Unidirectional Glass Fiber Composites Research Articles

Micro-buckling resistant unidirectional glass fiber composites with excellent transverse and longitudinal flexural properties from cross-linking by nano-/micro-aramid fibers

This study aims at obtaining an effective structural design strategy for stronger and more reliable Unidirectional glass fiber (UD-GF) composites by in-situ formed cross-linking from randomly distributed nano-/micro- Aramid pulp (AP) fibers. The flexural strength and stiffness have shown substantial improvements in both the transverse and longitudinal directions due to the AP cross-linking and the “brick-slurry” structure. With AP of 8 g/m2, up to 60–70 % improvement in flexural strengths (both transverse and longitudinal directions) and up to 20–37 % improvement in “effective modulus” have been observed. The noticeable longitudinal improvements have been attributed to the resin reinforcement and interlayer cross-linking provided by ultra-thin AP interlayers, and the resultant improvement in micro-buckling resistance under compression. Since sparsely distributed nano-/micro-AP can be readily incorporated in pultrusion and pre-preg manufacturing processes, this study is not only important for micro-mechanism study, but also provides a plausible improvement in manufacturing.

Finite element analysis of the thermal residual stress distribution in the interphase of unidirectional fiber-reinforced resin matrix composites

By means of three-dimensional finite element method (FEM) which is based upon the micro-mechanical model of fiber-reinforced composites, this paper selects representative volume elements and studies the effect of the five factors, namely, cooling rate, matrix elasticity modulus, fiber elasticity modulus, interphase elasticity modulus and fiber volume fraction, on the interphase thermal residual stress and its distribution law in epoxy resin NPEF-170/unidirectional glass fiber composites. The results indicate that thermal residual stress is mainly distributed on the fiber and the matrix of neighboring interphase; the thermal residual stress on the fiber and the matrix declines as the distance to the interphase layer grows; and it tends to zero at the distance of 1.5 times the radius of the fiber away from the interphase. The increase in any of the four factors, namely, cooling rate, matrix elasticity modulus, fiber elasticity modulus, and fiber volume fraction would trigger the rise of thermal residual stress in epoxy resin NPEF-170/unidirectional glass fiber composites. The additional flexible interphase layer can eliminate and transfer thermal residual stress effectively, whose effectiveness mainly depends on the difference between interphase elasticity modulus and fiber elasticity modulus.

Effect of the environmental conditions on the ultimate load of damaged unidirectional of natural hemp fiber/epoxy composite

Natural continuous natural fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibers. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fiber composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of natural fibers compared to glass fibers. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). The present paper proposes a strength reliability model for unidirectional composites with natural fibers in a hexagonal array. The model assumes that, a central core of broken fibers flanked by unbroken fibers which are subject to stress concentrations from the broken natural fibers. Thermal and hygroscopic residual stresses are neglected because they haven’t more effect when the composite was subjected to tensile than transverse loading. The approach of the model consists of using a modified shear lag model to calculate the ineffective lengths and stress concentrations around the broken fibers. In this paper, we attempt to incorporate in the proposed model the unidirectional composite property variation with temperature and moisture in order to predict even composite strength degradation. Strength degradation is often seen as a result of changes in ineffective lengths at natural fiber breaks and the corresponding stress concentrations in intact neighboring fibers.

Micro-crack detection and assessment with embedded carbon nanotube thread in composite materials

Carbon nanotube thread has shown strong promise to be built into or onto composite materials for strain and damage monitoring via the material’s piezoresistive property. It has been found that a distinguishing feature of sensing thread incorporated in these materials is the detection of micro-cracking. This study articulates how embedded carbon nanotube thread in unidirectional glass fiber composites can identify the onset of matrix cracking, track crack growth, and differentiate between crack breathing and closing states. This information is obtained by analyzing the resistance response of the thread with a low-speed data acquisition system and a simple Wheatstone bridge circuit. A digital optical microscope was utilized to verify that a micro-crack was indeed present in the structure at the location of the sensor thread. Additionally, to demonstrate the effectiveness of this crack detection approach compared to past crack detection approaches, a comparison against foil-type strain gauges and piezoelectric accelerometers was made. Finally, a simple crack model is presented for the sensor thread.

