The Influence of Normal Load and Sliding Speed on Frictional Properties of Skin

The study of frictional properties of human skin is important for medical research, skin care products and textile exploi- tation. In order to investigate the influence of normal load and sliding speed on the frictional properties of skin and its possible mechanism, tests were carded out on a multi-specimen friction tester. When the normal load increases from 0.1 N to 0.9 N, normal displacement and the friction coefficient of skin increase. The friction coefficient is dependent on the load, indicating that both adhesion and deformation contribute to the friction behaviour. The deformation friction was interpreted using the plough model of friction. When sliding speed increases from 0.5 mm·s^-1 to 4 mm·s^-1, the friction coefficient increases and ＂stick-slip＂ phenomena increase, indicating that hysteretic friction contributes to the friction. The hysteretic friction was in- terpreted using schematic of energy translation during the rigid spherical probe sliding on the soft skin surface, which provides an explanation for the influence of the sliding speed on the frictional characteristics of the skin.

Effect of Tension on Friction Coefficient Between Lining and Wire Rope with Low Speed Sliding

In order to obtain the exact friction coefficient between lining and wire rope,the tension of wire rope is stud-ied as a factor which affects this coefficient. A mechanical model of a wire rope subjected to axial load was established to determine the torque of the wire rope. The contact motion between lining and wire rope was regarded as a screw rota-tion and the axial force of the lining resulting from the torque of the wire rope was analyzed. Theoretical formulas re-lating tension of the wire rope and the friction coefficient was obtained. Experiments between lining and wire rope with low sliding speed were carried out with friction tester made by us. Experimental results show that increment of the fric-tion coefficient is proportional to that of the tension of the wire rope with a low sliding speed. The experimental results agree with the theoretical calculation; the errors are less than 6%,which proves the validity of the theoretical model.

Application and development of friction stir welding technology in high speed EMU manufacturing

The application status of friction stir welding technology is introduced,and the application research of advanced FSW technology in the manufacture of high-speed electric multiple unit aluminum alloy body is analyzed,including the application of traditional friction stir welding technology to a combination of a lap joint and butt joint,and butt joint of large thick plate by both sides process,also introduces bobbin-tool friction stir welding to a butt joint and three-dimensional space curved friction stir welding to T-joint. At the same time the future development trend of new FSW technology derived from traditional FSW technology in rail vehicle manufacturing industry is put forward. As the fast developing of critical technology used on railway vehicles,quick applying advanced welding technology will affect products quality,manufacturing cost and production cycle time directly,and it certainly will be one of the approaches to develop markets by all the railway vehicles manufacturing enterprises.

Dividing the transit wind speeds into intervals as a favorable methodology for analyzing the relationship between wind speed and the aerodynamic impedance of vegetation in semiarid grasslands

In grassland ecosystems,the aerodynamic roughness(Z0)and frictional wind speed(u*)contribute to the aerodynamic impedance of the grassland canopy.Thus,they are often used in the studies of wind erosion and evapotranspiration.However,the effect of wind speed and grazing measures on the aerodynamic impedance of the grassland canopy has received less analysis.In this study,we monitored wind speeds at multiple heights in grazed and grazing-prohibited grasslands for 1 month in 2021,determined the transit wind speed at 2.0 m height by comparing wind speed differences at the same height in both grasslands,and divided these transit wind speeds at intervals of 2.0 m/s to analyze the effect of the transit wind speed on the relationship among Z0,u*,and wind speed within the grassland canopy.The results showed that dividing the transit wind speeds into intervals has a positive effect on the logarithmic fit of the wind speed profile.After dividing the transit wind speeds into intervals,the wind speed at 0.1 m height(V0.1)gradually decreased with the increase of Z0,exhibiting three distinct stages:a sharp change zone,a steady change zone,and a flat zone;while the overall trend of u*increased first and then decreased with the increase of V0.1.Dividing the transit wind speeds into intervals improved the fitting relationship between Z0 and V0.1 and changed their fitting functions in grazed and grazing-prohibited grasslands.According to the computational fluid dynamic results,we found that the number of tall-stature plants has a more significant effect on windproof capacity than their height.The results of this study contribute to a better understanding of the relationship between wind speed and the aerodynamic impedance of vegetation in grassland environments.

