A tensile strength model for unidirectional fiber-reinforced brittle matrix composite

A model for the ultimate tensile strength of unidirectional fiber-reinforced brittle matrix composite is presented. In the model, transverse matrix crack spacing and change in debonding length between the fiber and the matrix is continuously monitored with increasing applied load. A detailed approximate stress analysis, together with a Weibull failure statistics for fiber fracture, are used to determine the probability of fiber fracture and fiber fracture location in the composite. Results of the model are consistent with experimental data. It is suggested from the results that the strength and toughness of the composite are significantly influenced by the Weibull modulus of the fiber and the fiber/matrix interfacial shear stress. A higher fiber Weibull modulus results in a lower composite strength while a higher fiber/matrix interfacial shear stress results in a composite with higher strength but lower toughness. A moderate variation in matrix strength and fiber/matrix interfacial shear strength does not significantly affect the strength of the composite.     

Hierarchical multiscale modeling of failure in unidirectional fiber-reinforced plastic matrix composite

A hierarchical multiscale approach is applied to study the tensile strength of fiber-reinforced composites. The approach is carried out in three scales: micro, meso and macro-scales, which are linked by information transfer from small to large-scale. In micro-scale, a 3D column model was established to calculate the residual stresses, which is fed into mesoscale for interfacial friction stress; in mesoscale, a representative volume (RVE) with a central broken fiber and four neighbor fibers is modeled, where matrix plastic hardening is considered. Local stress distribution in RVE is simulated by shear-lag model, and transferred into macro-scale for progressive damage simulation. In macro-scale, Monte Carlo simulations with the present shear-lag model were then conducted to obtain the ultimate tensile strength. Through this hierarchical multiscale simulation, composite macro-performance can be predicted by micro-scale parameters, this relationship will give a reference for composite design and optimization.     

单向纤维增强陶瓷基复合材料界面滑移规律

建立了分析单向纤维增强陶瓷基复合材料力学特性的界面摩擦模型。采用基体应变能准则对基体损伤状态进行预测; 由Weibull分布模型拟合出纤维断裂分数; 将界面磨损处理为纤维/基体相对滑移历程的函数τ_i=τ_i (Δδ), 很好地表征了不同位置纤维/基体相对滑移历程不同所引起的界面磨损程度的区别。运用该模型分析了准静态加载和拉-拉循环载荷下的应力-应变特性, 预测结果与实验数据吻合较好。最后采用此模型研究了任意载荷历程下界面的滑移规律。     

单向纤维增强陶瓷基复合材料界面滑移规律

建立了分析单向纤维增强陶瓷基复合材料力学特性的界面摩擦模型.采用基体应变能准则对基体损伤状态进行预测;由Weibull分布模型拟合出纤维断裂分数;将界面磨损处理为纤维/基体相对滑移历程的函数τi=τi(△δ),很好地表征了不同位置纤维/基体相对滑移历程不同所引起的界面磨损程度的区别.运用该模型分析了准静态加载和拉-拉循环载荷下的应力-应变特性,预测结果与实验数据吻合较好.最后采用此模型研究了任意载荷历程下界面的滑移规律.     

An interface element for the simulation of delamination in unidirectional fiber-reinforced composite laminates

Unidirectional fiber-reinforced composite laminates are widely used in aerospace industry for a great variety of structural parts. In order to enhance the exploitation of material reserves, there is a need for the integration of progressive damage scenarios in the design phase. Due to their hazardous effects on the load-carrying capacity of composite structures, this work focusses on the simulation of delaminations. A finite element based on a cohesive zone approach is developed. Two constitutive laws are proposed. One is characterized by linear degradation after delamination onset, the other is governed by exponential softening response. The damage process is history-dependent leading to an irreversible stiffness degradation in damaged zones. The practicability of the proposed model and the assets and drawbacks of the two material laws are shown by some numerical examples.     

Effect of friction between fiber and matrix on the fracture toughness of the composite interface

A method of evaluating the interfacial fracture toughness using a single-fibre composite test is proposed. In contrast with the existing techniques, the method takes into account the phenomenon of friction between the fibre and matrix in the debonding zone. A general mathematical solution of the problem and modelling of the friction phenomenon are presented. Finite-element analysis using a contact statement is utilized for numerical evaluation of the stress–strain state. The influence of the coefficient of friction and interfacial debonding length is analysed in detail. It is shown that the friction reduces the calculated value of the elastic strain energy release rate for a given debonding length, relative to that obtained when friction is neglected. The magnitude of the difference depends on the coefficient of friction, the elastic properties of the fibre and matrix, and the characteristics of the debonding mechanism. Experimental data on debonding in a series of glass-epoxy single-fibre composites are analysed using the proposed numerical technique to obtain the effects of fibre surface treatments and fibre strain-to-break on the interfacial fracture toughness. © Kluwer Academic Publishers     

The transverse coefficient of thermal expansion of a unidirectional composite

Various analytical models of the effective thermal expansion coefficients of unidirectional fibre-reinforced composite materials predict for certain fibre-matrix combinations an increase in the transverse coefficient of thermal expansion over that of its constituents at low fibre volume content. This effect is especially noticeable if the composite is fabricated with fibres of high modulus and low thermal expansion coefficient in matrices of low modulus and high thermal expansion coefficient. An experimental investigation was therefore conducted to study this behaviour in Textron fibre (SCS-6)-reinforced Hercules 3501 -6 epoxy matrix. Numerical calculations for this material system have shown that increases of the order of 20% over the matrix expansion coefficient is possible for fibre volume fraction in the range 3%–4%. Experimental measurements of the effective thermal expansion coefficients are seen to be in favourable agreement with the theoretical predictions. A parametric study is also undertaken to examine the influence of constituent properties on the effective composite behaviour. It is shown that the axial restraint of the fibre is responsible for a peak in the behaviour of the transverse expansion coefficient.     

