Mathematical simulation of high-porosity fibrous materials and determination of their physical properties

A statistical mathematical model of fibrous high-porosity composite material is described. This model is used for studying the basic radiative and thermophysical properties of material. It is demonstrated how the use of the model helps significantly extend the capabilities of experimental methods and enables one to compare the calculation and experimental results and obtain parameters of material which were previously hardly accessible for determination.     

Continuous transverse permeability of fibrous media

The compaction of composite preforms and the flow of resin through the fibrous network take place simultaneously during the Resin Film Infusion process. Therefore there is a coupled loading of the porous reinforcements. A new experimental device to impose combinations of hydraulic and mechanical loadings (Hydro-Mechanical loadings) to fibrous preforms is used to evaluate the transverse permeability in a continuous manner (under Hydro-Mechanical conditions) for a flax mat, a flax non-crimped fabric, a carbon plain weave and a glass satin weave. For a 0.5 mm/min compression speed the continuous technique gives values similar to the “classical” technique for the four composite reinforcements of very different nature. This suggests that it is possible to use the continuous technique to evaluate the transverse permeability behaviour of fibrous reinforcements (permeability vs. fibre volume) with a time reduction of about 8–10. Increasing the compression speed gives slowly decreasing continuous transverse permeability values.     

Longitudinal tensile failure of unidirectional fibrous composites

This paper presents a theoretical treatment of the tensile strength of a unidirectional fibrous composite, subjected to a tensile load in the fibre direction. The fibres are treated as having a statistical strength distribution which results in fibre failure prior to composite failure. The failure geometry of the model is similar to the observed geometry of fractured glass/epoxy and glass/polyester composites. Failure criterion is established and the strength is shown to decrease as the length of the specimen is increased. This size effect is very small.     

Creep of fibrous composite materials

Models are presented for the creep behaviour of fibrous composite materials with aligned fibres. The models comprise both cases where the fibres remain rigid in a creeping matrix and cases where the fibres are creeping in a creeping matrix. The treatment allows for several contributions to the creep strength of composites. The advantage of combined analyses of several data sets is emphasized and illustrated for some experimental data. The analyses show that it is possible to derive creep equations for the (in situ) properties of the fibres. The experiments treated include model systems such as Ni + W-fibres, high temperature materials such as Ni + Ni₃Al + Cr₃C₂-fibres, and medium temperature materials such as Al + SiC-fibres. For the first two systems reasonable consistency is found for the models and the experiments, while for the third system too many unquantified parameters exist and further studies seem necessary.     

The reflectivity discrepancy method for the determination of thepermittivity and permeability of complex materials

The possibility of distinguishing whether the losses in a material under investigation are due to either its dielectric or magnetic properties, or both, is tested by treating a single spectrum of a reflection simulated by computer. To determine the frequency bandwidth and range of sample thickness sufficient to measure both the permittivity and the permeability at any frequency, the reflection spectrum of a sample with magnetic and dielectric properties is analyzed. If the reflectivity is measured over a frequency band several times wider than the absorption line of the sample and the sample is semitransparent in the middle of the line, the magnetic and dielectric spectra may be determined. Insertion of an air gap between the sample, and the plane of the short circuit has been found to facilitate the measurements. The validity of this approach is shown with permittivity and permeability measurements of a composite substance containing iron     

Stationary transfer in fibrous composite materials

The effective heat-conduction coefficients are calculated for fibrous materials of different structure.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 36, No. 5, pp. 828–834, May, 1979.     

Boundary layers in fibrous composite materials

Many studies in the theory of composite materials are based on the homogenization approach, which consists of the substitution of the original heterogeneous medium by a homogeneous one with certain effective properties. Though this procedure works well for the entire composite solid, it cannot be applied in the vicinity of the outer boundary. The transmission of an external load applied at the boundary to the inner domain of the material occurs by a redistribution of stresses between the constitutive components (inclusions and matrix) and involves strong singularities in the local stress field, which may result in microscopic failure of the composite structure. In the present paper, we propose an approximate analytical procedure, allowing determination of the stress?Cstrain field in the vicinity of the outer boundaries of fibre-reinforced composite materials. It is also shown that controlled decrease in bonding between the components leads to a more uniform redistribution of local stresses, which can essentially reduce the risk of failure.     

Dynamic overstresses in fibrous polymeric materials

We have considered the problem of the dynamic stress redistribution in fibrous materials brought about by the failure of a single element. Our model consists of a number of aligned adjacent chains of harmonically coupled masses. Neighboring masses on adjacent chains are also coupled by shear-carrying springs. Depending upon the scope and assignment of parameters, the model might apply to aligned polymer chains in a crystal or amorphous environment, to filaments in a fiber, or to a fibrous composite material. At time t = 0, an intra-chain spring is broken and the motion of the masses in the local environment is followed by molecular dynamics. The dynamic loads carried by bonds in chains neighboring the defect oscillate with decreasing amplitude, ultimately reaching asymptotic values that can also be calculated exactly by harmonic analysis. We find that the maximum of the excess load carried by a spring in a chain adjacent to a defect is about two times the asymptotic value for two-dimensional arrays of chains and somewhat larger for three-dimensional arrays. The dependence of these quantities on the magnitude of the shear carrying spring constant is explored. These results have implications for a number of theoretical approaches to fiber and composite strength which use load-sharing models based on asymptotic values.     

