Mechanical and thermal behavior of non-crimp glass fiber reinforced layered clay/epoxy nanocomposites

Mechanical and thermal properties of non-crimp glass fiber reinforced clay/epoxy nanocomposites were investigated. Clay/epoxy nanocomposite systems were prepared to use as the matrix material for composite laminates. X-ray diffraction results obtained from natural and modified clays indicated that intergallery spacing of the layered clay increases with surface treatment. Tensile tests indicated that clay loading has minor effect on the tensile properties. Flexural properties of laminates were improved by clay addition due to the improved interface between glass fibers and epoxy. Differential scanning calorimetry (DSC) results showed that the modified clay particles affected the glass transition temperatures (T_g) of the nanocomposites. Incorporation of surface treated clay particles increased the dynamic mechanical properties of nanocomposite laminates. It was found that the flame resistance of composites was improved significantly by clay addition into the epoxy matrix.     

Enhancement of mechanical performance of epoxy/carbon fiber laminate composites using single-walled carbon nanotubes

Carbon nanotubes (CNT) in their various forms have great potential for use in the development of multifunctional multiscale laminated composites due to their unique geometry and properties. Recent advancements in the development of CNT hierarchical composites have mostly focused on multi-walled carbon nanotubes (MWCNT). In this work, single-walled carbon nanotubes (SWCNT) were used to develop nano-modified carbon fiber/epoxy laminates. A functionalization technique based on reduced SWCNT was employed to improve dispersion and epoxy resin-nanotube interaction. A commercial prepregging unit was then used to impregnate unidirectional carbon fiber tape with a modified epoxy system containing 0.1 wt% functionalized SWCNT. Impact and compression-after-impact (CAI) tests, Mode I interlaminar fracture toughness and Mode II interlaminar fracture toughness tests were performed on laminates with and without SWCNT. It was found that incorporation of 0.1 wt% of SWCNT resulted in a 5% reduction of the area of impact damage, a 3.5% increase in CAI strength, a 13% increase in Mode I fracture toughness, and 28% increase in Mode II interlaminar fracture toughness. A comparison between the results of this work and literature results on MWCNT-modified laminated composites suggests that SWCNT, at similar loadings, are more effective in enhancing the mechanical performance of traditional laminated composites.     

Layered silicates nanocomposite matrix for improved fiber reinforced composites properties

Three-phase glass fiber reinforced composites (GFRP) consisting of traditional woven glass fiber and polyamide-6 (PA6) matrix dispersed with organically modified layered silicates were prepared and investigated in this study. The fabrication of GFRP with different weight percentages of layered silicates was successful when the matrix contains less than 5 wt% of the layered silicates. The improvement due to the high aspect ratio and high stiffness of the layered silicates is illustrated through the matrix-controlled properties of the GFRP. The results showed that the GFRP with 5 wt% layered silicates offer the largest improvement of approximately 30% increase in both flexural strength and compressive strength at elevated temperatures. On the other hand, the in-plane shear properties measured from [±45]_s laminates revealed that the layered silicates help improved both the in-plane shear strength and modulus appropriately. By utilizing a nanocomposite matrix, improvement of stiffness and strength, as well as thermal and barrier properties is obtained without any change in processing temperature of the fiber composites.     

Review of the mechanical properties of carbon nanofiber/polymer composites

In this paper, the mechanical properties of vapor grown carbon nanofiber (VGCNF)/polymer composites are reviewed. The paper starts with the structural and intrinsic mechanical properties of VGCNFs. Then the major factors (filler dispersion and distribution, filler aspect ratio, adhesion and interface between filler and polymer matrix) affecting the mechanical properties of VGCNF/polymer composites are presented. After that, VGCNF/polymer composite mechanical properties are discussed in terms of nanofibers dispersion and alignment, adhesion between the nanofiber and polymer matrix, and other factors. The influence of processing methods and processing conditions on the properties of VGCNF/polymer composite is also considered. At the end, the possible future challenges for VGCNF and VGCNF/polymer composites are highlighted.     

