Einfluß trockener Friktionsbeanspruchungen auf Schädigungsverhalten,Restfestigkeit und Oberflächeneigenspannungen von Al2O3

Influence of dry frictional loading on damage behaviour, bending strength and surface residual stresses of Al₂O₃ ceramics The present contribution gives results of dry friction tests of an Al₂O₃‐plate against pellets made of grey cast iron. The influence of the frictional loading on the damage and fracture behaviour as well as on the surface residual stresses within the ceramic plate are presented and discussed. Furthermore FEM simulations of the friction system are given. The damage of the Al₂O₃ is mainly determined by the friction induced heat flux dQ/dt. At dQ/dt values higher than 1.3 kJ/s small radial surface cracks within the friction area occur. The location of crack initiation as well as the direction of crack propagation are predicted correctly by the FEM analysis. Increasing dQ/dt lead to additional tangential cracks connecting the initial radial cracks and, consequently, leading to a fragmentation of the alumina plate. The thermally induced radial cracks show a transcrystalline, the secondary radial cracks a mainly intercrystalline crack path. The occurrence of fine radial cracks is connected with a decrease of the bending strength of the Al₂O₃ of about 20%. After one friction cycle a bimodal strength distribution is observed. In spite of the fact that cyclic frictional loading does not alter the macroscopic damage behaviour with respect to one cycle, it leads to a more unimodal distribution of the residual strength. The residual stress state changes from compressive surface stresses in the as‐received state to biaxial tensile residual stresses with strong gradients after frictional loading.     

Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

基于新型自复位摩擦耗能支撑的RC框架结构地震残余变形控制

基于Cu-Al-Be合金丝,研制了一种新型自复位摩擦耗能支撑,通过试验研究了该支撑的滞回性能,并利用OpenSEES平台建立了分析模型,将其应用于RC框架结构的地震残余变形控制。结果表明:摩擦力越大,该支撑耗能性能越好,但相应会增大残余位移;利用OpenSEES分析模型计算的滞回曲线与试验曲线吻合较好,表明所建立的分析模型具有良好的计算精度;在RC框架结构地震残余位移较大处布置该支撑,能够经济有效地控制结构整体的残余位移。     

岩体结构面切向循环加载本构关系研究

回顾岩体结构面切向循环加载的力学试验和数值模型,并建立新的本构模型。在粘接状态下,结构面切向本构关系表示为由双曲线和直线段组成的滞回曲线。在接触状态由粘接变为滑移的过程中,峰值摩擦角以双曲线函数逐渐降低至残余摩擦角,体现出峰值剪切特性。考虑结构面切向与法向耦合的剪胀关系,建立相应的接触刚度矩阵,并引入初始剪胀角和残余剪胀角来建立分段抛物线形式的剪胀曲线。循环加载导致的磨损对结构面的摩擦和剪胀特性均产生影响,通过以切向塑性功指数函数表示摩擦角和剪胀角的磨损过程。模型在物理意义上反映了切向循环加载的特性,计算结果能较好地拟合试验曲线。     

Werkstoffanstrengung bei Wälzbeanspruchung – Einfluß von Reibung und Eigenspannungen

Material Stressing under Rolling Contact – Influence of Friction and Residual Stresses Material stressing of parts in rolling contact is in the main made up by the normal load to be transmitted, by surface friction due to slippage of the body in rolling contact and by residual stresses. The effects of varying slippage rates are described as well as the additional influence of residual stresses on magnitude, position and direction of load. By the aid of mere mechanical reflections the formation of flat and steep White Bands can be interpreted which are observed in long-lived ball bearings subjected to high stresses. The report shows that residual stresses and friction must not be neglected when describing the material stressing by rolling contact. This especially applies to residual stresses originating from rolling contact.     

固定短芯棒拔管残余应力分布规律研究

为了研究固定短芯棒拔管残余应力的产生机理和分布规律,采用弹塑性有限元法模拟拉拔过程,讨论模具形状参数、拉拔变形参数、摩擦条件对残余应力的影响.结果表明,当模具半锥角α较小时,随着α的增加,残余应力增大;在减径率εs一定的条件下,随着减壁量ΔS的增加,钢管内外表面的残余应力逐渐减小,在ΔS一定而εs增加时,内外表面上残余应力呈先增大后减小变化趋势;同时,轴向残余应力随着拉拔速度v和摩擦系数μ的增加而增大.     