A bimodal plasticity theory of fibrous composite materials

Summary It is shown that elastic-plastic response of metal matrix composites reinforced by aligned continuous fibers can be described in terms of two distinct modes. In the matrix-dominated mode, the composite deforms primarily by plastic slip in the matrix, on planes which are parallel to the fiber axis. In the fiber-dominated mode, both phases deform together in the elastic and plastic range. Constitutive equations are derived for the matrix-dominated mode of deformation in composites with elastic-perfectly plastic matrices. Response in the fiber-dominated mode is approximated by the self-consistent and Voigt models. The two deformation modes give different branches of the overall yield surface which identify the state of stress that activates a particular mode, and indicate the conditions for mode transition in a given composite system. The matrix-dominated mode is found to exist in systems reinforced by fibers of large longitudinal shear stiffness, such as boron or silicon carbide. Systems reinforced by more compliant fibers, such as graphite, appear to deform exclusively in the fiber-dominated mode. The results show good agreement with experimental data, and with predictions obtained from a more accurate material model. They also help to reconcile several different plasticity theories of fibrous composites, and suggest limits of their validity.With 9 FiguresPrepared for the Symposium on Plasticity: Foundations and Future Directions. In Memory of Aris Phillips. January 28–30, 1987. University of Florida, Gainesville, U.S.A.     

Failure of fibrous anisotropic materials under combined loading

A failure criterion for anisotropic materials under combined stress is developed and demonstrated. The generality and accuracy of the present theory are illustrated by examination through the use of material systems under various loading conditions. Calculated results are compared with the experimental data throughout four quadrants. It agrees with observation quite well at high values of shear stress, where the Tsai–Hill theory becomes too conservative. The present criterion is also compared with other criteria. The comparison shows that this criterion has a good agreement with the experimental data even when the shear stress component is greater than the shear strength.     

Functionalized fibrous materials for the removal of dyes

Wastewater effluents from textile industry mainly contain dyes used in the dyeing or printing of textiles yarns or fabrics. A lot of technologies can be adopted for dye removal from wastewaters, including biological treatments based on activated sludge, adsorption on activated carbon, or membrane processes. Nevertheless, none of these methods is performing toward all classes of dyes; treatment plants of great dimensions and difficult handling can be required, while cost can be prohibitive. In the present study, dye adsorption was performed on low-cost fibrous cellulose materials, submitted to a cationization process using a quaternary ammonium reagent. These materials were characterized by FTIR–ATR spectroscopy, while the treatment efficiency was investigated through exhaustion and kinetic adsorption tests toward anionic dyes, such as acid, direct, reactive, and a cationic dye. Comparisons were carried out between treated and untreated samples, and with activated carbon. Best results were obtained with cationized cotton, both in linters or “tulle” fabric form, showing good performance toward all the investigated dyes. Tests in continuous flow confirmed the good behavior of these materials, but tulle fabric was better as filtration medium for a lower pressure drop. Finally, regeneration tests by bleaching were carried out finding that regenerated material maintained good adsorption power.     

An analytical model for the longitudinal permeability of aligned fibrous media

This paper presents an analytical series solution of the longitudinal fluid flow in porous media consisting of aligned rigid fibers, which, in turn, is used to establish a relationship for the longitudinal permeability as a function of the fiber packing geometry, fiber volume fraction and the fiber radius. The analytical series solution is developed for rectangular and staggered packing arrangements of the fibers using the boundary collocation method where the constants in the series solution are solved for numerically. As the number of boundary collocation points is increased, the analytical solution is shown to converge with a finite element solution of the identical flow situation for all fiber volume fractions and packing arrangements. In addition, the permeability results are presented in a dimensionless form as a function of the fiber volume fraction and fiber packing arrangement for a general applicability and easy use of the results for predicting the longitudinal permeability of fiber tows consisting of aligned rigid fibers.     

A reference specimen for permeability measurements of fibrous reinforcements for RTM

The capability to simulate the flow of resin through a porous reinforcement by FE modelling has become very important for designing RTM production processes for high-performance composite parts. The key parameter in RTM flow simulations is the fibre reinforcement permeability. The measurement of this material parameter is still not standardized and many different set-ups have been proposed. Recently, a stereolithography technique was used to produce a textile-like solid specimen with anisotropic permeability, designed as a reference for calibration and comparison of permeability measurement set-ups and for validation of numerical permeability computation software. Unlike real textiles, the permeability properties of such reference specimens do not vary from test to test. Excellent repeatability of the experiments is achieved. When used for benchmarking, any discrepancy between different measurements on this specimen must be attributed to the set-up and data processing.This paper presents the first experimental measurements of the permeability of such reference specimens, obtained with a 2D central injection rig. The measured values, with principal components of the 2D permeability tensor of 2.47 ± 0.09 × 10^?9 m² and 5.44 ± 0.22 × 10^?9 m², are in good agreement with the values predicted using numerical permeability computation software. Statistics from a series of measurements using the same set-up show that the precision of the permeability identification depends on the data processing procedure. An approach which incorporates a numerical model that is also valid after the fluid reached an edge of the reinforcement performs significantly better than the approach, based on an analytical approximation, which was used before.