铁基形状记忆合金防松螺母防断机理的数值分析

由于铁基形状记忆合金材料本身的特殊性能,由其制成的螺母所组成的螺栓联接中,不仅螺纹副之间的自锁摩擦力矩有所提高,而且轴向载荷在各螺纹齿间分布的均匀性也得以改善,从而有效地防止了螺纹联接松动、松脱及疲劳断裂等失效现象的发生。主要对这种新型螺母的防断机理进行了分析,建立了螺栓联接载荷分布的计算模型,并通过与普通螺栓联接的比较,对这种新型螺母的防断性能进行了评价。     

The effect of thread dimensional conformance on yield and tensile strength

This paper presents the results from tests that investigate the effect of thread dimensional conformance of fasteners on yield and tensile strength. Test specimens include combinations of bolts and nuts within dimensional conformance as specified by ASME Standard B1.1-1992, as well as bolts with undersized pitch and major diameters and nuts with oversized pitch and minor diameters. Tensile tests were performed in accordance with ASTM F606-95b. Data from the tests show reduced yield and tensile strength for the fastener combinations with undersized pitch and major bolt diameters or oversized pitch and minor nut diameters, compared to fastener combinations within conformance. Variations in bolt pitch diameter were found to affect the yield and tensile strength by about an order of magnitude more than variations in bolt major diameter or nut pitch and minor diameters. The mean tensile strength for conforming product was found to be as much as 20% greater than the tensile strength for nonconforming product.     

Experimental results and finite-element predictions of the effect of nut geometry, washer and Teflon tape on the fatigue life of bolts

The effect of nut geometry, curved spring washer and a sealing material (Teflon tape) on the fatigue life of M12 and M16 ISO bolts was investigated. This was accompanied by the study of the axial and bending stress distribution in threads by numerical simulation of bolt and nut connections using the finite‐element method. The experimental results showed that the highest fatigue life is achieved for a slotted tapered nut. The presence of a spring washer also increases the fatigue life, providing that the correct tightening torque is applied to produce a pretension in the bolt. The use of Teflon tape as a filling material between engaging threads of the bolt and nut is shown to significantly increase the fatigue life. On the whole, the highest fatigue life is obtained for a slotted tapered nut using washer. Four fracture mechanisms were observed for bolt–nut connections during the fatigue tests.     

A COMPARATIVE STUDY OF METHODS FOR ESTIMATING BOLT FATIGUE LIMITS

The approach to predicting fatigue limits from calculated stress concentration factors, using thread load distributions obtained from analytical theories is examined. In particular combinations of the methods of Shigley, Otaki, Heywood, Birger, Bluhm and Flanagan, Sopwith and a modification to Sopwith's theory are assessed against fatigue test data and photoelastic results for a range of bolt and nut geometries. The Snow-Langer-Cook, the Goodman and the Gerber methods of allowing for the effect of mean stress were also examined. It was concluded that mean stress effects are significant, that none of the theories are sufficiently sensitive to changes in geometry, and that they seriously overestimate the fatigue limit when the geometry is more severe than the standard. The use of Sopwith's theory of load distribution combined with Heywood's method and the Snow-Langer-Cook technique was found to be the safest for design purposes, but not the most accurate.     

Determining the life cycle of bolts using a local approach and the Dang Van criterion

The fatigue behaviour of bolts under axial load has always been considered from the component point of view for which fatigue limit is usually taken equal to 50 MPa, and few results are available to designers for limited lifetimes. Here, we take up this problem from a material point of view using a local approach. For each case of fatigue testing, using finite‐element (FE) model of the bolt, we determine the stabilized local stress at the root of the first thread in contact with the nut. To characterize bolt behaviour with these numerical results, we use Dang Van multiaxial fatigue criterion for which we extend application to the medium fatigue life. These results can be correlated with the experimental numbers of cycles to failure to determine material parameters of the generalized criterion. Using statistical Gauss method, we can make lifetime predictions for any level of risk of failure. In addition, we propose an analytical model to rapidly determine the local stress condition from nominal loading data (mean stress and alternating stress). This model dispenses us from a new modelling if the bolt is stressed in the same manner as the bolts used for behaviour characterization. Using this model and the generalized criterion, it is extremely easy to make lifetime predictions whatever the risk considered.     

