Sliding state stepping algorithm for solving impact problems of multi-rigid-body system with joint friction

Impact dynamics of multi-rigid-body systems with joint friction is considered. Based on the traditional approximate assumption dealing with impact problem, a general numerical method called the sliding state stepping algorithm is introduced. This method can avoid difficulties in solving differential equations with variable scale and its result can avoid energy inconsistency before and after impact from considering complexily of tangential sliding mode. An example is given to describe details using this algorithm.

Identification of robot dynamic model and joint frictions using a baseplate force sensor

Dynamic models play an important role in robot control and applications.The accurate identification of dynamic models has become crucial to meeting increasing performance requirements.Owing to the inertial forces and the joint frictions coupling,the identification first requires a parametrized friction model.However,the joint frictions are strongly nonlinear and vary with many factors including posture,velocity and temperature.Hence,all friction models have some deviation from the real values,which reduces the identification accuracy.This paper proposes an identification approach using a baseplate force sensor.It identifies the inertial parameters first and then computes the joint friction values by subtracting the inertial torques from the joint torques.This method has the advantage that it does not require a priori friction model.It can choose or construct a proper model to fit the real values and is thus expected to achieve high performance.Experiments on a 6-DoF robot were conducted to verify the proposed method.

Effect of intermetallic compounds on the fracture behavior of dissimilar friction stir welding joints of Mg and Al alloys

Joining Mg to Al is challenging because of the deterioration of mechanical properties caused by the formation of intermetallic compounds(IMCs) at the Mg/Al interface. This study aims to improve the mechanical properties of welded samples by preventing the fracture location at the Mg/Al interface. Friction stir welding was performed to join Mg to Al at different rotational and travel speeds. The microstructure of the welded samples showed the IMCs layers containing Al12Mg17(γ) and Al3Mg2(β) at the welding zone with a thickness(< 3.5 μm). Mechanical properties were mainly affected by the thickness of the IMCs, which was governed by welding parameters. The highest tensile strength was obtained at 600 r/min and 40 mm/min with a welding efficiency of 80%. The specimens could fracture along the boundary at the thermo-mechanically affected zone in the Mg side of the welded joint.

Corrosion behavior of the friction-stir-welded joints of 2A14-T6 aluminum alloy

The corrosion behavior of friction-stir-welded 2A14-T6 aluminum alloy was investigated by immersion testing in immersion exfoliation corrosion（EXCO） solution. Electrochemical measurements（open circuit potential, potentiodynamic polarization curves, and electrochemical impedance spectroscopy）, scanning electron microscopy, and energy dispersive spectroscopy were employed for analyzing the corrosion mechanism. The results show that, compared to the base material, the corrosion resistance of the friction-stir welds is greatly improved, and the weld nugget has the highest corrosion resistance. The pitting susceptibility originates from the edge of Al-Cu-Fe-Mn-Si phase particles as the cathode compared to the matrix due to their high self-corrosion potential. No corrosion activity is observed around the θ phase（Al2Cu） after 2 h of immersion in EXCO solution.

Modeling joints with clearance and friction in multibody dynamic simulation of automotive differentials

Defects in kinematic joints can sometimes highly influence the simulation response of the whole multibody system within which these joints are included. For instance, the clearance, the friction, the lubrication and the flexibility affect the transient behaviour, reduce the component life and produce noise and vibration for classical joints such as prismatics, cylindrics or universal joints.In this work, a new 3D cylindrical joint model which accounts for the clearance, the misalignment and the friction is presented. This formulation has been used to represent the link between the planet gears and the planet carrier in an automotive differential model. C 2013 The Chinese Society of Theoretical and Applied Mechanics. [doi：10.1063/2.1301303]

Fatigue properties of friction stir welded butt joint and base metal of MB8 magnesium alloy

The fatigue properties of friction stir welded （FSW） butt joint and base metal of MB8 magnesium alloy were investigated. The comparative fatigue tests were carried out using EHF-EM2OOK2-070-IA fatigue testing machine for both FSW butt joint and base metal specimens. The fatigue fractures were observed and analyzed using a scanning electron microscope of JSM-6063LA type. The experimental results show that the fatigue performance of the FSW butt joint of MB8 magnesium alloy is sharply decreased. The conditional fatigue limit （2 x 106） of base metal and welded butt joint is about 77.44 MPa and 49. 91 MPa, respectively. The conditional fatigue limit （2 x 106 ） of the welded butt joint is 64.45% of that of base metal. The main reasons are that the welding can lead to stress concentration in the flash area, tensile welding residual stress in the welded joint（ The residual stress value was 30. 5 MPa）, as well as the grain size is not uniform in the heat-affected zone. The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface, the fracture type of the FSW butt joint belongs to a brittle fracture.