Friction-stir-welded overaged 7020-T6 alloy joint: an investigation on the effect of rotational speed on the microstructure and mechanical properties

Commercial A7020-T6 plates in the overaged state were subjected to friction stir welding with four different tool rotational speeds of 500, 710, 1000, and 1400 r/min and a single traverse feed rate of 40 mm/min. The resultant changes in the welding heat input, microstructure, and the mechanical properties of the joints were investigated. The changes were related to the processes of growth, dissolution, and re-formation of precipitates. The precipitate evolution was examined by differential scanning calorimetry, and the microstructural analysis was conducted using optical, scanning, and transmission electron microscopes. The results showed that the grain size in the stirred zone(SZ) decreased substantially compared with the base metal, but increased with tool rotational speed because of the rise in temperature. We found that the width of the heat-affected zone increased with tool rotational speed. The hardness and the tensile strength in the SZ increased with increasing heat input compared with the base metal in the overaged condition. This recovery in mechanical properties of the joints can be attributed to the dissolution and re-formation of precipitates in the SZ and the thermomechanically affected zone. This process is referred to as an "auto-aging treatment."

Influence of welding speed on corrosion behaviour of friction stir welded AA5086 aluminium alloy

The plates of AA5086 aluminium alloy were joined together by friction stir welding at a fixed rotation speed of 1000 r/min various welding speeds ranging from 63 to 100 mm/min.Corrosion behavior of the parent alloy(PA),the heat affected zone(HAZ),and the weld nugget zone(WNZ)of the joints were studied in 3.5%(mass fraction)aerated aqueous Na Cl solution by potentiodynamic polarization and electrochemical impedance spectroscopy(EIS).The corrosion susceptibility of the weldments increases when the welding speed increases to 63 and 100 mm/min.However,the value of corrosion rate in the weldments is lower than that in the PA.Additionally,the corrosion current density increases with increasing the welding speed in the HAZ and the WNZ.On the contrary,the corrosion potential in the WNZ appears more positive than in the HAZ with decreasing the welding speed.The WNZ exhibits higher resistance compared to the HAZ and the PA as the welding speed decreases.The results obtained from the EIS measurements suggest that the weld regions have higher corrosion resistance than the parent alloy.With increasing the welding speed,the distribution and extent of the corroded areas in the WNZ region are lower than those of the HAZ region.In the HAZ region,in addition to the pits in the corroded area,some cracks can be seen around the corroded areas,which confirms that intergranular corrosion is formed in this area.The alkaline localized corrosion and the pitting corrosion are the main corrosion mechanisms in the corroded areas within the weld regions.Crystallographic pits are observed within the weld regions.

Friction Wear Property of Brake Materials by Copper-based Powder Metallurgy With Various Brake Speeds

The experiment is conducted on MM-1000 friction test machine, which tests friction wear property of copper-based brake materials by powder metallurgy at different brake speeds. It shows that the coefficient of friction and wear volume are greatly influenced by brake speed. When the brake speed is 4000 r/min, which is a bit higher, the material still has a higher coefficient of friction with 0.47. When the brake speed is over 4000r/min, the coefficient of friction decreased rapidly. When the brake speed is 3000r/min, the material’s wear is in its minimum. That is to say no matter how higher or lower the brake speed is the wear volume is bigger relatively. With the brake speed of the lower one it mainly refers to fatigue wear; while of higher one it mainly refers to abradant and oxidation wear.

Uphill anomalous transport in a deterministic system with speed-dependent friction coefficient

We investigate the transport of a deterministic Brownian particle theoretically, which moves in simple onedimensional, symmetric periodic potentials under the influence of both a time periodic and a static biasing force. The physical system employed contains a friction coefficient that is speed-dependent. Within the tailored parameter regime, the absolute negative mobility, in which a particle can travel in the direction opposite to a constant applied force, is observed.This behavior is robust and can be maximized at two regimes upon variation of the characteristic factor of friction coefficient. Further analysis reveals that this uphill motion is subdiffusion in terms of localization（diffusion coefficient with the form D（t） -t-（-1） at long times）. We also have observed the non-trivially anomalous subdiffusion which is significantly deviated from the localization; whereas most of the downhill motion evolves chaotically, with the normal diffusion.

