发动机机体微动疲劳实验机构设计与分析

针对发动机机体与主轴承盖接触面之间存在的微动疲劳,采用接触面模拟件进行微动疲劳实验时,发现试件和方足桥的接触面不能很好的完全接触以及由于左右加载不同步导致试件产生额外弯矩等一系列问题。针对实验中出现的问题,考虑装夹与加载的方便性和准确性,采用PRO/E自顶向下的设计方法设计了一套进行发动机机体微动疲劳实验的实验件及实验机构。对实验件进行了微动疲劳实验仿真,对实验机构的关键部位进行了有限元分析校核。结果表明:所设计的实验件及实验机构满足实验要求,为提高微动疲劳实验的准确性提供了理论依据。     

钛合金TC11微动疲劳特性研究

以航空发动机榫连接结构微动疲劳问题的简化模型为研究对象,设计和制造了一套采用液压加载方式来实现微动疲劳法向载荷施加的试验装置,用于研究钛合金TC11微动疲劳的损伤过程,并对试验过程中在接触区域萌生裂纹的试件断口进行观测。研究结果表明:保持法向载荷恒定不变,增加轴向载荷将减少微动疲劳寿命。同样,保持轴向载荷恒定不变,法向载荷对微动疲劳寿命影响不如轴向载荷显著。另外,等效应力和滑移幅值是微动疲劳寿命的主要影响因素。     

压装轴微动疲劳主裂纹萌生位置的分析

为研究压装轴微动疲劳主裂纹的萌生位置,进行由锁紧环、压装垫环和压装轴试样组成的过盈配合结构的旋转弯曲加载条件下的微动疲劳试验,观察不同名义弯曲应力对应的试样的主裂纹萌生位置,发现主裂纹位于比张开区更深的接触内部。针对试验加载条件,采用有限元软件ANSYS,进行弹性有限元仿真分析,运用Ruiz法预测不同名义弯曲应力下试样的主裂纹萌生位置,并将Ruiz法的预测结果与疲劳试验的测量结果进行比较。结果表明,随着名义弯曲应力的增加,预测误差大幅度的增加。研究发现,接触边缘处发生的接触面张开现象是引起预测误差的主要原因;基于Ruiz法预测压装轴微动疲劳裂纹萌生位置时,需要考虑在接触边缘处接触面张开区宽度的影响,特别是对于名义弯曲应力与接触压力的比值较大的压装工况。     

微动疲劳加载装置的设计及其在典型合金上的应用

以疲劳试验机为平台,设计开发了一套微动疲劳加载装置,进行了钛合金TC11与TC11接触、单晶高温合金DD3与粉末高温合金FGH95接触以及定向凝固高温合金DZ125与FGH95接触等三种不同材料配对的微动疲劳试验,并对TC11、DD3和DZ125三种材料的微动疲劳性能进行比较。结果表明:三种材料的疲劳裂纹均出现在微动接触区域,说明所设计的试验装置性能良好,法向载荷加载稳定,可用于微动疲劳试验;在相同的几何尺寸、载荷条件以及表面加工状况的试验条件下,沿晶体生长方向上的各向异性材料DD3和DZ125的微动疲劳寿命比各向同性材料TC11的高。     

交变法向载荷作用下2024铝合金的微动疲劳试验研究

针对法向交变载荷作用下的2024铝合金微动疲劳失效问题开展了试验研究。设计了可施加交变法向载荷与远端载荷的双轴加载微动疲劳试验系统。通过Abaqus有限元分析，对试验台法向加载装置进行分析计算，考察了法向加载时力传导误差与微动垫摆角对试验精度的影响，验证了试验机加载的精度。进而研究了在交变法向载荷作用下，0°、45°、90°三种不同相位差对2024铝合金疲劳寿命的影响。试验结果表明，随着相位差的增加，2024铝合金试件的寿命逐渐增加。通过对磨损状态的分析发现，0°与45°相位差下，磨损的影响较小，寿命随着相位差的增大而增长；90°相位差时磨损较为严重，在磨损与疲劳损伤的共同作用下，90°相位差状态下试件的疲劳寿命进一步增加。试件微动疲劳寿命在交变法向载荷作用下的整体趋势为：随着相位差的增加寿命延长，随着相位差的增加，磨损增加。     

