Volume 22, Issue 3

Carbon nanotubes (CNTs) with different aspect ratios were modified by a silane coupling agent. The modified CNTs were embedded into a silicone elastomer (SE) matrix to obtain the composites. The dielectric behaviors of the composites with different frequencies of outer electric field were investigated. The results showed that the CNTs were modified by a silane coupling agent to improve the interaction with the SE matrix. The dielectric loss and permittivity of the SE were enhanced by the addition of CNTs. The CNTs with larger aspect ratios had a more effective dielectric loss and permittivity-enhancing effect. There were three existent polarization mechanisms in the composites, and the response times of the three were different. When the field frequency increased, a polarization mechanism or two would not catch up with the increase, and the dielectric loss and permittivity would obviously be affected. The CNTs with larger aspect ratios had a more effective dielectric loss and permittivity-enhancing effect. According to the Debye equation, CNT/SE composites have a similar function as the parallel RLC circuit.

In this study, structural damping and toughness characteristics of epoxy-based nanocomposite containing four different mechanisms of reinforcing investigated. These mechanisms are using multiwall carbon nanotube (MWCNT) as nanofiber, clay as nanolayer, SiO 2 as particulate nanofiller, and high impact polystyrene (HIPS) for polymeric alloying. The epoxy resin used was diglycidyl ether of bisphenol A (DGEBA) cured by cycloaliphatic polyamine. Natural frequency and damping coefficient are computed from the Stochastic Subspace Identification-Data Driven (SSI-Data) method. These parameters measured by Laser Doppler Vibrometer. In addition, impact resistance of these four types of composites is tested. The test results are compared between different weight percent loading of each reinforcement and also between different mechanisms. Also, morphological properties of DGEBA/CNT, DGEBA/clay, DGEBA/SiO 2 , and DGEBA/HIPS were studied using scanning electron microscopy (SEM).

Mass production of tires as well as its difficult storage or elimination is a real environmental problem. Various methods for recycling tires are currently used, such as mechanical crushing, which puts vulcanized rubber, steel, and fibers apart. The rubber may be used in several industrial applications such as flooring, insulations, and footwear. The present paper focuses on finding a new application for old used tires [ground tire rubber (GTR)]. To this end, tires dust has been mixed with various thermoplastic polymers such as polyvinyl chloride (PVC), high-density polyethylene (HDPE), ethylene vinyl acetate (EVA), and polypropylene (PP). We have checked the maximum GTR concentration values admitted by these compounds while keeping dielectric properties within acceptable values and therefore remaining suitable for industrial applications in the manufacturing of insulators for electrical wires. In particular, tires dust with particles size p<200 μm has been mixed with the polymers in four different GTR concentrations of 5%, 10%, 20%, and 50% to establish its performance through dielectric tests performed within a range of temperatures from 30°C to 120°C, and with frequencies from 1×10 -2 to 3×10 6 Hz, and analyzing conductivity, permittivity, dielectric loss factor, and relaxations. At last, the fracture surfaces of the composite samples have been evaluated using scanning electron microscopy (SEM).

This thesis is a discussion on obtaining a vacuum brazing joint of W-Cu alloy and 18-8 stainless steel with Cu-Mn-Co filler metal under the process parameter of a brazing temperature of 1060°C and holding time of 1 h. Four-point bending strength was tested at room temperature. Microstructure, microhardness, and element distribution near the interface of the brazed seam were observed via micro sclerometer, scanning electron microscope (SEM), and energy disperse spectroscopy (EDS) methods. Finally, analysis of the existence for phase composition near the interface was conducted using X-ray diffraction. Corresponding to a four-point bend strength of 780 MPa, there was a plain-interface, dense microstructure without obvious microdefects that formed. Microhardness values increased near the interface of the brazed joint and decreased gradually to the center of the brazed seam. Element distribution indicated that elements (Cu, Mn, Co) from the filler metal diffused into the interface or even substrates, and Fe and Cu element, together with a little of W element, from the substrates dissolved into the brazed seam region. Fracture morphology of the W-Cu/18-8 steel joint was identified as a mixed feature of fracture type. Moreover, the brazing seam contained mainly a Cu-rich phase, W 0.4 Cu 0.6 , α-Fe, and simple substance of W, together with a little of Co 0.7 Fe 0.3 .

