Volume 25, Issue 5

The natural and fiber-reinforced resin matrix composites have been increasingly and widely applied in society in order to improve the environment and promote sustainable development. In the first part of this article, the major foci of recent research on natural fiber composites are reviewed in detail, including fiber surface treatment, fiber agglomeration and dispersion, interfacial transcrystallinity, impact strength, foaming technique, inflaming retardance, biodegradable resin matrix and nanofiber reinforcement techniques. The fiber surface treatment and fiber dispersion are important factors in improving the mechanical properties of the natural fiber composites. The interfacial transcrystallinity can be generated on natural fibers owing to the rough surfaces. The natural fiber composites have a low impact strength. In order to reduce the density and improve the flame retardant effect of the composites, both the foaming technique and flame-retardant technology are studied in detail. The biodegradable resin matrix and natural fibers can be mixed to prepare fully-degradable composite materials. Using the natural nano fiber reinforcements in reinforcing resins is another important research area. In the second part of this article, the applications of these composites are summarized systematically, especially those involving the automotive industry, household appliances, construction materials, packaging materials and boards of musical instruments. Currently, the automotive industry and the production of construction materials are the most important application areas of these composites.

The paper presents a numerical procedure to evaluate the mechanical properties and predict the damage initiation of random multiwalled carbon nanotube-reinforced polymer nanocomposites (MWCNT-RPNC). The Hashin-Shtrikman (H-S) random prediction model is used to compute the properties of the reinforced polymer matrix, whereas the Chamis model is used to compute the lamina properties and the Hashin progressive damage model within the ABAQUS environment is used as a finite element analysis (FEA) tool to predict the damage initiation in the reinforced composite material. Experimental testing is employed to validate the numerical results and to adjust the H-S prediction model for MWCNT-RPNC.

This paper discusses the formation of matrix cracking and induced delaminations in a [ϕm(2)/ψn(1)]s$[\phi _m^{(2)}/\psi _n^{(1)}]s$ laminate subject to arbitrary in-plane loading using a variational approach. To this end, the effects of delaminations coming from the tips of transverse cracks in the middle sublaminates are considered to specify the critical crack density points and finally understand which modes of damage are prospects for various lay-ups in the presence of transverse cracks as well as induced delamination. After deriving a very good approximation of the principle of minimum complementary energy by considering complex equations, the stress fields are provided. In this analysis, a unit cell in the ply level of a composite laminate containing both matrix cracking and delamination is considered. Then, the values of the admissible stresses and compliance of cracked laminate are employed to evaluate the energy release rate of each mentioned damage mode. Eventually, the graphs for different lay-ups in such symmetric laminate are drawn, which indicate important points for designers in practical applications. Afterwards, the effects of thickness on the dislocation of critical crack density points are checked. It can be emphasized that the current approach opens a new insight for perceiving the composites’ structural behavior in vulnerable positions.

This paper reports the influence of process parameters on the size and distribution of in situ titanium diboride (TiB 2 ) particles within the aluminium (Al) matrix. TiB 2 particles were formed as a result of the in situ reaction of potassium hexafluorotitanate (K 2 TiF 6 ) and potassium tetra fluoroborate (KBF 4 ) with molten Al. Two process parameters, namely, the addition time (AT) and holding time (HT) of precursor salts, were considered. The Al/TiB 2 composites were produced by allowing the in situ reaction to occur at various ATs (10, 20, and 30 min) and HTs (20, 30, and 40 min). Results showed that the formation of TiB 2 was confirmed by XRD analysis. The microstructure, TiB 2 particle size, hardness, yield strength (YS), and ultimate tensile strength (UTS) were strongly affected by the said process parameters. The variations in hardness and UTS were highly consistent with those found in the microstructure of the composites. Compared with the Al parent material, the increase in the average hardness and UTS of the composite were 51% and 44%, respectively. This improvement was achieved for the composite sample fabricated with 20 min of AT and 30 min of HT. At this condition, the composite displayed near-uniform particle distribution.

