基于非线性连续疲劳损伤的再制造毛坯剩余寿命评估

在考虑裂纹闭合效应对疲劳损伤影响的情况下,对Chaboche提出的非线性连续疲劳累积损伤模型进行了修正,得到了疲劳累积损伤及再制造毛坯剩余寿命评估模型,并由对称循环拉压疲劳试验数据得出了修正模型的相关参数。通过二级加载(高低加载和低高加载)拉压疲劳试验对再制造毛坯剩余寿命评估模型进行了验证,结果表明模型计算值和试验值吻合良好,证明修正后的模型能够准确预测再制造毛坯的剩余寿命。     

Fatique design of machine elements using the “Bagci line” defining the fatigue failure surface line (mean stress diagram)Calcul de fatigue des elements mecaniques par emploi des “Lignes Bagci” definissant la ligne de surface de defaut par fatigue (epure des efforts moyens)

Recently proposed fatigue failure surface line of Bagci that updates the Modified Goodman's, Gerber's, and Kececioglu's mean stress diagrams for the fatigue design of machine elements subjected to cyclic combined stresses having nonvanishing mean stress is presented [1]. It fits the most recent fatigue data well and defines the two design zones in the mean stress diagram in one zone limiting the zone by the yield strength of the material, thus, providing a unified fatigue design equation for machine members. Its incorporation with the basic stresses according to different strength theories, such as the maximum distortion energy theory, maximum shearing stress theory, maximum total strain energy theory, and Coulomb-Mohr theory is given. The method of determining theoretical geometric stress concentration factor for combined state of stress—which is used along with the notch sensitivity of the material to determine a fatigue strength reduction factor—is also given. Applications of the Bagci line to the fatigue design of machine members are shown with numerical examples.     

高速列车车体加速寿命试验载荷谱编制及分析

铝合金车体是高速列车的关键核心部件,设计中需要满足超长疲劳寿命要求。工程中,为避免实际运行中发生疲劳失效,同时降低试验成本,常采用加速寿命试验方法。采用车辆系统动力学方法计算获得车体随机载荷谱,对载荷谱进行处理,得到载荷频次图;基于FKM标准,编制三种载荷比下1×10⁷次循环对应的车体加速载荷谱;采用有限元法,分别施加三种载荷比条件下的载荷谱,得到对应的加速系数;分别采用线性及非线性损伤累积理论,分析载荷块谱的加载顺序对车体损伤累积的影响。得出结论：在原始载荷谱和加速载荷谱作用下,车体结构各点疲劳损伤均小于1,满足设计要求;分别提高载荷比至P=1/3和P=2/3,对应的加速系数为12.75和218.65;载荷谱加载顺序对车体疲劳累积损伤有影响：以P=2/3为例,高-低顺序载荷谱循环632次时,损伤值累积达1,而对应相同的损伤值,低-高顺序载荷谱仅需要循环614次。结果表明车体承受载荷存在低应力幅占优的特点,即低-高加载顺序具有更好的加速效果。对车体加速寿命试验载荷谱的编制及分析方法研究,为车体台架疲劳试验提供理论依据及科学指导。     

Evaluation of an energy-based fatigue approach considering mean stress effects

In this paper, an attempt is made to extend the total strain energy approach for predicting the fatigue life subjected to mean stress under uniaxial state. The effects of means stress on the fatigue failure of a ferritic stainless steel and high pressure tube steel are studied under strain-controlled low cycle fatigue condition. Based on the fatigue results from different strain ratios, modified total strain energy density approach is proposed to account for the mean stress effects. The proposed damage parameter provides convenient means of evaluating fatigue life with mean stress effects considering the fact that the definitions used for measuring strain energies are the same as in the fully-reversed cycling (R = ?1). A good agreement is observed between experimental life and predicted life using proposed approach. Two other mean stress models (Smith-Watson-Topper model and Morrow model) are also used to evaluate the low cycle fatigue data. Based on a simple statistical estimator, the proposed approach is compared with these models and is found realistic.     

Numerical analysis for prediction of fatigue crack opening level

Finite element analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.     

Calculated load capacity of non-newtonian lubricants in finite width journal bearings

Starting from the most general type of fluid flow equation connecting cubic shear stress to rate of shear for non-Newtonian lubricants, a modified form of Reynolds' equation was derived for steady finite width journal bearings. The finite difference technique with successive over relaxation was used incorporating Reynolds' boundary conditions for pressure to obtain the pressure distribution and hence the load capacity and the attitude angle. It is shown that the flatter pressure profile and higher load capacity at low eccentricity ratios have practical advantages. The apparent viscosity loss at higher rates of shear decreases the load capacity at higher eccentricity ratios but it is expected that the flatter viscosity temperature slope of the polymer thickened non-Newtonian lubricants will compensate for their apparent viscosity decrease.     