Embedded carbon nanotube thread piezoresistive strain sensor performance

Purpose – Carbon nanotube (CNT) thread ' s piezoresisitive strain sensing properties of gauge factor, linearity, hysteresis, consistency, temperature stability, and bandwidth were evaluated. This evaluation was motivated by little information in literature combined with the need to understand these properties for commercial use. The paper aims to discuss these issues. Design/methodology/approach – The study here analyzes as-spun CNT thread built into unidirectional glass fiber composites and mounted onto aluminium beams with epoxy to evaluate strain sensing properties. The analyses utilize known sensor parameter definitions to quantify sensor performance. Findings – CNT thread can provide reliable and robust strain measurements for composite and metallic structures. The strain sensor performance meets or exceeds other strain sensors in performance. Research limitations/implications – CNT thread ' s piezoresistive effect is not well understood in terms of Poisson ' s ratio and nanotube contact. More research needs to be carried out to better understand this relationship and optimize the sensor thread. Practical implications – CNT thread can be utilized as a robust strain sensor for composite and metallic structures. It can also be built into composite materials for embedded strain and damage monitoring. By monitoring composite materials with the sensor thread, reliability will significantly increase. In turn, this will lower safety factors and revolutionize inspection methods for composite materials. Originality/value – This paper is the first to comprehensively evaluate key strain sensing properties of CNT thread. With all this strain sensor information in one spot, this should help expedite the use of this technology in other research and industry.

Tensile and impregnation behavior of unidirectional hemp/paper/epoxy and flax/paper/epoxy composites

Natural continuous bast fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibres. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fibre composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of bast fibres compared to glass fibres. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). In this work, unidirectional hemp/paper/epoxy and flax/paper/epoxy composites are manufactured by adding one or two sheets of paper at the surface of a unidirectional layer of hemp or flax fibres before molding. The composites are tested under tensile loads and the results compared with those obtained for UD composites made without paper. The results show a significant increase in strength and modulus repeatability when the paper layer is present. A significant increase in strength and modulus was also obtained compared to the base epoxy properties. The resin permeability of the flax–paper reinforcement has also been evaluated and compared to that of a standard glass fabric. Globally, the results suggest that adding thin sheets of paper at the surface of reinforcement layers stabilizes and increases the composite strength but slightly reduces the Young’s modulus. However the permeability of the flax–paper reinforcement was much lower than that of the glass fabric. Modifications to the paper architecture will be required to improve this aspect.

Quasi-UD glass fibre NCF composites for wind energy applications: a review of requirements and existing fatigue data for blade materials

Quasi-unidirectional glass fibre non crimp fabric composites are routinely used in wind energy applications. An important factor to consider in this type of application is the fatigue behaviour of the materials. The present paper gives an overview of the current approach to material fatigue testing and design in the wind energy industry, as well as a review of available material data for the fatigue of unidirectional glass fibre composites. Both “pure” unidirectional and quasi-unidirectional (NCF-based) materials are considered.

Quantitative study on the statistical properties of fibre architecture of genuine and numerical composite microstructures

A quantitative study is carried out regarding the statistical properties of the fibre architecture found in composite laminates and that generated numerically using Statistical Representative Volume Elements (SRVE’s). The aim is to determine the reliability and consistency of SRVE’s for representation of the composite microstructure as well as investigate the effect of a varying fibre radii distribution on the fibre architecture. Based on digital image analysis, the fibre architecture of unidirectional glass fibre composites with varying fibre content is recognised. The fibre architecture found is compared to a numerical microstructure generator using Monte Carlo simulations. It is shown that the numerical microstructure generator produces fibre arrangements that are statistically similar to the observed, which indicates a reliable and consistent SRVE. The microstructural effects of a parametric variation of the parameters for fibre radii distribution are simulated, and the influence on the fibre architecture is investigated.

Mechanical Behavior of Unidirectional Curaua Fiber and Glass Fiber Composites

AbstractSummary: This work intends to promote the use of natural fibers by comparing the behavior of isophthalic polyester matrix composites reinforced with unidirectional curaua fibers with that of unidirectional glass fiber composites. The composites were produced varying the reinforcement angle (0°, 15°, 30°, 45°, 60°, 75° and 90°) with the aim of studying the fiber orientation effect on composite strength. Composites were also made varying the fiber volume fraction (10%, 20%, 30%, 40% and 50%). The efficiency of an alkaline (5% NaOH) surface treatment of the curaua fiber was also evaluated. The unidirectional composites were characterized using tensile, flexural and short beam tests as per ASTM standards. The properties of a lamina reinforced with either glass or curaua fibers were also studied using theoretical micromechanical approach available in commercial software. The curaua fiber alkaline treatment produced higher tensile strength results compared with untreated fibers. The increase in reinforcement angle significantly decreased strength and modulus of the composites, as expected, and the glass fiber composites showed a more pronounced dependence with fiber orientation. Although the glass fiber laminas showed the best mechanical performance, the results obtained with the curaua fibers were considered similar for angles greater than 45°.

