Volume 24, Issue 1

In this study, a finite element method was developed based on X-ray computer tomography to predict the elastic modulus of needle carbon-fiber-reinforced ceramic matrix composites with voids randomly existing in the material. In these pictures, every pixel point contains all of the information of the components that we need, including voids. Using this information, the mechanical properties of components can be obtained, then a finite element model with voids was built and the predicted results fit well with the experiments. In addition, a volume average method was developed to determine the proper representative volume element size to reduce the computing time without losing the accuracy.

The increased use of composite materials and structures in many engineering applications led to the need for a more accurate analysis and design optimization. While methods of stress-strain analysis developed faster, optimization techniques have been lagging behind. As a result, many designed structures do not fulfill their full potential. The present study demonstrates the major achievements in recent years in an application of evolutionary algorithms to the design optimization of fiber-reinforced laminated composite structures. Such structures are of much interest due to high structural design sensitivity to fiber orientations as well as complex multidimensional discrete optimization problems. Using an anisotropic multilayered cylindrical pressure vessel and an exact elasticity solution as an example, we show how the optimum, or near–optimum, solution can be found in a more efficient way.

The research on impurity in the lithium iron phosphate has been a hot topic. Especially when prepared by the solid state method, the impurities occurred easily through high-heat sintering. But some impurity is not completely bad for the cell performance, such as Fe 2 P. In this paper, the influence of Fe 2 P has been researched. Using the magnetic separation method, the high and low contents of Fe 2 P in lithium iron phosphate are obtained and then compared with the primary sample. Results show that the Fe 2 P phase helps to improve the rate and cycling performances, but a too high content will decrease the specific capacity of the sample due to the low content of active material. It is proven with the electrochemical measurement that the Fe 2 P phase could enhance the electrical conductivity of cathode, but it gives electrochemical inactivity. It can be concluded that the high rate or high capacity types LiFePO 4 could be obtained by controlling the content of Fe 2 P through adjusting the preparation process.

The control of magnesia (MgO) hydration ratio is a key factor in refractory slurry processing. The volumetric expansion that follows this reaction can result in MgO product cracks as MgO greatly tends to react with water. Thus, the antihydration of MgO is a key concern in refractory suspension processing. The performances of ammonium phosphate (AP) and ethylene diamine tetraacetic acid (EDTA) as antihydration additives were evaluated by using hydration tests, and ionic conductivity, pH, and particle distribution of MgO powders in water and ethanol were used to provide information related to the MgO powder in different liquids. The results showed that the antihydration additives AP and EDTA can inhibit MgO hydration to some extent; however, AP was more effective in preventing volume expansion.

The effect of fiber characteristics from old corrugated container (OCC) paper on the strength properties of OCC/polypropylene composites was evaluated. Fibers with different contents of lignin (2.8%, 3.8%, 5.3%, and 7%) were produced using soda pulping. Wettability, tear, and tensile strength of the fibers were measured as the indication factors to assess the strength of reinforcing component in the composites. The weight portions of the OCC fibers, polypropylene, and maleic anhydride-grafted polypropylene (MAPP) were selected at 20%, 77%, and 3% of the total weight of the composite, respectively. The composite compounds were formed using a counter-rotating twin screw extruder, and the specimens were made in an injection molding machine. The interaction of fiber characteristics and fiber lignin content on the mechanical properties of composite was investigated. The results revealed that with lower fiber lignin content, both flexural and tensile properties were increased. Consequently, by forming better fiber dispersion and by reducing stress regions in the composite, impact strength was also improved. Lower lignin content resulted in better mechanical properties than fiber characteristics.

This study presents a new micromechanical model to investigate the effects of thermal residual stresses and interfacial properties on the transverse behaviors of SiC/Ti composites with different microstructures. In this model, the fiber-matrix interface is modeled by the bilinear cohesive zone model. The interface model is introduced into the generalized method of cells, which has the advantage of computational accuracy and efficiency. At the same time, the generalized method of cells is extended to consider thermal residual stresses within the fiber and matrix phases. Thermal residual stresses are found to have a significant influence on the transverse behaviors of the composites. Compared with the perfect interface, the transverse behaviors of the composites with weak interface bonding are much lower. Moreover, with the increase of fiber fraction, the stiffness of the composites increases before debonding occurs while the saturation stress decreases. The predicted results using the circular fiber model and considering thermal residual stresses are more consistent with the experimental values compared with the results using the square or elliptical fiber model. When the stress concentration factor is considered and the interface is weakly bonding, the strength predictions are much better than the results using the perfect bonding.