Influences of tool plunge speed and tool plunge depth on friction spot joining of AA5454-O aluminum alloy plates with different thicknesses

AA5454-O aluminum alloy plates with the thicknesses of 1.4 and 1.0 mm were friction-spot-joined (FSJed).The plunge speed of the joining tool was changed in a range of 100 500 mm/min under a constant rotation speed of 500 r/min.The plunge depth was ranged from 1.6 mm to 2.2 mm.The tool plunge speed did not make a remarkable effect on the surface appearance and macro-structure of the FSJed zone.The average hardness of the FSJed zone was greater than or equal to that of the base metal.However,there was no remarkable tendency in the average hardness change of the FSJed zone in spite of the variation in the tool plunge speed and tool plunge depth.The increase of the tool plunge depth resulted in the increase of the tensile shear load.However,the change of the tool plunge speed did not lead to the remarkable variation in the tensile shear load of the FSJed plates.It was noteworthy that the FSJed plate exhibited the highest tensile shear load of about 4.0 kN.

Constraints on rupture speed of the 2001 MS 8.1 West Kunlun Mountain Pass earthquake by co-seismic surface rupture slip displacements based on the slip-weakening mechanism with frictional undershoot involved

With co-seismic surface rupture slip displacements provided by the field observation for the 2001 MS8.1 West Kunlun Mountain Pass earthquake, this paper estimates the rupture speed on the main faulting segment with a long straight fault trace on the surface based on a simple slip-weakening rupture model, in which the frictional overshoot or undershoot are involved in consideration of energy partition during the earthquake faulting. In contrast to the study of Bouchon and Vallée, in which the rupture propagation along the main fault could exceed the local shear-wave speed, perhaps reach the P-wave speed on a certain section of fault, our results show that, under a slip-weakening assumption combined with a frictional undershoot (partial stress drop model), average rupture speed should be equal to or less than the Rayleigh wave speed with a high seismic radiation efficiency, which is consistent with the result derived by waveform inversion and the result estimated from source stress field. Associated with the surface rupture mechanism, such as partial stress drop (frictional undershoot) associated with the apparent stress, an alternative rupture mechanism based on the slip-weakening model has also been discussed.

Effect of rotary direction and speed on mechanical properties in friction stir spot welding of A6061 sheets e

Friction stir spot welding of A6061 sheets was conducted using a tool with thread pin. The hook geometries, hook formation and mechanical properties of the joints welded with different rotary directions and speeds were investigated. The results show that the hook in the joint welded in clockwise was curved upwards and that in anticlockwise was curved downwards. The hook formation was related to the plastic material flow in the joint. With increasing the rotary speed in clockwise direction, the hook moved upwards and far way from the center of the keyhole, resulting in an increase in the effective weld width and a decrease in the effective sheet thickness. Three types of fractuces were observed and they were affected by the hook geometries. The tensile shear load increased firstly and then decreased when the rotary speed increased in clockwise direction, which was related to the hook geometries.

Effect of pin rotating speed on lap shear strength of stationary shoulder friction stir lap welded 6005A-T6 aluminum alloy

Stationary shoulder friction stir lap welding （SSFSLW） was successfully used to weld 6005A-T6 aluminum alloy in this paper. Effect of pin rotating speed on cross section morphologies and lap shear strength of the SSFSLW joints were mainly discussed. Results show that joints without flash and shoulder marks can be obtained by the stationary shoulder. Cross section of the SSFSLW joint presents a basin-like morphology and little material loss. By increasing the rotating speed from 1 000 rpm to 1 600 rpm, both effective sheet thickness and lap width increase, while lap shear failure load firstly decreases and then increases. The maximum failure load of 14. 05 kN /s attained when 1 000 rpm is used. All SSFSLW joints present shear fracture mode.

