Volume 24, Issue 3

In recent years, guided wave propagation in piezoelectric-piezomagnetic composite (PPC) structures are paid much attention for the design and optimization of PPC transducers. Previous investigations are mainly limited in horizontally infinite flat plates, axially infinite hollow cylinders, and so on. They are all one-dimensional model structures, i.e. structures having variable displacement field in only one direction, and the other two displacement fields are both constant. In this paper, a double orthogonal polynomial series approach is proposed to solve the guided wave in two-dimensional model structures, namely, a functionally graded piezoelectric-piezomagnetic (FGPP) bar with a rectangular cross section. The validity of the double polynomial approach is illustrated by the comparison with the available reference results for a pure elastic homogeneous rectangular bar. The guided wave characteristics, including dispersion curves and mechanical displacement distributions, are discussed by calculating various FGPP rectangular bars.

A poly(vinylidene fluoride) (PVDF) nanocomposite incorporated with polydopamine-coated reduced graphene oxide (rGO@PDOPA) nanosheets has been developed with high dielectric permittivity and low loss for electricity storage applications. Structural analysis indicates that PDOPA was successfully anchored onto rGOs through non-covalent linkage under mild conditions. The presence of PDOPA layer between rGO and PVDF can not only prevent the agglomeration and direct contact of original rGOs but also enhance the interaction between PVDF and rGO and microcapacitor formation. Compared to rGO/PVDF composites, higher dielectric permittivity and lower loss factor were achieved simultaneously in rGO@PDOPA/PVDF nanocomposites at low and moderate frequencies. The PDOPA interface layers are effective in modifying the dielectric characteristics of the composites to increase the dielectric permittivity without the introduction of loss mechanisms. This study demonstrates that PDOPA is an ideal interface layer for the development of new percolative dielectric composites with high dielectric permittivity and low loss.

This paper presents the design and analysis of a novel biaxial double-negative (DNG) metamaterial for electromagnetic cloaking operation in the microwave range. The proposed metamaterial exhibits DNG characteristics for the three axes ( x , y , and z axes) wave propagation through the material. For the z -axis wave propagation, it shows resonance in the X-band and shows DNG characteristics there. Similarly, for the x -axis wave propagation, the material displays resonances in the multi-band (S-, C-, and X-bands) microwave frequency ranges with DNG characteristics at the S-, C-, and X-bands. The material exhibits DNG properties at the S- and C-bands for y -axis wave propagation as well. In the basic design, a new metamaterial structure was developed that was split into two arms. The commercially available finite-difference time-domain (FDTD)-based computer simulation technology (CST) Microwave Studio software was adopted to obtain the reflection and transmission parameters of the unit cell. The metamaterial was then used in designing a rectangular electromagnetic cloaking device where a metal cylinder was perfectly cloaked in the C-band region of the microwave spectra. The metamaterial shows near-zero refractive-index property as well in the cloaked zone. This is a novel and promising design in the electromagnetic paradigm for its biaxial DNG characteristics and rectangular cloaking operation.

In this study, copper-coated multi-walled carbon nanotubes (MWCNTs) were prepared using electroless deposition process with an efficient bath composition. As-received MWCNTs underwent acid treatment and subsequent sensitization with tin and activation with palladium before the copper coating process. The electroless deposition process was performed in a bath, which contained copper sulphate pentahydrate as a copper precursor, potassium sodium tartrate as a complexing agent, and formaldehyde as a reducing agent. Surface modifications of MWCNTs were studied by Fourier transform infrared spectroscopy, imaging techniques (scanning electron microscopy, transmission electron microscopy), and X-ray diffraction. On the basis of the results, it was observed that the coating layer covers most surfaces of the nanotubes. Furthermore, copper oxide is also deposited on the surfaces of MWCNTs. The total amount of metals after each sensitization, activation, and metallization step was determined by thermo gravimetric method and induced coupled plasma atomic emission spectroscopy. Coating of MWCNTs was accomplished successfully through adjusting proper chemical ratios and concentrations. The calculated bath efficiency was about 99%.

In this study, Al2014 matrix composites reinforced with nano-SiC particles were successfully fabricated by semi-solid stirring combined with hot extrusion. The obtained composites exhibited refined α-Al grains, reasonable nano-SiC particles distribution, good interface bonding, and high dislocation density. The addition of 0.5 vol.% nano-sized SiC particles significantly improved the tensile strengths of the Al2014 alloy without sacrificing the ductility. Compared with the Al2014 alloy, the yield strength, ultimate tensile strength, and fracture strain of 0.5 vol.% nano-SiC p /Al2014 composites increased from 242 MPa, 460 MPa, and 17.1% to 306 MPa, 532 MPa, and 17.7%, respectively.

