有砟梯式轨枕轨道减振性能的理论分析与试验研究

为实验室铺设有砟、无砟两种梯式轨枕轨道,用自由落锤激励装置对两种梯轨减振性能进行对比研究。建立有砟梯轨、有砟普通轨道的数值模型,利用试验测试结果对模型进行验证。通过在模型中施加货运列车荷载对比两种有砟轨道减振性能。结果表明,有砟梯轨减振性能不及浮置式梯轨,在低频段有一定减振能力,20 Hz以下振动经道砟平均衰减25 dB,对控制重载货运列车环境振动具备潜在优势;梯式轨枕对分散列车冲击振动、降低时域内峰值有明显优势。     

Microwave absorption response of nickel/graphene nanocomposites prepared by electrodeposition

In this study, nanostructures of nickel have been successfully deposited on graphene nanosheet by direct electrochemical deposition. The morphology, nickel content, and magnetic properties of the graphene as well as composites were examined by scanning electron microscopy, transmission electron microscopic, elemental analysis, and vibrating sample magnetometer, respectively. Their relative complex permeability and permittivity were also measured, and reflection loss values were calculated at given thickness layer according to transmit line theory in the range 2–18 GHz. The results reveal that with the increasing of the thickness of the samples, the matching frequency tends to shift to the lower frequency region, and theoretical reflection loss becomes less at the matching frequency. When the absorbing thickness is 1 mm, the maximum absorption value of graphene is ?6.5 dB at about 7 GHz. After decorating graphene sheet with magnetic nickel nanoparticles, the composites were shown to efficiently promote microwave absorbability. When the thickness is 1.5 mm, the absorption value of the composites exceeds ?10 dB in the 5 GHz absorbing bandwidth and the maximum absorption value is ?16.0 dB at 9.15 GHz.     

Effect of carbonization temperature on microwave absorbing properties of polyacrylonitrile-based carbon fibers

The pre-oxidized fibers were carbonized at the temperature ranging from 400 to 1300 °C for 1 h. The microwave absorption properties of carbon fibers (CFs) were examined in the frequency range of 2–18 GHz. It is found that the reflection loss characteristics are highly sensitive to the carbonization temperature. At a thickness of 2 mm, the CFs obtained at 710 °C exhibit the best microwave absorbing ability with a maximum reflection loss of ?22.9 dB at 15 GHz, and a bandwidth exceeding ?10 dB in the range 12.4–18 GHz. Results indicate that dielectric loss in cooperation with better matched characteristic impedance results in the excellent microwave absorption of CFs. Low temperature makes ?′ and ?″ too small to consume the energy of microwave, while over high temperature makes ?′ and ?″ too large to transmit the microwave into the CFs.     

PEPT Investigation of Particle Separation in a Novel Vertically Vibrated Particle Separator

Separation of particle mixtures in micron size range is of concern to many industrial processes. The experimental work presented in this article has looked into the operational assessment of a novel vertical vibration driven particle separator by using a positron emission particle tracking (PEPT) technique. In addition to PEPT, a smoke blanket visualization technique was used to track the intestinal air movements during the course of vibration induced particle separation. Three different finely sized glass and bronze particle mixtures that formed an average particle bed heights of 20, 40, 60, 80, and 100 mm in separation cells of thickness 20 and 40 mm were used to investigate the particle separation behavior. For a range of operating conditions, the results showed in favor of low particle bed heights (below 40 mm), lower vibration frequency (30 ± 10% Hz) and a small partition separation gap size of 5 mm for optimum separation of bronze particles in the presence of air.     

