Band 19, Heft 3

The aim of this contribution was to investigate the microstructure of in situ Al-Si functionally graded materials produced by traveling magnetic field. The research shows that the composition and associated microstructural feature of Al-Si alloys processed by this method changes from the outer of samples to the inner, respectively, from Al-Si hypereutectic with particles of primary Si to Al-Si eutectic to hypoeutectic composition with a great number of primary Al dendrites. Moreover, the hardness, the wear resistance of samples and the volume fraction of primary Si particles all have obviously gradient characteristics in the samples.

A series of polyamide (PA)/montmorillonite nanocomposites containing pyrazine moiety in the main chain were synthesized by a convenient solution intercalation technique. PA 3 as a source of polymer matrix was synthesized by the direct polycondensation reaction of pyrazine-2,3-dicarboxylic acid 1 with 4,4′-diaminodiphenyl sulfone 2 in the presence of triphenyl phosphite, CaCl 2 , pyridine, and N -methyl-2-pyrrolidone. The resulting nanocomposite films were characterized by Fourier transform infrared spectra, X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis (TGA). The results showed that organo-modified clay was dispersed homogeneously in the PA matrix. TGA indicated an enhancement of thermal stability of new nanocomposites compared with the pure polymer.

In this study, TiC/316L stainless steel composites were fabricated by combining warm compaction and microwave sintering (WMS). X-ray diffraction analysis was used to identify the phases in the material. Microstructure characteristics of composites were performed by means of scanning electron microscopy (SEM). Density, microhardness, wear resistance, and corrosion resistance were evaluated at room temperature. Results revealed that TiC/316L composites with higher relative density and finer properties can be obtained by WMS, compared with conventional powder metallurgy process. For all the tested species, the hardness of the composites prepared by WMS is higher than that of the composites prepared by conventional powder metallurgy (P/M). Wear tests showed that the WMS species exhibited significantly good wear resistance. Furthermore, it is found that the composites prepared by WMS were not only has high mechanical properties but also better corrosion resistance compared with the conventional P/M species.

The properties of particle-reinforced aluminum composites depend on the microstructure and evolved uniformity distribution of particles in the matrix. The distribution of Al 2 O 3 particles in the matrix and microstructure properties of varying volume fraction of particles up to 15% Al 2 O 3 particle-reinforced Al metal matrix composites produced by the mechanical alloying technique was investigated. Alloying was performed in a planetary ball mill using a milling time varying from 0 h to 7 h, a ball-to-powder ratio of 10:1, and a ball mill velocity of 400 rpm. The produced compositions were cold-pressed at 700 MPa with a single action and sintered at 600°C for 3 h under Argon gas atmosphere. The relative density (both pressed and sintered), porosity and hardness of composites were also examined. The results of mechanical alloying processing were investigated with scanning electron microscopy (SEM), X-ray diffraction and laser particle size analyzer. SEM observations showed that relatively homogeneous distribution of Al 2 O 3 reinforcement in the matrix could be obtained by mechanical alloying after 5 h. Moreover, the variation in hardness of the composites with alumina volume fraction and milling time was observed to be a strong function of the work hardening mechanism.

A new type of construction material named ultra-high performance cementitious composite (UHPCC) was developed in order to reduce the production cost and energy consumption of reactive powder concrete (RPC). Six different mixture proportions of UHPCC were designed, and their properties, including workability (slump test), static mechanical properties (compressive strength and flexure strength), and dynamic mechanical properties (dynamic compressive strength, toughness and fracture pattern), were investigated. Finally, the microstructure of the UHPCC was studied. The experiment result indicate that the compressive and flexural strength of UHPCC can reach 150–200 MPa and 15–50 MPa under standard curing conditions, respectively, which is close to RPC200. Besides, the steel fiber has significant improvement on mechanical properties of UHPCC. The dynamic impact test results show that UHPCC has obvious strain rate effects. Besides, UHPCC exhibits excellent properties to resist dynamic impact. The microstructure investigation showed that UHPCC has dense microstructures and very strong bonds of the interfacial transition zone between cementitious paste and fine aggregate steel fiber.