The current experimental study investigates impact response features of reinforced concrete (RC) beams with dominant shear failure mode. Six built nonstrengthened and strengthened RC beams with a shear span-to-depth ratio of 2.5 were subject to repeated low-velocity impacts. Projectile energy is equal to the amount required for collapse. Retrofit consists of carbon fiber-reinforced polymer (CFRP) sheets attached to the exterior surface in vertical-unidirectional and cross-ply orientation. Laser linear variable differential transformer (LVDT) and micro-electro-mechanical systems (MEMS) accelerometers record the resultant deflection and acceleration to inspect the relationship between retrofit and damage. Fast Fourier transform (FFT) and continuous wavelet transform (CWT) extract the response content specimens to monitor the frequency change for each case. According to extracted results, CFRP skin provides notable stiffness on the upward shift of active frequency range and peak spectrum amplitude maintained for impact repeats. The active domain in FFT spectrum is sensitive to failure type (shear or flexural). CWT analysis ridges show a clear difference between failure type samples. CFRP retrofit controls cracking and deflection, which is shown as increased acceleration in records, especially for the cross-ply wrap. Effective stiffness recovers during initial impacts and then starts degrading because of the rupture of horizontal fibers in cross-ply retrofit, but the stiffness is stable for unidirectional retrofit during all impacts.

An analysis of dense cements, such as polymer cement concrete, is made to produce original innovative components for different types of constructing materials. These materials present good functional properties (ageing resistance, crack formation resistance, hardness, and stability of mechanical modules) and can be used for various applications. In this paper, experimental tests on Portland cement with added γ-Al 2 O 3 and redispersible dry polymer performed using small angle neutron scattering are reported. The objective of the investigation was to assess the key parameters of the material (e.g., porosity, fractal dimensions, and size distribution) at the nanoscale level as well as to obtain useful structural information for expanding the possibility of applications. The results obtained can contribute to the optimisation of the consistency of the material, the design of operating conditions of elements of structures and facilities, and the design of the procedures that support ecological criteria and enhance quality and safety levels.

A compact ultra-wideband (UWB) antenna is presented in this paper with a partial ground plane on epoxy woven glass material. The study is discussed to comprehend the effects of various design parameters with explicit parametric analyses. The overall antenna dimension is 0.22×0.26×0.016 λ. A prototype is made on epoxide woven glass fabric dielectric material of 1.6 mm thickness. The measured results point out that the reported antenna belongs to a wide bandwidth comprehending from 3 GHz to more than 11 GHz with VSWR<2. It has a peak gain of 5.52 dBi, where 3.98 dBi is the average gain. Nearly omnidirectional radiation patterns are observed within the operating frequency bands. A good term exists between simulation and measurement results, which lead the reported antenna to be an appropriate candidate for UWB applications.

In this paper, polyimide-graphite (PG) composite was prepared and used as a solid lubricant at elevated temperatures. PG composite was characterized by X-ray diffraction, and its thermal stability was evaluated based on thermogravimetric/differential thermal analysis and Fourier transform infrared spectrometry analysis. The results show that in PG composite, there was some interaction between the graphite (GR) and the polyimide (PI) matrix. Consequently, the PG composite exhibited better thermal stability than PI. The friction behavior of the PG composite sliding against steel at elevated temperatures (300–600°C at an interval of 100°C) was evaluated with a ring-on-ring tribometer. It was found that the PG composite exhibited excellent friction-reducing ability and good load-carrying capacity in the temperature range of 300–600°C and applied loads of 300–700 N. Moreover, this excellent lubricating performance of the PG composite might be attributed to its good thermal stability as a result of the interaction between the GR and the PI matrix. Therefore, PG composite could be used as an excellent solid lubricant under the high-temperature conditions.

Experiments and simulation analysis for tensile strength of Z-pinned laminates were conducted in both room temperature and dry and hot-wet environments. The results of the experiments show that the fracture of [0] 6 specimen with Z-pins was longitudinally torn, and there was a row of Z-pins at the fracture of [(±45)/(0,90)] 2S specimen with Z-pins. It could be observed that the hot-wet effect on the tensile strength of the Z-pinned laminates increased with percentage of 0-direction fibers decreasing in the laminates from the experiments of the unidirectional tape laminates and the woven cloth laminates. When the volume content of Z-pin increased, the hot-wet effect on tensile strength of Z-pinned [45/0/-45/90] 2S laminates with Z-pins would become smaller and smaller. The simulation method of the hot-wet effect on tensile strength of the laminates was combined with the experiment and the finite element analysis. It used 14 nonlinear springs to simulate the longitudinal tearing of [0] 6 specimen with Z-pins. The simulation results showed that Z-pin played a role of fulcrum, and because of the lever principle, the fiber bundles were torn at both tips in resin-rich area.

The free vibration analysis of symmetrically laminated composite circular plates with curvilinear fibers is performed using the first-order shear deformation theory along with a curved hierarchical square finite element. The blending function method is used to describe accurately the geometry of the circular plate. The hierarchical shape functions are constructed from Legendre orthogonal polynomials. The element stiffness and mass matrices are integrated numerically by means of the Gauss-Legendre quadrature. The equations of motion are derived using Lagrange’s method. Results for the fundamental frequency are obtained for clamped and soft simply supported laminated composite circular plates with E-glass, graphite, and boron curvilinear fibers in epoxy matrices. The element is validated by means of the convergence test and comparison with published data for isotropic and laminated composite circular plates with rectilinear fibers. Contour plots of frequency as a function of fiber orientation angles for laminated composite circular plates with curvilinear fibers are presented. The fiber material and boundary conditions are shown to influence the distribution of frequency throughout the design space. Frequency curves as a function of fiber orientation angles for the first five modes of laminated composite circular plates with curvilinear fibers are also presented. They reveal that none of the first five modes of clamped and soft simply supported laminates is affected by crossing but modes 3 and 4 of clamped graphite/epoxy and boron/epoxy laminates are affected by veering.