Influence of tool pin profile on the interface migration of friction stir lap joints＊

Friction stir lap joints of LY12 aluminum alloy plates with a thickness of 3 mm were fabricated using several tools with different pin profiles. The effects of tool pin profile on the interface migration of friction stir lap joints were investigated with the comparison of weld morphologies. The results show that the screw thread of the pin plays an important role in the migration of weld interface in the thickness direction. The interface between the sheets will move upwards to the top of the plate when the pin with left hand thread was used. Conversely, the interface will move downwards to the tip of the pin when the pin with right hand thread was used： As for a stir pin with smooth surface was used, the upward or downward migration of the weld interface was largely reduced, but the extension of weld interface to the weld center line from the retreating side becomes more serious. By analyzing the force on the pin according to the sucking-extruding theory for the weld formation, the obtained results have been well explained.

Design of Robust Controller for Flexible Joint Robot with Nonlinear Friction Characteristics

A robust controller method for flexible joint robot considering the effect caused by nonlinear friction was presented.The nonlinear friction was denoted as inverse additive output uncertainty relative to the nominal model in our work,based on which the describing function was analyzed in frequency domain,and the weighting function of nonlinear friction was further calculated as well. By combining the friction uncertainty,the mixed sensitivity H∞optimization was proposed as the benchmark for controller design, which also leaded to good performance of robustness. Furthermore,unstructured perturbation to the system was analyzed so that the stability was guaranteed. Simulation results show that the proposed controller can provide excellent tracking and regulation performance.

Effect of tool pin profile on microstructure and tensile properties of friction stir welded dissimilar AA 6061e AA 5086 aluminium alloy joints

Joints between two different grades of aluminium alloys are need of the hour in many light weight military structures.In this investigation,an attempt has been made to join the heat treatable(AA 6061) and non-heat treatable(AA 5086) aluminium alloys by friction stir welding(FSW)process using three different tool pin profiles like straight cylindrical,taper cylindrical and threaded cylindrical.The microstructures of various regions were observed and analyzed by means of optical and scanning electron microscope.The tensile properties and microhardness were evaluated for the welded joint.From this investigation it is founded that the use of threaded pin profile of tool contributes to better flow of materials between two alloys and the generation of defect free stir zone.It also resulted in higher hardness values of 83 HV in the stir zone and higher tensile strength of 169 MPa compared to other two profiles.The increase in hardness is attributed to the formation of fine grains and intermetallics in the stir zone,and in addition,the reduced size of weaker regions,such as TMAZ and HAZ regions,results in higher tensile properties.

Comparison and analysis of two Coulomb friction models on the dynamic behavior of slider-crank mechanism with a revolute clearance joint

The objective of this study is to investigate the effects of the Coulomb dry friction model versus the modified Coulomb friction model on the dynamic behavior of the slider-crank mechanism with a revolute clearance joint. The normal and tangential forces acting on the contact points between the journal and the bearing are described by using a Hertzian-based contact force model and the Coulomb friction models, respectively.The dynamic equations of the mechanism are derived based on the Lagrange equations of the first kind and the Baumgarte stabilization method. The frictional force is solved via the linear complementarity problem(LCP) algorithm and the trial-and-error algorithm.Finally, three numerical examples are given to show the influence of the two Coulomb friction models on the dynamic behavior of the mechanism. Numerical results show that due to the stick friction, the slider-crank mechanism may exhibit stick-slip motion and can balance at some special positions, while the mechanism with ideal joints cannot.

Defects of friction stir welded 1060 aluminum alloy and their effects on joint tensile property

9. 6 mm thick 1060-H24 aluminum alloy plates were friction stir welded and the influencing factors on groove and tunnel defects were examined. Results show that the welding speed range for achieving a groove-free joint is enlarged with increasing the rotating speed. The tunnel size decreases with decreasing the welding speed under the same rotating speed. Excessive or insufficient shoulder plunge depth will cause defective joints. At a relatively low rotating speed and high welding speed, the tool having a larger shoulder diameter has a larger range of processing parameters to obtain a groove-free joint. Moreover, the tensile fracture behaviors of the defective and defect-free samples are different.