普通螺母及自锁螺母对螺栓标定特征曲线的影响

采用纵波法并使用自制的内螺纹夹具,分别选择普通螺母和自锁螺母来标定螺栓,通过不同的拧紧策略和标定方法,分析普通螺母和自锁螺母标定螺栓特征曲线的差异.结果表明:采用普通螺母和自锁螺母标定螺栓因方法不同会获得不同的标定特征曲线,自锁螺母的锁紧扭矩使螺栓温度升高导致超声波声时差增加,因此获得的标定特征曲线会平行右移.     

Bearing strength of autoclave and oven cured Kevlar/epoxy laminates under static and dynamic loading

The pin bearing behaviour of woven Kevlar fibre-reinforced epoxy laminates, prepared by autoclave and oven curing methods, was examined under static and dynamic loading. Static bearing strength was determined as a function of bolt constraint and specimen geometry with particular attention to failure modes. The performance was very sensitive to lateral bolt tightening: a bolt with only a ‘finger-tight’ nut produced ∼100% improvement in bearing strength, compared with the performance of a pin-loaded hole. Some specimens were hygrothermally conditioned and indicated a 10% deterioration in the bearing strength of the bolt-loaded hole. However, this was well below the magnitude of variation in strength which resulted under different methods of production. The fatigue endurance limit (N ∼ 10⁶) at 1 Hz was only achievable at stress levels equivalent to 25% of the maximum static bearing strength.     

Influence of the accumulated error in thread pitch on the stress–strain state and cyclic strength of threaded joints

Using the finite element method, the analysis of influence of errors in pitch during the thread production on the behavior of stress distribution in stud–nut joints is performed. The value of maximum local stresses in studs of a threaded joint is shown to substantially depend on errors in thread pitch of a stud and a nut. On the basis of the analyses of stresses and strains under cyclic loading, fatigue curves are plotted for the threaded joints having deviations in thread pitch. It is shown that, with a rational selection of the deviation in pitch, the cyclic strength of threaded joints can be considerably increased. The results of analysis agree satisfactorily with the data of fatigue tests of the M39 × 3 threaded joints.     

Failure analysis on fracture of worm gear connecting bolts

The worm gear connecting bolts of refueling machines of a nuclear power plant, with implementing standard of ANSI/ASME B18.3 and ASTM A574-08 and strength grade of 10.9, fractured at the thread neck position after running for about 10 years, and means such as macro examination, chemical compositions analysis, hardness testing, metallographic examination and fracture analysis, were used to analyze the fracture property and reasons of the bolts. The results show that the fracture of the bolts is due to two-way bending fatigue fracture. Surface decarburization of the bolts and stress concentration at the bolt thread neck decreased the fatigue strength of this position and resulted in the initiation of fatigue cracks. By comprehensive analysis and stress estimating, it was concluded that the main reason for fracture of the bolts is that there was a big gap between the bolts and the bolt holes, which resulted in fatigue fracture of the worm gear connecting bolts.     

蜗轮连接螺栓断裂失效分析

装卸料机上的蜗轮连接螺栓材料为35钢,强度等级为10.9级,在设备运行大约10a后发生断裂。对断裂螺栓进行宏观、化学成分、硬度、金相、能谱和断口分析后得出,该螺栓的断裂性质为双向弯曲疲劳断裂,螺栓表面的脱碳和螺纹颈部的应力集中降低了该部位的疲劳性能。通过综合分析和螺栓受力估算后得出,螺栓断裂的主要原因是螺栓和内齿轮螺栓孔之间存在较大的间隙,使螺栓的受力状态和受力大小过早地发生了变化,造成连接螺栓疲劳断裂。     

