基于微观模型分析的承压粗糙界面接触状态超声评价方法

提出一种用于承压界面接触状态超声评价的粗糙界面微观模型。通过对承压粗糙表面微观特性理论分析,建立粗糙界面微观形貌参数与承压界面超声响应之间的关系。开展三种不同粗糙度承压铝界面超声透射试验,得到加载和卸载过程界面透射系数随压力变化关系,进而获得界面接触刚度和非线性系数。研究表明,在较小压力情况下,超声非线性系数对承压界面接触状态评价具有更高的灵敏度。通过对试验检测数据进行非线性最小二乘拟合处理,确定出接触界面微观形貌特征参数,进而计算出接触界面声学特性指标。理论计算得到声学特性与试验结果吻合较好,证明粗糙表面微观模型用于非完好界面声响应预测的有效性。     

基于多孔介质理论的双粗糙面金属静密封泄漏模型

粗糙面微观几何形貌是影响静密封泄漏特性的重要因素。应用粗糙面的三维点云数据,将粗糙面进行离散化处理,进而将由双粗糙面构成的密封界面等效为三维逾渗栅格模型,基于多孔介质理论计算得到密封界面的孔隙率和渗透率,从而建立了一种双粗糙密封界面的泄漏率模型。搭建金属静密封泄漏率测量试验台,通过对环面金属静密封泄漏特性的试验研究,验证了该泄漏模型的有效性。利用该模型分析了表面纹理方向、粗糙面波动频率与材料特性对金属静密封泄漏特性的影响。结果表明：各向异性粗糙面构成的密封界面具有较好的密封性能;粗糙面波动频率越大,密封性能越好;低硬度材料易于实现有效密封;在重载情况下,粗糙面微观几何形貌对孔隙率与泄漏率的影响不显著。     

以粗糙集近似逼近理论提取发动机振动故障特征

主要探索一种从庞杂数据中挖掘有用信息的方法。首先介绍了粗糙集的基本理论与计算近似精度的方法，简述了粗糙集理论的特点及与模糊集理论、证据理论的区别与联系，然后将经过预处理的发动机振动信号进行实数离散．运用粗糙集的下近似、上近似及粗糙逼近理论，计算属性等价类对决策等价类的逼近精度。计算结果表明，采用等频率和等量间隔相结合的方法离散实数能保留数据中良好的自然分类特性，采用粗糙集的近似逼近理论能有效地提取出发动机故障特征。     

超声波固态耦合声特性研究

针对采用固体耦合方式的超声波对粗糙表面工件进行探伤,分析了固体耦合界面反射系数、表面粗糙度和界面压力之间的关系。提出了一种可以推导出干耦合界面刚度的方法,结合准静态刚度弹簧模型计算反射系数,通过模型对比与实验分析验证了理论的正确性。将模型预测值与实验测量值进行比较,最大相对误差为3.56%。此模型可以为干耦合超声检测中,在一定粗糙度表面下达到理想耦合效果所需给定压力的估算值提供一个可靠的理论数据。证明了固体耦合界面粗糙会显著降低耦合效果,增加固体界面间的压力可以提高耦合性能。通过建立的随机粗糙界面模型,可以将检测界面与界面压力的影响进行量化分析。     

基于应力波理论的新型冲击攻丝机的研究

讨论了一种最新的螺纹加工方法—冲击攻丝,研究了冲击攻丝装置的能量传递方法,运用应力波理论讨论了冲击构件中两物体碰撞界面处反弹系数,应用应力波在界面处的反射、透射的波动理论,近似得出两物体碰撞后的反弹系数。     