纤维缠绕复合材料管道双轴疲劳寿命预测研究

对纤维缠绕复合材料管道的双轴疲劳行为进行了实验与理论研究。根据多轴疲劳试验的要求,设计了一种新的纤维缠绕成型复合材料薄壁管型试样,通过对不同缠绕角度试样在不同双轴载荷比条件下的双轴疲劳实验,得出了玻璃纤维/环氧树脂薄壁管的应力/寿命曲线。参考金属材料的多轴疲劳失效准则,结合复合材料各向异性的特点,提出了复合材料多轴疲劳失效准则,对不同缠绕角复合材料管道在不同载荷条件下的疲劳寿命进行统一。结果表明:本文提出的失效准则可以把不同缠绕角复合材料在多种载荷条件下的多轴疲劳寿命统一起来,从而根据一个给定缠绕角度试样在某种试验条件下疲劳寿命来预测其它缠绕角度条件下的疲劳寿命,具有非常重要的理论研究价值和工程应用前景。     

Stability behaviour of finite MHD journal bearings

The instability of a rigid rotor supported on MHD journal bearings under the influence of an axial magnetic field and a radial electric field is theoretically analysed in this paper. The MHD Reynold's equation is solved by using the finite element method and stiffness and damping coefficients are obtained. The stability is determined by Routh's criteria. The analysis reveals that the threshold of instability is extended with low values of dimensionless currents at which the load carrying capacities are relatively low. For a bearing with length equal to diameter operating at an eccentricity ratio of 0.6, and Hartmann number of 2, the threshold speed increases by 11% for a dimensionless current of ?1, while there is decrease of 18% when the dimensionless current is ?5, when compared with the hydrodynamic case.     

Effect of stress ratio on fatigue life of GFRP composites for WT blade

Fatigue life of GFRP (glass-fiber reinforced plastic) composites used in wind turbine rotor blades has been evaluated considering the glass fiber orientations. Three different laminate composites with the respective laminating orientation of 0°, ±45°, and 0°±45° were prepared using vacuum infusion method. Tensile properties and S-N curves for these composites were experimentally determined at room temperature. From the tensile tests, it was found that tensile properties were greatly dependent upon the fiber orientation and the tensile strength of unidirectional composite was the largest and bidirectional (45°) composite was the weakest among three composites. The fatigue properties were determined under constant amplitude load control at different stress ratios, R, of 0.5, 0.1 and ?0.2. The properties also show the dependency of stress ratios and fiber orientation. The fatigue life diagrams of these three composite were relatively well presented with the double logarithmic S-N curve. The linear slopes of the respective S-N curves for three composite were not greatly different. The fatigue limits for the composites were evaluated and predicted with linear Goodman and Gerber diagrams.     

Probabilistic fatigue life prediction methodology for notched components based on simple smooth fatigue tests

The methodology of probabilistic fatigue life prediction for notched components based on smooth specimens is presented. Weakestlink theory incorporating Walker strain model has been utilized in this approach. The effects of stress ratio and stress gradient have been considered. Weibull distribution and median rank estimator are used to describe fatigue statistics. Fatigue tests under different stress ratios were conducted on smooth and notched specimens of titanium alloy TC11. The proposed procedures were checked against the test data of TC11 notched specimens. Prediction results of 50 % survival rate are all within a factor of two scatter band of the test results.     

Corrosion fatigue of steel under cavitation erosion generated intermittently in 3% salt water (results of two-stage tests with and without cavitation erosion)

Two-stage fatigue tests with and without cavitation erosion were performed in 3% salt water. When corrosion fatigue with cavitation erosion (erosion fatigue) is changed to corrosion fatigue during a test, the fatigue life decreases remarkably and the total number of stress cycles to failure becomes smaller than that for tests involving erosion fatigue only. In the reverse case, when corrosion fatigue conditions are changed to erosion fatigue conditions, the fatigue life increases slightly compared with the life expected from the linear damage law (Miner's law). The variation in fatigue strength can be explained by the characteristics of crack propagation in each environment. From these results, it is presumed that the corrosion fatigue strength of the material exposed intermittently to cavitation erosion decreases remarkably.     

316不锈钢材料的统计热疲劳模型

通过对316不锈钢材料在450℃,600℃和700℃中温环境应力控制下的低周疲劳试验,得到有实际意义的试验结果,对这些试验结果进行分析和研究,得出316不锈钢材料在中温环境和应力控制下的低周热疲劳行为,并且提出一个低周热疲劳寿命预测模型。在所建立的低周热疲劳寿命模型中,Manson通用斜率方程被用于疲劳寿命与拉伸性能的试验整合。通过对疲劳试验数据与不同的数学模型进行的拟合以建立温度和其他参数之间的函数关系,从而对316不锈钢材料在中温低周疲劳环境下进行寿命预测,为以后的寿命评估模型提供依据。     

