Abstract: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 relea… Show more
“…Fiber-matrix debonding is a micromechanical failure mechanism that follows the tensile fracture of a fiber in a unidirectional composite and dissipates the elastic energy stored in the fiber by creating debonded surfaces, and by frictional sliding between the fiber and the surrounding matrix at the debonded regions. [33][34][35][36] Microdroplet experiments on S glass and DER 353 epoxy resin with GPS sizing (i.e. same constituents used in this study) have been carried out by the authors 37,38 in determining the traction-separation law for mode II dominated fiber-matrix interface debonding.…”
Section: Modeling Fiber-matrix Debonding Using Zero-thickness Cohesivmentioning
Punch shear in unidirectional composites is induced by transverse shear loading that progressively perforates the laminate within a narrow shear annulus. At lower micromechanical length scales, punch shear loading creates unique micromechanical damage mechanisms dominated by transverse fiber shear failure, fiber-matrix interphase debonding and large inelastic deformation and cracking of the matrix. A new punch shear experimental method has been developed to test unidirectional S glass/DER353 epoxy composite ribbons at sub-millimeter length scale. The experimental data consist of a statistical measurement of the continuum response (load-deformation and punch shear strength) and the characterization of micromechanical damage modes. A simplified 2D micromechanical finite element model incorporating Weibull fiber strength distribution has been developed and correlated with the experimental data. The 2D micromechanical finite element model can simulate the punch shear failure of the ribbon incorporating mixed mode fiber fracture, and fiber-matrix debonding mechanisms using zero thickness cohesive elements. Results from stochastic simulations of punch shear experiments show that an equivalent 2D micromechanical finite element model can predict the micromechanical damage mechanisms and the statistical distribution of punch shear strength of the continuum with favorable correlation with the experiments. This paper presents a combined experimental and computational approach in simulating the stochastic non-linear progressive punch shear behavior of unidirectional composites for the first time in the literature.
“…Fiber-matrix debonding is a micromechanical failure mechanism that follows the tensile fracture of a fiber in a unidirectional composite and dissipates the elastic energy stored in the fiber by creating debonded surfaces, and by frictional sliding between the fiber and the surrounding matrix at the debonded regions. [33][34][35][36] Microdroplet experiments on S glass and DER 353 epoxy resin with GPS sizing (i.e. same constituents used in this study) have been carried out by the authors 37,38 in determining the traction-separation law for mode II dominated fiber-matrix interface debonding.…”
Section: Modeling Fiber-matrix Debonding Using Zero-thickness Cohesivmentioning
Punch shear in unidirectional composites is induced by transverse shear loading that progressively perforates the laminate within a narrow shear annulus. At lower micromechanical length scales, punch shear loading creates unique micromechanical damage mechanisms dominated by transverse fiber shear failure, fiber-matrix interphase debonding and large inelastic deformation and cracking of the matrix. A new punch shear experimental method has been developed to test unidirectional S glass/DER353 epoxy composite ribbons at sub-millimeter length scale. The experimental data consist of a statistical measurement of the continuum response (load-deformation and punch shear strength) and the characterization of micromechanical damage modes. A simplified 2D micromechanical finite element model incorporating Weibull fiber strength distribution has been developed and correlated with the experimental data. The 2D micromechanical finite element model can simulate the punch shear failure of the ribbon incorporating mixed mode fiber fracture, and fiber-matrix debonding mechanisms using zero thickness cohesive elements. Results from stochastic simulations of punch shear experiments show that an equivalent 2D micromechanical finite element model can predict the micromechanical damage mechanisms and the statistical distribution of punch shear strength of the continuum with favorable correlation with the experiments. This paper presents a combined experimental and computational approach in simulating the stochastic non-linear progressive punch shear behavior of unidirectional composites for the first time in the literature.
“…The major difference between the quasi-static and the transient dynamic solution of the single fiber break within a composite lies in the consideration of the inertial term in the governing equation of motion. 10,18 where σ is the Cauchy stress tensor, ρ denotes the mass density and a denotes the acceleration vector at a given material point.…”
Section: Problem Formulationmentioning
confidence: 99%
“…This is the same material system that was considered in our previous work where we had assumed that the matrix was purely linear elastic. 18…”
Section: Problem Formulationmentioning
confidence: 99%
“…The dynamic progression of events after a fiber break for the case of an elastic matrix has been described in Ganesh et al. 18 and also applies to the case of an elastic–plastic matrix. We assume that the central fiber (Fiber 0) breaks at the x = 0 plane at a break strength of σb.…”
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.
“…via acoustic emission) would thus be required in order to identify the sequence of individual ibre-break events leading to cluster formation. This is particularly important because most direct numerical simulations tend to predict a more progressive formation of clusters than what is typically seen experimentally[8,23]; this mismatch might be due to ignoring dynamic effects, as recent numerical simulations suggest that dynamic stress ields created during ibre-breakage are signi icantly different from those created under quasi-static conditions[70].De ining the link between progressive damage accumulation and specimen failure. By the nature of the in situ CT observation of ibre breaks (with inite time-scales for imaging and small imaging volumes), it has not yet been possible to conclude whether ultimate failure results from the overall accumulation of ibre breaks and broken clusters distributed over a volume of mate-rial, or from the formation of a critical cluster with a universal size.…”
Several modelling approaches are available in the literature to predict longitudinal tensile failure of ibre-reinforced polymers. However, a systematic, blind and unbiased comparison between the predictions from the different models and against experimental data has never been performed. This paper presents a benchmarking exercise performed for three different models from the literature: (i) an analytical hierarchical scaling law for composite ibre bundles, (ii) direct numerical simulations of composite ibre bundles, and (iii) a multiscale inite-element simulation method. The results show that there are signi icant discrepancies between the predictions of the different modelling approaches for ibre-break density evolution, cluster formation and ultimate strength, and that each of the three models presents unique advantages over the others. Blind model predictions are also compared against detailed computed-tomography experiments, showing that our understanding of the micromechanics of longitudinal tensile failure of composites needs to be developed further.
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