Two-stage identification of interlayer contact loss for CRTS Ⅲ prefabricated slab track based on multi-index fusion

To accurately identify the potential contact loss of the China railway track system(CRTS)III prefabricated slab track,a finite element model with contact loss of self-compacting concrete(SCC)under transient impact was established.Then the vertical accelerations near impact points on the track slab surface were extracted to obtain damage-sensitive indices in the time and frequency domains.The indices were initially normalized to obtain independent items of evidence before the Dempster-Shafer(D-S)evidence theory was used to fuse these into one.Finally,a two-stage identification was performed to identify the damaged SCC area,comprising a rough identification(Stage I)and a precise identification(Stage II).The research results show that the damage indices extracted based on the transient impact response change abruptly at the damage location,and that can be used for damage identification.However,the use of a single index to determine the damage of the impact point may be misjudged.In Stage I,five damage indices of acceleration were fused to magnify the difference between the damaged point and undamaged point,thereby improving the accuracy of finding damage.In Stage II,in the area where more impact points were added,a fusion of three indices of acceleration response,that is,the absolute mean of the time domain,the maximum amplitude of the frequency domain,and the power density ratio,further narrowed down the area where damage exists.As a result,when the contact loss of SCC is greater than 50%along the thickness direction,the identification accuracy can be as high as 70%to 80%.The two-stage identification method proposed in this study can greatly improve the efficiency of interlayer damage detection of slab tracks and is expected to provide effective technical support for damage identification of track structures in the future.

Stress-Induced Uphill Diffusion with Interfacial Contact Loss in Solid-State Electrodes

We simulate the mechanical-chemical coupling during delithiation and relaxation of a cathode in a solid-state lithium-ion battery.Contact loss at the interface between the active particle and the solid electrolyte is considered.Uphill diffusion is observed during delithiation and relaxation.This phenomenon is explained by analyzing the total chemical potential and its two components.Contact loss at the interface greatly influences the stress and stress gradient in the active particle.As delithiation continues,the stress and stress gradient grow considerably,and the mechanical part of the total chemical potential becomes dominant over the chemical part of it.In the latter stage of delithiation,the influence of the incomplete interfacial constraint on the stress becomes dominant,while the effect of the concentration gradient becomes negligible.After relaxation,the concentration and stress gradients increase in a particle with contact loss.The influence of the degree of contact loss on the mechanical-chemical coupling is investigated.The overall tensile stress in the active particle increases with decreasing contact loss,causing a sharp decrease in local concentration.We also check the effect of the elastic modulus of the solid electrolyte on the coupling of the active material.A rigid solid electrolyte with a higher elastic modulus more strongly restricts the active particle,leading to a higher tensile stress,a larger stress gradient,and a greater concentration gradient.

Critical Velocity of Short Floating Slab Track Using Alterable Element Method Considering Wheel-Rail Contact Loss

In actual line operation,the critical velocity is one of the key physical quantities of rail design owing to its great infuence on the riding comfort and safety of vehicles due to the wheel-rail contact loss caused by the abrupt change of rail foundation rigidity,rail wear,or abruptness irregularities on rail.In this study,the short floating slab track(SFST)structure is regarded as a double-layer system.The Euler beam and the rigid body model are adopted for the rail and the floating slab,respectively.and the dispersion equation and the theoretical critical velocity of the rail structure under ideal conditions are deduced.Besides,this st udy considers the implementation of the SFST in the vehicle-structure coupling system.The alterable element method is introduced for accurately simulating the change of the whee-rail contact state and coding a vehicle-structure dynamic analysis program(VSDAP)to calculate the critical velocity of rail structures from the dynamic response of vehicles and rail structures.The principle of its design at the beginning of the design is given on the basis of the theoretical value of the critical velocity and the simulation of the dynamic response,which can provide reference for practical engineering design.