考虑制动、荷载转移、纵坡的雨天小客车弯道安全车速模型

为了确定降雨环境下小客车弯道安全车速,基于瞬时轴线,建立了考虑纵坡、制动加速度及轮胎荷载转移的小客车侧滑速度模型;通过实地采集不同水膜厚度下的摩擦系数,得到了这二者之间的回归方程;结合摩擦椭圆原理,得到了考虑制动、驾驶人行为特性的综合摩擦系数分配方法;提出确定弯道车速的计算流程图,得到了不同降雨强度下以横向侧滑和纵向停车为指标的弯道车速。研究结果表明:当水膜厚度为0时,得到的侧滑速度值比点质量模型低35%左右;当水膜厚度小于2 mm时,摩擦系数降低最快,对车速影响最大;当水膜厚度大于2 mm时,摩擦系数变化幅度小且规律不明显;降雨对小客车弯道安全车速影响显著,仅小雨时就会导致车速平均降低15%左右;取一般最小半径计算时,不同降雨强度下的纵向停车速度均小于横向侧滑速度。

列车刹车闸片冶金材料成分配比优化研究

为了促进我国列车刹车闸片粉末冶金材料的发展,研究分析了不同增强组元对粉末冶金材料制备的摩擦材料性能影响。研究分析的2种增强组元分别是羰基铁粉和磷铁粉,粉末冶金材料的基体为超细铜粉,润滑组元为镀铜石墨,摩擦组元为铬铁粉。结果显示,时速低于275 km/h时,磷铁粉材料的摩擦系数更高,时速高于275 km/h时,羰基铁粉的摩擦系数更高,时速为350 km/h时,羰基铁粉材料磨损量为0.625 mg/kJ,磷铁粉材料磨损量为1.634 mg/kJ,研究还探讨了磷铁粉及羰基铁粉对列车刹车材料摩擦性能的影响,对我国粉末冶金材料、制备列车刹车闸片的发展具有指导意义。

水下高速外流减摩擦阻力功效测试方案设计与验证

为了支撑高速水下航行器减摩擦阻力(摩阻)方案的选型和优化,本文提出较好的减摩阻试验研究技术方案和特征参量表达方法以提高测量精度的技术途径,并以聚氧化乙烯(PEO)减摩阻实际试验演示上述方案和途径的可行性。相关结果可为高速水下航行器减摩阻方案的选型和优化提供支撑。

双排高立式尼龙网阻沙障与草方格固沙障的优化配置及防沙效益

[目的]针对双排高立式尼龙网阻沙障与草方格固沙障联合应用,开展风洞试验研究,以优化联合沙障中尼龙网阻沙障孔隙度配置及探究各孔隙度下联合沙障的防沙效益。[方法]基于风洞模拟试验数据,开展不同孔隙度(40%,45%,55%)联合沙障,在不同风速下的风速流场、空气动力学粗糙度、摩阻风速、输沙率随高度和阻沙率方面的变化特征研究。[结果](1)相同风速下尼龙网阻沙障孔隙度40%时,联合沙障弱风区面积最大;(2)随着尼龙网阻沙障孔隙度的增大,空气动力学粗糙度和摩阻风速减小,相同风速和相同位置时,40%孔隙度下联合沙障空气动力学粗糙度和摩阻风速值最大;(3)风速增加地表输沙率也增加,联合沙障内部与背风侧输沙率随高度增加呈指数型递减,输沙主要集中在0~12 cm高度内;(4)风速大小与沙障阻沙率呈现负相关变化;阻沙率与孔隙度之间也呈负相关关系,孔隙度为40%的联合沙障具有较高阻沙率;双排高立式尼龙网阻沙措施与草方格固沙措施联合之间还存在阻沙率叠加。[结论]40%孔隙度的双排高立式尼龙网和草方格的阻固联合可以实现较优的配置与防沙效益。

BFe10-1-1摩擦塞补焊接头组织及性能研究

采用顶锻式摩擦塞补焊方法对5 mm厚的BFe10-1-1铜镍合金进行匙孔补焊实验,探讨了不同主轴转速下塞补焊接头的微观形貌、显微硬度、温度场分布及其力学性能。结果表明:主轴转速对铁白铜塞补焊成形有重要的影响,随着转速提高,接头底部缺陷逐渐缩小,塞棒区金属摩擦流线明显,晶粒呈破碎细化特征,接头从上到下,热影响区范围缩小明显。由于细晶强化作用,塞棒区显微硬度高于母材,且塞棒上侧显微硬度高于中下侧。在转速为2 100 r/min时得到接头拉伸强度最高,达到母材78%。接头表现出韧性断裂特征,随着转速提高,接头弱连接缺陷逐步下移,并不断缩小。