高强度铸造铝合金微动疲劳特性研究

用ANSYS对高强度铸造铝合金的微动疲劳特性进行仿真模拟,得到接触面上的应力、应变分布规律;基于SWT临界面法预测微动裂纹的萌生位置,用实验值拟合得到微动疲劳寿命预测参数并用临界面法预测微动疲劳寿命。结果表明：在法向夹紧力不变时,微动疲劳寿命会随着轴向力的增大而减小,且轴向力存在一个临界值,超过这个临界值,构件寿命会急剧下降。在HYS 100型微动疲劳试验机上对高强度铸造铝合金的微动裂纹萌生位置及寿命进行实验验证。结果表明,SWT临界面法预测裂纹萌生位置与试件实际断裂位置一致,预测寿命与实际寿命在误差允许范围内。     

Ti-6Al-4V燕尾榫结构微动疲劳裂纹萌生及扩展行为研究

针对Ti-6Al-4V钛合金燕尾榫连接结构在不同载荷下的微动疲劳现象,采用榫形微动疲劳试验进行研究,并对裂纹萌生扩展、微动磨损及断口进行分析。结果表明,微动疲劳使构件疲劳寿命显著降低约70%;疲劳载荷对微动裂纹扩展的影响比对裂纹萌生的影响更大;微动疲劳裂纹起始于接触面边缘,与接触表面约成45°角,裂纹扩展到60~150μm后转向与接触表面垂直;微动疲劳断口形貌表面在微动磨损区具有多个裂纹源点,但只有一个主裂纹形成。     

Al-Li 8090 和 Ti-6Al-4V 的 20 kHz 微动疲劳研究

用超声疲劳试验技术研究了Ａｌ－Ｌｉ８０９０铝锂合金和Ｔｉ－６Ａｌ－４Ｖ钛合金在２０ｋＨｚ时的微动损伤现象。试验结果表明，在极高频率下，也有微动损伤发生，并可引发疲劳裂纹的萌生和扩展，导致微动疲劳破坏。     

关于微动磨损与微动疲劳的研究

微动磨损与微动疲劳是２种主要的微动模式,造成的损伤在工业中相当普遍,并可能引发灾难性的后果。主要研究了们移幅度、压力和疲劳应力３个基本微动参数,并以获得的微动区域、微动图为基础,分析了微动磨损与微动疲劳的运行机制和破坏规律。为更好地了解微动磨损与微动疲劳之间的内在联系,进一步探讨了接触磨损与局部疲劳、局部疲劳与整体疲劳之间的竞争机制。     

微动疲劳参数对钢丝微动疲劳磨损演化的影响*

微动疲劳易引起钢丝表面磨损和横截面积损失,进而造成钢丝断裂失效并缩短钢丝绳使用寿命。不同微动疲劳参数（接触载荷、疲劳载荷、钢丝直径和交叉角度）引起差异的钢丝微动疲劳磨损特性,故研究微动疲劳参数对钢丝微动疲劳磨损演化规律影响至关重要。基于摩擦学理论和Marc仿真软件构建钢丝微动疲劳磨损模型,探究接触载荷、疲劳载荷、交叉角度和钢丝直径对钢丝微动疲劳磨损演化的影响规律。结果表明：钢丝微动疲劳磨损体积主要与接触载荷和疲劳载荷有关;疲劳钢丝的磨损深度、磨损率及磨损体积随着接触载荷的增加而增大,且不同接触载荷下疲劳钢丝磨损体积均随着循环次数的增加而呈线性增加;随疲劳载荷幅值的增加,疲劳钢丝的磨损深度、磨损率及磨损体积均呈增加趋势;在不同疲劳载荷范围下疲劳钢丝的磨损体积均随着循环次数的增加而呈线性增加;当接触载荷、疲劳载荷及钢丝间摩擦因数相同时,不同交叉角度和不同加载钢丝直径下疲劳钢丝的磨损体积相同。     