Nano nitrocellulose/cyclotetramethylene tetranitramine (NC/HMX) composite particles were precipitated from their co-solutions by the spray drying method. The nano composite samples were characterized by scanning electron microscope, transmission electron microscope and X-ray diffraction. Impact sensitivity and thermal decomposition properties of nano composites were also measured and analyzed. Results show that the product particles are close to spherical in shape and range from 0.5 μm to 5 μm in size. In the product particles, β-HMX particles with size ranging from 50 nm to 100 nm are uniformly and discretely dispersed in NC binders. The drop height of nano NC/HMX composite particles (66.1 cm) is more than triple as high as that of raw HMX (21.6 cm), exhibiting considerably low impact sensitivity. Moreover, nano composite particles are easier to decompose and decomposed more rapidly than do raw HMX under the thermal stimulus due to the lower peak temperature and activation energy and higher reaction rate. Therefore, nano composites are expected to be candidates filled in high burning rate propellants.

AA8090, a certain type of Al-Li alloy, has been widely used in the aerospace industry because of its low density and high strength after heat treatment. In this research work, the AA8090/6% SiC p composite was produced by modified stir casting and then subjected to hot extrusion treatment. One part of the extruded rod was used to prepare the wear test samples and the remaining part was converted to strips with 2 mm thickness via hot rolling process. Three processes, including precipitation hardening with conventional heating, age hardening with rapid heating, and precipitation after directional quenching, were done on samples. The results of the mechanical properties revealed that precipitation hardening after directional quenching reduced the peak of hardness time from 16 to 6 h, increased the peak of hardness by 20%, and improved the tensile strength, yield stress, and elongation by 16%, 16%, and 40%, respectively. The results of wear test indicated that, at the load of 10 N, abrasive wear was the dominant wear mechanism. Increase in load showed delamination, and at the load of 30 N, a transition from mild wear to severe wear occurred due to a change in wear mechanism to adhesive wear. At this load, the samples without heat treatment showed the lowest wear rate in comparison to the samples subjected to heat treatment. By and large, the specimens heat treated with directional quenching among the other heat-treated specimens showed the higher wear resistance at all of loads.

The design and absorption analysis of a unit cell of a new multiband split-S-shaped metamaterial is presented in this paper. The computer simulation technology (CST) software based on finite-difference time-domain method was used for the design of the unit cell and its S-parameter calculations. The proposed design shows the resonance frequency within the S-band, X-band, and K u -band of the microwave spectra. In addition, the proposed material can be used in ε -negative, μ -negative, near-zero refractive index, and double-negative applications as well. The measured result is presented, which shows good conformity with the simulated result. The material shows nearly the same characteristics with bit shifted transmittance at the higher frequency side after reducing the coupling capacitance in the y - or z -axis of the proposed metamaterial. Moreover, it is evident from the investigation that, for shifting the lower ring in the z -axis, 15% more absorption can be achieved for the proposed metamaterial. The simple design, multipurpose applications, and compact size have made the design novel in the electromagnetic paradigm.

In this study, the effect powder size and matrix temperature on the physical and mechanical properties of SiO 2 -reinforced Al 7075 matrix composites were investigated. It was observed that with increasing powder size and temperature, infiltration height was increased. Optimum parameters of full infiltration that were determined for particle size and temperature were d 50 =150 μm and 800°C, respectively. It was also observed that the porosity of the produced composites changed in the range of 3.2–14.6%, and the lowest porosity was obtained from the composite having 105 μm SiO 2 particle size. The highest fracture strength (263 MPa) was obtained from the composite produced at a matrix temperature of 800°C and a particle size of 420 μm. It was concluded that particle size and temperature are effective parameters to reach full infiltration, and this method is more suitable for producing the composites that have high reinforcement volume fractions than conventional casting methods.

Nanocapsules using epoxy and urea formaldehyde as core and shell materials, respectively, were prepared by in situ polymerization. The effects of processing conditions on the properties of epoxy nanocapsules were systematically investigated based on w (core) and average diameter of nanocapsules through the method of orthographic factorial design, and the optimum processing conditions were concluded. The results indicated that the key influencing factors on w (core) was agitation rate; on average, diameter of nanocapsules was emulsifier. The analysis of mechanical properties and thermal stability indicated that nanocapsules prepared in the optimum conditions are suitable for storage and the optimum content of nanocapsules was 10%. Scanning electron microscopy indicated that nanocapsules were well encapsuled and presented uniform spheres with rough surface. The broken nanocapsule indicated that the shell of the nanocapsule was thin and could coat more epoxy resin. The analysis of finite element method proved that nanocapsules prepared in the optimum conditions with an average of 110 nm shell thickness were suitable for self-healing materials.