法向载荷对紫铜的微米划痕测试的影响

使用Rockwell C金刚石圆锥压头对紫铜进行微米划痕实验,研究了法向载荷对样品的微米划痕测试的影响。结果表明:随着法向载荷的增大,压入深度和残余深度均线性增加,弹性恢复率线性减小;划痕宽度随压入深度的增加先非线性地增大,之后趋于线性增加。当法向载荷在0.08~0.11N的范围内时,摩擦力线性增大,摩擦系数趋于一个常数,摩擦机制为粘着摩擦;当法向载荷在0.11~17N的范围内时,摩擦力和摩擦系数非线性地增大,摩擦机制为犁沟摩擦;当法向载荷在17~28N的范围内时,摩擦系数趋于一个常数,摩擦力线性增大,摩擦机制为微切削。     

搅拌头尺寸变化对搅拌摩擦焊接的影响

采用有限元方法模拟了搅拌摩擦焊接过程中材料的流动行为以及残余应力的分布情况,结果显示,材料的切向流动构成搅拌头周围材料流动的主要形式,并且搅拌头周围材料的运动有较为明显的分层现象,随着搅拌头直径的不断增加,这种分层流动的现象变得更加明显.纵向残余应力的最大值产生在热影响区的边界,残余应力呈现典型的双峰特征,搅拌头直径的增加会导致高水平拉伸残余应力的分布区域变大.     

摩擦耗能型SMA杆自复位梁柱节点滞回性能分析

在梁柱节点中引入NAO（非石棉）摩擦耗能器和超弹性形状记忆合金（SMA）杆,形成摩擦耗能型SMA杆自复位梁柱节点（NAO-SMA-SC）,可有效解决传统梁柱节点震后残余变形较大和耗能较低的问题。该文分析了其构造和工作机理,针对节点中SMA杆大应变需求,基于Lagoudas模型提出SMA杆的应变强化段改进本构模型,并将其嵌入到有限元软件OpenSees;建立了NAO-SMA-SC节点的杆系分析模型,考察了循环加载机制下SMA杆、NAO摩擦耗能器、间隙单元随节点转动时的受力行为;系统分析了耗能器摩擦力F_f、SMA杆直径D、SMA杆预应变P等关键参数对NAO-SMA-SC节点滞回性能的影响。结果表明:在节点中引入摩擦耗能器,能明显地提高节点抗弯能力和耗能能力,但同时会增大节点残余变形;随着SMA杆直径的增大,节点抗弯能力和自复位性能均显著提高;SMA杆预应变的施加能有效降低节点的残余变形。     

云母和纤维组合增强聚丙烯复合材料Ⅱ.界面性能研究

用单丝拔出法测定了云母和纤维组俣增强聚丙烯合材料中纤维与基体之间的界面结合强度，测定了云母和纤维组合增强聚丙烯复合材料的结晶温度，动态储存模量和线膨胀系数，并计算了纤维因基体体积收缩而形成的径向热残余压缩应力和基体与纤维之间的摩擦系数，研究了云母对纤维和基体界面性能的影响，结果表明，云母的加入使纤维受到的残余应力增加，摩擦系数显著下降，导致基体与纤维界面的结合强度随云母含量的增加先上升后下降。     

转速对 7075-T651 铝合金搅拌摩擦焊接件内部残余应力分布的影响

目的掌握铝合金搅拌摩擦焊接板内部残余应力的分布,为控制焊接残余应力、改进焊接工艺和提高焊接件质量。方法以13.6 mm厚的7075-T651铝合金为研究对象,用短波长X射线衍射技术,对在不同搅拌头转速下搅拌摩擦焊接板内部的残余应力进行了无损测试,并对焊接接头在板厚中心层上的微观组织和显微硬度进行了研究。结果在垂直于焊缝截面上的显微硬度均呈"W"型分布,焊核区的显微硬度高于其两侧的热机械影响区和热影响区,但低于母材区的硬度;随着转速的增大,接头硬度的最小值减小,低硬度区的范围越大。横向残余应力绝对值整体小于焊接方向;焊核区为正应力,热机械影响区残余应力减小且变化梯度最大;残余应力的极大值位于热影响区和热机械影响区的交界处;残余应力极大值与硬度最小值的位置重合。结论通过残余应力的无损检测分析,不仅可以直接获得加工件内部应力分布,还可以间接获得加工件内部的加工缺陷情况,为改进加工工艺、提高成形精度提供依据。     