Flexural and compression response of woven E-glass/polyester–CNF nanophased composites

A significant improvement in fiber reinforced polymeric composite (FRPC) materials can be obtained by incorporating a very small amount of nanofiller in the matrix material. In this work, an ultrasonic liquid processor was used to infuse carbon nanofiber (CNF) into the polyester matrix which was then mixed with catalyst using a mechanical agitator. Both conventional and CNF-filled glass-fiber reinforced polyester composites (GRPC) were fabricated using the vacuum assisted resin transfer molding (VARTM) process. Excellent dispersion of CNFs into the polyester resin was observed from the scanning electron microscopy (SEM) micrographs. Flexural and quasi-static tests were performed for investigating the mechanical responses. Fracture surface was examined using optical microscopy (OM) and SEM. Flexure tests performed on the conventional GRPC, 0.1–0.4 wt.% CNF-filled GRPC showed up to 49% and 31% increase in the flexural strength and modulus, respectively, compared to the conventional one with increasing loading of CNFs up to 0.2 wt.%. Similar trend was seen in quasi-static compression properties. SEM evaluation revealed relatively less damage in the tested fracture surfaces of the nanophased composites in terms of matrix failure, fiber breakage, matrix–fiber debonding, and delamination, compared to the conventional one. This might be the result of better interfacial interaction between matrix and fibers, due to the presence of CNFs.     

Fire retardancy of clay/carbon nanofiber hybrid sheet in fiber reinforced polymer composites

In this paper, two kinds of clay/carbon nanofiber hybrid sheets containing 0.05 wt% and 0.20 wt% of Cloisite Na⁺ clay, were fabricated through a high-pressure filtration system. These sheets were integrated onto the surface of laminated composites like traditional continuous fiber mats through vacuum-assisted resin transfer molding process. The fire performance of the laminated composites was evaluated with cone calorimeter tests under an external radiant heat flux of 50 kW/m². Their residues were analyzed with scanning electron microscopy and thermal gravimetric analyses. It was found that the clay/nanofiber hybrid sheets survived on the combustion surface of composites and significantly reduced the heat release rate by 60.5%. The protective clay layer reduces the heat release rates and the nanofiber network reinforces the clay layer against the air bubbling and melt flow of the products degraded from the polymer resin. The clay/carbon nanofiber hybrid sheet combines the barrier and insulator effects of the clays with the re-emitting heat effect of carbon nanofibers on the combustion surface of composites.     

The effect of multiwall carbon nanotubes on the in-plane shear behavior of epoxy glass fiber reinforced composites

In this study, we investigate how multi-wall carbon nanotubes (MWCNTs) affect the in-plane shear mechanical behavior of glass fiber/epoxy composite. These multi-scale composites are fabricated using vacuum infusion: pristine MWCNT and amino-functionalized MWCNT are incorporated into epoxy resins at concentrations of both 0.1 and 0.3 wt.% and are subsequently evaluated. The MWCNT are mixed into the resins by mechanical stirring and sonication prior to resin infusion, and the MWCNT distribution in the cured laminate is then evaluated by performing a heat conduction assessment. Monotonic and cyclic quasi-static room temperature in-plane shear tests are performed following the ASTM D 4255 standard. The initial shear modulus, the deterioration of the shear modulus during plastic deformation and material hardening are evaluated. Incorporating MWCNT into the resins did not affect the parameters investigated under the imposed conditions.     

Pristine and amino functionalized carbon nanotubes reinforced glass fiber epoxy composites

Multi-phase composites have been studied by incorporating carbon nanotubes (CNTs) as a secondary reinforcement in an epoxy matrix which was then reinforced with glass fiber mat. Different types of CNTs e.g. amino functionalized carbon nanotubes (ACNT) and pristine carbon nanotubes (PCNT) were homogeneously dispersed in the epoxy matrix and two-ply laminates were fabricated using vacuum-assisted resin infusion molding technique. The issues related to CNT dispersion and interfacial bonding and its affect on the mechanical properties have been studied. An important finding of this study is that PCNT scores over ACNT in composites prepared under certain conditions. This is a very significant finding since PCNT is available at a much lower cost than ACNT.     