基于赫兹接触模型的发动机封严涂层碰磨力计算与优化

目的提高航空发动机的推进效率,在压气机机匣上喷涂可以减少叶尖径向间隙的封严涂层。叶尖与涂层之间的碰磨力会导致涂层脱落,且会击伤叶片,需要对碰磨力进行分析。方法采用大气等离子喷涂技术制备4种不同硬度的AlSi-PHB(聚苯酯)封严涂层,通过表面硬度测试、弹性模量测试和高速碰磨试验,分别评价封严涂层的硬度、弹性模量,以及在高速碰磨过程中不同工况下涂层受到的碰磨力;基于赫兹接触模型对叶尖与涂层之间的碰磨力进行计算,通过激光共聚焦显微镜和扫描电子显微镜对碰磨后的涂层和叶尖进行分析,同时根据接触面形态特征和温度特征对赫兹模型进行优化。结果碰磨力与涂层的硬度、叶尖转速、叶尖切入速率有关,复杂的接触表面形貌和摩擦升温会导致理论计算值与实验值之间出现偏差。结论通过优化叶尖和涂层的接触系数,同时考虑摩擦升温对涂层弹性模量的影响,可将不同工况下碰磨力计算值与测量值之间的偏差控制在1%~11%,这项研究对于指导航空发动机封严涂层的设计具有重要意义。

道面平整度对湿滑道面-飞机轮胎相互作用影响分析

湿滑条件下,跑道不平整使积水厚度随道面起伏而变化,影响飞机起降安全。因此,引入国际平整度指数(international roughness index,IRI),搭建基于IRI的轮胎-积水-道面有限元模型,通过仿真模拟,分析不同IRI的道面对飞机临界滑水速度的影响;结合IRI,对比不同飞机轮胎速度、积水厚度条件下道面视摩擦因数较平整道面的降低幅度,分析IRI对道面摩擦性能的影响;并依据某机场视摩擦因数实测试验结果,对仿真实验结果进行修正,探究IRI对飞机刹车效应的影响。结果表明:道面平整段积水厚度一定时,飞机临界滑水速度随IRI增加而降低,当IRI=5时,飞机临界滑水速度较平整道面降低约30%;飞机轮胎速度一定时,视摩擦因数随道面平整度等级降低而减小,当IRI=5时,视摩擦因数较平整道面降低幅度高达98%,严重影响飞机运行安全;道面平整度等级为“好”时,相同平整度段积水厚度下,飞机轮胎与道面视摩擦因数随IRI的增加而减小;综合考虑IRI和经试验数据修正后的视摩擦因数对飞机刹车效应产生的影响发现,随着平整度指数与平整段积水厚度逐渐增加,飞机刹车效应会产生降级。国内某机场道面在1.5<IRI≤2,平整段积水厚度≥7.66 mm的道面条件下,飞机刹车效应最大降低了3级,即由“中到差”降至“差”,此时飞机方向操纵与减速能力严重下降;积水道面平整度评价为“中”和“差”时,飞机刹车效应降级幅度突增,导致道面条件无法保障飞机运行安全;平整段积水厚度为7.66~13 mm时飞机刹车效应受IRI变化的影响较大,降级变化明显,在维护过程中须尤为注意。

焊接速度对Q&P980钢搅拌摩擦焊接头组织和性能的影响

通过搅拌摩擦焊接对Q&P980钢进行焊接,固定转速为400 r·min^(-1),焊接速度分别为50 mm·min^(-1)(低焊速)和600 mm·min^(-1)(高焊速),采用光学显微镜、扫描电子显微镜、电子背散射衍射、显微硬度和拉伸性能测试等手段对焊接接头的微观组织和力学性能进行了表征,研究了焊接速度对接头微观组织和力学性能的影响。结果表明:相较母材,接头整体残余奥氏体含量降低,搅拌区晶粒明显细化和均匀化,有大量马氏体生成;热影响区晶粒略微粗大,显微硬度降低。与低焊速接头相比,高焊速接头峰值温度降低,材料流动范围较窄,热力耦合作用减弱;热影响区宏观尺寸减小,残余奥氏体含量增加,显微硬度提高,接头具有更好的变形协调性,接头抗拉强度和伸长率分别从1074 MPa、6.0%提高至1268 MPa、15.3%。