Thermal decomposition and fire behavior of glass fiber–reinforced polyester resin composites containing phosphate-based fire-retardant additives

The thermal degradation and the fire behavior of a polyester resin containing phosphate-based fire-retardant additives and its corresponding glass fiber composites were investigated. An unsaturated commercial polyester resin was modified by the addition of three phosphate-based fire retardants: ammonium polyphosphate, silane-coated ammonium polyphosphate, and melamine pyrophosphate, at 35% w/w. The effects of the fire retardants on resin thermal decomposition and small-scale fire behavior were studied using dynamic thermogravimetric tests at different heating rates and microcalorimetric measurements according to ASTM D7309-07. Different modes of degradation with different activation energy levels for the neat resin and the phosphate-loaded resins were identified by analyzing the thermogravimetric data through the Kissinger method. Since the ammonium polyphosphate-containing resin showed greater thermal and fire performance than the other systems, it was used to manufacture unidirectional glass fiber composites by a vacuum infusion process. The oxidative pyrolysis and fire behavior of the composites produced were studied using thermogravimetric and cone calorimeter tests that demonstrated improvement of their thermal stability and fire performance.

Electrical anisotropy in multiscale nanotube/fiber hybrid composites

This letter reports an experimental and theoretical study on the electrical properties of carbon nanotube/glass fiber composites. Experimental measurements on unidirectional glass fiber composites with nanotubes dispersed in the polymer matrix show a high degree of anisotropy. The composites, manufactured with a vacuum infusion technique, do not show any significant process-induced anisotropy. Theoretical modeling reveals that the microstructure of the fiber composite plays a dominant role in the electrical behavior due to alteration of percolating paths in the carbon nanotube network.

Effect of Hygrothermal History on Water and Mechanical Properties of Glass/Vinylester Composites

The effects of hygrothermal aging on the mechanical and viscoelastic properties of vinylester (VE)/unidirectional glass-fiber composites are reported. The studies are done by means of a dynamometer (INSTRON) and a dynamic mechanical thermal analyzer (Perkin Elmer). The studies are carried out at three different temperatures: 20, 40, and 60 C. The adhesion between the fibers and matrix is studied by scanning electron microscopy (SEM) microphotographs. Flexural strength, flexural modulus, storage modulus in the glass state (Tg - 50 K), in rubber state (Tg + 50 K), and the glass transition temperatures are also investigated. These parameters are determined in the case of the matrix and composites materials, for the initial and final states. The diffusion coefficients are calculated by using the simplest Fick's equation.

Damping Analysis of Unidirectional Glass Fiber Composites with Interleaved Viscoelastic Layers: Experimental Investigation and Discussion

The article investigates the damping of unidirectional glass fiber composites with a single or two interleaved viscoelastic layers. The experimental damping characteristics are derived from flexural vibrations of cantilever beams as a function of the fiber orientation. The experimental results are analyzed considering the modeling developed in a previous article, Berthelot, J.-M. Damping Analysis of Orthotropic Composites with Interleaved Viscoelastic Layers Modelling, Journal of Composite Materials (in press). The analysis considers the variations of Young’s modulus and damping of viscoelastic layers with the frequency. Finally, the article presents the effects of Young’s modulus and the damping of a viscoelastic layer interleaved in the middle plane of unidirectional laminate. The results are also reported for the case of external viscoelastic layers.

Temperature effect on the damping properties of unidirectional glass fibre composites

The paper presents the analysis of the temperature effect on both the stiffness and the damping of glass fibre composite materials. The properties of these materials are derived from flexural vibrations of free-clamped beams. The experimental results show that the dynamic properties are appreciably kept up to the temperature of glass transition, where damping increases sharply in a little interval of temperature. This paper also presents an evaluation of the damping as function of the temperature using the Ritz method.

Tensile creep behavior of unidirectional glass-fiber polymer composites

The tensile creep behavior of unidirectional glass-fiber polymer composites was studied at three different temperatures, namely 298, 333, and 353 K. Testing was performed on the pure epoxy matrix, the 0° specimens as well as off-axis at 15, 30, and 60 degrees in respect to the axis of tension. The creep strain rate was negligible at room temperature, while it was considerable at the higher temperatures examined. The materials exhibit nonlinear viscoelastic behavior, and the creep response of the composites was treated as a thermally activated rate process. The creep strain was considered to include an elastic, a viscoelastic and a viscoplastic part. The viscoplastic part was calculated through a functional form, developed in a previous work, assuming that viscoplastic response of polymer composites arises mainly from the matrix viscoplasticity. The model predictions in terms of creep compliances were found to be satisfactory, compared with the experimental results. POLYM. COMPOS. 26:287–292, 2005. © 2005 Society of Plastics Engineers.