Effects of thermal treatment on the tribological characteristics of thermoplastic polymer film

Friction tests were carried out using a microtribometer to investigate the effect of thermal treatment on microscale friction and wear between poly(methyl methacrylate) (PMMA) film and a fused silica lens. Two films were examined: one that was baked at 413 K for 2 min and one that was baked at 433 K for 24 h. The friction forces on the PMMA films were measured under atmospheric conditions as the temperature of the films was increased from 300 K to 443 K. The contact area between the films and the lens was also examined. As the temperature increased, the friction force increased for both films. The slope of the friction force with temperature and the contact area varied, depending on the state of the film surface; glassy, rubbery, and viscous flow states. The baking conditions also affected the slope, contact area, and wear generation. For temperatures at which the samples were in a glassy state, wear particles were not generated on the sample baked for 24 h. The results demonstrate that the tribological characteristics can be altered by the thermal treatment of the PMMA film as well as the temperature. When the film contains some residual solvent, the residual solvent in the PMMA film can diffuse to the PMMA surface due to heating and thus decrease the friction force under room-temperature conditions.     

Artgleiche und artverschiedene reibrührgeschweißte Verbindungen aus AA2124+25 % SiC und AA2024

Similar and dissimilar friction stir welded joints made from AA2124+25 % SiC and AA2024 An aluminium matrix composite (AMC) consisting of an AA2124 matrix reinforced by 25 vol.% SiC particles was used to produce similar AMC+AMC and dissimilar AMC+2024‐T3 joints by friction stir welding. When the particle reinforced composite was located on the retreating side, material mixing was less intense for dissimilar joints. Nevertheless, a higher strength has been determined for this arrangement due to a hook‐like interlocking of both materials. Tensile test and S‐N fatigue behaviour is shown to be compromised by alignment of the reinforcement particles perpendicular to loading direction already in the particle reinforced base material. Welding residual stresses were determined through the cut‐compliance method in terms of stress intensities acting at the crack tip. The underlying residual stress distribution in the un‐cracked structure was calculated by the weight function method. Longitudinal tensile residual stresses were found to be higher in the monolithic material as compared to the particle reinforced composite. This held true both for similar and within dissimilar joints. Growth behaviour of cracks crossing the joint line was described and correlated with residual stresses for similar joints.     

基于热力耦合模型铝合金搅拌摩擦焊接顺序优化仿真分析

基于所建立的搅拌摩擦焊接过程热输入模型和热力耦合有限元分析模型,对带圆孔铝合金板的搅拌摩擦焊接顺序进行了有限元研究,并得到了焊接过程中不同焊接方案下的温度、瞬态应力变化曲线以及焊后残余变形。通过综合比较可以看出,从两边向中间焊的方案2优于从中间向两边焊的方案1和从左向右的顺序焊接的方案3。此外,有限元模型求解结果和实验测量数据具有相当好的吻合。     

Plain fatigue and fretting fatigue behaviour of plasma nitrided Ti-6Al-4V

Fatigue tests with and without fretting against unnitrided fretting pads were conducted on unnitrided and plasma nitrided Ti-6Al-4V samples. Plasma nitrided samples exhibited higher surface hardness, higher surface compressive residual stress, lower surface roughness and reduced friction force compared with the unnitrided specimens. Plasma nitriding enhanced the lives of Ti-6Al-4V specimens under both plain fatigue and fretting fatigue loadings. This was explained in terms of the differences in surface hardness, surface residual stress, surface roughness and friction force between the unnitrided and nitrided samples.     

Effects of friction on roller expanded tube–tubesheet joint strength

Roller expansion of new tubes in enlarged heat exchanger tubesheet holes requires higher rolling torques which may result in over-thinning of the tube wall. This affects adversely the tube–tubesheet joint strength, which is measured in terms of residual contact pressure between the expanded tube and tubesheet. The axial force required to cause the mechanical failure of the tube–tubesheet joint has therefore been considered as an indication of the integrity of the joint. This force is influenced by several parameters such as the type of tube and tubesheet materials, the initial clearance and coefficient of friction. In the present work, an axisymmetric finite element (FE) model is used to evaluate the combined effects of friction between tube and tubesheet, initial clearance and tube material strain hardening on the strength of the tube–tubesheet joint. The FE results show that the increase in friction between tube and tubesheet results in higher residual contact stress and lower cutoff clearances. The residual contact stress also increased linearly with increasing tube material strain hardening level for all friction coefficients.     

