Compressibility and relaxation of fiber reinforcements during composite processing

In many reinforced composite manufacturing processes it is necessary to compact the fiber materials to obtain the desired fiber/resin ratio in the finished part. Detailed knowledge of applied surface force versus material fiber volume is particularly important in processes such as pultrusion, resin transfer molding, and compression molding. The force required to compact a stack of reinforcing material is strongly dependent on the type of fiber used and its material form. Complicated interactions are possible, particularly when mixtures of unidirectional, oriented cloth and random fiber mats are used. This paper will present results of an experimental and analytical investigation of the response of various dry reinforcing materials subjected to compressive forces applied normal to their principle plane. Experiments were conducted by applying up to 8.6 MPa normal force to thick stacks of E-glass, graphite cloth, mat and unidirectional material and combinations of two different fiber orientation. Pressure versus fiber volume data were generated for both individual materials and various combinations. Experimental results were compared to analytical predictions. Data showed that the force versus deformation is very strongly dependent on the details of the fiber form or forms being used. There is structural relaxation during fiber compression. Relaxation is very related to fiber orientation, span length, and fiber breakage during compaction. Relaxation behavior decreases with fiber alignment. Random mats and 0/90 cloth show much more relaxation than unidirectional fibers. Data of relaxation is very well fitted with a Maxwell-Wiechert viscoelastic model.     

Simultaneous measurements of permeability and capillary pressure of thermosetting matrices in woven fabric reinforcements

A simple apparatus was designed and constructed capable of measuring the unsteady-state permeability and the capillary pressure simultaneously in a simulated composite impregnation experiment. It was found that the Kozeny-Carman equation used to describe the permeability of composites during impregnation adequately described experimental results for woven fabric preform up to porosity values of 0.5. Above this value, observed deviations were attributed to interfacial effects between adjacent woven fabric layers. For woven fabric preforms made of T-300 carbon fibers, a maximum capillary pressure of 3.7 × 10⁴ Pa (=5.4 psi) was observed at low porosity values. Thus, the capillary pressure may compete with other pressure sources in low pressure processes, such as the prepregging process. The woven fabric preform used in this study is observed to have a permeability similar to a unidirectional fibrous preform along the transverse direction. Furthermore, an existing modeling methodology capable of predicting permeability and capillary pressure through different preforms was found to be valid for fibrous preforms of complex orientation.     

Compaction of textile reinforcements for composites manufacturing. II: Compaction and relaxation of dry and H2O-saturated woven reinforcements

Previous analysis of published experimental results on compaction and relaxation of textile reinforcements allowed the effects of some processing parameters on the mechanical behavior of the reinforcements to be identified. However, a limited number of relaxation results are available; also, the effect of some parameters on compaction received limited attention, and the behavior observed with fluid-saturated reinforcements has not been investigated. In this paper, the results of a structured experimental program of compaction and relaxation performed on three woven reinforcements are reported. In half the trials, a relaxation period was imposed on the samples. In the other half, samples saturated with distilled H₂O were compacted. The selection of the processing parameters was found to be as important as the selection of the reinforcement itself for the definition of a manufacturing operation. The processing parameters governing the compaction and relaxation were seen not to be the same, and the fiber reorganization that occurs during the compaction phase was found to have a different effect on successive compaction cycles than the reorganization occurring during the relaxation.     

A comparison of the unidirectional and radial in-plane flow of fluids through woven composite reinforcements

The in-plane flow of fluids through dense fibrous woven reinforcements was studied to aid the development of constitutive models for use in simulations of composite fabrication by resin transfer molding. As a first part of this effort, both one-dimensional and radial flow experiments were conducted with Newtonian fluids in several woven glass fabrics. Analysis of the one-dimensional flow experiments shows that the two experimental techniques are often, but not always, consistent, and both techniques suggest a relationship between the physical structure of the reinforcement and the mathematical structure of the permeability tensor. Preform features at the laminar and interlaminar scales were hypothesized to influence the experimental results.     

In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique

This report discusses the advantages and drawbacks of the multi-cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably- between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.     