基于8.8级螺栓疲劳性能的预紧力优化

测量了8.8级螺栓的拉伸性能,根据螺栓材料的强度极限和屈强比研究了预紧力分别为强度的10%、30%和50%的极限条件下材料的疲劳性能。结果表明,当8.8级螺栓的预紧应力从10%强度极限提高到50%强度极限时,其疲劳极限由370 MPa降低到263 MPa。根据有效应力(σˉσˉ)参数法处理预紧应力对8.8级螺栓疲劳曲线的影响,得到了疲劳极限处的有效应力(σˉ10^7=562.75MPaσˉ10^7=562.75MPa)。当有效应力σˉ<σˉ10^7σˉ<σˉ107时预紧的8.8级螺栓不会发生疲劳失效,由此得到了8.8级M6和M27两种螺栓在不同应力比下所对应的最大预紧力和预紧扭矩曲线。     

Investigation into the loosening mechanism of bolt in curvic coupling subjected to transverse loading

It has been widely observed that fasteners turn loose when subjected to dynamic loads in the form of shock, vibration or transverse cyclic loading. This reduces the preload force of bolt and leads to joint failure. Such failures can be catastrophic in safety critical applications. In this paper, the self-loosening of bolts in curvic coupling is analyzed based on the self-rotation of nut in the cases of cyclic torque loads on discs after the preload of bolts. The three-dimensional finite element model for curvic coupling and threads is established in commercial finite element software ANSYS to study the details of the self-loosening process of bolt. Such processes are characterized by microslip at the curvic, the bolt head and the thread contact surfaces. It is found that due to the application of the cyclic transverse load, the nut rotation can occur for only localized slip without complete slip at the bolt head contact surface and the thread surface. However, the curvic surface always shows complete slip under all external loads. The microslip on all contact surfaces are identified to be the major mechanisms responsible for the self-loosening of a curvic coupling. The results obtained agree quantitatively with the experimental observations.     

影响钢结构用高强度大六角头螺栓连接副扭矩系数的因素

从自身质量及检测过程两方面入手,讨论了影响钢结构用高强度大六角头螺栓连接副扭矩系数的因素,并通过对比试验,就各种因素对扭矩系数的影响规律进行了初步研究。结果表明:影响因素主要有螺纹完好状况、螺母和垫圈油污情况、连接副锈蚀情况、检测环境、检测仪器精度、检测细节和连接副保证期等。     

螺栓杆应力集中系数的研究

螺栓球节点栓杆上的应力集中是导致栓杆发生疲劳破坏的主要原因。该文通过理论推导及数值分析的方法研究了螺栓杆的应力集中系数。在忽略螺纹升角的情况下,根据变形协调条件研究了螺栓杆的轴力梯度变化,采用数值计算确定常用螺栓球的螺纹位移系数及螺纹力计算公式。采用局部建模,通过迭代计算的方法确定螺栓杆应力集中系数。与完整模型的计算结果比较表明该方法的计算结果具有较高的精度,且计算效率高。     

发动机气缸体螺纹联接强度有限元分析

针对发动机设计中螺纹联接强度计算的问题,采用有限元通用程序ABAQUS/STANDARD,研究气缸体螺纹区域强度准确计算的方案,即通过简化后的装配体模型计算螺栓在工作载荷下的受力,然后将获得的螺栓受力做为载荷边界条件施加在带螺纹的局部模型中,计算螺栓和缸体螺纹区域的应力。网格划分时,在局部模型中将缸体简化,并将螺柱分为两部分,之后皆用六面体单元进行网格划分以减小计算量、提高准确性。最后通过实例对某气缸体螺纹区域在预紧状态、峰值燃烧压力作用下的应力进行计算与分析,验证该计算方案的可行性。该方案为今后发动机的螺纹联接设计和计算提供一条可行的路径,且对相关的工程实际应用具有很强的指导意义。     