Microwave-absorbing properties of silver nanoparticle/carbon nanotube hybrid nanocomposites

Silver (Ag) nanoparticles fabricated by chemical reduction process were grafted onto the surface of carbon nanotubes (CNTs) to prepare hybrid nanocomposites. The Ag/CNT hybrid nanomaterials were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The Ag/CNT hybrid nanomaterials were then loaded in paraffin wax, and pressed into toroidal shape with thickness of 1 mm to evaluate their complex permittivity and complex permeability by scattering parameters measurement method in reflection mode using vector network analyzer. The reflection loss of the samples was calculated according to the transmission line theory using their measured complex permittivity and permeability. The minimum reflection loss of the Ag/CNT hybrid nanocomposite sample with a thickness of 1 mm reached 21.9 dB (over 99 % absorption) at 12.9 GHz, and also exhibited a wide response bandwidth where the frequency bandwidth of the reflection loss of less than ?10 dB (over 90 % absorption) was from 11.7 to 14.0 GHz. The Ag/CNT hybrid nanocomposite with thickness of 6 mm showed a minimum reflection loss of ~?32.1 dB (over 99.9 % absorption) at 3.0 GHz and was the best absorber when compared with the other samples of different thickness. The reflection loss shifted to lower frequency as the thickness of the samples increased. The capability to modulate the absorption band of these samples to suit various applications in different frequency bands simply by manipulating their thickness indicates that these hybrid nanocomposites could be a promising microwave absorber.     

Effect of reinforcement type on high velocity impact response of GRP plates using a sharp tip projectile

High velocity impact performance of glass reinforced polyester (GRP) resin composite plates with different type of reinforcement was investigated. The projectile used was a sharp tipped (30°) conical head with total length of 30 mm and shank length of 15 mm with weight of 9.74 g. Five different types of E-glass fiber reinforcement were used, including chopped strand mat (CSM), plain weave, satin weave, unidirectional and cross-ply unidirectional fiber reinforcements. A smooth barrel gas gun was used to conduct high velocity impact tests in the velocity range of 80–160 m/s. Composite plates with size of 15 cm × 15 cm were prepared in 3 and 6 mm thickness. Results showed higher ballistic limit velocity (velocity at which samples fully penetrated the target plates with zero residual velocity) for 3 mm GRP plates with cross-ply unidirectional reinforcement followed by unidirectional reinforcement and plain weave, the plates with satin weave and CSM reinforcements were almost in same level. The thicker specimens (6 mm), plates with plain weave reinforcement showed better ballistic performance towards sharp tipped conical projectile impact, followed by cross-ply unidirectional, satin weave, unidirectional and CSM reinforced plates. Experimentally determined ballistic limit velocity for all specimens correlate well with estimated ballistic limit values obtained in full perforation tests. Damage assessment conducted on all specimens indicated fiber tension and shear failure for thin-walled and sever delamination for the thick-walled specimens as the dominant failure modes.     

Synthesis and microwave absorbing properties of Mn–Zn nanoferrite produced by microwave assisted ball milling

In this paper, well dispersed spinel Mn_xZn_1?xFe₂O₄ (x = 0.3,0.5 and 0.7) were obtained by microwave assisted ball milling at 2.45 GHz through only one step. The synthesized products were characterized by X-ray diffraction, high resolution transmission electron microscope, vibration sample magnetometer, and vector network analysis. Synthesized Mn–Zn nanoferrite showed the saturation magnetization reached 84.91emu/g when the x was 0.7 and the largest magnetic loss tangent at the frequency of 2.45 GHz. Microwave absorbing properties of these composites were studied at the frequency range of 2–18 GHz. Two microwave reflection loss peaks appeared for all the spinel ferrite. When x was 0.5, the minimum reflection loss appeared at the highest frequency. When x was 0.7, these two minimum reflection loss peaks, ?17.36 and ?48.13 dB, were calculated with the ?10 dB bandwidth at the frequency ranges of 2.24–5.04 and 13.28–14.88 GHz, respectively. Resonance reflection loss peaks shifted to lower frequencies when the matching thickness increased.     