A microstrip line-fed monopole antenna is proposed for super wideband (SWB) applications printed on an epoxy-resin-reinforced woven-glass material. The reported SWB antenna has been made of a rectangular partial ground plane with an L-type slit and a heart-shaped radiating patch. This antenna is connected precisely by a tapered feed line that provides SWB greater than ultra-wideband. The heart-shaped radiating patch and the partial ground plane containing a gap and an L-type slit on ground plane also play paramount roles to procure wide impedance bandwidth. It is determined from measurements that this antenna contains SWB characteristic [voltage standing wave ratio (VSWR) ≤2] spanning from 1.30 to 40 GHz (187.41%), with a bandwidth dimension ratio (BDR) of 5544.66 and a ratio bandwidth of 30.77:1. Simple construction, sharp surface current flow, much impedance bandwidth, nearly omnidirectional radiation patterns, stable peak gain (2.20–6.06 dBi), time domain performance, and considerable BDR (5544.66) make it a promising candidate for SWB applications.

In this paper, the effect of casting speed on fabricating Al-1%Mn and Al-10%Si alloy clad slab was investigated. Before the trials with the semi-continuous casting process were conducted, a series of simulation with the engineering software FLUENT was made to forecast the influence of the casting speed on the temperature field and the liquid fraction of the clad slab. The simulation results indicated that the increase of the casting speed reduces the time in the cooling zone, thus the thickness of the Al-1%Mn solidification shell under the dividing plate gets thinner. Based on the simulation results, the Al-1%Mn alloy and Al-10%Si alloy clad slab was successfully produced by semi-continuous casting process. The interface of clad ingots was investigated by methods of metallographic and electron probe microanalysis. The tests on the interface confirmed the simulation results and showed that a clad slab of two different aluminum alloys with excellent metallurgical bonding was achieved by semi-continuous casting. According to the result of the tensile tests, the strength of the specimens remains at 110 MPa, and the fracture position located in the Al-1%Mn alloy indicates that the strength of the interface is higher than that of the Al-1%Mn alloy.

Sepiolite and hollow glass microspheres (HGMs) are used as binary fillers to prepare thermal insulating composites to improve the protecting durability of epoxy composites on Mg alloy. Sepiolite can be evenly dispersed inside epoxy at around 10% (vol.%), while HGMs are also found evenly distributed in this matrix with content up to 45% (vol.%), as shown in the scanning electron microscope pictures. Their thermal stability and thermal insulation ability improve simultaneously with the increase of the inorganic filler, as shown in the thermal gravimetric analysis results and thermal insulting tests. According to the three-dimensional finite element analysis using ANSYS software (ANSYS Inc., USA), their thermal conductivity coefficients are found to decrease linearly with the increase in HGMs volume fractions. The tested temperature changes on the opposite side of the Mg alloy coated with sepiolite/HGMs/epoxy composites match well with those on the stimulant curves, indicating the reliability of the models and the thermal insulating functions provided by the fillers.

In the following study, the dynamic behaviour of polymer laminates reinforced by glass and carbon fibres with different orientations was considered for several excitation frequencies and elevated temperatures. The obtained dynamic moduli, loss factors and glass-transition temperatures were used for the evaluation of damping capabilities and the self-heating phenomenon. In order to evaluate the influence of a fibre type, additional studies for a pure matrix were carried out and used in the analysis as the reference. The obtained results show significant differences both in thermal and dynamic mechanical properties with respect to a fibre type and its orientation, which has a direct influence on the analysed self-heating and damping phenomena. The results of the present study could be used for the design of composite properties with respect to thermal degradation resulting from coupled thermomechanical loading conditions. Several hypotheses on the influence of a heating rate on estimation of the thermomechanical properties as well as structural degradation of composites in such conditions were presented with appropriate discussion and argumentation. The present study has a preliminary character. Furthermore, it is planned to perform such tests for specimens with a low cross-linking degree in order to analyse its influence on the resulting thermomechanical response of fibre-reinforced composite structures.

In this study, the dynamic stability of an embedded viscoelastic composite cylindrical shell reinforced by boron nitride nanotubes (BNNTs) is investigated. The composite cylindrical shell is coated by a viscoelastic piezomagnetic layer and subjected to combined magneto-electro-mechanical loads. The composite polymer matrix and the coating layer are made of polyvinylidene fluoride (PVDF) and iron oxide (CoFe 2 O 4 ), respectively. The composite cylindrical shell conveys pulsating fluid flows, which results in harmonic oscillations. The equivalent characteristics of composite are determined using micro-electro-mechanical models. Considering the magneto-electro-mechanical coupling, motion equations are obtained using Hamilton’s principle. Results show the influences of fluid velocity, geometrical parameters of shell, viscoelastic foundation, orientation angle and percentage of BNNTs on the resonance frequency and stability of a PVDF-coated nanocomposite shell. The result of this study may be used for the design of rotating machines, hydraulic systems and motors.

