Subramani Sockalingam

Recent advances in modeling and experiments of Kevlar ballistic fibrils, fibers, yarns and flexible woven textile fabrics – a review

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 and 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 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.

Fiber-Matrix Interface Characterization through the Microbond Test

Fiber reinforced polymer matrix composites are widely used to provide protection against ballistic impact and blast events. There are several factors that govern the structural response and mechanical properties of a textile composite structure, of which the fiber-matrix interfacial behavior is a crucial determinant. This paper reviews the microbond or microdroplet test methodology that is used to characterize the fiber-matrix interfacial behavior, particularly the interface shear strength (IFSS). The various analytical, experimental, and numerical approaches applied to the microbond test are reviewed in detail.

Modeling the fiber length-scale response of Kevlar KM2 yarn during transverse impact

In this study, transverse impact of a cylindrical projectile onto a 600 denier Kevlar KM2 yarn (400 individual fibers) is studied using a fiber length-scale three-dimensional finite element model to better understand projectile–fiber and fiber–fiber contact interactions on wave propagation and fiber failure within the yarn. A short time scale response indicates significant transverse compressive deformation in the fiber that increases with impact velocity. Fiber-level modeling predicts a flexural wave that induces curvatures in the fibers significant enough to induce compressive fiber kinking and fibrillation. A spreading wave normal to the direction of projectile impact develops and spreads the fibers at high velocity. The models predict bounce velocities of the individual fibers within the yarn that varies based on spatial location. These mechanisms result in non-uniform loading and progressive failure of fibers within the yarn. In addition, the models show a gradient in the axial tensile stress in the fiber cross-section at the location of failure. Current state-of-the-art experimental capabilities in yarn/fabric impact testing do not have the spatial resolution to track individual single-fiber micron length-scale deformations in real time. These fiber-level mechanisms may explain the experimentally observed lower breaking speed for yarns better the classic Smith solution, which assumes yarns are homogenous (i.e. individual fibers and their interactions are not considered) and loaded uniformly in tension (multi-axial loading and stress gradients are neglected).

Influence of multiaxial loading on the failure of Kevlar KM2 single fiber

The role of multiaxial loading on the failure of Kevlar® KM2 ballistic fibers during transverse loading is not well understood. Quasi-static experiments reported by Hudspeth M, Li D, Spatola J, et al. (2015) on single KM2 fiber subjected to transverse loading by three indenter geometries over a wide range of loading angles exhibit significant reductions in the average axial tensile failure strain. In this study, a three-dimensional finite element model is developed to predict the degree of multiaxial loading present at the location of fiber failure in these experiments. The model predicts an axial tensile strain concentration within the indenter–fiber contact zone. The fiber is also subjected to multiaxial stress/strain states within the contact zone consisting of axial tension, axial compression, transverse compression and interlaminar shear that can degrade axial tensile failure strain. For a round indenter with a radius much higher than the fiber diameter, the axial tensile strain concentration and multiaxial strain in the fiber are negligible. In the case of a fragment-simulating projectile and a razor indenter, significant axial tensile strain concentrations (2.2–5.9) are predicted and the localized transverse loading results in extensive inelastic deformation within the fiber cross-section. Based on the results, a maximum axial tensile strain failure criterion incorporating the multiaxial loading degradation effects is developed. The failure criterion correlates well with the experimental measurements reported by Hudspeth et al. for all three indenters. Modeling the experiments provides new insights into the tensile failure strain of high-performance ballistic fibers at extremely small gage lengths subject to transverse impact loading.

Dynamic effects of single fiber break in unidirectional glass fiber-reinforced composites:

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process that 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 two-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.

Transverse Compression Response of Ultra-High Molecular Weight Polyethylene Single Fibers

This work reports on the experimental quasi static transverse compression response of ultra-high molecular weight polyethylene (UHMWPE) Dyneema SK76 single fibers. The experimental nominal stress-strain response of single fibers exhibits nonlinear inelastic behavior under transverse compression with negligible strain recovery during unloading. Scanning electron microscopy (SEM) reveals the presence of significant voids along the length of the virgin and compressed fibers. The inelastic behavior is attributed to the microstructural damage within the fiber. The compressed fiber cross sectional area is found to increase to a maximum of 1.83 times the original area at 46 % applied nominal strains. The true stress strain behavior is determined by removing the geometric nonlinearity due to the growing contact area. The transverse compression experiments serve as validation experiments for fibril-length scale models.

Experimental characterization of tensile properties of epoxy resin by using micro-fiber specimens

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.

Dynamic effects of a single fiber break in unidirectional glass fiber-reinforced polymer composites: Effects of matrix plasticity:

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process, leading to stress concentrations in the matrix and neighboring fibers and debonding of the interface, which can propagate at high speed over long distances. In our previous work, a fiber break within a two-dimensional fiber array embedded in elastic epoxy matrix (with cohesive interface) was modeled to quantify the effects of these dynamic stresses. The results indicated that the elastic limit of the polymer matrix can be exceeded. In this study, the effects of matrix plasticity on dynamic stress concentrations due to a single fiber break are investigated. For the range of matrix yield stresses considered, the dynamic stress concentrations are significantly higher than corresponding values predicted by a quasi-static model with a pre-broken fiber. Based on the ratio of shear yield strength of the matrix and mode II peak traction of the interface cohesive law, two distinct regimes of damage are shown to exist. Only matrix yielding occurs when this ratio is less than 1.0, while both interfacial debonding and matrix yielding occur when it is greater than 1.0. At higher fiber break strengths, where the elastic matrix model predicts unstable interfacial debonding, reduction in matrix yield strength leads to a transition to stable debonding and arrest. Reducing the matrix yield strength also leads to a lowering of the peak dynamic stress concentrations in adjacent fibers, while spreading the stress concentrations over a larger volume of the composite microstructure.