Multiscale progressive failure analysis of textile composites

The experimental determination of stiffness and strength of textile composites is expensive and time-consuming. Experimental tests are only capable of delivering properties of a whole textile layer, because a decomposition is not possible. However, a textile layer, consisting of several fiber directions, has the drawback that it is likely to exhibit anisotropic material behavior. In the presented paper a finite element multiscale analysis is proposed that is able to predict material behavior of textile composites via virtual tests, solely from the (nonlinear) material behavior of epoxy resin and glass fibers, as well as the textile fiber architecture. With these virtual tests it is possible to make predictions for a single layer within a textile preform or for multiple textile layers at once. The nonlinear and pressure-dependent behavior of the materials covered in the multiscale analysis is modeled with novel material models developed for this purpose. In order to avoid mesh-dependent solutions in the finite-element simulations, regularization techniques are applied. The simulations are compared to experimental test results.     

Shape memory and thermo-mechanical properties of shape memory polymer/carbon fiber composites

In this study, chopped carbon fiber reinforced trans-1, 4-polyisoprene (TPI) was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk. Specimens of the developed shape memory polymer (SMP) composites were fabricated with carbon fiber weight fraction of 5%, 7%, 9%, 11% and 13%, respectively. Measured are the effects of chopped carbon fiber and temperature on: (a) shape recovery ratio and rate; (b) stress–strain relationship; (c) maximum tensile stress, strain and Young’s modulus; and (d) maximum stress and residual strain under a constant strain cyclic loading. In addition, SEM micrographs were also presented to illustrate the fracture surface. The present experimental results show that the SMP with 7% carbon fiber weight fraction appears to perform best in all the tests. This indicates that the 7% carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.     

Augmented fatigue performance and constant life diagrams of hierarchical carbon fiber/nanofiber epoxy composites

We report the results of an extensive multi-stress ratio experimental study on the axial fatigue behavior of an all-carbon hierarchical composite laminate, in which carbon nanofibers (CNFs) are utilized alongside traditional micron-sized carbon fibers. Primary carbon fibers were arranged in matrix-dominated biax ±45° lay-ups in order to establish matrix and matrix/fiber interaction based performance. CNFs were matrix dispersed by three-roll calender milling. Results indicate that the CNF-reinforced composites collectively possess improved fatigue and static properties over their unmodified counterparts. Large mean lifetime improvements of 150–670% were observed in fully compressive, tensile and tensile dominated loadings. Enhancements are attributed to the high interface density and damage shielding effect of the CNFs within the matrix. Further improvements are believed to occur when the nanofibers arrest and redistribute small scale, slowly propagating matrix cracks at low applied stresses. These results highlight the ability of a nanometer-sized reinforcing phase to actively participate and enhance matrix properties while moving toward a cost effective alternative to current material solutions.     

Tensile and thermomechanical properties of short carbon fiber reinforced polyamide 6 composites

In this study, carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by using melt mixing method. Effects of fiber length and content, on the mechanical, thermal and morphological properties of CF reinforced PA6 composites were investigated. Fiber length distributions of composites were also determined by using an image analyzing program. It was seen that the maximum number of fibers were observed in the range of 0–50 μm. Mechanical test results showed that, increasing CF content increased the tensile strength, modulus and hardness values but decreased strain at break values of composites. DSC results showed that T_g and T_m values of composites were not changed significantly with increasing CF content and length. However, heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. DMA results revealed that storage modulus and loss modulus values of composites increased with increasing CF content.     

Torsion fatigue behavior of unidirectional carbon/epoxy and glass/epoxy composites

This paper presents results of the feasibility of carbon/epoxy composites (CFRP) as a future helicopter flexbeam material. Torsional behaviors of unidirectional CFRP and glass/epoxy composites (GFRP) with the same resin matrix were investigated. The initial torsional rigidity of CFRP was almost identical to that of GFRP. The torsional rigidities calculated using finite element analyses (FEA) agreed with the experimental results: the torsional rigidities are governed mainly by the material’s shear stiffness. Torsion fatigue tests were also conducted by controlling the angle of twist of the sinusoidal wave under a constant tensile axial load. No catastrophic failure occurred with either GFRP or CFRP, although decreased amplitudes of torque and torsional rigidities were observed according to the number of cycles. Results of X-ray CT inspections and numerical calculation by FEA revealed that degradation of a torsional rigidity is caused mainly by splitting crack propagation along the fiber direction. The torsion fatigue life of CFRP was superior to that of GFRP. Consequently, results confirmed that CFRP exhibits excellent properties as a torsional element of a helicopter flexbeam in terms of torsional rigidity and tension–torsion fatigue behaviors.     