Nickel (Ni) matrix reinforced with boron carbide (B 4 C) has been produced by conventional furnace sintering at various temperatures. A uniform Ni layer on B 4 C powders was deposited prior to sintering using electroless plating technique, allowing close surface contact between particles. A composite consisting of quaternary additions, a ceramic phase, B 4 C, within a matrix of Ni and AstaloyCr-M powders has been prepared at a temperature range of 1000°C–1200°C under Ar shroud. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) study, compressive testing, and hardness measurements were employed to characterize the properties of the composite. Experimental results showed that the best properties as compressive strength σ max and hardness (HB) were obtained at 1150°C. Wear resistance was relatively higher than cast iron.

In this research, compressibility and the effect of cold compaction pressure on sintered density of tungsten (W)-15% wt copper (Cu) and W-75% wt Cu composite powders were investigated. The powders were prepared by milling and reduction of WO 3 /CuO powder mixtures. The crystalline size and lattice strain of WO 3 and CuO were determined using Debye-Scherrer’s formula after mechanical milling. Heckel and Panelli-Ambrosio equations were used to evaluate cold compaction behavior of the reduced powders. The results showed that Heckel equation represents better correlation between compaction pressure and relative density of W-75% wt Cu composite powders. However, it was confirmed that the preferred relation for evaluating the compaction behavior of W-15% wt Cu composite powders is Panelli-Ambrosio equation. The green compacts were sintered at 1000°C. It was shown that, by increasing the cold uniaxial pressure prior to sintering, the difference between green and sintered densities of W-15% wt Cu composite powder compacts was decreased. However, the W-75% wt Cu powders showed a different manner and higher compaction pressures led to higher densification during sintering.

Flexural fracture analysis with an emphasis on microstructural assessment of self-compacting concrete (SCC) containing class F fly ash (FA) and titanium dioxide (TiO 2 ) nanopowder has been carried out in this paper. For this purpose, Portland cement was replaced by low volumes of FA as 5, 10, and 15 wt% and nanopowder as 1–5 wt%. Flexural and compressive strengths were determined at different ages and empirical relationships were obtained to predict flexural strength based on compressive strength at different ages. Microstructures of different concrete mixtures were also investigated by scanning electron microscopy (SEM). It was found that addition of FA could lead to flexural strength decrease at earlier ages; however, improvement of the flexural strength could be achieved at higher ages. TiO 2 nanopowder as a partial replacement of cement up to 4 wt% could accelerate C-S-H gel formation as a result of increased crystalline Ca(OH) 2 amount at the early age of hydration and hence increase flexural strength of concrete specimens. TiO 2 nanopowder could also improve the microstructure of concrete by shifting the distributed pores to finer and less harmful pores.

The use of resources is increasing due to continuous increase in world population and rapid industrialization, while natural resources are being exhausted day by day. Usage of waste materials or by-products in highway construction has substantial environmental and economic benefits. In this study, the usage of cullet and waste glass bottle dust as mineral filler material in hot mix asphalt as an alternate to traditional crushed stone dust was investigated. Optimum bitumen content was determined by the Marshall mix design method by using six different bitumen contents (4.0%, 4.5%, 5.0%, 5.5%, 6.0%, and 6.5%). With the optimum bitumen content, three different mineral filler types (cullet, glass bottle waste, and stone dust) and six different filler ratios (4%, 5%, 6%, 7%, 8%, and 9%) were used to prepare asphalt mixture samples. Samples were performed using the Marshall stability test, and the results were compared. It is concluded that cullet and glass bottle waste can be used in asphalt mixtures as a mineral filler alternate to crushed stone dust if the economic and environmental factors favor it.

Glazed hollow bead, cement, fly ash, and latex powder were used to prepare a glazed hollow bead thermal insulation material by way of compression molding, and the effects of redispersible latex powder on the mechanical properties and water resistance performance of the material were studied. In addition, the action mechanism of latex powder was analyzed. The surface of alkali-resistant glass fibers was treated by styrene-acrylic emulsion, and the effects of glass fibers on the mechanical properties of glazed hollow bead thermal insulation materials before and after treatment were studied, respectively. Moreover, the fracture morphology of the samples was observed and analyzed to explore the reinforced mechanism of fiber. The results show that when the dosage of latex powder is 4%, compared with blank samples, the sample’s flexural and compressive strengths increase by 48% and 20.83%, respectively, and the 2-h and 24-h water absorption of the samples is reduced by 71.37% and 66.94%, respectively. When the dosage of surface-treated fibers is 1.0%, the flexural strength of the samples increases by 35.71% and the compressive strength of the samples increases by 8.34% compared with samples that were mixed with untreated fibers.

