Variation Of Fiber Volume Fraction Research Articles

Effect of spatial inhomogeneity in fiber packing on the strength variability of Al–matrix composites

Abstract Nonuniform packing of fibers in Al2O3 fiber-reinforced Al–matrix composite (AMC) arises due to the pressure infiltration processing technique used for producing these composites. The effect of spatial variation in fiber volume fraction, fL, on the strength variability of a Al–2Cu alloy matrix reinforced with 65 vol.% Nextel™–610 Al2O3 fibers was investigated. Measurements indicate a variance of ∼7% in the fL, much larger than the measured variance of ∼3% in the specimen-to-specimen fiber volume fraction, fG. An analytical model to account for the variance in fG and fL in predicting the Weibull modulus and the mean strength variation with the volume of AMCs is presented. This model incorporates the characteristic link concept and utilizes the theoretical predictions for their strength variability under local load sharing conditions. Comparisons of the model predictions with the experimental results of three- and four-point flexure, and tensile tests show excellent agreement.

Failure modes of fibre reinforced composites: The effects of strain rate and fibre content

The many aspects of high speed response of fibre reinforced composite materials have received the attention of a large number of investigators. Nevertheless, the understanding of the mechanisms governing failure under high speed loadings remain largely unknown. The effect of rate and fibre content on failure mechanisms was investigated by viewing fractured surfaces of tensile specimens using a scanning electron microscope (SEM). Tensile tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. A second laminate (with random continuous glass reinforcement) was tested in tension at varying fibre volume fractions in order to ascertain the relationship between fibre content and failure mechanisms. The results suggest a brittle tensile failure in fibres of the woven laminate. In addition, the matrix was observed to play a greater role in the failure process as speed was increased, resulting in increased matrix damage and bunch fibre pull-out. The results also indicated that increasing the fibre volume fraction increased the likelihood of a matrix dominated failure mode.

Experimental and theoretical assessment of the longitudinal tensile strength of unidirectional SiC-fiber/titanium-matrix composites

The tensile strength of unidirectional SiC-fiber/titanium composites (SCS-6/Ti-1100) of varying fiber volume fraction (0.15–0.35) was measured in the as-produced condition. Additionally, fibers were etched from the panels and tensile tested to determine their strength distribution, interface properties were measured with fiber push-out tests, and the tensile properties of the matrix (without fiber) were also determined. From the measured constituent properties, the composite strength was predicted by using both global and local load-sharing (GLS and LLS) models, and these predictions were compared with the measured values. The composite strength was lower than predicted by Curtin's GLS model, but greater than predicted if it is assumed that the first fiber failure initiates composite failure (i.e. severe LLS). Metallographic examination of the fractured specimens revealed that the failure was consistent with a non-cumulative (localized damage) failure mode, which, as suggested by previous theoretical and experimental studies, is promoted by matrix plasticity.

A method for measuring residual strains in fiber-reinforced titanium matrix composites

A new method for measuring the residual fiber strains in fiber-reinforced titanium matrix composites has been developed. The method involves selectively etching the matrix over a prescribed length of composite and subsequently measuring the extension of the relaxed fibers relative to neighboring fibers that are still embedded within the matrix material. The extensions are measured using confocal microscopy. The method is demonstrated on three unidirectionally reinforced composites with varying fiber volume fractions. The effects of specimen tilt and fiber splaying following dissolution on the measured fiber extensions are analyzed. The residual fiber strains are rationalized on the basis of the thermoelastic properties of the constituents through a concentric cylinder model. The current method can be applied to other metal matrix composites reinforced with large diameter (monofilament) fibers, provided the matrix can be selectively etched.

Three-dimensional modeling of woven-fabric composites for effective thermo-mechanical and thermal properties

This paper presents effective thermo-mechanical and thermal properties of plain-weave fabric-reinforced composite laminates obtained from micromechanical analyses and a two-scale asymptotic homogenization theory. A unit cell, enclosing the characteristic periodic repeat pattern in the fabric weave, is isolated and modeled. The orthotropic tensors for effective mechanical stiffness, coefficient of thermal expansion and thermal conductivity are obtained by numerically solving appropriate microscale boundary value problems (BVPs) in the unit cell by the use of three-dimensional finite element analyses. Further, analytical models consisting of series-parallel thermal resistance networks are developed in order, to obtain orthotropic thermal conductivity. The numerical and analytical models are explicitly based on the properties of the constituent materials and three-dimensional features of the weave style. Results obtained from the models are compared with experimental values and with models available in the literature. Non-linear mechanical constitutive behavior due to resin stress/strain non-linearity and to transverse yarn damage under in-plane uni-axial loads are also investigated. Parametric studies are conducted to examine the effect of varying fiber volume fractions on the effective thermal properties.