旋转轴唇形密封圈的建模与仿真研究

为了进一步研究唇形密封圈的密封机制,建立唇形密封的理论模型。基于流量因子分析轴向泵汲效应,建立泵汲流量方程;运用圆周平均雷诺方程描述密封界面流场,采用G-W模型近似描述唇轴粗糙峰互相接触下的接触力与径向变形;定量分析密封界面的周向摩擦力,并给出流体摩擦表达式;对以上各因素进行强耦合分析。结合船舶桨轴密封圈的实际应用工况及结构参数进行仿真计算,得出其方向角、膜厚、压力分布,并得到净流量随转速和粗糙度的变化关系。研究结果表明:净流量随转速增加而增加,但增速逐渐变缓;净流量随粗糙度近似呈线性增加,但高粗糙度会使泄漏量增大和导致表面更容易被磨损,因此实际唇口粗糙度的选取应综合考量多种因素。     

软X射线界面传输特性及应用研究

对软X射线与界面相互作用的规律进行初步研究,给出了软X射线界面散射的一级近似一般理论推导,从中给出了粗糙界面软X射线的散射传输特性,表明在粗糙表面上软X射线的镜向反射系数,需要做与粗糙度有关的数学修正。作为形式上的一种方法,本文还给出了粗糙表面对软X射线全反射的动力学模型,以及一维软X射线散射测量粗糙度原理。通过实验测量了λ=243Å,256Å时具有超光滑表面的Mo,Si样品之反射率和λ=44.6Å时石英超光滑样品的反射率。采用镀碘化铯的方法,使“真空紫外-软X光反射率计”的可测量短波长从200Å降至10Å.对44.6Å进行了散射测量,可以明显看到软X射线的非正常反射现象;还对积分散射测粗糙度方法进行了初步研究。最后给出了X线聚焦装置的理论公式和设计实例。     

双粗糙面滑动摩擦热力耦合有限元分析

建立了双粗糙分形表面滑动摩擦的热力耦合模型,综合考虑了随温度变化的材料性能、材料的弹塑性变形及摩擦副的磨损失效等因素,以摩擦材料的性能参数及设定的材料损伤参数为实例对双粗糙分形表面滑动摩擦全过程的温度场、应力场及磨损进行了数值模拟,分析得到了滑动摩擦过程中摩擦界面最高接触温度、接触应力的分布、磨损率及其变化规律,实现了对双粗糙面摩擦磨损情况的模拟及预测。     

含多粗糙峰涂层等效应力的有限元分析

研究刚性平面与含粗糙峰涂层在二维与三维模型下的弹性接触问题,采用有限元法分析涂层弹性模量比、涂层厚度、粗糙峰间距、刚性平面压下深度对涂层粗糙峰表面、涂层/基体界面分布及基体等效应力分布的影响。计算结果表明压下深度对三维涂层粗糙峰表面最大应力的影响最大,涂层厚度、涂层/基体弹性模量比、粗糙峰间距的变化对应力值影响逐渐减小;增大涂层厚度、减小压下深度和粗糙峰间距、降低弹性模量比会使得三维接触模型最大等效应力值显著降低;增加涂层粗糙峰数和涂层厚度、同时降低涂层弹性模量有助于提高涂层/基体界面结合强度。相对于二维接触模型来说三维接触模型在粗糙峰表面的等效应力增大,造成这种变化的主要原因是由于涂层表面粗糙峰之间的等效应力叠加引起的。该研究为涂层粗糙峰及涂层/基体界面强度的应力分析提供依据。     

双粗糙面滑动过程摩擦因数的变化分析

采用W-M函数建立具有分形特征的三维双粗糙面接触模型,考虑了接触界面间的黏着效应,在滑动速度、法向载荷及界面剪切强度等参数变化下,运用有限元方法探讨了粗糙体在滑动过程中摩擦因数的变化情况。结果显示,滑动速度、法向载荷及界面剪切强度等参数对摩擦因数的变化有一定的影响,边界润滑工况下平均摩擦因数为0.28,无润滑工况下平均摩擦因数为0.713,最大界面剪切强度时的平均摩擦因数为0.73;随着界面剪切强度的减小、法向载荷的增大、滑动速度的增加,滑动摩擦因数有所减小。与相关文献结论或实验结果进行比较,证明了上述结果的正确性。分析结果可为摩擦学设计和摩擦材料的制备提供理论参考。     