Analyzing the mechanisms of fatigue crack initiation and propagation in CRH EMU brake discs

A significant number of high-speed electric multiple units’ (EMU) brake discs, manufactured from forged steel, showed thermal cracks during work and NDT. There exist three kinds of cracks on the friction surface; namely, the crackle, radial crack and circumferential crack. Macro-morphologies of the friction surface indicate that the cracks appeared in the interior and edges of the hotspots. Crack growth methods include the single crack propagation and multiple crack connectivity. A finite element analysis (FEA) was performed to determine temperature and stress distribution in the brake disc as well as to estimate stress distribution during braking. Simulation results indicate more significant residual, circumferential tensile stress on the external friction surface after emergency braking. The maximum residual circumferential tensile stress is 200 MPa after 300 km/h emergency braking. In addition, there is only the circumferential compressive stress on a section which is a certain distance from the exterior of the friction surface, and the distance depends on braking conditions. Therefore, not taking into account thickness reduction of the friction surface due to wear, it can be concluded that when the cracks run along the thickness direction to the specified distance, they will cease to run along this direction and begin propagating mainly in the direction of the radius. In addition, based on the simulation results, a measure was presented to prevent and inhibit the crack propagation.     

搅拌摩擦焊残余应力研究现状及展望

搅拌摩擦焊是一种新型的固相焊接技术,目前国内外的研究主要集中在工艺、组织、力学性能等方面,而有关搅拌摩擦焊缝内部残余应力的研究相对较少。由于检测设备的限制,国内主要采用有损检测方法,无损检测方法却很少。详细综述了国内有关搅拌摩擦焊件内部残余应力测试的钻孔法、云纹干涉法,以及国内外无损检测方法中的X射线衍射法、中子衍射法和高能同步辐射法的研究概况,讨论了各种检测方法的优缺点,对未来搅拌摩擦焊残余应力的研究作了展望。     

Monte Carlo calibration of avalanches described as Coulomb fluid flows

The idea that snow avalanches might behave as granular flows, and thus be described as Coulomb fluid flows, came up very early in the scientific study of avalanches, but it is not until recently that field evidence has been provided that demonstrates the reliability of this idea. This paper aims to specify the bulk frictional behaviour of snow avalanches by seeking a universal friction law. Since the bulk friction coefficient cannot be measured directly in the field, the friction coefficient must be calibrated by adjusting the model outputs to closely match the recorded data. Field data are readily available but are of poor quality and accuracy. We used Bayesian inference techniques to specify the model uncertainty relative to data uncertainty and to robustly and efficiently solve the inverse problem. A sample of 173 events taken from seven paths in the French Alps was used. The first analysis showed that the friction coefficient behaved as a random variable with a smooth and bell-shaped empirical distribution function. Evidence was provided that the friction coefficient varied with the avalanche volume, but any attempt to adjust a one-to-one relationship relating friction to volume produced residual errors that could be as large as three times the maximum uncertainty of field data. A tentative universal friction law is proposed: the friction coefficient is a random variable, the distribution of which can be approximated by a normal distribution with a volume-dependent mean.     

轨道车辆型材结构搅拌摩擦焊残余状态研究

目的 研究轨道车辆中典型的箱型和V筋结构型材搅拌摩擦焊残余状态。方法 采用顺序热力耦合方法进行了搅拌摩擦焊残余应力及变形的仿真研究。结果 给出了上下壁板焊缝区域的残余应力大小及分布规律,阐述了两种结构的横向和纵向残余变形特征。对V筋结构型材,分析了不同筋板角度对焊接残余状态的影响。结论 残余应力方面,主要表现为纵向残余应力,上下壁板两条焊缝的残余应力峰值相近,高应力分布区域宽度差别较大。残余变形方面,箱型结构呈马鞍形,V筋型材呈下凹形。V筋型材筋板夹角的变化对残余应力没有显著影响。筋板角度增大,纵向残余变形减小而横向残余变形增大。