Influence of unwoven roving and woven fabric carbon fiber reinforcements on reaction-to-fire properties of polymer matrix composites

The influence of carbon fiber orientation on reaction-to-fire characteristics of layered polymer matrix composites is investigated in detail. M18-1/G939 as woven fabric and M18-1/G947 as unwoven roving prepregs with identical, epoxy based matrix composition were laid-up to give 1 to 8 mm thick unidirectional and quasi-isotropic laminates. Fundamental reaction-to-fire properties of these composites are interpreted on basis of the matrix components: epoxy resin and polyetherimide, as well as contained flame retardants magnesium hydroxide and zinc borate. Cone calorimetry and temperature distributions through the laminate show, that the velocity of combustion is influenced by fiber orientation for a given resin. For an unwoven roving it is confirmed that a quasi-isotropic fiber orientation leads to faster ignition, due to preferred delaminations, but retards burning processes more effectively than a unidirectional lay-up. This fundamental principle is extended to woven fabric reinforcements, however with less pronounced effects. Migration velocities of the pyrolysis zone are measured and additionally, decomposition of carbon fibers is considered. A comparison with a similar epoxy based matrix system without toughener and flame retardants: RTM6/G939 is carried out, in order to assess the effects by fiber orientation.     

Shear properties of three‐dimensional woven composite reinforcements

This paper presents a comprehensive experimental study and detailed mechanistic interpretations of the shear deformation of three‐dimensional (3D) reinforcements. Six types of 3D angle interlock glass fiber preforms (3DAP) were fabricated using a range of weave parameters including the fabric density, fabric layer, and yarn linear density. A modified picture frame was developed to ensure a pure shear load during the test. Through a series of comprehensive tests, our results demonstrated that the fabric density played a key role in the mechanical properties of 3DAP and that the reinforcements with low fabric density and yarn linear density were easy to shear. The shear deformation mechanism was analyzed based on the meso‐structure. It is expected that this research will provide preliminary work for building a theoretical model of 3D woven preform. POLYM. COMPOS., 38:244–251, 2017. © 2015 Society of Plastics Engineers     

Reusing textile waste as reinforcements in composites

Polyester (PET) has wide applications in textile industries as textile fiber and its share continues to grow. Substantial quantities of cotton/polyester blend fabrics are disposed every year due to technical challenges, which pose a big environmental and waste‐dumping problem. The aim of this study is to evaluate the potential of discarded cotton/PET fabrics as raw materials for composites. If their inherent reinforcement properties can be used in composites, an ecological footprint issue can be solved. In this study, we investigate three concepts for reuse of cotton/PET fabrics for composites: compression molding above the T_m of PETs, use of a matrix derived from renewable soybean oil, use of thermoplastic copolyester/polyester bi‐component fibers as matrix. All three concepts have been explored to make them available for wider applications. The effects of processing parameters such as compression temperature, time and pressure are considered in all three cases. The third concept gives the most appealing properties, which combine good tensile properties with toughness; more than four times better tensile strength than the first concept; and 2.2 times better than the second concept. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40687.     

Measurement of in-plane permeability of anisotropic fiber reinforcements

Three of the most common methods (two with parallel flow and one with radial flow) for determination of the in-plane permeability tensor are studied both theoretically and experimentally. An error analysis shows that the difference between the methods is negligible if the error levels are equal. However, the radial flow method is found to be susceptible to large errors from mold deflection in an experimental comparison between the methods. Additional experiments with the radial flow method in a stiffer mold show that the method gives the same values for the permeability tensor as the other two methods. A new method with multiple cavities in parallel is proposed that combines the simplicity of the radial flow method with the stiff mold of the parallel flow method. Only mass and time need to be measured in one experiment and it eliminates the need to measure fluid viscosity, temperature, and injection pressure. The method depends on the availability of a reference material with known permeability.     