Enhancing absorption properties of Mg–Ti substituted barium hexaferrite nanocomposite through the addition of MWCNT

M-type barium ferrite with Mg–Ti substitution and MWCNT addition was synthesized using high-energy ball milling. The prepared sample was further analyzed using X-ray diffraction, field emission scanning electron microscope (FESEM), vibrating sample magnetometer and vector network analyzer. The results showed that the particle size had a wide range of distribution, and a hexagonal structure was formed in the sample. The sample was observed to have lower saturation magnetization and coercivity after Mg–Ti was substituted with MWCNT and added into the barium hexaferrite. Reflection loss was studied as a function of frequency and thickness of the sample. For Mg–Ti substituted barium hexaferrite composite with a thickness of 2.0 mm, the reflection loss peaked at ?28.83 dB at a frequency of 15.57 GHz with a bandwidth of 6.43 GHz at a loss of less than ?10 dB. The microwave absorption primarily resulted from magnetic losses caused by magnetization relaxation, domain wall resonance, and natural resonance. FESEM micrograph demonstrated that carbon nanotubes were attached to the external surface of the ferrite nanoparticles. The investigation of the microwave absorption indicated that with an addition of carbon nanotubes, the real and imaginary parts of permittivity and reflection loss had enhanced to ?34.16 dB at a frequency of 14.19 GHz with a bandwidth of 5.72 GHz.     

Dynamic material properties of wood subjected to low-velocity impact

This article studied the effects of low-velocity impact on the failure stresses and stiffness using a pendulum test. The specimens were of variable depth (20, 30, and 40 mm), a width of 50 mm, length of 650 mm, and span-length of 480 mm. The smallest specimen depth was similar to specimen sizes tested in the literature used to create the duration of load curve, while the largest specimen depth are considered structural size specimens. The impact was predicted using a numerical approach with Euler–Bernoulli beam, as well as Timoshenko beam theory, with a plastic contact law. The models were validated for impact from a low release-angle (where the beam remained elastic), but could use improvement for the force prediction at a high incidence velocity. The measured force signals were used as forcing functions to obtain the dynamic failure stresses for all of the evaluated specimens, and the Timoshenko–Goens–Hearmon Method to derive the dynamic E. The resulting strain rates ranged from 9.11?×?10^?5 s^?1 for the quasi-static specimens up to 25 s^?1 for the greatest incidence velocity. The results from this study suggest different duration of load factors than the Madison Curve, influencing the design of structures subjected to dynamic loading.     

FBG sensor array-based-low speed impact localization system on composite plate

A fiber Bragg grating (FBG) sensors-based impact localization system on composite structure and a novel localization algorithm independent of wave velocity were proposed. Six FBG sensors constitute two isosceles right triangle FBG arrays. Impact signals were detected by a high-speed FBG interrogation system. Morlet wavelet transform was employed to extract time differences of impact signals. The straight lines equations, which are through impact source and FBG sensors of right-angled vertices of FBG arrays, can be obtained by the time differences. The coordinate of impact source is the intersection of straight lines. Testing experiments were carried out on composite plate within 400 mm × 400 mm monitor area. The experimental results showed that the maximum and average errors were 20.92 and 8.67 mm, respectively. This article provides a simple and stable impact source localization system independent of wave velocity.     

Low-Velocity Impact Response and Finite Element Analysis of Four-Step 3-D Braided Composites

The low-velocity impact characters of 3-D braided carbon/epoxy composites were investigated from experimental and finite element simulation approaches. The quasi-static tests were carried out at a constant velocity of 2 mm/min on MTS 810.23 material tester system to obtain the indentation load–displacement curves and indentation damages. The low-velocity tests were conducted at the velocities from 1 m/s to 6 m/s (corresponding to the impact energy from 3.22 J to 116 J) on Instron Dynatup 9250 impact tester. The peak force, energy for peak force, time to peak force, and total energy absorption were obtained to determine the impact responses of 3-D braided composites. A unit cell model was established according to the microstructure of 3-D braided composites to derive the constitutive equation. Based on the model, a user-defined material subroutine (VUMAT) has been compiled by FORTRAN and connected with commercial finite element code ABAQUS/Explicit to calculate the impact damage. The unit cell model successfully predicted the impact response of 3-D braided composites. Furthermore, the stress wave propagation and failure mechanisms have been revealed from the finite element simulation results and ultimate damage morphologies of specimens.     