ZrO 2 -toughened Al 2 O 3 (ZTA, 40%) ceramic particles reinforced by high chromium cast iron (HCCI) matrix composites were fabricated by casting infiltration. The interaction between the ZTA ceramic particles and the matrix by the interface transition layer (ITL) was investigated. From the perspective of interfacial bonding, the ceramic particles and HCCI experienced metallurgical bonding by the ITL. Electron probe microanalysis results demonstrated that Cr, Fe, and Mn diffused from the matrix to the ITL. Transmission electron microscopy revealed that the main phases in the transition layer were glassy. The wear resistance of the composites with the transition layer was better than those without, as indicated by the three-body abrasive wear tester.

In this work, the rolling process was employed to fabricate polylactic acid/multi-walled carbon nanotube (PLA/MWCNT) composites at room temperature. The effects of the rolling conditions on the mechanical and electrical properties of the fabricated composites were investigated. The evolution processes of the internal molecular structures, i.e. changes in molecular orientation and crystallinity, were examined by X-ray diffraction, differential scanning calorimetry, and density method. The results suggested that the molecular orientation improved; however, the crystallinity decreased when the rolling ratio increased. The analysis of the mechanical properties revealed that the rolled composites displayed anisotropy during the rolling process. In the rolling direction, after adding 1 wt.% MWCNTs, the tensile strength increased from 58.6 to 94.3 MPa with the rolling ratio, whereas the fracture strain sharply increased to 131.5% at the rolling ratio of 60%. In addition to the mechanical properties, electrical resistivity was also investigated; notably, this property was not significantly affected by the rolling process. Furthermore, the MWCNT dispersion and morphology were investigated by scanning electron microscopy. These findings offer a simple and effective method to fabricate conductive composites with excellent mechanical properties.

Bone damage patients may suffer from metal toxicity resulting from an adverse reaction. To avoid the need for a second operation, we set out to identify a material that can be used as a substitute for metal in small fragment plates, which is compatible with the human body. Thus, in this study, we set out to study the development of a material that can be applied to the small fragment plate, based on a hydroxyapatite (HA)-polylactic acid (PLA) composite. This study examined three main factors, namely, the ratio of the PLA to the HA, injection temperature (T) and injection pressure (P). Based on the ASTM standard, the best results (Code 4 and Code 1) obtained from the mechanical property tests (tension and flexural) were 44.02 MPa and 63.97 MPa, respectively. When compared to HA-HDPE, our material offers both strength and biodegradable/biocompatible advantages. By inspection with scanning electron microscope (SEM) and energy-dispersive spectrometry (EDS), we could identify the components of the HA and distribution pattern. In terms of biocompatibility, Code 1 is promising. To maximize the composite desirability, optimal condition was mathematically calculated. In addition, finite element analysis confirmed that the proposed bone fixation plate would not be damaged when the wrist is impacted.

In the present study, dielectric and thermal properties of poly(aryl ether ketone) (PAEK)-nanocomposites with varying weight percentage of magnesium oxide (MgO) have been reported. The thermal properties of PAEK/MgO nanocomposites were studied using differential scanning calorimetry, thermogravimetric analysis, thermo-mechanical analysis and thermal conductivity. Transmission electron microscopy of the developed nanocomposites shows agglomerate-free dispersion of MgO nanoparticles in PAEK matrix. From the dielectric properties, dielectric constant of 13 was achieved for 10 wt% PAEK/MgO nanocomposite at 230°C. Further, minimum tan δ and maximum volume resistivity were found for 10 wt% PAEK/MgO nanocomposite. Data from thermal studies indicate that the incorporation of MgO into PAEK significantly enhanced the glass transition temperature and slightly deteriorated the thermal stability, char yield, and flame-retardant properties. Also, highest thermal conductivity and lowest coefficient of thermal expansion were achieved at 10 wt% of the MgO content. An excellent combination of both dielectric and thermal properties of the PAEK/MgO nanocomposites developed in the present study finds its potential application in microelectronics and electrical insulation in power equipments.