Experimental studies were made on isotropic skew plates made of aluminum 7075-T6 and laminated composite skew plates under uniaxial compression with unloaded edges completely free and one loaded edge restrained completely and the other loaded edge restrained except translationally in the direction of loading. Experimental values of the buckling load have been determined using five different methods. The buckling load has also been determined using CQUAD8 finite element of MSC/NASTRAN. Comparison is made between the various experimental values of buckling load and the finite element solution. The effects of the skew angle and the aspect ratio on the critical buckling load of isotropic skew plates made of aluminum 7075-T6 have been studied. The effects of the skew angle, aspect ratio, and the laminate stacking sequence on the critical buckling load of laminated composite skew plates have also been studied. The critical buckling load is found to increase with the increase in the skew angle and decrease with the increase in aspect ratio. Method IV yields the highest value for critical buckling load and Method III the lowest value for critical buckling load. Among the various experimental values, the one given by Method IV is closest to the finite element solution, and the discrepancy between them is less than about 5% in the case of isotropic skew plates and about 10–15% in the case of laminated composite skew plates.

The static tensile properties in the form of ultimate failure stress, ultimate failure strain and Young’s modulus of a cross-ply glass fiber-reinforced polymer (GFRP) composite laminate [90 4 , 0 4 ] s and an unconventional angle-ply GFRP composite laminate [+67.5 4 , -67.5 4 ] s were investigated using the netting analysis, the laminate mixture rule (Hart-Smith 10% rule) and the classical laminate theory (CLT). The findings were then compared to experimental results to determine the accuracy of each analytical technique. It was found that the netting analysis was the best overall method for estimating the cross-ply laminate tensile properties, whereas neither the CLT nor the 10% rule were appropriate for estimating the tensile properties of the unconventional ply angle laminate.

Within the scope of experiments, the effect of aramid and glass yarns as stitch and biaxial (warp and weft) yarns in the biaxial weft knitted (BWK) composite was compared. After production of four types of composite panel using the hand lay-up method, the tensile and bending properties of the BWK composites were investigated both experimentally and numerically. The composite with the glass stitch and biaxial yarns exhibited higher tensile and bending properties than did the composite with the aramid stitch and biaxial yarns. The good agreement between the experimental results and the numerical results validated the applicability of the finite-element method for the BWK composites. The laminate beam theory was utilized as another modeling method for calculation of the bending modulus.

A methodology of laminate design rules is proposed to incorporate constraint in genetic algorithm (GA). It can provide optimization result that conforms to the composite design rules. Several methods such as replacement approach, swap approach, combined approach, and zero fitness approach are introduced to execute the laminate design rules concept. Effects are analyzed in terms of fitness value and convergence rate. Using rule operators, the convergence speed of the GA is increased. The proposed solutions will be more suitable than standard GA for some particular application with their conformance to the design rules. As a result, these methods are successfully applied and a practical optimal stacking sequence is obtained with the most appropriate for design rules.

The humeral heads of rabbits were replaced with individualized C/C composite implants coated with hydroxyapatite (HA) via rotation plasma spraying. The effect of the implants was evaluated after 5 and 150 days by using histology, scanning electron microscopy, X-ray diffraction, and so on. Results showed that the coating interface of C/C composites facilitated phosphorite deposition and osseous tissue growth. Furthermore, the humeral bone and remaining bone tissues were morphologically similar and possessed enhanced bonding strength. These results indicated that individualized C/C composite implants with HA coatings enabled the humeral head to adapt to dynamic requirements and accelerate healing after bone transplantation.

Efforts are needed to develop innovative and environmental friendly materials in order to reduce greenhouse gas emissions. An experimental investigation on the flexural behavior of thin cementless composite panels reinforced with welded rectangular wire mesh and chicken mesh with varying number of mesh layers as well as varying concentration of alkaline solution is presented. A total of 30 panels have been tested under flexural loading. The size of the panel is 1000 mm (length)×200 mm (width)×25/35 mm (thickness). The parameters studied in this investigation include varying the concentration of NaOH (8, 10, 12, 14 m ) and thickness of composite panels. In this work, cement is replaced by geopolymer mix to bind the ferrocement skeletal framework and its flexural behavior is studied. It is concluded that the first crack and ultimate loads increase with the increase in the thickness of the element and the concentration of alkaline solution. From the studies, it is observed that the load-carrying capacities, energy absorption, and deformation at ultimate load are high in the case of geopolymer ferrocement element. Further, it is observed that there is reduction in crack width, and increase in number of cracks in the case of geopolymer ferrocement indicates delay in crack growth.