Microwave absorption properties of FSS-impacted composites as a broadband microwave absorber

The development of a cost-effective microwave absorber with wide bandwidth corresponding to reflection loss (RL)?≤??10 dB is still a very challenging task. A sugarcane bagasse-based agricultural waste composite has been analyzed for its elemental contents. The combination of elements is suitable for its possible usage as a cost-effective microwave absorbing material. Therefore, this composite has been subjected to morphological and electromagnetic studies to analyze its microwave absorbing behavior. The frequency dependent complex dielectric permittivity and complex magnetic permeability values were obtained using a transmission/reflection waveguide approach in the X-band. Furthermore, the effect of the Minkowski loop frequency selective surface (FSS) was studied over the absorption capability of the composite. It was found that the application of FSS leads to a reduction in thickness up to 2.9 mm and an enhancement in absorption bandwidth up to 3.6 GHz. The FSS patterned composite shows a remarkable performance with peak RL of ?28.4 dB at 10.7 GHz and absorption bandwidth of 3.6 GHz.     

Low Temperature Solution Synthesis and Microwave Absorption Properties of Multiwalled Carbon Nanotubes/Fe3O4 Composites

Multiwalled carbon nanotubes (MWCNTs)/Fe₃O₄ nanocomposites were synthesized via a simple low temperature solution method. The phase structures and morphologies of the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the Fe₃O₄ spheres of about 150 nm were linked with MWCNTs. The microwave absorption properties of the MWCNTs/Fe₃O₄ nanocomposites were measured by vector network analysis (VNA). A wide region of microwave absorption was achieved due to dual magnetic and dielectric losses. When the matching thickness is 2 mm, the reflection loss (RL) of the sample exceeding ?10 dB was obtained at the frequency range of 9.9–12.4 GHz, with an optimal RL of ?29.8 dB at 11.04 GHz. A possible mechanism of the improved microwave absorption properties of the composites was discussed.     

Fabrication and microwave absorption performances of hollow-structure Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/PANI microspheres

The hollow polyaniline (PANI) microspheres were prepared by controlling the mass ratio of the aniline to polystyrene (PS) via a template method, and Fe₃O₄/PANI composite microspheres have been fabricated by blending the hollow PANI microspheres with Fe₃O₄ magnetic particles. The effects of the mass ratio of aniline/PS on the microwave absorption performances of Fe₃O₄/PANI microspheres were investigated. It was found that the value of minimum reflection loss (RL_min) of the microspheres were respectively ?14.06, ?22.34 and ?24.3 dB, corresponding to the mass ratio of aniline/PS of 1:1.5, 1:3, and 1:6. In addition, when the mass ratio of aniline/PS was 1:6, with the thickness of 1.5 and 2.0 mm, the bandwidth below ?10dB were respectively 2.48 GHz (15.52–18 GHz) and 4.64 GHz (11.04–15.68 GHz), indicating that the Fe₃O₄/PANI microspheres could be a potential electromagnetic wave absorbing material in X (8–12 GHz) and Ku (12–18 GHz) bands.     

