Experimental study and numerical simulation of the impact of under-sleeper pads on the dynamic and static mechanical behavior of heavy-haul railway ballast track

Laying the under-sleeper pad(USP)is one of the effective measures commonly used to delay ballast degradation and reduce maintenance workload.To explore the impact of application of the USP on the dynamic and static mechanical behavior of the ballast track in the heavy-haul railway system,numerical simulation models of the ballast bed with USP and without USP are presented in this paper by using the discrete element method(DEM)-multi-flexible body dynamic(MFBD)coupling analysis method.The ballast bed support stiffness test and dynamic displacement tests were carried out on the actual operation of a heavy-haul railway line to verify the validity of the models.The results show that using the USP results in a 43.01%reduction in the ballast bed support stiffness and achieves a more uniform distribution of track loads on the sleepers.It effectively reduces the load borne by the sleeper directly under the wheel load,with a 7.89%reduction in the pressure on the sleeper.Furthermore,the laying of the USP changes the lateral resistance sharing ratio of the ballast bed,significantly reducing the stress level of the ballast bed under train loads,with an average stress reduction of 42.19 kPa.It also reduces the plastic displacement of ballast particles and lowers the peak value of rotational angular velocity by about 50%to 70%,which is conducive to slowing down ballast bed settlement deformation and reducing maintenance costs.In summary,laying the USP has a potential value in enhancing the stability and extending the lifespan of the ballast bed in heavy-haul railway systems.

The Effects of Stacking Sequence on Dynamic Mechanical Properties and Thermal Degradation of Kenaf/Jute Hybrid Composites

This research focused on the dynamic mechanical and thermal properties of woven mat jute/kenaf/jute(J/K/J)and kenaf/jute/kenaf(K/J/K)hybrid composites.Dynamic mechanical analysis(DMA)and Thermo-gravimetric Analysis(TGA)were used to study the effect of layering sequence on the thermal properties of kenaf/jute hybrid composites.The DMA results;it was found that the differences in the stacking sequence between the kenaf/jute composites do not affect their storage modulus,loss modulus and damping factor.From the TGA and DMA results,it has been shown that stacking sequence has given positive effect to the kenaf/jute hybrid composite compared to pure epoxy composite.This is because kenaf and jute fibre has increased the Tg values of the composites,thus affect the thermal degradation.Results showed that the storage modulus for kenaf/jute hybrid composites increased compared with pure epoxy composites with increasing temperature and the values of remained almost the same at glass transition temperature(Tg),the hybrid composite perhaps due to the improved fibre/matrix interface bonding.The preliminary analysis could provide a new direction for the creation of a novel hybrid composite which offers unique properties which cannot be accomplished in a single material system.

Dynamic Mechanical and Thermal Properties of Cross-linked Polystyrene/Glass Fiber Composites

Cross-linked polystyrene/glass fiber composites were fabricated using cross-linked polystyrene （CLPS） as matrix and E-glass fiber as the reinforcement. Surfaces of E-glass fibers were modified by vinyl triethoxysilane （VTES）, vinyl trimethoxysilane （VTMS） and γ-methacryloylpropyl trimethoxysilane （MPS）. The treated glass fibers were analyzed by fourier transform infrared spectroscopy （FTIR）. Dynamic mechanical thermal analysis （DMTA） and thermo-gravimetric analysis （TGA） were employed to investigate the effect of glass fibers surface modification on viscoelastic behavior and thermal properties. The morphology of fracture surfaces of various composites was observed by scanning electron microscopy （SEM）. The results revealed that these coupling agents were connected to the surfaces of the fibers by chemical bonding. Dynamic mechanical properties as well as thermal stability of the composites were improved considerablely, but to varying degrees depending on the fiber modification. The diversities of improvement of properties were attributed to the different interfacial adhesion between CLPS matrix and the glass fibers.

