基于机车车轮的动态疲劳可靠性研究

 随着火车速度的不断提高,对火车走行部件的疲劳可靠性要求也越来越高．根据疲劳寿命预测理论,建立了某机车车轮受力危险点在不同路况下的疲劳寿命曲线．以实测数据和有限元分析所得数据为基本参数,采用线性累积损伤法则和Monte-Carlo模拟,对车轮在等幅载荷和随机载荷谱共同作用下进行疲劳寿命预测,得到了车轮在不同路况下可靠度随路程变化的关系曲线．为火车的安全性维护提供了理论依据,对火车的安全运行有重大的实际应用价值．     

基于数字疲劳传感器的疲劳损伤在线评估系统

为了长期有效预测大型结构的疲劳损伤,以实现在线评估,本文介绍了基于数字疲劳传感器的疲劳监测系统,该系统可以利用电阻等效原理与变幅加载等效及双疲劳计建立的计算模型得到时程载荷谱;并根据Miner线性疲劳损伤累积理论计算出桥梁测点的剩余寿命.本文以上海某大桥疲劳监测子系统为例,根据疲劳传感器采集到的数据估算测点的疲劳剩余寿命,从而实现对大型结构可靠有效的在线健康评估.     

基于WIM数据分析的上海长江大桥拉索疲劳应力谱研究

拉索是斜拉桥的关键承重构件,拉索的疲劳状态直接关系到斜拉桥的正常运营,交通载荷导致的索力变化是拉索疲劳损伤和寿命评估的关键因素.本文利用上海长江大桥动态称重系统采集到的车辆载荷相关数据,依据等效疲劳损伤原理,推导出代表实际运营车辆的等效载荷谱,将载荷谱加载到ANSYS模型中获得拉索应力历程,采用泄水法,建立拉索应力谱,为拉索疲劳损伤及寿命评估提供可靠的依据.     

复合材料单钉接头疲劳累积损伤破坏分析

基于时间增量原理 , 推导了层合板接头疲劳加载累积损伤应力2应变分析的虚功方程。同时 , 引入Hashin三维疲劳失效准则进行材料的损伤判定 , 并结合建立的疲劳加载材料退化模型、 4种基本损伤机制相互关联作用的材料性能退化方法及复合材料接头最终失效判据 , 建立了层合板接头疲劳载荷作用下三维累积损伤分析的寿命预测方法。最后 , 对层合板接头拉2拉疲劳载荷作用下的损伤累积扩展与失效规律进行了仿真分析 , 并与试验结果进行了对比 , 结果表明 : 本文中建立的寿命预测方法能够很好地预测层合板接头的寿命以及损伤发生、扩展及最终失效。      

变幅载荷下纤维金属层板的疲劳与寿命预测

文章建立了纤维金属层板等幅疲劳载荷下的疲劳裂纹扩展速率与寿命预测模型。在此基础上对玻璃纤维-铝合金层板（GLARE）的疲劳裂纹扩展与分层扩展行为进行了试验研究,探讨了层板过载疲劳行为的机理,提出了纤维金属层板变幅载荷下疲劳寿命预测的等效裂纹闭合模型,并在GLARE层板上得到了验证。     

一般尺度法的振动疲劳强化系数分析

在Dirlik模型的基础之上,结合一般尺度法（General scaling law）提出适用于平稳宽带随机过程的疲劳强化系数模型。一般尺度法认为结构在原始载荷和强化后载荷下的应力响应功率谱各阶谱矩成简单的线性关系;与Dirlik宽带疲劳损伤模型结合,便得到适用于平稳宽带随机过程的疲劳强化系数模型。为了验证模型的有效性,分别在原始加速度功率谱和经一般尺度法强化后的加速度功率谱载荷下;对悬臂梁采用雨流计数(Rainflow counting)和Miner损伤准则进行疲劳寿命预测,得到它们的疲劳强化系数,并与疲劳强化系数模型得到的疲劳强化系数进行对比。结果表明：提出的疲劳强化系数模型精度为99.7 %,由此可验证疲劳强化系数模型的有效性。     

概率局部应力应变法

实际工程中的结构件往往具有多个不确定因素,包括材料、几何、载荷等。这些不确定因素导致构件的局部应力应变响应和疲劳寿命响应具有随机性。因此对低周疲劳分析中的局部应力应变法进行了概率分析。通过基本随机变量将诺伯法中的循环应力应变曲线(迟滞回线)和诺伯公式表示为概率曲线,基本随机变量反映了构件的不确定因素。通过建立近似拟合多项式的方法,求得局部应力应变的随机响应。将应变寿命曲线视为概率曲线,采用随机累积损伤理论,通过同样方法得到疲劳寿命的随机响应。算例表明结果与蒙特卡罗模拟的结果十分接近。该方法是一个简单有效的疲劳寿命概率分析方法。     