Hierarchical structures based on Li<Subscript>0.35</Subscript>Zn<Subscript>0.3</Subscript>Fe<Subscript>2.35</Subscript>O<Subscript>4</Subscript>/polyaniline nanocomposites: synthesis and excellent microwave absorption properties

For the first time, the hierarchical structures of Li_0.35Zn_0.3Fe_2.35O₄(LZFO)/polyaniline nanocomposites were successfully synthesized by interfacial polymerization. Firstly, the LZFO particles were prepared by the sol–gel method, and subsequently the PANI nanorods, composed of nanoneedle-like PANI, were grafted on the surface of the LZFO. A novel microtopography, urchin-like, of LZFO/PANI was prepared by a simple, efficient and controllable two-step method. The crystal structure, chemical bonding states and morphology of samples were characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Scanning and Transmission electron microscopy (SEM/TEM). The bandwidth of reflection loss exceeds 10 dB in the frequency was 5.56 GHz (3.36–8.48, 10.32–10.76 GHz), and the maximum reflection loss can reach ??49.4 dB at 4.96 GHz with the thickness of 5.1 mm. The enhanced microwave absorption properties of LZFO/PANI nanocomposites are mainly ascribed to the multi-level structure and the improved impedance matching, and make it a potential candidate for microwave absorption materials.     

Influence of Ho substitution on structural, magnetic and microwave absorption properties of PbM-type hexaferrites nanoparticles

Ho substituted lead hexaferrites, PbCo_0.5Sn_0.5Ho _x Fe_11?x O₁₉ (x = 0.00–0.75) were successfully prepared by a citrate combustion process. Saturation magnetization and coercivity were measured from MH-loops taken on vibrating sample magnetometer. X-ray analysis reveals that in all samples M-type structure exist with few secondary phases. The results of X-ray diffraction analysis show that the particle size is inversely related to the dopant content. Microwave absorption properties of the nanocrystalline lead ferrite (PbCo_0.5Sn_0.5Ho _x Fe_11?x O₁₉) for single-layer investigated in a frequency range of 2–18 GHz. Results indicated that the substituted Ho can greatly improve the microwave absorption properties of PbCo_0.5Sn_0.5Fe₁₁O₁₉ hexaferrites. For sample with x = 0.75, a minimum reflection loss of ?33 dB was obtained at 16.5 GHz for a layer of 1.8 mm in thickness. The microwave absorption performance of PbCo_0.5Sn_0.5Ho_0.5Fe_10.5O₁₉ ferrite gets significant improvement: The bandwidth below ?15 dB expands from 12 to 16.5 GHz, and the peak value of reflection loss decreases from ?18 to ?37 dB. The minimum reflection loss reaches ?34.2 dB when the substitution amount of the Ho³⁺ ions is 0.25. Also results show that the peak positions of the minimum reflection loss shift toward higher frequency band.     

Prediction of displacement induced by tilting trains running on ballasted tracks through measurement of track impact factors

In the track design and behavior prediction, the variable contribution of wheel load was evaluated by considering the design load for the tracks. Further, the track impact factor was used to calculate the dynamic wheel load as a single value, which was chosen according to the rail type (i.e., continuous welded rail or joint rail) and the design speed but did not take into consideration the track conditions (i.e., the ballast condition good or bad), train type (i.e., tilting train and EMU), and track components (i.e., sleeper type and fastening type). In this study, the measured track impact factor was applied to the time history function of the FE analysis in order to predict the displacement of ballasted tracks under real conditions, which included curved and the deteriorated tracks, thus increasing the train speed by approximately 20–30% of the existing train speed. Therefore, the dynamic wheel load and the rail and sleeper displacement were measured for two different trains running on four conventional curved track sections with two different sleeper types. The track impact factor was estimated from the measured dynamic wheel load, and the empirical dynamic wheel load was calculated using the measured track impact factor at each site. The measured track impact factors were used for simulating the dynamic wheel loads applied on the derived time history function for fast trains. A finite element analysis model using the derived time history function based on the empirical dynamic wheel load was used to predict the train-induced track displacement. The numerical simulations and field test results were compared with German and Japanese regulations for train-induced track displacement, and the speedup effect of a tilting train was compared with that of a general train (EMU).The empirical track impact factors were 17–18% higher in the R400 sections than in the R600 sections. As the track curvature decreased, the impact on the track increased. Therefore, the empirical track impact factors were 21–23% higher in the WT sections than in the PCT sections. At 70 km/h, the impact on the track from a tilting train was 7–11% less than that from EMU. Although the tilting train sped up by 30% (90 km/h), its response level was similar to or less than that of the EMU. The analytical results reproduced the experimental results well within about 2–5% difference in the values. Therefore, the derived time history function based on the measured track impact factors is considered to provide sufficiently reliable FEA results in the investigation of the behavior of ballasted tracks. The difference between the maximum displacements for both train types on all the sections was about 15–20%.The analytical results show that the speed limits of the tilting train were higher than those of the EMU by approximately 8–23 km/h at each test site. Therefore, the speed limit of each test site was estimated by considering the limit of sleeper displacement. It was shown that the time history function derived using the measured track impact factor on a small track curvature with wooden sleepers was higher than that in other test sections. Therefore, it would be advantageous to increase the weight of sleepers on existing lines to increase the train speed through the speedup effect without increasing the track curvature. The increase in the speed of a tilting train with a small track curvature was much better than that by a track curvature increased by approximately 10%.     