Pore network modeling of permeability for textile reinforcements

The homogenized permeability of textile reinforcement is computed using pore network modeling. The model takes as input a 3D image of the reinforcement, reduced to an array of points, each of them having a binary value designating whether the point lies inside a yarn or in a pore. The array can be acquired by X‐ray micro‐computerized tomography. The calculations proceed in two steps. First, the 3D image is analyzed to create a representation of the pores in the reinforcement by a system of interconnected pipes. Each pipe is characterized by its hydraulic resistance. Then the hydraulic resistance of the system of pipes is computed using the Kirchoff theory to yield a value for homogenized permeability of the reinforcement. The model is applied to a glass woven laminate, producing results in good agreement with experimentally measured values.     

The permeability and compressibility of aligned and cross-plied carbon fiber beds during processing of composites

Two of the most important input parameters needed to simulate the processing of continuous fiber laminated composites are the fiber bed permeability and the portion of the autoclave load borne by the consolidating fiber network (compressibility). In this study we have experimentally examined how both these parameter change with resin volume fraction as pressure is applied and consolidation proceeds. For a unidirectional fiber bed, the Kozeny-Carman equation can be used to predict both the transverse (perpendicular to the laminate plies) permeability (Kozeny constant, K′_z = 11) and the axial (parallel to the fibers) permeability (Kozeny constant, K′_X = 0.57). The axial permeability was found to be dependent on the surface tension of the permeant. For a unidirectionally aligned fiber, the measured transverse permeabilities varied from 1.1 × 10^?10 cm² to 12. × 10^?9 cm² while the axial values varied from 2.1 × 10^?9 to 4.4 × 10^?8 cm² for a liquid volume fraction range of 0.25 to 0.5. Axial permeability measurements indicate that the permeability decreases with increasing off-axis angle × (measured from the laminate axial direction). The off-axis permeability behavior can be described by a modified Kozeny-Carman equation. The fiber network compressibility can be described with a logarithmic relation which has been found valid for a large number of consolidated soils.     

Performance of hybrid reinforcements in PVC composites. II: Use of surface-modified mica and different cellulosic materials as reinforcements

The effect of the composition of various wood fibers and surface-treated mica as well as different surface treatments of cellulosic materials on the mechanical properties of PVC composites has been evaluated. Cellulosics were surface modified by prior coating with maleic andydride (MA), mixtures of MA and Na-silicate and isocyanate. The filler concentration was fixed at 25 wt%. Both tensile strength and modulus of composities filled solely with mica are superior to those of non-treated wood fiber-filled composities. while the reverse is true for impact strength (except for bagasse-filled composities), ultimate elongation, and tensile toughness. Moreover, the mechanical properties of composities, with the exception of modulus, filled only with mica and/or non-treated wood fibers are inferior to those of unfilled PVC. Compared to non-treated fiber-filled composites, properties improved when surface-modified wood fibers were used alone, or along with mica. Isocyanate-coated wood fibers ranked best with regard to the mechanical properties of the composities. Properties also changed with the change of wood species and compositions of mica and wood fibers. Experimental results indicate good compatibility between surface-treated wood fibers/mica and PVC composities.     

A note on permeability simulation of multifilament woven fabrics

A conventional approach for modeling permeability of multifilament fabrics is to consider their warps and wefts to be individual thick filament made of homogeneous porous media and solve the flow equations for such monofilament fabrics. In this work, for the first time, the full 3-D geometry of an idealized multifilament woven fabric, wherein the filaments are packed in hexagonal arrangement, is generated to compute its permeability and compare with the homogeneous anisotropic lumped model of Gebart (1992. Permeability of unidirectional reinforcements for RTM. Journal of Composite Materials 26(8), 1100-1133). While a relatively good agreement is obtained, our results indicate that Gebart's model slightly underestimate the permeability of multifilament fabrics even at high yarn's solid volume fractions.     