Failure Modes of SiC‐Fiber/Si3N4‐Matrix Composites at Elevated Temperatures

An investigation of composite failure modes as a function of temperature and fiber‐volume fraction was carried out in SiC‐fiber‐reinforced Si3N4‐matrix composites fabricated in our laboratories. Mechanical testing was carried out at temperatures from 25° to 1500°C. Matrix‐cracking stress and ultimate strength of the composites were measured from load‐displacement curves. They were both found to decrease with increasing temperature, but their temperature sensitivity decreased with increasing fiber‐volume fraction. The tendency for noncatastrophic failure increased with fiber‐volume fraction, while the tendency for catastrophic failure increased with temperature. The failure mode was demonstrated experimentally to be determined by the fiber bundle strength, Sfb, and the matrix cracking stress, σc. These two parameters, in turn, were shown to be controlled by the fiber‐volume fraction, f, and the temperature. Failure at various temperatures was noncatastrophic when Sfb > σc, and catastrophic when Sfb < σc. Transition in composite failure mode between noncatastrophic and catastrophic failure was controlled via the variation of fibervolume fraction and testing temperature. Catastrophic failure at high temperatures was found to be mainly a result of fiber strength loss.

The nonisothermal viscoplastic behavior of a titanium-matrix composite

The thermomechanical fatigue (TMF) response of a unidirectional SCS-6/Timetal 21S composite is determined using the unified inelastic-strain model of Bodner and Partom for the matrix response. The Bodner-Partom model captures the strain-rate sensitivity and time-dependent behavior of Timetal 21S for a wide range of temperatures (25–815°C) and strain rates (10 −3-10 −7s −1). For nonisothermal conditions, special terms are added to the inelastic-strain-rate expressions that account for changes in the temperature-dependent material parameters with changing temperatures. This viscoplastic model is implemented into the finite element package ADINA through user-defined subroutines. The unidirectional composite is represented by a concentric cylinder geometry formulated from axisymmetric elements. Numerical simulations predict an increase in residual stresses as the cooling rate from consolidation is increased. The composite response under in- and out-of-phase TMF loading compares well with experimental measurements. Compared to out-of-phase TMF, in-phase TMF shows considerably more ratchetting. For in-phase TMF, both the response and fatigue lives are more sensitive to variations in fiber-volume fraction than the out-of-phase case. The numerical model is consistent with the experimental observations in that the fibers control the in-phase TMF behavior, while the matrix controls the out-of-phase TMF behavior.

Effective Thermomechanical Behavior of Plain-Weave Fabric-Reinforced Composites Using Homogenization Theory

A micromechanical analysis is presented to obtain the effective macroscale orthotropic thermomechanical behavior of plain-weave fabric reinforced laminated composites based on a two-scale asymptotic homogenization theory. The model is based on the properties of the constituents and an accurate, three-dimensional simulation of the weave microarchitecture, and is used for predicting the thermomechanical behavior of glass-epoxy (FR-4) woven-fabric laminates typically used by the electronics industry in Multilayered Printed Wiring Boards (MLBs). Parametric studies are conducted to examine the effect of varying fiber volume fractions on constitutive properties. Nonlinear constitutive behavior due to matrix nonlinearity and post-damage behavior due to transverse yarn failure under in-plane uniaxial loads is then investigated. Numerical results obtained from the model show good agreement with experimental values and with data from the literature. This model may be utilized by material designers to design and manufacture fabric reinforced composites with tailored effective properties such as elastic moduli, shear moduli, Poisson’s ratio, and coefficients of thermal expansion.

The role of a polyamide interphase on carbon fibres reinforcing an epoxy matrix

Carbon fibres were pretreated by means of an interfacial in-situ polyamidation technique resulting in a polyamide coating of the fibres. The procedure followed involved successive application to the carbon fibres of two immiscible solutions of hexamethylenediamine and adipoyl chloride. Previous work conducted in our laboratory on chrysotile/epoxy composites coated with Nylon 6,6 provided interesting results when the tensile performance was evaluated. This behaviour was attributed to the improved affinity between matrix and asbestos fibres as a result of the well-known compatibility between polyamide and the epoxy phase. In this study emphasis is given to the coating of long carbon fibres such as are used for the preparation of unidirectional carbon/epoxy composites. The mechanical properties of uniaxial laminates with varying fibre volume fractions and polyamide contents are then compared with those of uncoated carbon fibres.