Contact force and mechanical loss of multistage cable under tension and bending

A theoretical model for calculating the stress and strain states of cabling structures with different loadings has been developed in this paper. We solve the problem for the first- and second-stage cable with tensile or bending strain. The contact and friction forces between the strands are presented by two-dimensional contact model. Several theoretical models have been proposed to verify the results when the triplet subjected to the tensile strain, including contact force, contact stresses, and mechanical loss. It is found that loadings will affect the friction force and the mechanical loss of the triplet. The results show that the contact force and mechanical loss are dependent on the twist pitch. A shorter twist pitch can lead to higher contact force, while the trend of mechanical loss with twist pitch is complicated. The mechanical loss may be reduced by adjusting the twist pitch reasonably. The present model provides a simple analysis method to investigate the mechanical behaviors in multistage-structures under different loads.

Internal Energy Losses and Contact Duration About a Circular Plate Colliding to a Half-Plane Target

Collisions between multibody systems are irreversible processes which cause loss of internal energy by a stress wave that propagates in the impacting bodies away from the region of impact. A coefficient of restitution relating to approach velocity is introduced to denote the losses of translational kinetic energy. A parameter β involved in internal energy losses has been obtained to calculate the coefficient of restitution. As a result, the internal energy losses caused by elastic stress waves and the contact duration in metals can be calculated numerically for the collision between circular cylinder and half plane. The metals include aluminum alloys, steel-mild 1020, steel-stainless austenitic 304, tungsten alloys, copper alloys, nickel alloys and titanium alloys. By introducing a coefficient of velocity-frequency, an exponential aggression equation related the normalized oscillating frequency and normalized approach velocity has been obtained by the numerical method.

装配式混凝土梁桥预应力摩阻损失研究

装配式预应力混凝土梁桥以其施工效率高、构件质量好等优势应运而生,混凝土构件在工厂可以标准化生产确保质量,而预应力损失却难以把控。现行规范中对于弯曲孔道预应力摩阻损失的计算在接触压力、弯曲夹角等方面存在不适用性。基于弹性接触理论,揭示该公式在推导过程中的缺陷,对某线路装配式混凝土梁桥进行预应力沿程损失试验,验证预应力混凝土梁桥的有效预应力不足来源于计算公式缺陷。

非均布压力作用下纯弯曲孔道预应力摩阻损失

为获得更准确的孔道预应力摩阻损失计算方法,保证桥梁结构设计的有效预应力和桥梁结构的安全运行。根据赫兹接触理论,综合考虑摩擦力、接触压力与张拉力间的相互作用,导出了纯弯曲孔道在非均匀压力作用下摩阻损失公式。通过有限元软件分析不同参数对摩阻损失计算值的影响。通过实际工程的摩阻试验探究纯弯曲孔道在非均匀压力下预应力摩阻损失的具体分布规律,并通过规范公式、推导公式的理论计算值与现场实测数据的偏差,验证公式的合理性。结果表明:弯曲角度小于120°、摩擦系数为0.2~0.4范围内,推导公式计算值更接近实际情况;随着张拉应力和弯曲角度的增加,利用推导公式计算得到的摩阻损失值更接近实测结果。

人工种植牙邻接触丧失应对策略的专家共识

人工种植牙邻接触丧失(proximal contact loss,PCL)所引发的问题是近年研究热点,学者们一致致力于其原因和相关因素分析,旨在寻求解决与PCL相关问题的方案。前驱力作用、颌骨及牙槽窝等终身改建以及种植牙的骨整合特性是PCL的主要原因。一方面,下颌的闭口运动通过后牙牙尖产生了将牙齿往近中移位的前驱力,使上下颌后牙、下颌前牙近中漂移,同时,可使上下颌前牙唇向移位;另一方面,颌骨、牙槽窝、牙根的改建以及咀嚼肌的前向水平分力、下颌向前向上的动力分量以及牙尖斜面产生的向前分力使天然牙向近中漂移;此外,天然牙终身具备水平、垂直向移位以及旋转的能力,以适应口颌系统的改建和维持口腔的功能,而种植牙骨整合缺乏天然的牙周膜,缺乏近中漂移生理基础,垂直向平均动度微小,种植牙骨整合沉默性不具备天然牙整体漂移特点,增加了PCL发生的可能性。PCL高发生率与戴牙时间和近中位置有明显相关性,但也受咬合力大小、对颌牙状况、邻牙情况、修复方式、牙位、上下颌骨、年龄、性别等因素影响。PCL与食物嵌塞呈显著相关性,但并不一一对应,不满足充分必要条件。PCL也与种植体周病变以及龋齿等有关。PCL的预防包括知情同意、定期检查、固位方式选择、接触点增强、配戴保持器以及应用多用途数字化牙冠。PCL的处理包括邻接触点增补、正畸牵引以及咬合调整。现有方法可以短期解决食物嵌塞情况,综合干预以寻求稳定的远期效果。对称、平衡的考量将拓展处理有关PCL引发的问题。