Surface damage of unidirectional carbon fiber reinforced epoxy composites under reciprocating sliding in ambient air

For many technical applications friction and wear are critical issues. Reinforced polymer-matrix composites are widely used under vibrating contact condition in various automotive and aerospace applications as well as in structural engineering. In this paper, the friction and wear of bulk epoxy and unidirectional carbon fiber reinforced epoxy composite have been investigated under reciprocating sliding against either alumina or stainless steel balls in ambient air. The effect of sliding direction with respect to the long and unidirectional carbon fibers has been studied. We demonstrate that the carbon fiber reinforcement greatly improves the tribological properties of the thermoset epoxy: it reduces the coefficient of friction and the debris formation. It was found that on sliding in the anti-parallel direction a more significant degradation takes place than in the parallel direction. The coefficient of friction measured on bulk epoxy sliding against either stainless steel or alumina is around 0.65, whereas the coefficient of friction measured on epoxy reinforced with carbon fibers is significantly lower, namely down to 0.11. It was found that sliding with a stainless steel ball in a direction parallel to the fiber orientation results in a lower coefficient of friction than sliding in anti-parallel direction. The reduced coefficient of friction is largely influenced by the carbon fiber reinforcement due to the auto-protecting film formed as a paste in the contact area and along the wear track edges. The relationship between friction and degradation of the composite material including surface wear and debris formation are discussed based on an in-depth analysis of the worn surfaces by optical and scanning electron microscopy, micro-Raman spectroscopy, and white light interferometry.     

Stress-dependence and time-dependence of the post-fatigue tensile behavior of carbon fiber reinforced SiC matrix composites

The major objective of this paper is to phenomenally report the stress-dependence and time-dependence of fatigue damage to C/SiC composites, and to tentatively discuss the effects of the fatigue stress levels and the fatigue cycles on the post-fatigue tensile behavior. Results show that compared with the virgin strength of the as-received C/SiC specimens, the tensile strengths of the as-fatigued specimens after 86,400 cycles were increased by 8.47% at the stresses of 90 ± 30 MPa, 23.47% at 120 ± 40 MPa, and 9.8% at 160 ± 53 MPa. As cycles continued, however, the post-fatigue strength of the composites gradually decreased after the peak of 23.47%, at which the optimal strength enhancement was obtained because the mean fatigue stress of 120 MPa was the closest to thermal residual stress (TRS), and caused TRS relieve largely during the fatigue. Most interestingly, there was a general inflexion appeared on the post-fatigue tensile stress-strain curves, which was just equal to the historic maximum fatigue stress acted upon the as-fatigued specimens. Below this inflexion stress the tensile curves revealed the apparent linear behavior with little AE response, and above that nonlinearity with new damage immediately emitted highly increase rate of AE activities. This ‘stress memory’ characteristic was strongly relevant to damaged microstructures of the as-fatigued composites in the form of the coating/matrix cracks, interface debonding/wear, and fiber breaking, which resulted undoubtedly in reduction of modulus but in proper increase of strength via TRS relief.     

Application of infrared thermography for the characterization of damage in braided carbon fiber reinforced polymer matrix composites

The focus of this study is to assess, using infrared thermography, the fatigue behavior and the corresponding damage states of a textile polymeric composite plate, as a prerequisite step in the development of damage based life prediction models for such advanced composite materials. Monotonic (quasi-static) loading test results confirmed that the dominant damage mechanism is cracking in the braider yarns, which was monitored using thermographic images and confirmed by edge replication microscopic observations. Fatigue results confirmed that the saturation of braider yarn cracks during cyclic loading corresponded to changes in the stiffness degradation rate as well as the surface temperature profile. This was confirmed by edge replication and scanning electron microscopic analysis. The reported results and observations provide an important step in the validation of thermography as a powerful non-destructive evaluation tool for monitoring the development of fatigue damage as well as predicting the damage states of laminated composite materials in general, and braided polymeric composite materials in particular.     