深中通道沉管隧道最终接头MGB滑轨摩擦系数试验研究

深中通道沉管隧道最终接头采用了水下推出式对接方式,推出过程中采用MGB滑轨作为底部滑行系统,为分析结构受力,建立了与施工现场荷载相同的试验结构,研究了深中通道沉管隧道最终接头MGB滑轨与防腐涂层在不同施工荷载和润滑状态下的摩擦系数,并与厂家提供的摩擦系数进行了对比。研究结果表明:MGB材料的摩擦系数随着荷载的增大而减小,干摩擦情况下的静摩擦系数范围为0.181~0.266,滑动摩擦系数范围为0.150~0.226,在水润滑条件下摩擦系数小于干摩擦,摩擦系数与接触界面有关,工程中建议根据实际荷载和摩擦界面情况对摩擦系数进行测试。

连接界面黏滑摩擦行为的非线性载荷重构方法

提出一种连接界面黏滑摩擦行为的非线性载荷重构方法。基于非线性动力学子结构建模方法,将复杂连接结构视作线性子结构和非线性连接两部分,通过虚拟非线性激励载荷进行关联。以线性子结构为研究对象,采用两次动刚度矩阵求逆的反转技术消除传递函数的病态奇异性,建立观测点自由度和非线性连接自由度之间的传递关系。考虑连接界面黏滑摩擦行为的影响,将观测的非线性动力响应进行谐波级数展开,直接辨识连接界面的局部非线性接触载荷。利用螺栓连接梁结构的数值仿真和实验结果进行验证。结果表明:重构的虚拟非线性激励载荷能够直接反映连接界面黏滑摩擦行为的影响,并且能够有效地识别螺栓连接界面预紧性能的变化。

柱端盖板型滑移摩擦节点分层装配钢框架滞回性能研究

分层装配式钢框架是一种能充分体现快速装配性的新型结构形式。基于低损伤抗震概念,针对柱端盖板型滑移摩擦节点(CPSFJ)分层装配式钢框架的滞回性能进行了试验研究和数值分析。研究发现,通过在柱端设置CPSFJ,可以有效实现在大震作用下以滑移摩擦耗能代替结构构件的塑性损伤耗能,进而保护结构主体构件免受损伤。试验结果表明,该结构体系在低周往复加载下,滞回曲线呈现出饱满的弓形,且骨架曲线表现为三段式平顶型,显示出良好的能量耗散能力和稳定的滞回性能。为解决梁端局部刚度不足的问题,提出了梁端加劲肋布置改进方案,能显著改善梁端应力集中现象,降低塑性损伤的可能性。本研究为分层装配式钢框架结构设计提供了新的抗震耗能方案,具有重要的理论意义和应用价值。

7075–O铝合金搅拌摩擦焊接头微观组织与力学性能

时效硬化态7075铝合金搅拌摩擦焊接头塑性较低,给进一步变形加工带来困难。本文采用3 mm厚的7075–O铝合金板进行搅拌摩擦对接焊,研究焊接速度对接头组织和强塑性的影响,为获得高塑性的搅拌摩擦接头提供解决方案。结果表明,7075–O铝合金接头焊核区为细小的等轴晶粒,随焊接速度的增加,焊核区平均晶粒尺寸呈先减小后缓慢增加的趋势,当焊接速度为40 mm/min时,焊核区晶粒尺寸最小,约为1.96μm。不同焊接速度下接头横截面显微硬度呈“凸”形分布,接头区域发生明显硬化,其中焊核区硬度最高,约为145HV。7075–O铝合金搅拌摩擦焊工艺窗口较宽,焊接速度在20~80 mm/min范围内接头强度均高于母材强度,焊接件伸长率达母材的82%,采用7075–O铝合金不仅能够克服时效硬化态7075铝合金搅拌摩擦焊接头的软化问题,而且能够获得更高的伸长率。接头弯曲试验结果表明,7075–O铝合金搅拌摩擦焊接头能够承受更大的弯曲角度,极限弯曲角达105°,具备更加出色的塑性变形能力。