Conservation of natural resources, healthy environments, and optimal utilization of waste materials are intimate needs of the present time, and this research work was carried out to fulfill these needs. In this experimental and analytical study, concrete was prepared by replacing natural fine aggregates with two types of used foundry sands by 10%, 20% and 30% (by volume). The properties of fresh and hardened concrete were investigated and compared with a replacement amount of fine aggregates from 0% to 30%. Compressive strength was evaluated after 7, 28 and 63 days of moist curing. Along with compressive strength, the modulus of elasticity was also investigated and a reduction in compressive strength and modulus of elasticity was observed with the increase in the amount of used foundry sand. A prediction formula was proposed to predict the compressive strength, and verified by current experimental observations and also with a large database that was also established in this work. The prediction formula may be considered as very helpful for predicting the potential of using used foundry sand as an aggregate in concrete.

Various nano- and micro-sized fillers can be integrated into polymers to enhance their flame-retardant performance. In this work, a diglycidyl-ether bisphenol A epoxy was used as the matrix and nanostructured silica aerogel (AG) and ammonium polyphosphate (APP) microparticles were investigated as fillers to improve the flame-retardant and thermal properties of the epoxy. The anti-flame, thermal, and mechanical properties of the composites were investigated for different volume fractions of filler particles. It was found that APP decreased the burning rate while significantly improving the thermal stability. To investigate the flame resistant properties of combined AG and APP, an optimized ratio of AG and APP was added to the epoxy, leading to a stable flame-retardant epoxy with a low thermal conductivity and improved glass transition temperature ( T g ). The synergy between the AG and APP in composite samples resulted in an interesting burning behavior where sample core was relatively less deteriorated compared with the samples containing only APP or AG. This was attributed to the decrease of thermal conductivity due to the addition of AG. Lastly, samples containing APP showed the highest limiting oxygen index percentage and it was found that only small amounts of APP are required to make the epoxy flame-retardant.

To improve the separation performance of the polydimethylsiloxane (PDMS)/bark biochar (BB) nanocomposite membranes used for alcohol/water separation, the preparation conditions of these composite membranes were analyzed and optimized. In this study, we investigated the following preparation parameters: the BB pyrolysis temperature, the weight ratio of the silane coupling agent (KH-550) to bark biochar (BB), and the BB loading amount. The regression equations were established between these three preparation parameters and the final pervaporation (PV) performance characteristics of the composite membranes. The membranes performed the best under the following optimal preparation conditions: a BB pyrolysis temperature of 407°C; a silane coupling reagent/BB weight ratio of 0.86, and a BB loading amount of 3.36 wt%. According to the results of the regression analysis, a maximum permeation flux of 221.2 g·m −2 ·h −1 and a maximum selective factor of 21.3 was obtained when the feed temperature for the 5 wt% alcohol solution was set at 40°C.

In this study, the mullite-quartz-based proppants were successfully prepared by using the coal gangue as the raw materials. Then, the effects of the additive and the sintering temperature on the composition, microstructure, and properties of the proppants were investigated. Results showed that the proppants sintered at 1250°C with the 10 wt% bauxite additive presented the best performance, which was very close to that of the quartz-proppant, and met the operational requirements of the 52 MPa coal bed methane wells. The viscous flow mechanism of the liquid phase formed during the sintering process also promoted the arrangement of the grains, thus benefiting the densification and the strength of the proppants.