Effect of epoxy coatings on carbon fibers during manufacture of carbon fiber reinforced resin matrix composites

The changes in oxygen and nitrogen during manufacture of the carbon fiber reinforced resin matrix composites were measured using the X-ray photoelectron spectroscopy method. The effects of the change in oxygen and nitrogen on the strength of the carbon fibers were investigated and the results revealed that the change of the tensile strength with increasing heat curing temperature was attributed to the change in the surface flaws of the carbon fibers because the carbon fibers are sensitive to the surface flaws. The effect of the surface energy that was calculated using Kaelble’s method on the strength of the carbon fibers was investigated. Furthermore, the surface roughness of the carbon fibers was measured using atom force microscopy. The change trend of roughness was reverse to that of the strength, which was because of the brittle fracture of the carbon fibers.     

Improving the flexural and thermomechanical properties of amino-functionalized carbon nanotube/epoxy composites by using a pre-curing treatment

A prior thermal (pre-curing) treatment of mixtures of epoxy monomer and amino-functionalized carbon nanotubes (CNTs) was used to promote a chemical reaction between the matrix and the reinforcement, favouring the formation of a strong interface. Samples of epoxy resin and different weight percentages of amino-functionalized multi-walled CNTs were prepared with and without the pre-curing treatment (150 °C, 1 h). The degree of dispersion of the nanofiller was better when this pre-curing treatment was used. This allowed a higher CNT content while keeping a high sample homogeneity. Without the pre-curing step, the addition of CNTs increases both the flexural strength and strain to failure by 45%. Moreover, with the pre-curing step, the nanocomposite with 0.25 wt.% CNTs presents an increase of flexural strength by 58% and strain to failure by 68% regard to neat epoxy resin.     

Interlaminar properties of carbon fiber composites with halloysite nanotube-toughened epoxy matrix

Halloysite nanotubes (HNTs), which are geometrically similar to multi-walled carbon nanotubes, can improve the impact strength of epoxy substantially, according to our previous work [1]. Using a HNT-toughened epoxy as the matrix, a set of hybrid composites was prepared with carbon fiber-woven fabrics. The interlaminar properties of the composites were investigated by a short-beam shear test, a double-cantilever-beam test and an end-notched flexure test. The results showed that the addition of HNTs to the composites improved the interlaminar shear strength and the fracture resistance under Mode I and Mode II loadings greatly. The morphological study of the hybrid composites revealed that HNTs were non-uniformly dispersed in the epoxy matrix, forming a unique microstructure with a large number of HNT-rich composite particles enveloped by a continuous epoxy-rich phase. A study of the fracture mechanism uncovered the important role of this special morphology during the fracturing of the hybrid composites.     

Fabrication and mechanical properties of carbon short fiber reinforced barium aluminosilicate glass–ceramic matrix composites

Dense BaSi₂Al₂O₈ (BAS) and Ba_0.75Sr_0.25Si₂Al₂O₈ (BSAS) glass–ceramic matrix composites reinforced with carbon short fibers (C_sf) were fabricated by hot pressing technique. The microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning and transmission electron microscopies, and three-point bend tests. The carbon fibers had a good chemical compatibility with the glass–ceramic matrices and can effectively reinforce the BAS (or BSAS) glass–ceramic because of associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects. Doping of BAS with 25 mol% SrSi₂Al₂O₈ (SAS) can accelerate the hexacelsian to celsian transformation and result in the formation of pure monoclinic celsian in C_sf/BSAS composites, which can avoid the undesirable reversible hexacelsian to orthorhombic transformation at 300 °C and reduce the thermal expansion mismatch between the fiber and matrix.