Analysis and Design of Thermally Actuated Micro-Cantilevers for High Frequency Vibrations Using Finite Element Method

Vibrational behavior of thermally actuated cantilever micro-beams and their mechanical response at moderately high frequency under a non-harmonic periodic loading is studied in this paper. Two different configurations are considered: 1) a straight beam with two actuation layers on top and bottom which utilizes the bimorph effect to induce bending;2) a uniform beam with base excitation, where the beam is mounted on an actuator which moves it periodically at its base perpendicular to its axis. Generally, vibrating micro-cantilevers are required to oscillate at a specified frequency. In order to increase the efficiency of the system, and achieve deflections with low power consumption, geometrical features of the beams can be quantified so that the required vibrating frequency matches the natural frequencies of the beam. A parametric modal analysis is conducted on two configurations of micro-cantilever and the first natural frequency of the cantilevers as a function of geometrical parameters is extracted. To evaluate vibrational behavior and thermo-mechanical efficiency of micro-cantilevers as a function of their geometrical parameters and input power, a case study with a specified vibrating frequency is considered. Due to significant complexities in the loading conditions and thermo-mechanical behavior, this task can only be tackled via numerical methods. Selecting the geometrical parameters in order to induce resonance at the nominal frequency, non-linear time-history (transient) thermo-mechanical finite element analysis (using ANSYS) is run on each configuration to study its response to the periodic heating input. Approaches to improve the effectiveness of actuators in each configuration based on their implementation are investigated.

磁场辐射下TNT晶体性能的变化特性

为了解强磁场辐射下三硝基甲苯(TNT)晶体性能的变化特性,借助扫描电子显微镜仪研究了磁场辐射(0,6 T)下TNT晶体形貌,利用X射线衍射仪和拉曼光谱分析了磁场辐射后TNT晶格的变化,采用差示扫描量热仪(DSC)获得了磁场辐射下TNT的热分解特性。基于分子动力学模拟,探讨了磁场辐射下(0,3,6 T和8 T)TNT的晶格常数、微观分布、力学性能和撞击感度变化情况。结果表明:TNT晶体受到6 T磁场辐射后,微观形貌由辐射前的鳞片针状结构转变为不规则的块状结构,热分解放热峰温度由289.6℃升高到304.1℃,但TNT的晶体相结构和晶格常数未发生明显改变。TNT晶格常数不受磁场辐射的影响,但磁场会改变TNT分子在晶体内部的分布。8 T磁场辐射能显著改善TNT晶体的延展性。通过比较最长引发键的占比,发现8 T磁场会增加TNT晶体的撞击感度。

单体中仲胺类化合物含量对长碳链尼龙1012性能的影响

以癸二胺和十二碳二元酸为原料制备尼龙1012盐,然后将双六甲基三胺作为含仲胺结构的化合物引入到尼龙1012盐聚合反应中制备尼龙1012,考察双六甲基三胺含量对尼龙1012性能的影响。结果表明:随着双六甲基三胺含量的增加,尼龙1012的相对分子质量逐渐增大,相对分子质量分布明显变宽,双六甲基三胺质量分数从0增加至2.0%,聚合物分散性指数从2.48上升到4.29;当双六甲基三胺质量分数大于1.0%时,尼龙1012的动态流变性能表现出弹性大于黏性的行为;随着双六甲基三胺含量的增加,尼龙1012的熔融温度和结晶温度均呈现逐渐降低趋势,而热分解温度逐渐上升;双六甲基三胺质量分数从0增加至1.0%,尼龙1012的拉伸强度逐渐降低,缺口冲击强度逐渐增大;双六甲基三胺的仲胺结构参与了聚合反应,形成了支化、交联结构,交联结构的增加有利于提高尼龙1012的拉伸强度。

热分析技术在弹性体材料中的应用

热分析技术是评价弹性体材料热行为的主要手段之一。通过热分析技术全面深入地研究弹性体的物化特性,对改善和提高弹性体的应用性能具有重要的现实意义和价值。本文介绍了常见的热分析方法,包括:差示扫描量热法(DSC)、热机械分析法(TMA)、动态热机械分析(DMA)、热重分析(TGA)技术及热分析联用技术在弹性体材料中的应用研究进展。综述了热分析技术表征弹性体材料的特点及优势,同时展望了热分析技术的应用前景。