Mechanisms of energy absorption in the creep fracture of woven ceramic composites

A micromechanical model is developed which accounts for the energy absorbed in the creep deformation and fracture of a 2.5D SiC/C/SiC composite, representative of the new generation of non-oxide CMCs. The model quantifies the influence of the geometrical, mechanical and material parameters and, in particular, is very sensitive to the interfacial sliding stress. The effect of the sliding stress on the contribution of the different energy absorbing mechanisms in the creep fracture of CMCs is described. It is concluded that, for all testing conditions, most of the energy absorbed in the creep fracture is controlled by fibre–matrix debonding and fibre pull-out.     

Internal geometry variability of two woven composites and related variability of the stiffness

In this work, the internal geometry measurements of two woven composites are analyzed: cross section of the yarns, fiber volume fraction, crimp, misalignment of the samples, thickness of the plates, nesting value, and so forth. The chosen textile composites are produced using the same resin reinforcement and same architecture, but have different yarn size (3K and 12K). The dispersion of measured geometrical parameters is introduced in a numerical multiscale modeling approach to evaluate the macroscopic stiffness values. A sensitivity analysis is performed for each geometrical parameter and laminate stiffnesses are derived. These are linked to the experimentally obtained elastic properties by tensile tests. Finally, the unit cell size scale effect for measured geometry variability and experimentally obtained elastic properties are evaluated. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

新一代三向织物及其复合材料

本文介绍国外碳纤维三向织物的发展，包括新一代碳纤维三向织物的制备、特点、性能及其在复合材料上的应用。     

Some features in processing carbon yarns and tows into woven fillers

Conclusions -- Processing carbon yarns of types Ural-N and UKN requires modernization of the weaving equipment.-- It is necessary to reduce the speed regime for all technological transitions.-- All the yarn-guiding elements should be made of pyroceram.-- Introduction of the fill-yarn into the orifice is desirably carried out with a punch.-- This structure of the woven filler should aid in maximum realization of the strength of carbon tows.Translated from Khimicheskie Volokna, No. 4, pp. 6–7, July–August, 1991.     

Characterization of polymeric fibers as reinforcements of cement‐based composites

In this study, three polymeric fibers (nylon 66, polypropylene, and acrylic) were used to improve the flexural and tension strength of cementitious materials. To characterize the performance of these fibers in a cement matrix, scanning electron microscopy, optical microscopy, dynamic mechanical analysis, tensile strength testing, and alkali resistance test were employed. The performance of cement‐based composites containing the fibers was evaluated with a flexural strength test. The results indicated that the flexural strength increased with an increasing number of interfacial interactions between the fibers and cement. This finding was supported by dynamic mechanical analysis data. This has great application potential for fibers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Determination of core shear properties of three-dimensional woven sandwich composites

Abstract

The core shear characteristics of a relatively new material have been investigated by two methods: three point bending tests and a shear test. The material is based on woven sandwich fabric preform, which offers important advantages related to the integral core–skin structure. The core shear moduli results from the block shear test and ASTM three point bending tests show very little difference. The results from the shear test obtained by measuring the crosshead displacement were verified by conducting additional shear tests, in which two linear variable differential transformers were used for strain measurement in the longitudinal and transverse directions. The ASTM block shear test and three point bending test were used to determine the core shear strength of the material. To obtain pure core shear failure leads to a correct core shear strength result; the three point bending test was modified by increasing the skin thickness and stiffness of the panel.     

Slow crack growth resistance of electrically conductive zirconia-based composites with non-oxide reinforcements

Slow crack growth (SCG) behavior of four zirconia-based composites reinforced with 40 vol% WC, TiC, NbC or TiCN were studied by means of double-torsion testing. Compared to monolithic zirconia, the composites had a higher resistance to fast fracture, i.e., higher fracture toughness. The extent of toughening depended on the reinforcement type, shifting the V-K_I (crack velocity versus stress intensity factor) curve parallel to higher K_I values. More importantly, these composites were less sensitive to SCG. Identical V-K_I/K_IC curves with steeper slopes compared to monolithic zirconia were observed for the investigated composites, independent on the reinforcement type. No rising R-curve was measured, at least in the crack-size domain investigated by SCG. Therefore, the higher SCG resistance of the composites was due to the intrinsic stress-assisted corrosion resistance of the covalent non-oxide secondary phase.