; Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr.
    In unidirectional carbon fiber reinforced plastic (CFRP) laminates, the distance between fibers can vary from submicron to micron length scales. The mechanical properties of the matrix at this length scale are not well understood. In this study, processing methods have been developed to produce high quality epoxy micro-fibers with diameters ranging from 100 to 150 um that are used for tensile testing. Five types of epoxy resin systems ranging from standard DGEBA to high-crosslink TGDDM and TGMAP epoxy systems have been characterized. Epoxy macroscopic specimens with film thickness of 3300 um exhibited brittle behavior (1.7 to 4.9% average failure strain) with DGEBA resin having the highest failure strain level. The epoxy micro-fiber specimens exhibited significant ductile behavior (20 to 42% average failure strain) with a distinct yield point being observed in all five resin systems. In addition, the ultimate stress of the highly cross-linked TGDDM epoxy fiber exceeded the bulk film properties by a factor of two and the energy absorption was over 50 times greater on average. The mechanism explaining the dramatic difference in properties are discussed and is based on size effects (the film volume is about 2000 times greater than the fiber volume within the gage sections) and surface defects. Based on the findings 3 presented in this paper, the microscale fiber test specimens are recommended and provide more realistic stress-strain response for describing the role of the matrix in composites at smaller length scales.
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    Recent Advances in Modeling and Experiments of Kevlar Ballistic Fibrils, Fibers, Yarns and Flexible Woven Textile Fabrics – A Review
    (Sage Publications, 2016-05-02) Sockalingam, Subramani; Chowdhury, Sanjib C.; Gillespie, John W. Jr.; Keefe, Michael; Subramani Sockalingam, Sanjib C. Chowdhury, John W. Gillespie Jr and Michael Keefe; Sockalingam, Subramani; Chowdhury, Sanjib C.; Gillespie, John W. Jr.; Keefe, Michael
    Ballistic impact onto flexible woven textile fabrics is a complicated multi-scale problem given the structural hierarchy of the materials, anisotropic material behavior, projectile geometry-fabric interactions, impact velocity and boundary conditions. Although this subject has been an active area of research for decades, the fundamental mechanisms such as material failure, dynamic response, multi-axial loading occurring at the lower length scales during impact are not well understood. This paper reviews the recent advances in modeling and experiments of Kevlar ballistic fibrils, fibers, yarns and flexible woven textile fabrics pertinent to the deformation modes occurring during impact and serves to identify topics worthy of further investigation that will advance the basic understanding of the phenomena governing transverse impact. This review also explores on aspects such as homogeneous versus heterogeneous behavior of yarns consisting of individual fibers and the inelastic transverse behavior of the fiber which is not considered in the previous review papers on this topic.
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    Transverse impact of ballistic fibers and yarns: fiber length-scale finite element modeling and experiments
    (University of Delaware, 2016) Sockalingam, Subramani
    Ballistic impact onto flexible textile fabrics is a complicated multi-scale problem owing to the structural hierarchy of the materials, anisotropic material behavior, projectile geometry, impact velocity and boundary conditions. While this subject has been an active area of research for decades, the fundamental mechanisms such as material failure, dynamic response and multi-axial loading occurring at lower length scales during impact are not well understood. This work provides new insights into the fundamental deformation and failure mechanisms during ballistic impact onto textile fabrics at the micron length scale. In this research, a hybrid computational-experimental systematic approach is adopted to understand the mechanisms and deformation modes of high performance polymer fibers, specifically Kevlar KM2, that is widely used in ballistic impact applications. Fiber length-scale 3D finite element (FE) models are developed to better understand and complement the complicated transverse impact experiments. The fiber length-scale study suggests that fibers are subjected to multiaxial stress states including transverse compression, axial tension, axial compression and transverse shear significant enough to cause fibrillation in the fiber during impact. A dispersive flexural wave mode is predicted by the model due to the finite longitudinal shear modulus of the fiber. The flexural wave induces curvature in the fiber significant enough to cause compressive kinking and, in turn, local fibrillation in the fiber. A fiber length-scale yarn model is developed by explicitly modeling all the 400 fibers in a KM2 600 denier yarn. The yarn transverse compression results show that fiber-fiber contact plays a significant role in the spreading and deformation of individual fibers that is consistent with experimental results. When subjected to transverse impact, the model indicates significant transverse compressive strains in the fiber that increase with impact velocity and a flexural wave that induces curvatures in the fibers significant enough to induce compressive kinking and fibrillation. In addition to the transverse wave, a spreading wave develops due to fiber-fiber contact interaction that spreads the fibers to a large extent resulting in non-uniform loading and progressive failure of fibers within the yarn. Guided by the computational models, single-fiber micromechanical experiments for axial compressive kinking and transverse compression deformation modes are developed. The average tensile strength of the kinked fibers is found to be reduced by 7% compared to the virgin fibers. An experimental methodology is developed to determine the single fiber constitutive behavior in quasi-static transverse compression by removing the geometric nonlinearity due to the growing contact area. The fibers exhibit nonlinear inelastic behavior under large compressive strains. The fibers subjected to 60% nominal strains (80% true strains) showed a 20% reduction in average tensile strength compared to the virgin fibers. A nonlinear inelastic constitutive model is implemented as a user defined material (UMAT) suitable for the commercial FE code LS-DYNA explicit analysis. During impact, the inelastic behavior results in a significant reduction in the fiber bounce velocity and a reduction in the projectile-fiber contact forces by 40% compared to an elastic constitutive behavior. The inelastic dissipation and reduced bounce leads to an inelastic collision rather than an elastic collision. The longitudinal shear modulus and the inelastic behavior are found to govern the failure response of the fibers during impact. Modeling the single fiber quasi-static multiaxial loading experiments indicate fiber failure may be initiated based on a gage length dependent maximum axial tensile strain in the fiber. Regardless of the material behavior (elastic or inelastic), fiber length-scale impact models show a gradient in the axial tensile strain (stress) in the fiber cross section at the location of failure consistent with multiaxial loading experimental observations. Fiber-level yarn breaking speed predictions based on a maximum axial tensile strain (stress) criterion are much lower than the breaking speed based on classical theory and they are consistent with experimental measurements. Therefore, the reduction in experimental yarn breaking speed compared to theoretical Smith solution is attributed to the stress concentration and property degradation mechanisms due to multiaxial stress states at the location of failure.