Modeling Clutter from Bosnian and Puerto Rican Rough Ground Surfaces for GPR Subsurface Sensing Applications Using the SDFMM

The Steepest Descent Fast Multipole Method (SDFMM) is used to analyze the distorting effect of random rough ground surfaces on scattered and transmitted electromagnetic waves. Two well-measured loamy soils: Bosnian and Puerto Rican clay loam are investigated, each with a variety of surface roughness. This study is important in understanding the effects of different soil properties and is meant to be an a priori phase of investigating scattering from buried targets under the rough ground. In this work, we investigated the scattering from rough soil ground without buried objects. The SDFMM is an integral equation-based fast algorithm that is well suited for two-dimensional penetrable rough surfaces (3-D scattering) in the frequency domain. The scattered and transmitted near electric field of an incident Gaussian beam are calculated at different locations above and below the mean plane of the dielectric rough interface. The receiver locations above are chosen to simulate GPR measurement protocols. The obtained numerical results show that the scattered field undergoes more distortion than the transmitted field from both soil types. Moreover, the transmitted fields into the higher dielectric constant Puerto Rican soil experience more distortion than those transmitted into Bosnian soil.     

SPECKLE METHOD EVALUATION OF THE ROUGHNESS OF ELECTRODISCHARGE MACHINED SURFACES

Abstract

This paper describes the applicability of the speckle method to evaluate the roughness of surfaces produced by non-traditional machining processes such as EDM, ECM or USM, where material removal occurs randomly. The method is based on the digital correlation of two speckle images produced by interference phenomena arising when a coherent light beam is incident on a rough surface. The basic principle of the speckle pattern correlation is presented and the theoretical analysis based on a hypothesis concerning the morphology of the machined surface is reported. An experiment has been set-up in order to assess the feasibility of applying the speckle method to evaluate the roughness of machined surfaces. In the experimental tests, two speckle patterns produced from the same rough surface under two different illumination conditions have been correlated. The two different conditions have been obtained by varying the angle of incidence of coherent light on the surface being analysed. The roughness of electrodischarge machined surfaces, as measured by the speckle method, is found to be in good agreement with that of a stylus instrument.     

An Elastic-plastic Adhesion Model for Contacting Fractal Rough Surface and Perfectly Wetted Plane with Meniscus

The strong stiction of adjacent surfaces with meniscus is a major design concern in the devices with a micro-sized interface.Today, more and more research works are devoted to understand the adhesion mechanism. This paper concerns the elastic-plastic adhesion of a fractal rough surface contacting with a perfectly wetted rigid plane. The topography of rough surface is modeled with a two-variable Weierstrass-Mandelbrot fractal function. The Laplace pressure is dealt with the Dugdale approximation. Then the adhesion model of the plastically deformed asperities with meniscus can be established with the fractal microcontact model. According to the plastic flow criterion, the elastic-plastic adhesion model of the contacting rough surfaces with meniscus can be solved by combining the Maugis-Dugdale (MD) model and its extension with the Morrow method. The necessity for considering the asperities' plastic deformation has been validated by comparing the simulation result of the presented model with that of the elastic adhesion model. The stiction mechanism of rough surfaces with meniscus is also discussed.     

