Circular Fiber Cross Sections Research Articles

Analytical modeling of fiber–matrix interface failure in unidirectional laminae subjected to in-plane shear loads

Unidirectional laminae subjected to in-plane shear loads present complex behavior with strong nonlinear response. A novel analytical model is proposed considering a unit cell with square symmetry and circular fiber cross-section taking into account the failure on the interface between fiber and matrix. The interface failure is a shear-driven phenomenon described by a constant value during the failure process. The model validation is performed using two different approaches: the analytical estimations are compared with experimental data, and finite-element simulations are employed to evaluate the influence of fiber volume fraction. The proposed analytical model captures the main features of experimental data showing its capability and reliability to represent the complex interface debonding propagation phenomenon. Numerical simulations show in-plane shear stress–strain curves for four distinct unidirectional laminae with different fiber volume fractions. Results attest the model capability to describe composite materials with composite failure, essentially characterized by the interface behavior.

Enhanced Modeling of Water Diffusion in Natural Fibers: Application to Diss Fibers

ABSTRACT Water diffusion through natural fibers represents an important aspect with regard to the integrity of biocomposites. Usually, diffusion model is defined assuming circular fiber cross-sections, while microscopic analysis findings revealed other geometries. This was found to affect the modeling of water transport through fibers and provide a gap versus experimental data. This work aims to present a numerical approach using finite element method to overcome the limits of use of analytical approaches relating to the morphological shape of vegetal fibers. The cross-section of the Diss fibers was observed by an optical microscope and simulated at an ellipsoidal shape after processing the images. Then, the average morphological parameters were determined. A numerical finite element model was implemented based on the observed geometry in order to determine the diffusion coefficient by an inverse approach compared to experimental results. The results showed that the numerical approach made it possible to raise the effect of fiber morphology, often assumed to be circular for plant fibers in analytical approaches, on the diffusion coefficient value, which was defined by a unique diffusion coefficient.

A vision based method for automated measurement of circular fiber cross-sections

Abstract There are many synthetic fibers with circular cross-sections, and characteristics of the fiber cross-sections have a major influence on the fiber properties. Direct measurements made with microscopic techniques on the fiber cross-sections can output maximum information on transverse dimensions. However, measuring parameters of the fiber sections in the microscopic images is a laborious task. In this paper, a hybrid method based on machine vision is proposed to measure the diameter and ellipticity of the circular fiber cross-sections. The hybrid method consists of modified Circle Hough Transform (CHT) and edge tracing algorithm, and the two sub-algorithms are performed successively. The modified CHT is used for detection and initial measurement of the fiber sections. Then the results are refined by the edge tracing algorithm, outputting elliptical results along with the ellipticity information of each fiber section. Experimental results show that the proposed method outperforms existing methods in circular fiber cross-section measurement.

FIBER SEGMENTATION IN CRACK REGIONS OF STEEL FIBER REINFORCED CONCRETE USING PRINCIPAL CURVATURE

This paper tackles the non-trivial image-processing task to segment hook-ended fibers in three-dimensional images. For this purpose, a novel segmentation method is presented that relies on the following observation: For a single fiber the configurations of principal curvatures that can occur on its surface are limited. Deviations from these configurations indicate potential overlaps of fibers. The method that was developed based on this observation is used to separate several simulated clusters of touching fibers as a proof-of-concept. Further, it is applied to two images of cracked steel fiber reinforced concrete specimens arising from a 4-point bending test. The method's performance is compared to manual separation. Overall, we can state that the proposed method yields satisfying results when data meets the following criteria: Low fiber volume density, circular fiber cross section and sufficient spatial resolution of fiber-fiber contacts.

Mechanical properties of natural fiber composites produced using dynamic sheet former

ABSTRACTComposites formed from wood fibers and man-made cellulosic fibers in PLA (polylactic acid) matrix, manufactured using sheet forming technique and hot pressing, are studied. The composites have very low density (due to high porosity) and rather good elastic modulus and tensile strength. As expected, these properties for the four types of wood fiber composites studied here improve with increasing weight fraction of fibers, even if porosity is also increasing. On the contrary, for man-made cellulosic fiber composites with circular fiber cross-section, the increasing fiber weight fraction (accompanied by increasing void content) has detrimental effect on stiffness and strength. The differences in behavior are discussed attributing them to fiber/ fiber interaction in wood fiber composites which does not happen in man-made fiber composites, and by rather weak fiber/matrix interface for man-made fibers leading to macro-crack formation in large porosity regions.