Torsional fretting fatigue strength of a shrink-fitted shaft with a grooved hub

Fretting causes considerable reduction in the fatigue strength of a shrink-fit assembly and failures through fretting are as numerous as failures from normal fatigue. The purpose of this investigation was to determine the effect of contact pressure and slip amplitude on the fatigue limit, and a favourable value for overhang of hub and fillet radius with constant diameter ratio, at which fretting failure can be avoided and the maximum normal fatigue strength will be obtained. The torsional fatigue strength of shrink-fitted shaft couplings was estimated by tests performed by varying the overhang of the hub, the fillet radius of the shaft and the contact pressure of the shrink-fitted assembly. Press-fitting of the hub overhanging the shoulder was used to increase the contact pressure. The tests were performed using a grooved hub. These experiments showed that fretting was reduced with an increase in contact pressure, because the slip amplitude decreased. The shaft was fractured just inside the end of the fit by fretting fatigue with low contact pressure, but if the contact pressure was very high, the shaft fractured at the fillet by normal fatigue. The fretting fatigue limit at a constant diameter ratio increases with an increase in the fillet radius, and reaches its maximum value at a certain radius using the grooved hub.     

中性腐蚀环境下钢丝的微动疲劳行为

在自制的微动疲劳试验机上开展中性腐蚀环境下单根钢丝的微动疲劳实验,考察在相同接触载荷下,不同振幅对钢丝的微动疲劳行为的影响,并用扫描电子显微镜观察疲劳钢丝的磨痕和断口形貌,研究钢丝微动疲劳断裂机制.结果表明:在较大的振幅下,钢丝的微动区均处于滑移状态,而在较小振幅下,钢丝的微动区从滑移状态逐渐转变为黏着状态;磨损机制主要为磨粒磨损、疲劳磨损、腐蚀磨损和塑性变形;钢丝疲劳寿命随着微动振幅的增大而减小;钢丝的疲劳断口可分为3个区域,即疲劳源区、裂纹扩展区及瞬间断裂区.     

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

Most previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.     

Development of a test device for the evaluation of fretting in point contact

Fretting wear and fatigue may occur between any two contacting surfaces, wherever short‐amplitude reciprocating sliding is present for a large number of cycles. A test device has been developed for the evaluation of fretting fatigue and wear in partial and gross slip conditions. Three similar sphere‐on‐plane contacts run at the same time. Normal force, tangential force or displacement amplitude and constant bulk stress can be controlled and measured separately. Reciprocating tangential displacement is produced with rotational motion, the amplitude and frequency of which can be adjusted and controlled accurately by an electric shaker. The number of load cycles for crack initiation and growth is determined with strain‐gauge measurements near the fretting point of contact. The contact surfaces are measured with 3D optical profilometer before fretting measurements to determine actual contact geometry. The measurements were done with quenched and tempered steel. The initial results indicate that cracks are mostly formed in partial slip conditions, whereas fretting wear is more heavily involved in gross slip conditions. The initiation of a crack occurs near the edge of the contact in the slip direction, where the calculated cracking risk has its maximum value in partial slip conditions. The number of cracks increases as the displacement amplitude, i.e. friction force, increases in partial slip conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

TC4合金微动疲劳损伤研究

研究了TC4合金在柱面-平面接触务件下的微动疲劳行为，分析了其微动疲劳损伤机制。结果表明：在试验务件下，微动区边缘的损伤特征以粘着磨损为主，而微动区中部则以磨粒磨损和接触疲劳为主。疲劳裂纹易于在微动区．特别是在蚀坑处萌生和扩展。促使微动疲劳裂纹萌生的因素：一是法向应力和切向摩擦力引起的材料表层塑性变形，二是微动磨损破坏了材料的表面完整性，造成了缺口应力集中效应。     

Some Observations on Frictional Force During Fretting Fatigue

Frictional force behavior during fretting fatigue and its interdependence on other fretting variables are investigated. Both coefficient of static friction and the normalized frictional force (i.e., the ratio of frictional force and normal contact load) increase during the earlier part of a fretting fatigue test and then both reach to a stabilized value. The variation of temperature in the contact region and normalized frictional force with increasing cycle numbers and bulk stress show similar trend implying that normalized frictional force represents the average friction in the contact region during a fretting fatigue. An increase in bulk stress, relative slip, and hardness of pad material results in an increase of the normalized frictional force, while an increase in contact load, frequency and temperature decreases the normalized frictional force. The normalized frictional force is also affected by the contact geometry. On the other hand, coefficient of static friction increases with an increase in the hardness of mating material, temperature and roughness from shot-peening treatment, but is not affected by contact geometry and displacement rate. Further, the normalized frictional force is not affected by the contact geometry, roughness and applied bulk stress level when fretting fatigue test is conducted under slip controlled mode, however it increases with increasing applied relative slip and decreasing contact load in this case.