C/SiC复合材料螺栓螺牙承载能力

为了对C/SiC复合材料螺栓螺牙的承载能力进行评估,采用有限元法和刚度折减方法对C/SiC复合材料螺牙抵抗拉脱的能力进行了研究。结果表明:当齿合螺牙数大于6时,再增加齿合螺牙数已不能有效地提高螺牙的初始拉脱载荷;增大螺距会降低螺牙初始拉脱强度,因而也不能显著地提高螺牙的初始拉脱载荷;在螺距与螺栓直径之比保持常数的情况下,螺牙的初始拉脱载荷与螺栓直径的平方成正比;而螺牙的极限拉脱载荷则近似正比于齿合螺牙数、螺距及螺栓直径。     

Experimental characterization of the bending fatigue strength of threaded fasteners

The fatigue strength in bending of pre-stressed steel bolts is investigated and compared to the fatigue strength in axial tension. The strength is measured in terms of maximum engineering stress amplitude, neglecting any stress concentration in the threads. The experimental results reveal that the fatigue limit is 76% higher in bending than in axial tension. A finite element model is used to compute the stress state in the threaded region for both axial tension and bending. It allows fitting a volume based weakest link model to the experimentally observed failure probabilities. Based on the good fit of the weakest link model it is argued that randomly distributed defects in the highly stressed thread root determine the fatigue strength.     

高强度螺栓疲劳缺口系数的有限元分析

螺栓球节点网架结构在悬挂吊车作用下的疲劳是工程界和学术界关注的热点,该结构的疲劳关键是高强度螺栓的疲劳。该文借助ANSYS有限元软件;选取常用的M14、M20、M24、M30、M33、M39、M52、M60共8种规格40Cr制高强度螺栓为分析对象;采用20节点的SOLID95单元类型进行三维实体建模;在重点定量探讨螺栓直径、螺纹升角、螺纹牙根圆角半径及螺栓球四个主要影响因素的基础上,建立了适用于各种规格高强度螺栓疲劳缺口系数的通用计算公式,数值区间为4.35―4.89。该文的研究结论揭示了应力集中和疲劳源的关系,为进一步以热点应力或热点应力幅为参量建立螺栓球节点网架结构疲劳计算方法奠定了理论基础。     

Tightening Tests and Friction Coefficients Definition in the Steering Shaft of Front Motorbike Suspension

Abstract: The steering shaft of front motorbike suspensions is, usually, tightened up the fork by means of a nut and a locknut. The aim of this study is to evaluate the actual relation between the bolt torque and the preloading force of the steering shaft by performing some tightening tests. The bolt torque is given by a torque wrench whereas the preloading force has been evaluated by means of a strain gauge located on the steering shaft. The steering shaft has a hollow section with an external thread so that the strain gauge has been applied on its internal surface. In relating the bolt torque to the bolt tension, the friction coefficients have been accurately calculated: the friction coefficient values increase notably after the first set of tightening tests mainly because of the spoiling of the contact surfaces.     

Preparation and thermodynamic analysis of the porous ZrO2/(ZrO2 + Ni) functionally graded bolted joint

Ceramic–metal functionally graded materials (FGMs) have been extensively used in aerospace engineering where high strength and excellent heat insulation materials are desired. In this paper, the thermodynamic behavior of the Thermal Protection System (TPS) used bolted joints made up of porous ZrO₂/(ZrO₂ + Ni) FGMs is investigated by finite-element (FE) modeling. The bolted joint is subjected to reentry heating corresponding to the Access to Space Vehicle. Thermodynamic simulations are carried out to yield the transient response of the porous ZrO₂/(ZrO₂ + Ni) functionally graded bolted joint (FGBJ). The effects of the preload on the thermomechanical behavior and service reliability of the bolted joint are numerically analyzed in detail by ABAQUS codes. It is found that the preload relaxation of the bolted joint occurs at elevated temperature, and the preload has significant influence on service reliability of the bolted joint under transient thermomechanical circumstances. With the increase of the preload, stress concentration which occurs at the root of the first thread of the bolt increases rapidly and predominates in service reliability. Proper preload is thus defined to balance the service reliability and tightness of the bolted joint. Further studies show that the shape of the nut has a great effect on the stress concentration of the thread, the optimized nut is designed to reduce the stress concentration of the thread, and thus the reliability of the bolted joint is also improved.