Correction Factors for the Analytical Transverse Stiffness of Unidirectional Fiber Composites

An analytical expression for the transverse elastic stiffness of unidirectional fiber composites is developed in this paper. Based on a basic solution adopting 1D analysis of square fiber cross-section, analytical results from (i) simplified 3D analysis using square fiber cross-section, and (ii) 1D analysis using circular fiber cross-section lead to Poisson’s ratio and geometrical correction, respectively. The Poisson’s ratio correction factor is shown to be a factor to the Young’s Modulus of matrix material while the geometrical correction factor is shown to be a factor to the fiber volume fraction. A geometrical correction function, which describes the fiber-matrix boundary with respect to the representative volume element, is identified. This function transforms results based on square fiber to those of circular fiber cross-section. Comparison of the Poisson’s ratio and geometrically corrected transverse moduli to experimental results reveal good agreement.

Comparison of tensile, thermal, and thermo‐mechanical properties of polyester filaments having different cross‐sectional shape

AbstractOne of the most important morphological features of fibers is their cross‐sectional shape. Nowadays, the circular fiber cross‐section is the most common shape of melt‐spun man‐made fibers. Other shapes are beginning to emerge for a variety of reasons such as performance, comfort, pilling propensity, bulkiness, tactility, processing etc. The filaments' cross‐section can be easily varied by changing the spinneret hole shape. Synthetic fibers that are predominantly spun by the melt spinning method with spinnerets having the noncircular hole geometry are called profiled or noncircular fibers. Modifications of the fiber cross‐section allow designing surface properties in yarn and fabric. However, the effect of profiled fibers on yarn properties has not been well documented yet. In this article, the influence of different filament cross‐section geometry on fiber properties was studied. PET (polyethylene terephthalate) filament yarns having two different cross‐sectional shaped filaments, circular and cruciform, were manufactured by melt spinning. Differences in tensile properties of filament yarn and as well as of individual filament depending on the cross‐sectional type were studied and revealed. More over, thermal and thermomechanical properties of filament yarn of both the cross‐sections were studied and revealed by DSC and TMA method, respectively. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Application of Fizeau fringes with bending technique to determine the shear modulus of the cladding material of single-mode optical fibers through the mechanically induced change of refractive index

In the present study, multiple-beam Fizeau fringes combined with the bending technique are used to determine the shear modulus of the cladding material of a single-mode optical fiber with accuracy of ±1×10 −4 for the index measurements. The described method requires no precise polarizing optics, or complicated mechanical equipment, or variation of angle of incidence, or accurate light intensity comparisons. A mathematical expression describing the modulus in terms of the fringe shift that is due to the induced-birefringence in the bent fiber has been derived. An asymmetric distribution of the shear modulus in the two cladding sides is seen experimentally. Applying the forward scattering technique confirmed that the asymmetry distribution in the modulus value due to asymmetric index profile could be attributed to a shift in the fiber centroid rather than a deviation in the circular fiber crosssection to a deformed elliptical crosssection under the effect of bending. The determined value of the shear modulus in the tensile side of the cladding is 3.4±0.2×10 4 N/ mm 2 while its value in the compressed side is 4.3±0.2×10 4 N/ mm 2 . The modulus mean value, in the present study, is in good agreement with the literature. As far as the author is aware, it is first time that the radial profiles of the shear modulus of the cladding material of an optical fiber have been determined interferometrically.

Effect of external electromagnetic field and bias voltage on the chemical vapor growth of the carbon micro-coils and their properties

Carbon micro-coils were obtained by the chemical vapor deposition using a Ni-catalytic pyrolysis of acetylene at 770°C with hydrogen sulfide as impurity, with and without an external electromagnetic field (EM) and applying a DC and/or AC bias voltage to the substrate. The effects of the EM field and bias voltage on the growth rate, morphology and properties of the as-grown carbon micro-coils were examined. A pale red-purple light emission was observed during the reaction. The EM field resulted in full decomposition of acetylene and coil yield of 1.6–2.3 times higher than that without the external EM field was obtained. The coil yield obtained by the application of bias voltage increased linearly with the increasing of bias voltage. The coil yield obtained with 400 V DC bias and 1000 V AC bias with the external EM field were 2.2 and 2.3 times higher than that without the EM field and no applied bias, respectively. The very regular-coiled carbon fibers with a flat fiber cross-section and irregular-coiled carbon fibers with a circular fiber cross-section were obtained with the EM field and without the EM field irrespective of the bias voltage, respectively.