In the present study, aluminium alloy (A380) composites containing 3, 6, 9 and 12 weight percentage of fly ash particles were fabricated by liquid metallurgy technique. Three different size ranges of fly ash particles (50–75 μm, 75–103 μm and 103–150 μm) were used. The composites were evaluated for hardness, tensile strength, density, dry sliding wear and frictional behaviour. Pin-on disc apparatus was used to conduct wear tests at loads of 20, 30 and 40 N at a sliding speed of 3 m/s for a constant period of 10 min. Results showed that hardness, ultimate tensile strength, wear resistance and coefficient of friction were superior in composites reinforced with coarse fly ash particles (103–150 μm) compared to composites with fine particles. The hardness, ultimate tensile strength and wear resistance increased, whereas the coefficient of friction and density decreased with the increase in weight percentage of fly ash. Wear resistance and coefficient of friction decreased with increase in applied load. Scanning electron microscope investigations of worn-out samples were carried out to study the progress of wear.

In this work, electroactive polyvinylidene fluoride (PVDF) was a matrix for dispersing 10 wt% ferromagnetic nickel (Ni) particles. The composites were formed into its shape by using a spin coater with varying spinning speeds from 750 to 3750 rpm to obtain the thicknesses between 68 μm and 40 μm on printed circuit board substrates. Magnetic hysteresis loops of the composites were dependent on the spinning speed because the spin regulated the dispersion and size distribution of Ni particles in the PVDF matrix. Owing to the Ni agglomeration, the composites prepared by 750, 1500 and 2250 rpm spinning clearly displayed soft ferromagnetic properties but the capacitance was decreased with the decrease in spinning speed. In every sample, the capacitance was reduced with the increase in frequency from 1 kHz to 100 kHz and remained minimal up to 1 MHz. These changes in capacitance can be explained by the polarization of surface charges of Ni clusters of varying sizes from different spinning speeds. The dissipation factor was, by contrast, sensitive to the frequency and the dispersion of Ni only at frequencies higher than 60 kHz.

In this study, the effects of aggregate type on the coefficient of thermal expansion of self-consolidating concrete (SCC) produced with normal and lightweight (porous) aggregate (SCLC) were investigated. In experiments, three aggregate types, gravel, volcanic tuff, and diatomite, were used. Different combinations of water/cement ratio and superplasticizer dosage levels were prepared for the SCC and SCLC mixtures. Thermal tests were performed to accurately characterize the coefficient of thermal expansion (CTE) of SCC and SCLC aged 28 days using the dilatometer. The CTEs of SCC and SCLC were defined by measuring the linear change in length of concrete specimens subjected to a range of temperatures from 20°C to 1000°C. The results, in general, showed that SCLC has a lower CTE than that of SCC above 100°C. Moreover, CTE values of SCC and SCLC were decreased with increase in porous structure. The aggregate type has significant influence on the thermal properties of SCC.

The aim of this paper is to analyze the effects of electromagnetic (EM) energy absorption at 900 and 1900 MHz when the human head’s dielectric properties are varied. This radiation is measured in terms of specific absorption rate (SAR). The characteristics of the helical antenna and its substrates with variation of the human head dielectric properties were simulated via the finite-difference time domain (FDTD) method using the CST Microwave studio. The human head dielectric properties are manipulated by increasing and decreasing 10% and 20% of each dielectric property. In this study, SAR values increase with increase of the human head’s conductivity, while increase of permittivity and densities decreases it. A helical antenna with a substrate of FR4 resulted in higher SAR values in all frequencies. The head SAR values are higher in the upper frequency exposures. The helical antenna with a substrate of Rogers RO3006 (loss free) was found to be the best-tested substrate, which contributed toward much lower SAR values in all GSM frequency bands.

Sugar cane bagasse ash (SCBA), a by-product of sugar and alcohol production, is one of the potential pozzolanic material that can be blended with Portland cement. In this study, SCBA with particle sizes <45 μm was used to replace type I ordinary Portland cement with various dosages (10%, 20%, and 30%) by weight of binder. The water/cementitious material (w/cm) and sand/binder ratios were kept at constants of 0.55 and 2.75, respectively. Composites were mixed, and effects of SCBA on properties were investigated by conducting flow test, water absorption test, initial surface absorption test, drying shrinkage test, compressive strength test, rapid chloride penetration test (RCPT), thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM). Experimental results show that the flow spread of fresh mortars would decrease with an increase of SCBA replacement. The specimens with 10% SCBA have the superior performance on compressive strength, drying shrinkage, water absorption, initial surface absorption, and chloride ion penetration, TGA, and SEM at the age of 56 days. It indicates that 10% cement replacement of SCBA may be considered as the optimum limit.