Fail-safe light transmitting SiC fiber-reinforced spinel matrix optomechanical composite

A light transmitting Optomechanical Composite was fabricated by incorporating SiC (SCS-6) fibers into optically transparent MgAl₂O₄ ceramics. The fibers were aligned unidirectionally in the matrix with a spacing of 1 to 4 mm to allow light transmittance between the fibers: resulting fiber volume content was from 0.75 to 0.19 vol%. The total and in-line light transmittance of the matrix and the composite was measured in the visible/near-IR wavelength region. The light transmittance of the composite was found to be decreasing with increasing fiber volume fraction, however, even the composite with the highest fiber volume fraction has a light transmittance of 25–50% in the visible wavelength region. Although the optical transparency of the matrix becomes slightly lower by the incorporation of the opaque fibers, its catastrophic failure is prevented by the bridging effect of the intact fibers, introducing the fail-safe nature to the brittle ceramic material. To obtain this fail-safe mechanism, the minimum fiber volume fraction was 0.37 vol% in the present material system.     

Fabrication of transparent continuous oxynitride glass fiber-reinforced glass matrix composite

Unidirectional-aligned continuous SiCaAlON fiber-reinforced glass matrix composites have been fabricated and their light transmittance was measured. Optically transparent composites with the fiber volume fraction from 0.03 to 0.10 were fabricated by a hot-pressing method. The light transmittance of the composite perpendicular to the fiber axis in the wavelength range from 200 to 700 nm was measured, and found to decrease with the increase of the fiber volume fraction. This decrease is explained by the theory proposed by the authors (Hl and YK). The major source of a light transmittance loss of the composite originates from a phase change of transmitted light in the composite.     

Modeling of crack bridging in a unidirectional metal matrix composite

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches; the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, K _eff, was calculated using both models to correlate the measured crack growth rate in the composite. The calculated K _eff from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.     

The stress-strain behaviour of a porous unidirectional ceramic matrix composite

The stress-strain behaviour of a porous unidirectional ceramic matrix composite is investigated. The fibres are SiC (Nicalon) and the matrix is an aluminium phosphate based system. Microscopic observations indicate that pores and microcracks break up the continuity of the matrix to create an irregular array of discontinuities or ‘matrix cracks’. Load/unload tensile test results indicate an increase in compliance, permanent strain and hysteresis as the peak stress is increased. These features are attributed to sliding at the fibre-matrix interface. In the model developed, the product of the crack spacing and interfacial sliding stress, l_cτ, completely defines the stress-strain response provided the constituent properties, residual stresses and initial crack spacing are known. For the material investigated, the product l_cτ maintains a value of about 1.0 mm MPa throughout the tensile test. This result is corroborated by optical measurements of the pore/microcrack spacing and push-out test results.     

Release of fluoride from glass fiber-reinforced composite with multiphase polymer matrix

This study investigated the suitability of a fluoride containing monomer resin system for use as a copolymer of dental fiber-reinforced composite (FRC) materials. The purpose of the study was to measure the release of fluoride from the test materials. The monomer resin system was either light-polymerized, or light-polymerized and post-cured with heat at 130°C. The release of fluoride from FRC test specimens during 30 day storage periods was compared to the release of fluoride from unreinforced test specimens (n = 5). The fluoride release into distilled water was determined with an ionanalyzator. During the first week of water storage, the fluoride release was 0.31 wt % for the unreinforced specimens and 0.13 wt % for reinforced specimens. The post-curing had no influence on the fluoride release values. The results of this study suggest that fiber inclusion reduces fluoride release of reinforced specimens compared to unreinforced specimens because the amount of polymer was smaller in reinforced specimens. The results of this study showed that the fluoride containing monomer system could be incorporated into the polymer matrix of fiber-reinforced composites. © 2001 Kluwer Academic Publishers     

The tensile behavior of a silicon carbide fiber-reinforced titanium matrix composite

This paper summarizes the results of a study on the effects of composite microstructure and test temperature on tensile deformation and fracture behavior of a symmetric [0/90]_2s titanium alloy metal-matrix composite. Matrix microstructure is controlled by heat treatment, which is used to produce metastable or Widmanstatten + microstructures. The sequence of damage initiation and evolution at both room and elevated temperature (650°C) is identified using ex-situ scanning electron microscopy observations during incremental monotonic loading to failure. The nature, sequence and complexity of damage during uniaxial loading is presented and discussed in light of competing and mutually interactive influences of load level and deformation characteristics of the microstructure.     