The aviation, automobile, and consumer products industry requires long-life, durable, lightweight high-performance polymer matrix composites. Polymer fiber reinforced materials possess low weight and high specific quality along with high specific stiffness. The addition of carbon nanoparticles in the composites improves their mechanical properties, including wear enhancement, which leads to the utilization of these composites in different fields. The present work investigates the wear performance of glass fiber and carbon nanotube (CNT) reinforced hybrid polymer composites. Dry sliding tests for wear were conducted using a pin-on-disc wear tester by varying the load and speed. The worn surfaces were examined by utilizing scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and atomic force microscopy. The result shows that the increase in volume percentage of CNTs in glass fiber reinforced polymer composites decreases the wear rate. The result also clearly states that the coefficient of friction increases with an increase in the CNT percentage.

In the present research, polyaniline is used as a conducting polymer and polyvinyl alcohol is also used as a biopolymer, because of its mechanical properties and suitable processability. Also, silver nanoparticles are considered as a reinforcing agent of thermal stability, mechanical and antibacterial properties to prepare polyaniline-polyvinyl alcohol-silver nanocomposite. The synthesis of polyaniline-polyvinyl alcohol composite and polyaniline-polyvinyl alcohol-silver nanocomposite is performed through addition of polyaniline and silver in polyvinyl alcohol solution. In order to review thermal, mechanical and antibacterial properties of synthesized composite and nanocomposites, components with different weight rates are used. The obtained results from thermogravimetric analysis (TGA) tests also indicate promotion of thermal stability of polyaniline-polyvinyl alcohol-silver nanocomposite compared with pure polyvinyl alcohol in temperatures above 400°C. The results of Fourier-transform infrared (FTIR) spectroscopy revealed the presence of polyaniline, polyvinyl alcohol and silver in the structure of polyaniline-polyvinyl alcohol-silver triple nanocomposite film. The obtained results from a review of antibacterial properties showed that polyaniline-polyvinyl alcohol-silver nanocomposites have antibacterial effects on two different types of Gram-positive and Gram-negative bacteria. The obtained results from a review of mechanical properties of nanocomposites showed that the greatest value of tensile strength (13.8 MPa) belonged to polyaniline-polyvinyl alcohol-silver (88%/9%/3% w/w) nanocomposites. Therefore, this is determined as an optimal triple nanocomposite. In addition, scanning electron microscopy (SEM) coupled with an energy dispersive X-ray (EDX) system was used to characterize the composition and structure of polyaniline-polyvinyl alcohol-silver nanocomposite film.

Aluminum-based nanocomposites reinforced with carbon nanotubes have increased scientific attention in today’s life. The dispersion quality was the critical aspect, which decides the homogeneous distribution of CNTs within the Al matrix as starting precursors. In this study, a new attempt has been made to obtain a uniformly dispersed Al-0.75% CNT precursor via combining ultra-sonication, cubic tumbler rod milling, and spray drying. This process was integrated with organic deflocculant (formulator) in specific proportion to transform as a semi-wet-based route. The effect of milling media on the morphology and interface structure of the as-produced composite precursor after all the processing steps was investigated through scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD) analysis, Raman spectroscopy, and Fourier transform infrared spectroscopy. The results reveal that the approach is effective in CNT dispersion in Al precursor, which shields the nanotube structure from damage for longer periods of milling time due to the organic formulator mixture, and also, the CNT retention in the Al precursor with minimum clustering is identified compared to the ball milling process. Carbon traces were confirmed in the as-produced composite precursor by this approach.

In recent years, natural fibers have received much attention from various industrial applications. As these fibers are lightweight, nonabrasive, low cost, ecofriendly and biodegradable, they can be sometimes considered as alternatives to synthetic fibers in lightweight composite structures. In this work, date palm fibers (DPFs) were treated by oxygen plasma at various plasma discharge power and exposure time. The effects of plasma treatment on tensile strength of DPF and interfacial adhesion between DPF and epoxy were determined by single fiber tensile test and microbond test, respectively. Scanning electron microscopy was used to investigate the surface morphologies of DPFs before and after the plasma treatment. The functional groups on the surface were studied by attenuated total reflectance-Fourier transform infrared spectroscope (ATR-FTIR). Decrease in hemicellulose and lignin content of DPF was indicated in ATR-FTIR spectra of the treated sample with plasma treatment. The results show that plasma treatment cleans the fiber surface and increases the surface roughness by etching effect. Moreover, fiber surface modification significantly improves tensile properties of DPFs and interfacial shear stress (IFSS) of fiber/matrix. However, the effects of plasma power and exposure time on tensile properties and IFSS values of DPFs are not found significant. Moreover, Weibull statistics show that plasma treatment could not decrease the variability in fiber strength due to the nature of fibers.