柔性转子弹性对行波旋转超声电机接触特性的影响

转/定子间动态接触过程决定行波旋转超声电机(TRUM)的输出性能,但是柔性转子弹性在以往界面接触特性研究中常被忽略.本文中基于建立的考虑柔性转子和摩擦层三维弹性的转-定子动力学耦合模型,从接触界面的接触状态、速度、接触应力、摩擦应力驱动和阻碍子区等空间分布角度,研究了柔性转子弹性对TRUM摩擦界面的动态接触行为和摩擦损耗的影响.结果表明:柔性转子在轴向、径向和切向均呈现出一定的振动速度,且在接触界面沿圆周方向与定子各向速度形成一定的相位关系,显著影响界面动态接触特性、摩擦特性和摩擦损耗;柔性转子与定子轴向位移分布的相位一致性表明,定子与摩擦层的接触界面具有类齿轮啮合机制;与以往中间驱动两端阻碍的分布结果不同,考虑转/定子动力学耦合的接触区摩擦应力分布为一端驱动和一端阻碍的新机制.计算结果表明在建模过程中忽略柔性转子弹性得到的界面摩擦损耗要比考虑柔性转子弹性获得的摩擦损耗大得多,主要原因:(1)柔性转子径向刚度小,摩擦力作用下接触区的运动方向与定子相同,从而减小了径向速度差和径向摩擦损耗;(2)柔性转子界面的切向振动特性,改变了接触界面黏-滑子区分布,使得行波远离端的摩擦应力阻碍子区消失,且摩擦层界面切向剪切变形长度增大,从而减小了切向滑动和摩擦损耗.研究结果将有助于更好地理解TRUM中的动态接触特性,并指导摩擦界面的设计.

双块式无砟轨道轨枕与道床板界面脱空损伤识别

为准确识别双块式无砟轨道轨枕与道床板连接面脱空损伤,建立了双块式无砟轨道冲击回波有限元模型,分析界面损伤对弹性波传播规律的影响。在室内浇筑含有不同损伤位置和不同损伤程度的混凝土复合试件,采用冲击回波法进行损伤识别。研究结果表明:脱空损伤会显著影响弹性波在界面处的传播,基于回波信号分析可识别损伤;弹性波频率主要受激振锤大小的影响,应结合结构易损位置和理论计算选取合适的激振锤;混凝土复合试件的试验厚度值与理论厚度值相差仅为1 cm,测试结果合理,可用于判断脱空损伤程度,但不能确定损伤区域;利用平面成像技术可准确识别轨枕与道床板界面脱空范围,为双块式无砟轨道结构养护维修提供一定参考,试验结果验证了冲击回波法用于双块式轨道层间脱空识别的可行性和合理性。

基于MFDC-SSD网络的接触网定位线夹缺陷识别

针对接触网应用环境复杂,定位线夹体积小、安装方向特殊、不易识别,传统的目标检测算法效果较差等问题,基于多尺度特征融合密集连接网络模型,提出了一种接触网定位线夹缺陷检测新方法。首先结合DenseNet与Inception模块对SSD模型的特征提取网络进行改进,在特定的特征层间共享上下文信息,提升特征提取能力。然后从网络深层到浅层,逐级采用FPN融合SSD检测的多尺度特征图,设计多特征融合的密集连接网络模型,最后将f GIoU作为边框损失函数,在训练中优化真实框和预测框的重合度。对采集的某段接触网定位线夹图像数据集进行检测识别。结果表明:该定位线夹缺陷检测方法可在复杂接触网背景下,对定位线夹脱落和松动进行检测,且在不同角度、亮度的图像中均具有较强的鲁棒性。