Thermal Cycling Response of Quasi-Isotropic Metal Matrix Composites

In this study, thermal cycling response of quasi-isotropic metal-matrix composite (MMC) with a stacking sequence of [0/ ± 45/90]s was investigated. The thermal cycles were imposed between temperatures of 316–649° C. Metallography of the samples at the edge has shown the presence of fiber-matrix debonding and ply-to-ply separation (delamination) in the 45 and 90 deg plies. The propensity for debonding was found to be greater when fibers are too close in any of these plies. Both closed form elastic analysis and linear finite element analysis using temperature dependent material properties were undertaken to evaluate the criticality of local stresses and strains in the constituent materials. Inelastic deformation around fiber-matrix interface leading to cracking in the 90 deg ply was best simulated by higher local variation in fiber volume fraction in the composite and the presence of initial process induced defect.

Structure and mechanical properties of oxide fibre reinforced metal matrix composites produced by the internal crystallization method

A new method of composite fabrication, called by the authors the internal crystallization method, was invented some years ago. The method is essentially based on growing fibres from a melt within the volume of a matrix. It has been successfully used to obtain a number of composites with a molybdenum matrix and various oxide fibres including those of sapphire, some oxide eutectics, complex oxides or rare earth metals and so on. A fibre can have either a single crystalline structure or a composite structure. The strength description of such fibres needs to take into account a variation of fibre cross-sectional shape with the variation of fibre volume fraction. A strength model of composites of the brittle fibre/metal matrix type suggested earlier is used to obtain statistical strength characteristics of fibres and to relate them to some details of the fabrication process. Creep strength of the composites is also studied and the results show superior performance of composites with eutectic fibres.

Mechanical properties of E-glass fibre reinforced nylon 6/6 resin composites

The tensile strength, tensile modulus, compressive strength, interlaminar shear strength and residual tensile strength of E-glass fibre reinforced nylon 6/6 resin composite with the variation of fibre volume fraction are characterized. The results are in line with the required limits of theoretical values.

Free-edge stress reduction through fiber volume fraction variation

An effective technique to reduce the free-edge interlaminar stresses in a symmetric laminate under uniform axial extension is presented. A pseudo three-dimensional finite element method is employed to calculate the interlaminar stresses. The reduction of the interlaminar stresses is achieved by varying the fiber volume fraction near the free edge. Numerical results show that the interlaminar normal and shear stresses near the free edges can be significantly reduced or even totally eliminated by the proposed technique.

Transition properties of pretreated asbestos-filled epoxy polymers

By means of interfacial in situ polymerization poly(hexamethylene adipamide) was deposited on chrysotile fibres to ensure a strong interphase bond. The filler treated fibres were incorporated into epoxy resin using dibutyl phthalate as diluent to overcome viscosity effects and the resulting composite material was examined. Transition properties were considered while varying fibre volume fraction and concentration of the polyamide coating.

Mechanical properties of model synthetic tendons.

Model synthetic tendons consisting of 20 vol % of texturized poly(ethylene terephthalate) fibers and of the water-swollen poly(2-hydroxyethyl methacrylate) matrix have the tensile modulus E = 1.5 +/- 0.1 GPa, strength and strain-at-break sigma b = 85 +/- 10 MPa and epsilon b = 0.08 +/- 0.02. The force required for breaking tendons with the diameters 2, 3, 4 mm is, respectively, 300, 500, and 960 N. By these properties model synthetic tendons closely imitate the properties of natural tendons. Long-term (100 min) and repeated short-term (30 times 1 min) creep shows that on loading model tendons lose some 10% of their stiffness, but that the whole deformation is reversible. The shape of the compliance vs. time dependence of synthetic tendons closely resembles the dependence determined for the parent fiber. The stiffness and strength of a tendon are given by those of the fiber bundle used; by varying fiber volume fraction, it is possible to adjust the required mechanical properties of tendons.

EPR study of the RKKY interaction in metallic Eu xGd 1−xB 6

By means of interfacial in situ polymerization poly(hexamethylene adipamide) was deposited on chrysotile fibres to ensure a strong interphase bond. The filler treated fibres were incorporated into epoxy resin using dibutyl phthalate as diluent to overcome viscosity effects and the resulting composite material was examined. Transition properties were considered while varying fibre volume fraction and concentration of the polyamide coating.

Stress Concentrations Near a Discontinuity in Fibrous Composites

Stress concentration factors are defined for the fiber and matrix in an axially loaded unidirectional composite which has a discontinuous fiber Effects of variations in fiber volume fraction, end-gap size, and modulus ratio are studied by using a linearly elastic finite-element analysis Results show that the fiber stress concentration factors reach a maximum value of 1.5 and then remain relatively unchanged The matrix stress concentration factors, however, are shown to increase rapidly with decreasing end-gap size and increasing modulus ratio.