Effect of external electromagnetic field and bias voltage on the vapor growth, morphology and properties of carbon micro coils

Carbon microcoils were obtained by the Ni-catalyzed pyrolysis of acetylene at 770°C using thiophene as an impurity with and without an external electromagnetic (EM) field and by applying or not applying a bias voltage to the substrate. The effects of the external EM field and bias voltage on the growth rate, morphology and some properties of the carbon microcoils were examined. It was observed that the external EM field accelerated the decomposition of acetylene resulting in a coil yield of 1.3–1.7 times higher than that without the external EM field. The application of the bias voltage also resulted in an increase of the coil yield; the yield linearly increased with increasing bias voltage. The coil yield obtained with a DC bias of 600 V and an AC bias of 2000 V with an applied external EM field was 1.9 and 2.1 times higher than that without the EM field and bias voltage, respectively. The very regularly-coiled carbon fibers with flat fiber cross-sections and irregularly-coiled carbon fibers with circular fiber cross-sections were obtained with the EM field and without the EM field irrespective of the bias voltage, respectively.

Modelling functional effects of muscle geometry

Muscle architecture is an important aspect of muscle functioning. Hence, geometry and material properties of muscle have great influence on the force–length characteristics of muscle. We compared experimental results for the gastrocnemius medialis muscle (GM) of the rat to model results of simple geometric models such as a planimetric model and three-dimensional versions of this model. The capabilities of such models to adequately calculate muscle geometry and force–length characteristics were investigated. The planimetric model with elastic aponeurosis predicted GM muscle geometry well: maximal differences are 6, 1, 4 and 6% for fiber length, aponeurosis length, fiber angle and aponeurosis angle respectively. A slanted cylinder model with circular fiber cross-section did not predict muscle geometry as well as the planimetric model, whereas the geometry results of a second slanted cylinder model were identical to the planimetric model. It is concluded that the planimetric model is capable of adequately calculating the muscle geometry over the muscle length range studied. However, for modelling of force–length characteristics more complex models are needed, as none of the models yielded results sufficiently close to experimental data. Modelled force–length characteristics showed an overestimation of muscle optimum length by 2 mm with respect to experimental data, and the force at the ascending limb of the length force curve was underestimated. The models presented neglect important aspects such as non-linear geometry of muscle, certain passive material properties and mechanical interactions of fibers. These aspects may be responsible for shortcomings in the modelling. It is argued that, considering the inability to adequately model muscle length–force characteristics for an isolated maximally activated ( in situ) muscle, it is to be expected that prediction will fail for muscle properties in conditions of complex movement with many interacting factors. Therefore, modelling goals should be limited to the heuristic domain rather than expect to be able to predict or even approach medical or biological reality. However, the increased understanding about muscular mechanisms obtained from heuristic use of such simple models may very well be used in creating progress in, for example, clinical applications.

Random Lattice Electrical Conductivity Calculations for a Graphite/Epoxy Ply of Finite Thickness

A single ply of unidirectional graphite/epoxy composite is modeled by both a two-dimensional Cartesian square and a close packed triangular lattice of fibers of infinite width and finite thickness. All fiber cross-sections are cir cular (of the same diameter) and those fibers with centers located at adjacent points on the lattice will contact at the midpoint. From these contacts con duction paths are formed across the latice. To generate randomness, the cir cular fiber cross-sections are removed at random from the lattice, leaving va cant sites of insulating matrix material, until a certain fiber fraction is reached. Lattice conductivities are calculated for both models at various fiber fractions and lattice thicknesses. Good agreement is obtained when the asymptotic conductivity versus thickness curves are compared with ex perimental data.

Percolation in a Thin Ply Of Unidirectional Composite

A unidirectional composite is modeled as a two-dimensional cartesian square lattice of infinite width and finite thickness. Circular fiber cross sections, each contained completely within a lattice square and touching on all four sides, are placed at random within the lattice according to the fiber volume fraction. Percolation, in this system, is shown to be very sensitive to the lattice thickness. To be regarded as infinitely thick a lattice must be at least 150 fiber diameters across, and for a typical ply thickness of 20 fiber diameters the critical fiber fraction for percolation is shifted by 26% from the infinite lattice value of 0.46 to a smaller value of 0.34.