横向载荷下复合材料层合板疲劳分析

疲劳破坏是结构失效的主要原因之一。基于复合材料各向异性层合板的宏观力学理论,分析在横向交变载荷作用下层合板的应力场。结合材料的S-N曲线和累积损伤原理,建立局部名义应力与结构整体寿命相关联的疲劳寿命预测模型,计算层合板的疲劳损伤和寿命。采用名义应力的方法估算复合材料层合板的疲劳寿命较为便捷和直观。     

基于损伤力学的疲劳裂纹萌生及扩展规律研究

针对金属构件疲劳裂纹的萌生、扩展寿命及其规律等问题,应用损伤力学理论与有限元相结合的方法,建立了计算疲劳裂纹全寿命的统一模型。引入附加载荷法,通过MATLAB编程计算,实现了对刚度矩阵的连续计算,并给出了编程的流程。通过对单个单元的损伤计算,得到了单元从无损到破坏过程中等效应力的变化;通过计算各个构件损伤单元寿命,进而给出了金属构件总体疲劳寿命。分析得到了微裂纹萌生及扩展寿命占总体疲劳寿命的80%以上,并应用有限元软件ANSYS模拟给出了缺口件裂纹萌生及扩展过程。理论计算的结果与试验数据对比基本一致,验证了本工作方法的准确性。     

风力机叶片载荷谱及疲劳寿命分析

研究了大型风力机玻璃钢叶片载荷谱和疲劳寿命的工程估算方法。运用片条理论分析了影响风力机叶片疲劳寿命的气动载荷分布;应用多体动力学方法,分析了旋转叶片动力刚化效应产生的原因及其对叶片振动模态的影响;根据模态叠加法,计算了叶片在气动力、重力和旋转惯性力等确定性载荷作用下的动应力响应;探讨了玻璃钢叶片的疲劳性能、破坏准则及疲劳寿命估计方法;提出了一种基于 Palmgren Miner 线性累积损伤法则的玻璃钢叶片安全寿命估计方法。通过所设计的 1.5MW 变速变桨距风力机叶片疲劳寿命估计的算例表明,该方法是可靠和实用的。     

Fatigue analysis of offshore platforms subject to sea wave loadings

The fatigue damage calculation for random loading on offshore platforms takes the form of a rainflow analysis of the dynamic response of individual members to various sea states. This procedure is lengthu and consequently this paper attempts to provide a theoretical method for determining random load fatigue damage. This dynamic response for many joints leads to a broad band random loading but despite this, previous theoretical methods have simplified the loading to narrow band. This has not been done in the present case; instead, an analysis based on broad band random loading has been produced. This theoretical approach gives a fatigue life estimate which is slightly (6.6%) more conservative, for a typical example, than a rainflow analysis.     

RAINFLOW CYCLES IN GAUSSIAN LOADS†

We present a new approximation of the expected damage for Gaussian loads. The approximation is based on an exact relationship between the rainflow cycle count and the range-pair exceedances count. This relationship links the intensity of rainflow amplitudes to expectations of marked crossings of the random load. The method is best suited for the fatigue life prediction for locally stationary Gaussian loads with slowly varying means and covariance functions. These results are illustrated with numerical examples.     

FATIGUE LIFE UNDER RANDOM LOAD HISTORY DERIVED FROM EXCEEDANCE CURVES USING DIFFERENT ALGORITHMS

Low cycle fatigue life and crack growth rates were analytically estimated for random load sequences, generated from three combat aircraft load exceedance curves using different algorithms, including simulated rainflow cycle count, extreme-to-extreme excursions, upper to lower bound excursions and unrestricted peak-trough excursions. Also, the response of a fatigue meter to a random load sequence was simulated. Fatigue damage for the different load histories was computed using material constants for an Al-Cu alloy. Computed fatigue damage was relatively insensitive to the algorithm used for load sequence generation from combat aircraft load exceedance curves. Fatigue meter data based damage estimates were, however, sometimes unconservative.     

Influence of correlations between stresses on calculated fatigue life of machine elements

The correlation between components of the random stress tensor and its influence on the calculated fatigue life of machine elements were analysed. Three covariance matrices of components of biaxial stress state were considered. They were determined from measurements of strains in an element of a vibrating screen for aggregate, in the back wall of a bus, and in a welded element of excavator fittings. Calculations were made according to four criteria of multiaxial random fatigue. Cycles were counted with the rainflow method, and damage was cumulated with the Palmgren-Miner hypothesis. It was found that convariances of random stress state components strongly influence the calculated fatigue life.     