基于空间设计的5089结构胶改性制备与应用策略研究

基于环境空间设计需要,对空间设计用5089胶粘剂进行了改性处理,对比分析了3种不同改性胶粘剂的动态热机械分析曲线和玻璃化转变温度-固化温度关系曲线。结果表明,与改性胶粘剂A相比,相同固化温度下改性胶粘剂B和改性胶粘剂C的储能模量都相对较高,且在固化温度为100~130℃时,改性胶粘剂A、改性胶粘剂B和改性胶粘剂C都兼具弹性和粘性;耐热性从高至低顺序依次为改性胶粘剂B、改性胶粘剂C、改性胶粘剂A,且改性胶粘剂B在固化温度100、110和130℃时的玻璃化转变温度都高于原始5089胶粘剂,表明改性胶粘剂B在此温度下的胶接接头耐热性能都优于原始5089胶粘剂。

金沙江下游高温天气的环流分型和诊断

金沙江流域作为我国重要的生态屏障和清洁能源输出地,其下游干热河谷地段是高温频发地区,针对该区域开展高温天气环流特征和诊断对提高高温灾害预报水平和提高该区域能源调度效率具有重要意义。利用1981—2020年逐日最高气温资料和欧洲中期天气预报中心(the European Centre for Medium-Range Weather Forecasts,ECMWF)第五代大气再分析数据集ERA5(Reanalysis v5),通过K-means聚类分析法对金沙江下游高温天气环流形势进行分型,并对各类高温天气的动力和热力因子进行诊断,提炼出各型高温的物理量阈值。结果表明:金沙江下游高温天气的环流背景可分为暖脊型、副热带高压-青藏高压型和青藏高压型3种类型。高温期间中高层均有≥0.25 Pa·s^(-1)的下沉气流,天空晴朗无云,晴空辐射强,且近地面受到≤800 hPa强度的热低压直接加热作用。热力诊断发现金沙江下游高温主要由非绝热加热引起,温度平流对局地升温的贡献为负,垂直绝热变化贡献接近于0,但下沉气流有利于地面接收更多太阳辐射,使近地层非绝热加热增加,从而导致金沙江下游出现高温天气。

A MULTI-COUPLED FINITE ELEMENT ANALYSIS OF RESISTANCE SPOT WELDING PROCESS

A two-dimensional axisymmetric finite element model is developed to analyze the transient thermal and mechanical behaviors of the Resistance Spot Welding （RSW） process using commercial software ANSYS. Firstly a direct-coupled electrical-thermal Finite Element Analysis （FEA） is performed to analyze the transient thermal characteristics of the RSW process. Then based on the thermal results a sequential coupled thermo-elastic-plastic analysis is conducted to determine the mechanical features of the RSW process. The thermal history of the whole process and the temperature distribution of the weldment are obtained through the analysis. The mechanical features, including the distributions of the contact pressure at both the faying surface and the electrode-workpiece interface, the stress and strain distributions in the weldment and their changes during the RSW process, the deformation of the weldment and the electrode displacement are also calculated.

Temperature Dependency of Dynamic Mechanical Properties of Cement Asphalt Paste by DMTA Method

We investigated the temperature dependency of the dynamic mechanical properties of cement asphalt paste by the dynamic mechanical thermal analysis（DMTA） method. The experimental results show that the dynamic mechanical properties of cement asphalt pastes are sensitive to temperature due to the inclusion of asphalt, and may go through different states within a temperature range of-40 ℃ to 60 ℃, which is different from that of pure cement and asphalt. As the temperature of the cement asphalt paste increases, a considerable change of dynamic mechanical properties, including storage modulus（E＇）, loss modulus（E＇＇） and loss factor（tand） is observed. Moreover, the influence of asphalt to cement（A/C） ratio on the temperature sensitivity of the dynamic mechanical properties of cement asphalt composites was investigated. The temperature dependency of cement asphalt composites is ascribed to the temperature dependency of the asphalt and its interaction with cement paste. A simple fractional model is proposed to describe the viscoelastic behavior of cement asphalt composites.

NONLINEAR VIBRATION AND THERMAL-BUCKLING OF A HEATED ANNULAR PLATE WITH A RIGID MASS

On the basis of Hamilton's principle and dynamic version of von Karman's equations, the nonlinear vibration and thermal-buckling of a uniformly heated isotropic annular plate with a completely clamped outer edge and a fixed rigid mass along the inner edge are studied. By parametric perturbation and numerical differentiation, the nonlinear response of the plate-mass system and the critical temperature in the mid-plane at which the plate is in buckled state are obtained. Some meaningful characteristic curves and data tables are given.