基于多角度下TC4钛合金表面多颗粒动态冲蚀仿真研究

为了准确地预测TC4钛合金材料固体颗粒冲蚀的损伤过程,采用显式动力学有限元分析手段,建立了一个弹塑性有限元模型来模拟三维结构中多颗粒模型的动态冲蚀过程,并结合了Johnson-Cook材料模型和应变累积失效模型。结果表明:最大塑性变形发生在第一次碰撞时,且随着碰撞次数的增加而减小。冲蚀角度和速度显著影响颗粒和靶材表面之间的接触时间,动能损失随着冲蚀速度和角度的增加而增加。在低冲蚀角下,动态冲蚀过程中切向接触力和法向接触力有多个极大值和极小值,冲蚀机理主要是塑性变形和切向剪切。在高冲蚀角下,球形颗粒能很好地模拟冲蚀过程中对材料的剪切和挤压作用。从微观机制上解释了入射角度和入射速度对多颗粒连续冲蚀行为的影响,明确了冲蚀坑的动态冲蚀机理和几何演化规律。

种植固定修复义齿邻接丧失的研究进展

邻接丧失是口腔种植修复后的常见并发症之一,其发病率较高,且具有独特的解剖和生理特征。邻接丧失容易引起食物嵌塞,引发种植体周围组织创伤及邻近牙齿的龋病或牙周组织炎症,且治疗方式有限,影响种植修复的预后和患者满意度,因此对种植义齿邻接丧失进行研究至关重要。

关合速度对SF_(6)断路器弧触头侵蚀的影响

SF_(6)断路器关合操作时产生的关合涌流对弧触头会造成严重的侵蚀。为了研究弧触头在不同关合速度下的侵蚀情况,文中设计了模拟的SF_(6)断路器灭弧室,对CuW弧触头进行了不同关合速度下的关合侵蚀实验并对不同关合速度下的弧触头受力分析进行了仿真。研究发现每焦耳的质量侵蚀率并不是随着电弧能量单调增加的,4号触头的电弧能量最小,但是质量损失率却高达40 mg·kJ^(-1)。在相同电流下随着关合速度的增加,虽然燃弧时间和电弧能量会变小,但是弧触头的受力会随着关合速度的增加而增加,摩擦磨损侵蚀会变大,从而每千焦的质量损失率会增大。文中的研究结果对触头关合侵蚀电寿命的分析具有一定的指导意义。

高速铁路弓网离线在线识别及应对策略

电气化铁路车辆运行过程中,受电弓与接触网之间的短暂分离(即弓网离线)易引发过电流问题,产生安全隐患、影响铁路运营秩序。文章基于既有动车组牵引电传动系统,介绍其主电路结构及网侧整流器的控制方法,分析弓网离线对系统的影响,着重研究弓网离线后恢复接触时产生过电流问题的原因,提出弓网离线在线快速识别及应对策略。文章搭建了包含电弧模型的牵引电传动系统全数字实时仿真模型,首先通过与实测波形对比证明模型的准确性,然后基于该模型验证文中提出的弓网离线在线识别及应对策略。

基片等离子处理对PTFE微波基板性能的影响

采用等离子处理对聚四氟乙烯(PTFE)基片进行表面处理,研究气体流量、放电功率、处理时间的变化对PTFE基片微观形貌和接触角的影响,进而研究对PTFE微波基板介质损耗、吸水率、剥离强度等综合性能的影响。实验结果表明,等离子体处理可以有效增加基片表面粗糙度,降低PTFE基片的接触角,提高其表面能,使覆铜箔层压板的铜箔与介质层之间结合强度提升,制备的PTFE微波基板的剥离强度可接近未处理之前剥离强度的3倍。