Electromagnetic Behaviors of Carbon Coated Permalloy Nanorods at GHz Frequencies

The carbon coated Permalloy (CPy) nanorods array films with a thickness of 0.5 and 1.5 μm were grown in anodic aluminum oxide porous templates with a diameter of about 15 nm. To verify the electromagnetic characteristics of CPy nanorods array in the GHz frequency region, the CPy nanorods were measured by the coplanar waveguide (CPW) method up to 10 GHz. The signal attenuations of the CPy nanorods exhibited about 1 dB/cm at 1 GHz (in pass-band frequency) and about 17–18 dB/cm at 10 GHz (in stop-band frequency), respectively. The wavelengths of the CPy nanorods were shortened about 50 % at 1 GHz in comparison with that of the CPW.     

地铁隧道内不同轨道结构振动测试与分析

通过对地铁隧道内普通整体道床、Ⅲ型轨道减振器、弹性短轨枕、梯形轨枕、钢弹簧浮置板道床的现场振动测试,进行时、频域对比,了解各种减振措施在不同频率范围内的减振效果差异。结果表明,轨道减振器、梯形轨枕、弹性短轨枕及钢弹簧浮置板可分别降低隧道壁VLZmax分别为4 dB,7.6 dB,7.8 dB,19.0 dB;无论何种轨道减振措施,高频减振效果高于低频减振效果, Z计权的振动加速度级明显小于不计权的振动加速度级减振效果;梯形轨枕、弹性短轨枕、轨道减振器对50 Hz以上振动减振效果明显,钢弹簧浮置板道床对12.5 Hz以上振动减振效果明显,对控制列车运行产生的二次噪声更有效。     

Enhanced electromagnetic wave absorption properties of polyaniline-coated Fe3O4/reduced graphene oxide nanocomposites

Fe₃O₄-reduced graphene oxide-polyaniline (Fe₃O₄–RGO–PANI) ternary electromagnetic wave absorbing materials were prepared by in situ polymerization of aniline monomer on the surface of Fe₃O₄–RGO nanocomposites. The morphology, structure and other physical properties of the nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, vibration sample magnetism, etc. The electromagnetic wave absorbing properties of composite materials were measured by using a vector network analyzer. The PANI–Fe₃O₄–RGO nanocomposites demonstrated that the maximum reflection loss was ?36.5 dB at 7.4 GHz with a thickness of 4.5 mm and the absorption bandwidth with the reflection loss below ?10 dB was up to 12.0 GHz with a thickness in the range of 2.5–5.0 mm, suggesting that the microwave absorption properties and the absorption bandwidth were greatly enhanced by coating with polyaniline (PANI). The strong absorption characteristics of PANI–Fe₃O₄–RGO ternary composites indicated their potential application as the electromagnetic wave absorbing material.