Preparation, Characterization, and Comparison of Thermal Properties of Aromatic Polyether Imides

Two aromatic polyimides were prepared by two- step way. Firstly, the poly（amic acid）s were synthesized from 1, 3 - bis（4 - aminophenoxy）benzene and two different aromatic dianhydrides. Secondly, polyimides were prepared via thermal imidization of poly（amic acid） precursors. The polyimides prepared were insoluble in common organic solvents such as tetrahydrofuran, chloroform and N, N- dimethyiformamide. The inherent viscosities of poly（amic acid）s were 1.82 dL/g and 2.67 dL/g. The high inherent viscosities were due to the strong intra- or intermolecular hydrogen interaction. The polyimides were characterized by mechanical and thermal analysis. It was found that the samples are of excellent thermal stability, higher glass-transition temperature and excellent dynamic mechanical and thermal properties.

考虑扣件系统非线性力学行为的车辆与轨道动力分析模型

为深入研究扣件系统的力学特性及其损伤失效对轮轨系统振动的影响,基于精细的扣件系统有限元模型揭示扣件弹条扣压状态和轨下胶垫支承状态的非线性力学特征,提出并建立考虑扣件系统非线性力学行为的车辆与轨道动力分析模型。通过实测结果验证本文模型的计算可靠性,并分析扣件失效类型对轮轨系统动力响应特性的影响。研究结果表明:弹条扣压力随弹程增大而近似呈双线性特征,单个弹条扣压刚度在“2点接触”状态时为0.675 kN/mm,在“3点接触”状态时为5.11 kN/mm;本文模型计算得到的钢轨加速度响应频率100 Hz以上的较传统模型明显增大,与实际情况更为接近;传统模型利用刚度属性折减或去掉弹簧单元来实现扣件弹条失效的模拟,会放大失效区域附近的轮轨系统动力响应;针对扣件失效的情况,包括弹条失效和胶垫失效,实际现场运营维护时不仅要关注失效区域内的轨道结构状态,对于后邻近区域也要及时加强关注。

进给系统角接触球轴承的热力耦合分析

对角接触球轴承热力耦合性能进行预测分析是保证机床工作精度的关键。基于赫兹接触理论结合热网络方法,建立滚珠丝杠进给系统成对安装轴承工作状态下的动态热力耦合模型,考虑了温升热效应下的球轴承内部润滑状态以及热膨胀,提出了轴承的动态热力耦合分析数值计算方法。探明了轴承内部接触参数和摩擦生热的时变规律。研究表明:转速、外部载荷等工况影响轴承的摩擦接触特性,轴承性能显著受热力学行为的影响。提出的轴承动态热力耦合分析模型可以有效预测轴承非线性热行为下的接触特性。

纤维素气凝胶骨架负载SiC提升环氧复合材料非线性电导特性与热力学特性

掺杂SiC颗粒的聚合物材料可以表现出优异的非线性电导特性,并广泛应用于电场梯度材料。然而,由于非线性复合材料中填料含量较高,优异的非线性电导特性以牺牲机械特性为代价。因此,如何在极低SiC填充量下实现非线性电导特性和机械特性的协同优化对实际工程应用而言尤为重要。该文首先利用硅烷偶联剂对SiC进行表面改性,然后在纤维素气凝胶骨架上成功实现自组装,并通过真空辅助浸渍法制备了非线性电导环氧复合材料,实现了其导热性能、机械性能和非线性电导特性的协同提升。研究结果表明:该复合材料在极低SiC填充量下(体积分数4.47%)表现出优异的非线性电导特性,且不同SiC体积分数下纤维素气凝胶骨架负载的SiC/环氧复合材料可以大大拓宽开关场阈值的范围。在体积分数10.58%下,该复合材料阈值场强为2.49 kV/mm,非线性系数为4.54。同时,导热特性和动态热机械特性测试表明,由于相互连接的3D结构,新型复合材料储能模量提高约100%,交联密度高达15.9×10^(–3 )mol/cm^(3),热导率提升至0.69 W/(m·K)。该方法为制备具有优异热力学性能的电场梯度复合材料提供了新的思路。

Thermodynamic analysis of a space station remote manipulator with a harmonic drive that considers an integrated thermal protection layer.