低频水下声信号调制下衍射光场的分布

建立了一个简单可行的光学系统用于探测低频水下声信号。水下声信号产生的低频声波通过水介质传到液体表面产生表面波,激光束入射到液面时,在光屏上观察到了清晰、稳定的衍射图样,发现在低频水下声信号的调制下衍射光场的分布具有明显的不对称性。实验中得到了衍射图样光斑光强与位置的分布,理论分析发现,结果与实验所观察到的现象是很吻合的。结果表明,在低频水下声信号调制下衍射光场的分布是非对称分布的。     

Effect of stiffness of multi-level hierarchical attachment system on adhesion enhancement

Geckos are known for their remarkable ability to cling on and detach from ceilings and walls using a unique attachment system. Their foot pads are covered by a large number of small hair (setae) that contain many branches per seta with a lower level of spatulae. This hierarchical structure gives the gecko adaptability to create a large real area of contact with rough surfaces. In this study, using the three-level hierarchical model recently developed to simulate a gecko seta contacting with random rough surface, the effects of spring stiffness and number of springs on the adhesion enhancement of multi-level hierarchical model are investigated. One- and three-level hierarchically structured spring models with different spring stiffnesses and number of springs on each level in contact with various rough surfaces are considered. The efficiency of attachment-the adhesion coefficient, the adhesion force, the number of contacts and the adhesion energy-for the three-level models with different stiffness is investigated in contact with different rough surfaces.     

A simple theory of asperity contact in elastohydro-dynamic lubrication

The Greenwood and Williamson theory of random rough surfaces in contact has been combined with established elastohydrodynamic theory to provide a theoretical approach to highly loaded lubricated contacts in which the load is shared between hydrodynamic pressure and asperity contact. It is shown that, provided a major part of the load is carried by elastohydrodynamic action, the separation between the two rough surfaces is given (to a first approximation) by the film thickness which would exist between two smooth surfaces under the same conditions of load, speed and lubricant. It then follows that the asperity pressure, both real and apparent, is determined primarily by the ratio of theoretical film thickness to the combined roughness of the two surfaces (h_o/σ). A corollary of this result is that an increase in total load, which has only a small influence on the film thickness, is carried by an increase in fluid pressure and only gives rise to a small increase in asperity contact pressure.     

Effects of fabricated error on transmission performance of double layer frequency selective surface configuration

1Introduction Thedoublelayerfrequencyselectivesurface(FSS)hasflattopsandsharpskirtsandhas beenwidelyappliedtotheantennareflectorfor satellitecommunicationandvariousradome.Up tothepresent,mostofthesingle layerand multi layerFSSsaredesignedbyusinginfinite planeperiodicresonantconfigurationmodel,whichsatisfiestheinfiniteperiodicboundary conditionofFloquettheorem.Thereforemany numericalanalysismethodsbasedonFloquetpe riodicmodelareemployedtosimulatethemulti layerFSSelectromagnetictransmissionp…     

Thermomechanical contact homogenization with random rough surfaces and microscopic contact resistance

We extend an earlier computational thermomechanical contact homogenization framework [Temizer ?, Wriggers P. International Journal for Numerical Methods in Engineering 2010; 83:27-58] to random rough surfaces generated through the random-field model based on the concepts of ensemble averaging and sample enlargement towards the effective limit. Additionally, the homogenization theory is revisited in order to incorporate thermal dissipation at the microscopic contact interface within a thermodynamically consistent approach that preserves dissipation across the scales. Large-scale three-dimensional computations were performed to demonstrate the effectiveness and feasibility of the computational framework for an accurate characterization of the macroscopic thermomechanical response of rough surfaces in contact.     

The elastic contact of a rough surface

The elastic contact of an isotropically rough surface with a plane is treated by approximating the summits of a random process model by paraboloids with the same principal curvatures and applying the classical Hertzian solution for their deformation. The errors in this approximation are computed in terms of the separation and of a bandwidth parameter α. Load and real contact area are derived as functions of separation. For large separation the fractional area of real contact is found to be half the bearing area fraction and the separation may be eliminated to give direct porportionality between load and area. For all separations the load is approximately proportional to the contact area. The constant of proportionality depends only on the Hertzian elastic modulus and the profile absolute mean slope. Experimental measurements of the latter and of α for a variety of surfaces show little variation in either, and the measured values of α are within the range of applicability of the model. The theory is critically compared with existing theories.