Fatigue life estimation under random loading using the energy criterion

A procedure for estimating the useful life of a component for a given (admissable) probability of fatigue fracture origination under random loading is presented. The method uses material constants obtained from the S/N and cyclic stress/strain curves, standard deviation and probability density distribution of the loading process and a macroblock of harmonic cycles obtained by applying the rainflow cycle counting method to the random loading process. Theoretical and experimental lives are found to exhibit good agreement.     

On fatigue cycle distribution in non-stationary switching loadings with Markov chain structure

The non-stationary loadings experienced by structural members can be often modelled as a sequence of stationary load states, with different mean and variance levels, where load switches are controlled by an underlying random process (regime process). The structural integrity of structures and mechanical components under such complex loadings requires the assessment of the statistical distribution of rainflow cycles counted within single load states, as well as of transition cycles caused by load state switches. Under the assumption of a constant mean for the switching loading, the authors showed in a previous paper that a linear combination of single loading spectra would provide a fairly good estimation of the overall loading spectrum of the switching loading. However, when mean value differences among load states are large, additional fatigue cycles caused by load state transitions would be present, besides cycles counted within load states. This work presents a comprehensive approach to assess the overall loading spectrum in switching loadings with variable mean value, when modelling the underlying regime process as a stationary Markov chain. For large relative mean value differences compared to load state variances, transition cycles are found by rainflow counting the regime process. The rainflow matrix for transition cycles is then estimated from the transition probability matrix of regime process by a method available in the literature. The distribution of the ranges of transition cycles is finally estimated from the statistical distribution of the largest peak and the lowest valley within load states. An illustrative example is finally discussed to show the accuracy of the proposed method.     

Fatigue life prediction for a vessel sailing the North Atlantic route

A method for calculating the wave load induced fatigue damage accumulated by a vessel sailing along the North Atlantic route (NAr) is presented. This method is based on the Palmgren–Miner additive rule and the rainflow cycle (RFC) count. For simplicity, the load the vessel experiences is assumed to be proportional to the encountered significant wave height process, H_s. The asymptotically normal character of the nominal damage is proved and used to derive the probability distribution of the fatigue life prediction. The proposed method improves on the already existing ones by making use of the information contained in the variance of the fatigue damage accumulated during the voyages. The method is illustrated through numerical examples.     

Extrapolation of load histories and spectra

In fatigue life assessments, both the material properties and the load characteristics are essential parameters. The life of a component can be experimentally found by performing fatigue tests. In order to get reliable predictions of the life in service, the tests should be performed using variable amplitude loadings that are representative for the service loads. This paper concentrates on the problem of extrapolating a measured load history to a longer time period, for example to a full design life. Using statistical extreme value theory, a new method for extrapolation in the time domain is presented. The extrapolated load spectrum, obtained from the extrapolated time signal, is compared to the result using a method for extrapolating the rainflow matrix.     

FATIGUE LIFE ESTIMATION OF WELDED JOINTS OF AN ALUMINIUM ALLOY UNDER SUPERIMPOSED RANDOM LOAD WAVES: APPLICATION OF A 2-DIMENSIONAL RAINFLOW METHOD

Abstract— Fatigue tests were carried out on welded T-joints of JIS A5083P-O Al-Mg alloy under constant amplitude and superimposed random load waves. Each random wave was generated by superimposing a secondary zero-mean random process on a constant amplitude pulsating trapezoidal wave, which simply simulated the GAG (ground-air-ground) stress cycles, or on a constant d.c. (direct current) component. It is proved that the 1-dimensional rainflow method is insufficient as the mean stress effects are not considered. From a procedure using a 2-dimensional rainflow method and the original and a modified Goodman diagram, it was concluded that the mean stress effects on secondary waves are larger in the case of GAG than d.c. Considering this, a life estimation procedure is proposed that gives consistent and acceptable estimation results.     

Fatigue life prediction based on natural frequency changes for spot welds under random loading

Fatigue tests were carried out on tensile-shear spot-welded specimens under random loading; at the same time, natural frequencies at different life stages were measured. Test results were compared with that under constant amplitude loading. The relationship between damage and natural frequency change ratio established under constant amplitude loading is here modified. The nonlinear damage evolution equation was established using the damage represented by frequency change for spot welds. The fatigue life of spot welds under random loading was predicted with this damage evolution relationship using the rainflow counting method results, as well as other life prediction methods with different counting methods. Comparison of life prediction results showed that the method using changes of natural frequency can be an on-line approach and the predicted lives have a good agreement with the experimental data.