干湿循环下节理砂岩细观能量演化规律研究

为探究干湿循环作用下的节理砂岩的细观损伤劣化规律,采用颗粒流软件开展了节理砂岩细观数值模拟,研究了不同干湿循环下节理砂岩的细观力学特性和能量演化机制。研究表明:随着干湿循环次数的增大,颗粒间的接触力和平行黏结力最大值呈线性递减规律;干湿循环下节理砂岩裂纹演化主要经历缓慢增加、均匀增加、阶梯增加及突变增加4个阶段,且裂纹数目(总裂纹、拉伸裂纹及剪切裂纹)与干湿循环次数呈正相关;细观边界能、平行黏结应变能及线性黏结应变能随干湿循环次数的增加呈逐渐减小趋势,节理砂岩中耗散能与微裂纹数大致呈正相关;干湿循环作用导致颗粒间抗断裂能力弱化,使岩样能量储存能力降低,这是造成节理砂岩干湿损伤劣化的能量机制。

带非对称摩擦连接的RCS梁柱节点抗震性能与构造参数研究

基于非对称摩擦连接(AFC)的耗能特点,提出了一种新型的钢筋混凝土柱-钢梁(RCS)梁柱节点,对3组采用AFC连接的RCS梁柱节点进行往复加载试验,考虑了螺栓预紧力、摩擦垫片材料等构造因素对节点抗震性能的影响.结果表明:3组带摩擦连接RCS试件均为梁铰机制破坏,满足“强节点,弱构件”的抗震设计要求;3组试件的滞回曲线具有明显的双平台耗能,滞回曲线饱满稳定,有利于实现两阶抗震设防目标;耐磨钢材料与黄铜材料的两组试件的承载力相近,滞回曲线特征相似,都具有良好的耗能能力;随着螺栓预紧力的增加,试件获得的承载力、刚度也随之大,耗能能力得到了提升,在实际应用中,可通过合理控制预紧力来满足预期所需的滑移荷载与耗能能力.

SS304和Q235异种钢搅拌摩擦焊接头腐蚀有限元模拟

针对异种金属焊接接头易于发生电偶腐蚀,基于腐蚀电化学原理,将动电位极化曲线参数和金相组织面积比例等作为边界条件,采用有限元分析软件COMSOL对Q235低碳钢和304不锈钢(SS304)异种钢FSW接头不同区域腐蚀行为进行了预测,研究了焊接接头不同区域宏观和微观电偶腐蚀电位分布和电流密度的变化规律。结果表明,在宏观电偶腐蚀中,腐蚀深度的最大值位于SS304后退侧(RS_(304))与搅拌区(SZ)的交界处,约为Q235低碳钢母材(BM_(Q235))的5倍,各区电流密度随腐蚀时间的增长而增大,在微观电偶腐蚀中,腐蚀深度的最大值位于SS304侧搅拌区(SZ_(304))与Q235侧搅拌区(SZ_(Q235))的交界处,约为BM_(Q235)的1.2倍,并且珠光体组织面积比例越大,耐蚀性越差。采用该有限元模型可以预测异种钢搅拌摩擦焊接头不同区域的腐蚀行为。

6082–T6/7075–T6异种铝合金搅拌摩擦焊接接头应力腐蚀行为

为提高航空、航天运载工具异种高强度铝合金零部件搅拌摩擦焊接接头在腐蚀环境下的服役可靠性,采用金相分析、电子显微分析和四点弯曲应力腐蚀法,对经过T6处理的6 mm厚6082–T6/7075–T6异种铝合金搅拌摩擦焊接接头应力腐蚀行为进行了研究。结果表明,搅拌头转速1200 r/min、焊接速度80 mm/min条件下,6082–T6/7075–T6母材获得了具有“洋葱环”结构的致密、成形良好的搅拌摩擦焊接接头组织。6082–T6和7075–T6母材搅拌摩擦焊接接头四点弯曲应力腐蚀主要发生在后退侧的7075–T6铝合金一侧,而前进侧6082–T6铝合金一侧表现出较好的耐蚀性。这主要是由于异种铝合金母材电极电位差异导致焊接接头发生应力作用下的电偶腐蚀所致。此外,α–Al基体与第二相形成微区电偶对,以及焊接接头热机械影响区(TMAZ)、热影响区(HAZ)和母材区(BM)存在的大角度晶界也是造成应力腐蚀的重要原因。