Microstructure and mechanical properties of graphite fiber-reinforced high-purity aluminum matrix composite

Industrial pure aluminum (0.5 wt% impurity elements) was utilized in many investigations of aluminum matrix composites at home and abroad. However, impurity elements in industrial pure aluminum may influence the interface during fabrication of composite at high temperature. Thereby, it is necessary to use high-purity aluminum (impurity elements less than 0.01%) as matrix to enable study the interface reaction between reinforcement and matrix. In this study, stretches of brittle Al₄C₃ at the fiber/matrix interfaces in Gr_f/Al composite were observed. The fracture surface of the composite after tensile and bending tests was flat with no fiber pull-out, which revealed characteristic of brittle fracture. This was related to Al₄C₃, as this brittle phase may break before the fiber during loading and become a crack initiation point, while the corresponding crack may propagate in the fiber and the surrounding aluminum matrix, finally resulting in low stress fracture of composites.     

Biaxial experimental determination of in-plane matrix fracture envelope of unidirectional composite

A test procedure for the determination of the in-plane fracture envelope of unidirectional fibre reinforced polymers (FRP) is presented. In particular, the determined fracture envelope covers combined in-plane shear and transverse (perpendicular to the fibre direction) matrix strength. The proposed test procedure allows the manufacture of specimens for material fracture characterisation in the same way that real composite structures are usually produced for the automotive industry. The biaxial testing is performed using a custom-made dual actuator test machine and keeping the ratio of transverse and shear load constant until fracture. The experimentally obtained transverse–shear strength relation can be well represented by the matrix fracture model by Puck. It is shown that the stress concentrations in the gauge section of the flat biaxial specimens can be avoided by the introduction of a thickness reduction, whereas the stress concentrations within biaxial specimens without such a thickness reduction lead to significantly lower strength.     

Strength of unidirectional glass/epoxy composite with silica nanoparticle-enhanced matrix

A technique was developed to improve the strength of unidirectional composites by enhancing the matrix properties through nanoparticles infusion. A commercially available standard DGEBA epoxy with silica nanoparticles (Nanopox F 400) was used as the matrix to make fiber composites. The silica nanoparticles in Nanopox were grown in situ via a sol–gel process resulting in a concentration of 40 wt% which was later diluted to 15 wt% particle loading. TEM images showed very uniform dispersion of silica nanoparticles with a size distribution of about 20 nm. Compression test revealed a substantial improvement (40%) in elastic modulus of the modified epoxy. A modified vacuum assisted resin transfer molding process was used to fabricate unidirectional E-glass fiber reinforced silica/epoxy nanocomposites. Inclusion of silica nanoparticles dramatically increased the longitudinal compressive strength and moderately increased the longitudinal and transverse tensile strengths. A microbuckling model was used to verify the compression testing results.     

Analysis of a unidirectional composite containing broken fibers and matrix damage

An analytical solution is developed for the determination of the stresses and displacements in a unidirectional fiber-reinforced composite containing an arbitrary number of broken fibers as well as longitudinal yielding and splitting of the matrix.The solution is developed using a “materials-modeling” approach which is based on a shear-lag stress transfer mechanism. The equilibrium equation in the axial direction gives a pair of integral equations which are solved numerically.Excellent agreement is shown to exist between the solution and experimental results for notched unidirectional boron/aluminum laminates without splitting. For brittle matrix composites (i.e. epoxy) equally good results are indicated for both matrix yielding and splitting.For yielding without splitting the fracture strength is found to depend on crack length while for large splitting it is crack length independent.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

The semi-interpenetrating polymer network matrix of fiber-reinforced composite and its effect on the surface adhesive properties

This aim of this study was to examine the effect of further-impregnation time of polymer pre-impregnated fiber-reinforcement on polymer matrix structure of the fiber-reinforced composite (FRC) used in dental applications. In addition, shear bond strength between the FRC and veneering composite after various length of further-impregnation was studied. Polymethyl methacrylate (PMMA) pre-impregnated glass fiber-reinforcement was further-impregnated with a diacrylate monomer resin by using five lengths of further-impregnation from 10 min to 24 h. The test specimens (n=5) from each five groups were treated with the solvent tetrahydrofuran and examined with a scanning electron microscope (SEM) to determinate the existence of linear PMMA in the polymer matrix of the FRC. The same lengths of further-impregnation were used to form an adhesive substrate for veneering composite and to measure the shear bond strength (n=8). The SEM examination showed that linear PMMA-polymer and cross-linked diacrylate polymer formed two independent networks for the polymer matrix of FRC. The highest mean shear bond strength value (18.7±2.9 MPa) was achieved when the fiber reinforcement was further-impregnated for 24 h. The shortest further-impregnation, 10 min, resulted in the lowest mean shear bond strength (12.7±2.9 MPa). A correlation between increased shear bond strength and longer further-impregnation was found (0.689, p<0.001). The results revealed that linear PMMA network of the polymer matrix of the FRC remained in the structure regardless of the various lengths of the further-impregnation with diacrylate resin.