A number of thin biomaterials were prepared via a wet formation method from seven different unbeaten plant fibers without using any chemical additives. It is noted that sheets made with short fibers (Flax-B: 0.32 mm and Straw-UB: 0.36 mm) and high fine content (Flax-B: 9.77% and Straw-UB: 10.64%) showed higher tensile index values (Flax-B: 27 Nm/g and Straw-UB: 25 Nm/g) than those made from long fibers. Despite having the longest fibers (1.11 mm) in studied samples, the tensile index values of sheet sample made from Pine-S-UB was just 12 Nm/g, which is just half of short-fibered (0.36 mm) straw sheets. Air permeability of sheets made from Pine-S-UB and Straw-UB were 5322 and 917 ml/min, respectively. Densities of the same sheets in the same order were 493 and 539 kg/m 3 . It is concluded that long cellulose fibers actually produce bulky materials with limited interfiber contacting areas, while short ones give biocomposites with higher density values indicating a firmly compacted internal structure in the case of utilizing plant fibers without beating and refining. Results would be usable when producing biocomposites from plant fibers via wet formation followed by various resins and polymer moldings.

This paper deals with frequency optimization of symmetrically laminated 4-layered angle-ply plates with one or two different intermediate line supports. The design objective is the maximization of the fundamental frequency and the design variable is the fiber orientation in the layers. The first order shear deformation theory and nine-node isoparametric finite element model are used for finding the natural frequencies of laminates. The modified feasible direction method is used for the optimization routine. For this purpose, a program based on FORTRAN is used. Finally, the numerical analysis is carried out to investigate the effects of location of the internal line supports, plate aspect ratios and boundary conditions on the optimal designs and the results are compared.

In this study, the influence of the impactor shape on the dynamic response of E-glass/epoxy laminates was investigated. Composite samples were impacted using steel 90° and 120° conical, 24 mm and 12 mm in diameter hemispherical, pyramidal impactors via a specially developed drop weight test machine. The impact tests were performed at impactor mass (20 kg) for impact velocity (2.5 m/s). Plate specimens consisting of 18 plies for tests (180×50 mm) with a nominal thickness (7 mm) were used. Contact force-time and contact force-deflection data obtained from the experiments were interpreted. It was found that the contact force was smaller in the 90° conical impactor while it was bigger in the 24 mm hemispherical one. Absorbed energies were also investigated. The absorbed energy obtained for pyramidal impactor was greater than that of the others. Furthermore, overall damage areas caused by the impactor were evaluated.

In this study, thermal buckling load optimization of symmetrically laminated composite folded plates subjected to uniformly distributed temperature load is investigated. The objective function is to maximize the critical temperature capacity of laminates and the fiber orientation is considered as a design variable. The first-order shear deformation theory is used to study thermal buckling response of the laminates. The modified feasible direction method is used as the optimization routine. For this purpose, a program based on Fortran is used for the optimization. Finally, the significant effects of crank angles, plate lengths and boundary conditions on the optimum results are demonstrated and the results are compared.

Cutouts such as circular, rectangular, square, elliptical, and triangular shapes are generally used in composite plates as access ports for mechanical and electrical systems, for damage inspection, to serve as doors and windows, and sometimes to reduce the overall weight of the structure. This paper addresses the effects of different cutouts on the buckling behavior of plates made of polymer matrix composites. To study the effects of cutouts on buckling, loaded edges are taken as fixed and unloaded edges are taken as free. Finite element analysis is also performed to predict the effects of different geometrical cutouts, orientations, and position of cutouts on the buckling behavior. The results show that fiber orientation angle and cutout sizes are the most important parameters on the buckling loads. For all types of cutouts the buckling loads decrease dramatically by increasing the fiber orientation angle. It is observed that minimum buckling load is reached when 45° fiber angle is used, and after this angle critical buckling load begins to increase. Also, it is concluded that while fiber orientation angle is 0°, elliptical cutout has the highest buckling load and while fiber orientation angle is 45°, circular cutout has the highest buckling load.