To eliminate anomalies and improve the performance of a space station remote manipulator(SSRM) used in a dynamically changeable thermal environment, we analyze the thermodynamic behavior of an SSRM that considers an integrated thermal protection system(ITPS). Solar radiative heat gain and loss become equally significant as conductive heat transfers through the interior of the SSRM on orbit. A thermodynamic model of the SSRM with a sandwich ITPS structure is established on the coupling between harmonic drive and changeable thermal environment. A motion precision is proposed to evaluate thermodynamic behavior under continuously changeable thermal circumstances. Simulation results indicate that the ITPS with a corrugated sandwich structure reduces the maximum amplitude of angular position errors to 41.6%, which helps improve the motion precision of the SSRM. The feasible regions for the SSRM in the Low Earth Orbit(LEO) and Geostationary Earth Orbit(GEO) are analyzed, which shows that the proportion of feasible region in LEO is significantly larger than that in GEO.

超临界汽轮机再热主汽阀门卡涩分析

汽轮机再热主汽阀是汽轮机组的重要辅机,在动态性能、密封性、可靠性方面都有特殊要求。该类汽轮机再热主汽阀是汽轮机组的重要辅机,在动态性能、密封性、可靠性方面都有特殊要求。该类型阀门机构在冷起动过程中经常发生卡涩,且随蒸汽参数升高,出现故障的概率增大且更加严重,严重影响机组的正常运行。针对机构故障现象与结构特点,对动态行为异常变动进行分析,研究表明冷起动过程中,不同零件物理性能差异导致零件间热量传递速度存在较大差异,引起动静配合要素位置和形状变化,并进一步导致径向间隙和轴向间隙变动,严重影响机构的动力学性能。有限元法用来定量分析机构在冷起动至稳态过程中动态配合要素之间的间隙变化,并分析间隙对阀门机构的动态行为影响,数值分析的结果与电厂中出现的问题是一致的,其实际处理方案则验证该方法的正确性。从改进公差设计和制造精度控制两个方面提出解决该类阀门卡涩的具体方案,对于解决该类工程问题具有重要理论价值和现实指导意义。

高温后花岗岩冲击破坏行为及波动特性研究

采用SRM-5N超声检测分析仪和高温分离式霍普森压杆(SHPB)系统装置,分别对不同高温后花岗岩的波动特性和动态力学特性进行试验研究,分析不同温度条件对花岗岩纵波波速、波形频谱的影响,研究高温后花岗岩的动态抗压强度、峰值应变以及冲击破碎形态的变化情况。试验升温等级设为25℃,100℃,200℃,400℃,600℃,800℃,1 000℃七个等级,升温速度为10℃/min。试验结果表明:(1)随着温度的增高,花岗岩试样的热损伤总体上呈逐渐增大趋势。但是100℃之前热损伤有所降低,出现负的热损伤,随后热损伤不断增加,直到600℃以后热损伤增幅开始变缓。(2)随着温度的升高,试样的动态抗压强度总体减小,峰值应变总体增大;但是在110℃左右,抗压强度有所增强,峰值应变有所减小;600℃之后抗压强度和峰值应变分别显著减小和增大。(3)推断110℃左右为花岗岩一个阈值温度,在这个温度之前,温度的对花岗岩产生负损伤,花岗岩强度增强;推断600℃～800℃范围内存在为花岗岩另一个阈值温度,超过这个温度花岗岩的力学性能发生显著变化。该方法和成果可为岩体工程施工、防火设计以及火灾后评估修复提供一定参考价值。

生物质热反应机理特性研究

通过对不同生物质在热解以及不同氧含量下燃烧过程的研究，详细探讨了生物质热反应机理。将OZAWA分析方法和动力学分析法结合，计算得到固体样品在热反应过程中的活化能分布特征，进而确定所对应的动力学反应机理。通过对纤维素热解过程的分析证明该方法的准确性，并对不同生物质在不同条件下热反应过程进行全面分析。结果表明，热反应过程可以分为两个阶段，第一阶段为挥发份释放阶段，动力学基本上符合R2即三维随机成核和核增长（除棉杆在N2：O2=54：1条件下的反应）；第二阶段为焦反应阶段，动力学机理变化较大，而该变化与颗粒相的结构有关，其动力学特征可以归纳为（1-a）^m，n的取值与颗粒结构特性相关。