Some variation in the topological distribution of fibers inside the matrix phase of fiber reinforced polymers (FRP) is inevitable. Such irregularities can accelerate moisture diffusion and adversely affect the life of FRP. This paper presents a hierarchical technique for characterization of clustered microstructures and their transient moisture diffusion response. The clustering descriptors are derived for different fiber volume fractions (dilute to dense) for the quantitative definition of a given fiber matrix architecture. The metrics are normalized to remove dependence on volume fraction. The microstructures are analyzed for Fickian moisture diffusion. Suggested descriptors show a good correlation with transient diffusion response in relation to saturation time. The results can be used to predict the time-dependent moisture diffusion response of FRPs for any given fiber volume fraction.

The mechanical properties and stress relaxation behavior of annealed aluminum-carbon nanotube composite were studied and compared with those of pure aluminum. The composite was prepared using an induction furnace, where 1.6 vol.% of multiwall carbon nanotubes were added in aluminum as strengthening material. It was found that the mechanical strength of the annealed composite was almost twice that of aluminum. The stress relaxation behavior of both materials was logarithmic in nature. However, the stress relaxation, hardening component, and intrinsic height of the thermally activated barrier were significantly influenced by the presence of nanotubes in the aluminum matrix. It was found that the stress relaxation rate of the composite was reduced (>30%) and the hardening component was increased (>100%) compared with that of aluminum. The calculated strengths of the thermally activated barriers for aluminum and the composite were 1.7 and 2.6 eV, respectively.

The present study is dedicated to investigate the liability of continuously reinforced concrete pavement (CRCP) cast with self-consolidating concrete (SCC) to restrained shrinkage cracking and the values of restraint stresses in these pavements. SCC, which is becoming increasingly popular due to its several superiorities over conventionally vibrated concrete (CVC), has higher amounts and rates of shrinkage compared to CVC. The higher risk of restrained shrinkage cracking of SCC is a great cause of concern in pavement construction as the penetration of water, chemicals, and salts increases the risk of corrosion of reinforcement. In the present study, an analytical restraint stress expression was developed for typical CRC pavements by modifying the restraint stress equation developed previously for RC beams. Using this equation, the restraint stresses induced to the longitudinal reinforcement by the rigid pavement, cast with CVC or SCC, were calculated for eight different example sections. These restraint stress values were found to reach up to 50% of the limit stresses of bars, allowed by the design guidelines, when the pavement is cast with SCC. The amounts of longitudinal reinforcement used in typical CRCP roads were found to be more critical when the pavement is cast with SCC.

In order to calculate the effective permittivity of a mixture with multilayer ellipsoids, this paper presents a self-consistent approximation (SCA) on the basis of the Bruggeman’s analytical model. The effective permittivity of a mixture with aligned multilayer ellipsoids is derived directly from the linear system of equations, which are built using the boundary condition of the electric field on the confocal ellipsoidal interface in the ellipsoidal coordinate system. Furthermore, for a mixture with multilayer ellipsoids oriented randomly, an effective ellipsoid is introduced to substitute the original multilayer ellipsoid, and the permittivity of the effective ellipsoid is derived by jointly solving the two linear systems of equations for the situation of the original multilayer ellipsoid and that of the effective ellipsoid, then the effective permittivity of the mixture can be calculated by the existing Maxwell-Garnett formula. After comparisons, it is revealed that there is a good agreement between this SCA method and existing theories.