Tensile strain-rate response of polymeric fiber composites

The tensile behavior of unidirectional glass-fiber polymer composites was studied at three different strain rates. Tests were performed on 0° specimens as well as off-axis specimens at 15°, 30°, 45°, and 90° with respect to the axis of tension. The nonlinear material behavior was modeled through a viscoplastic model based on a one-parameter plastic potential function developed elsewhere. An effective stress-effective plastic strain curve was constructed for each strain rate imposed and fitted with a power law. Thus, the tensile stress–strain curve could be predicted in a very accurate way for every strain rate examined and various types of off-axis specimens. The strain rate-dependent behavior is described through a scaling law, assuming that a model parameter is a function of the imposed strain rate. Predictions of the material response at strain rates different from those initially studied were found to be successful. POLYM. COMPOS., 26:572–579, 2005. © 2005 Society of Plastics Engineers

Damping analysis of unidirectional glass and Kevlar fibre composites

The paper presents an extensive analysis of the damping of unidirectional fibre composites as function of frequency and fibre orientation. Damping of glass and Kevlar composites is analysed experimentally using a cantilever beam test specimen and an impulse technique. Damping parameters are derived by fitting the experimental Fourier responses with the analytical motion responses. The experimental results are compared with literature models: Adams–Bacon analysis, Ni–Adams analysis and complex stiffness model. Next the paper presents an evaluation of the damping of unidirectional or orthotropic composites using the Ritz method, which takes account of the effect of the beam width.

Interlaminar fatigue crack propagation in continuous glass fiber/polypropylene composites

The mode II interlaminar fatigue crack propagation behavior of unidirectional continuous glass fiber (GF) composites with a polypropylene (PP) matrix obtained under three different molding conditions has been studied with the use of the end-notch flexure (ENF) geometry. The microstructure and mechanical performance, especially the interlaminar fatigue crack propagation, are strongly affected by the molding conditions. Comparative results reveal a major influence of the fiber–matrix interface and the matrix morphology on the crack propagation resistance. The distribution of the ductile amorphous PP phase in the semi-crystalline PP matrix appears to be the controlling parameter determining the fatigue crack propagation resistance of the PP/GF composite. Fractographic observations clearly showed the role of this phase.

Effect of the Molding Conditions on Mode II Interlaminar Crack Propagation in Continuous Glass Fiber/Polypropylene Composites

The effect of molding conditions on the resistance to Mode II interlaminar crack propagation under monotonic and cyclic loading of unidirectional continuous glass-fiber composites with a polypropylene (PP) matrix was studied. The distribution of the fibers and of the crystalline and amorphous components of the matrix phase, the melting temperature and the amount of crystallinity are related to the molding conditions employed and to the resulting flexural strength and modulus, apparent interlaminar shear strength and Young’s modulus. Mode II crack propagation, either cyclic or monotonic, is strongly affected by the fiber-matrix interface and matrix morphology. The distribution of the soft amorphous PP phase in the semi-crystalline PP matrix appears to be the controlling parameter determining the fracture and fatigue resistance of the composite. The fractographic features clearly show the role that this phase plays during crack propagation. A relationship between the shear cusp size measured on the fatigue surfaces and the cyclic strain energy release rate is proposed.

The effect of matrix toughness and loading rate on the mode-II interlaminar fracture toughness of glass-fibre/vinyl-ester composites

Glass-fibre-reinforced composites are increasingly used for structural applications. However, like high-performance carbon-fibre composites, they are susceptible to mode-II-dominated delamination. In response to this problem, this paper investigates the effect of matrix toughness and loading rate on the mode-II interlaminar fracture toughness ( G IIc) of unidirectional glass-fibre composites with brittle and rubber-toughened vinyl ester matrices. Mode II tests were conducted σn end-notch-flexure (ENF) specimens at test rates ranging from 1 mm/min to 3 m/s. The G IIc results were compared to the order of matrix G Ic. There was no significant effect of loading rate or matrix toughness on G IIc. The absence of a loading rate effect is consistent with the bulk of the experimental data in the literature, but the absence of a matrix effect is not. Microscopic examination of fracture surfaces shows similar matrix deformation in each composite. The through-thickness matrix deformation zone size is also similar. These observations suggest similar energy absorption in each composite and hence support the G IIc test results. It is concluded that failure is interface controlled, whereby unstable fracture is initiated after a similarly short period of crack growth in each composite, and before an increase in G IIc as a result of increased matrix toughness becomes apparent. The G IIc results indicate that the use of rubber-toughened vinyl ester matrices in glass-fibre composites will not improve resistance to impact-induced mode II-delamination. However, through-thickness impact damage in composite structures is likely to result from mixed-mode (I/II) loading. Therefore, suggestions for future work include investigation of the matrix effect on mixed-mode (I/II) interlaminar fracture toughness, and on delamination resistance of plate structures subjected to transverse low-velocity impact.