The buckling and layer failure characteristics of composite laminated cylinders subjected to hydrostatic pressure were investigated through finite element analysis for underwater vehicle application. The Tsai-Wu failure criteria were used as the failure criteria for the buckling analysis. A sensitivity analysis was conducted to research the influence of the number of elements on the critical buckling pressure. ANSYS, a finite element program, successfully predicted the buckling pressure with 5.3–27.8% (linear) and 0.3–22.5% (nonlinear) deviation from experimental results. The analysis results showed that the cylinders can carry more pressure after a slight decrease in pressure and recovery of the supporting load. For layer failure analysis, it was found that the failure that occurred in the 0° layer was more serious than that in the 90° layer within the neighboring layers at the inner layers (nos. 1–7) and outer layers (nos. 8–24).

A simple method of one-step pulse plating was used in the fabrication of core-shell SiO 2 /Ag composite spheres. Structural characteristics and morphologies of the prepared SiO 2 /Ag composite spheres are characterized by means of X-ray diffraction, scanning electron microscope, and transmission electron microscopy. The Ag shell is uniformly coated on the surface of SiO 2 spheres with the thickness of about 20 nm. Photoluminescence (PL) spectrum has revealed that PL of the core-shell samples is much stronger than that of bare SiO 2 spheres. Raman spectrometer measurements show that the SiO 2 /Ag composite spheres have excellent surface-enhanced Raman scattering performance. In addition, the current-voltage characteristic of SiO 2 /Ag composite spheres has improved at the same time.

In this paper, a longitudinal compression experiment of composites was conducted and the macroscopic failure mode was obtained. Also, the microscopic failure morphologies of longitudinal compression and kink band were observed by using scanning electron microscopy. It can be seen that, under compression, fibers bend and form a kink band, which is the most typical failure mode. Then a micromechanical model of fiber random distribution based on the random collision algorithm, which can reveal the progressive failure mechanism of longitudinal compression considering the kink-band deformation, was established, with two dominant damage mechanisms – plastic deformation and ductile damage initiation of the polymer matrix and interfacial debonding included in the simulation by the extended Drucker-Prager model and cohesive zone model, respectively. Through numerical simulation, the loading and failure procedures were divided into three stages: elastic domain, softening domain and fiber failure domain. It can be concluded that the kink band was a result of fiber instability (micro-bulking), which is caused by the elastic bending of fibers. The fibers rotate and break into two places, forming a kink band. Then the fibers rotate further until the matrix between the fibers fails and the kink-band breaks and, hence, the composite loses its load-bearing capability.

A three-dimensional heat transfer model for friction stir welding is presented in this paper. A moving coordinate was introduced to reduce the difficulty of modeling the moving tool. Heat input from the tool shoulder and the tool pin were considered in the model. The plastic deformation heat was introduced into the model, too. It is clear that the heat production increased owing to plastic deformation, and this process depends on the stress level. Temperature measurement experiments were done to validate the calculated results. The calculated results were in good agreement with the experimental results. Preheating the workpiece is beneficial to obtain a good weld seam.

Based on random crack core theory, a model for predicting the longitudinal tensile strength of three-dimensional (3D) four-directional composites with low braiding angle is established. The model carries out accurate theoretical predictions of the longitudinal tensile strength of 3D four-directional braided carbon fiber/resin composites. The average stiffness method is used to calculate elastic constants of an inner single cell of 3D four-directional braided composites. Meanwhile, the corresponding relationship between failure probability of a unidirectional composite fiber bundle and stress level is given based on the random crack core model of the longitudinal tensile strength of a unidirectional composite. Furthermore, strength algorithms of low braiding angle 3D four-directional composites under different damage modes are built on the basis of the Tsai-Hill criterion. In this paper, the dispersion of single fiber strength is also considered in the model, so the size effect of the composite strength can be reflected effectively. At last, the longitudinal tensile strength of 3D four-directional braided carbon fiber/resin composites is predicted and analyzed, and the result shows that this model has high prediction accuracy.

This article investigates the nonlinear stability of eccentrically stiffened moderately thick plates made of functionally graded materials (FGM) subjected to in-plane compressive, thermo-mechanical loads. The equilibrium and compatibility equations for the moderately thick plates are derived by using the first-order shear deformation theory of plates, taking into account both the geometrical nonlinearity in the von Karman sense and initial geometrical imperfections, temperature-dependent properties with Pasternak type elastic foundations. By applying the Galerkin method and using a stress function, the effects of material and geometrical properties, temperature-dependent material properties, elastic foundations, boundary conditions, and eccentric stiffeners on the buckling and post-buckling loading capacity of the eccentrically stiffened moderately thick FGM plates in thermal environments are analyzed and discussed.

Novel variable configuration spacecraft (VCS) can change its configuration by on-orbit assembly or the motion of the mechanism according to the requirements and missions, and the multibody dynamics analysis is performed to obtain the corresponding theoretical lowest values of the mechanical properties of force-bearing structures for the VCS module. In the meantime, to meet the actual use requirements for VCS, novel force-bearing structures are designed optimally and fabricated successfully. The corresponding experimental values of the mechanical properties for the force-bearing structures are also tested and investigated, which are compared to the corresponding theoretical values via multibody dynamics analysis. This paper is part of a large project on the development and preparation of the novel force-bearing structures applied in the next generation of VCS.