T-Scan分析单颗后牙种植修复体咬合对垂直骨吸收的影响

目的:利用T-Scan定量分析单颗后牙种植修复体在牙尖交错位(intercuspal position,ICP)时咬合接触情况,并探讨其对种植体周围垂直骨吸收的影响。方法:选取第一磨牙常规种植修复的40例无咬合高点等不良修复现象和磨牙症等不良习惯的患者。T-Scan记录种植修复体及对侧同名牙咬合接触情况的差异,分别按照咬合延迟时间、牙合力百分率比、咬合力峰值及咬合接触面积比进行分组,用影像学测量技术评估种植体周围骨组织的变化,从而分析咬合因素对种植体周围垂直骨吸收(vertical bone loss,VBL)的影响,并建立多元线性回归模型。结果:在咬合延迟时间和咬合力峰值分组比较中,平均VBL均存在显著性差异(P<0.05)。同时得出预测方程:Y=1.060-0.734X1+0.248X2-0.589X3,其中Y为平均VBL、X1为咬合延迟时间、X2为咬合力峰值、X3为咬合接触面积比。结论:在单颗后牙缺失常规种植修复时除考虑适当的咬合设计外,在调整种植修复体的咬合接触或定期咬合检查时,推荐应用T-Scan进行定量分析以确保ICP时咬合力峰值尽可能降低(即轻咬合接触),和适当延迟种植修复体首次咬合接触时间,对维持种植体周围骨的稳定性极为有益。

挠性印制电路板板边插头的优化设计

高频高速挠性印制电路板(FPCB)信号传输过程中易出现反射、串扰等影响信号完整性的损耗。从线路的仿真设计与实际测试差异着手分析,对影响FPCB板边插头信号完整性的损耗进行研究,从插入损耗波形出现谐振点的测试结果分析串扰的影响,提出降低信号损耗的设计理念,为解决因FPCB板边插头设计不合理,使得信号传输过程中插入损耗出现谐振点的问题提供改善思路。

Influence of graphite content on sliding wear characteristics of CNTs-Ag-G electrical contact materials

CNTs-Ag-G electrical contact composite material was prepared by means of powder metallurgical method. The influence of the graphite content on sliding wear characteristics of electrical contact levels was examined. In experiments, CNTs content was retained as 1% (mass fraction), and graphite was added at content levels of 8%, 10%, 13%, 15% and 18%, respectively. The results indicate that with the increase of graphite content, the contact resistance of electrical contacts is enhanced to a certain level then remains constant. Friction coefficient decreases gradually with the increase of graphite content. Wear mass loss decreases to the minimum value then increases. With the small content of graphite, the adhesive wear is hindered, which leads to the decrease of wear mass loss, while excessive graphite brings much more worn debris, resulting in the increase of mass loss. It is concluded that wear mass loss reaches the minimum value when the graphite mass fraction is about 13%. Compared with conventional Ag-G contact material, the wear mass loss of CNTs-Ag-G composite is much less due to the obvious increase of hardness and electrical conductivity, decline of friction surface temperature and inhibition of adhesive wear between composites and slip rings.

Occupational Allergic Contact Dermatitis in Tunisia: Epidemiology and Occupational Outcome

Background: Occupational allergic contact dermatitis (ACD) is one of the most frequent diseases on the workplace worldwide. Its national incidence and prevalence were not previously studied in Tunisia. Objective: To assess incidence of occupational allergic contact dermatitis and its occupational outcome in Tunisia. Methods: Descriptive epidemiological study of all cases of occupational allergic contact dermatitis reported from 2002 to 2012 in Tunisia and recognized by the medical commissions of the National Health Insurance Fund. Results: 315 cases of occupational ACD were collected representing an annual incidence of 31.65 cases/100.000 workers in the private sector in Tunisia. The mean age of our population was 42.70 ± 9.02 years with a male predominance (sex ratio 3.03). The most affected sectors were the clothing sector (14.3% of cases) and the cement industry (13.7% of cases). Patch tests were positive in 188 patients (59.67%). The most frequently positive allergen was potassium dichromate in 27.3% of the cases. Overall, 25.4% of patients had lost their jobs. After multiple binary logistic regression, job loss had a statistically significant relationship with localization of lesions on the face and on dominant hand. Conclusion: Occupational ACD is frequent in Tunisia and is responsible for an important rate of job loss. Preventive measures must be reinforced, especially the wearing of protective gloves.