高速列车转向架构架损伤、等效应力及寿命分布特性研究

为分析高速列车转向架构架损伤、等效应力及寿命分布特性,对构架疲劳关键测点进行动应力线路实测并对测点实测时域数据波形进行解析;基于实测应力时间历程及雨流计数法编制二维应力谱,利用Goodman等寿命方程将二维应力谱等效转换为一维应力谱;阐述线性累积损伤及非线性累积损伤模型的建立方法并对实测数据的线性累积损伤及非线性累积损伤进行了计算及对比分析;分别基于线性累积损伤理论及非线性累积损伤理论推导出各理论下的等效应力,基于实测数据对两种等效应力进行了计算及对比分析;通过结合非线性累积损伤及线性累积损伤理论计算的等效应力及不同可靠度下的材料S-N曲线计算并对比分析构架结构的疲劳寿命。研究结果表明,与非线性疲劳分析理论相比,线性疲劳分析理论对高速列车转向架构架的疲劳特性评估偏于保守。     

A study on the determination of closing level for finite element analysis of fatigue crack closure

An elastic-plastic finite element analysis is performed to investigate detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using 4-node isoparametric elements can predict fatigue crack closure behavior. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. The crack opening level for the constant mesh size increases linearly from initial crack growth. The crack opening level for variable mesh size, is almost flat after crack tip has passed the monotonic plastic zone. The prediction of crack opening level using the variable mesh size proportioning the reversed plastic zone size with the opening stress intensity factors presents a good agreement with the experimental data regardless of stress ratios.     

基于损伤力学的某飞机构件冲击疲劳寿命预估

某飞机起落系统中某重要构件在飞机往复起落中承受循环冲击载荷作用,需对该构件的冲击疲劳强度进行分析。损伤力学方法对冲击疲劳问题来说是一种新的分析方法。首先分析构件在静载荷作用下的细节应力,判明疲劳危险点,接着分析得到危险点在冲击载荷作用下的应力响应。然后构建冲击型损伤演化方程,并识别其中的材质参数,提出将应力响应视为载荷谱的应力应变场—损伤场解耦处理方式,发展了损伤力学—有限元法,使其可用于预估构件冲击疲劳裂纹萌生寿命。而后应用该方法对构件在冲击疲劳载荷与非冲击疲劳载荷作用下的疲劳裂纹萌生寿命进行预估,得到寿命预估结果,进而比较得到冲击疲劳载荷作用下和非冲击疲劳载荷作用下构件寿命的当量关系,为该构件的等效疲劳试验提供重要参考依据。     

基于接触应力的圆柱滚子轴承疲劳寿命分析

首先介绍了圆柱滚子轴承疲劳寿命的经典理论计算方法.然后运用Hertz接触应力理论计算出圆柱滚子轴承的最大接触应力,结合三参数幂函数公式确立疲劳寿命曲线方程计算出轴承的疲劳寿命值.通过改变轴承滚子数目、径向力大小以及径向游隙等参数,将基于接触应力得到的疲劳寿命结果与经典理论结果进行比较,证明了该方法的合理性.并分析了在考虑离心力作用下高速圆柱滚子轴承疲劳寿命与转速之间的关系.     

The effects of radial stresses on the strength of thin-walled filament wound GRP composite pressure cylinders

Filament wound glass fibre reinforced epoxy cylinders with thickness : internal diameter ratios of approximately 0.01, and winding angle ± 85° have been tested to failure under different combinations of axial load and internal pressure. The effect of the interaction coefficients in the Tsai—Wu failure criterion equation are considered. Results for the 17 specimens tested and published results for other winding angles have been used to assess some commonly used forms of failure criteria. It was shown that inclusion of the radial stress terms had no significant effect on the experimental results considered. The geometries and hoop to axial stress ratios for which radial stresses are likely to be important are stated.     

平均应力对ZG20SiMn和ZG0Cr13Ni4Mo腐蚀疲劳强度的影响

平均应力是对疲劳强度的重要影响因素之一,在不同平均应力下实验得到的疲劳极限图是进行疲劳设计的基础。在腐蚀环境中与在一般空气状态下,平均应力对疲劳强度的影响是不尽相同的,对这一问题应给予应有的重视。实验得出了ZG20SiMn和ZG0Cr13Ni4Mo在不同平均应力下淡水介质中的腐蚀疲劳（CF）极限,发现平均应力对这两种钢材腐蚀疲劳强度的影响非常显著,继续沿用空气中推荐的疲劳极限图可能导致不安全的设计,并对其可能的机理进行了探讨。     

两级载荷作用下的损伤累积规律及其试验验证

研究累积损伤规律的文献很多,但迄今为止,各种能反映加载顺序影响的累积损伤理论都认为试样在按由高到低顺序排列的多给载荷作用下疲劳破坏时必有累积损伤的结果。本文作者经过分析研究认为,这些理论只反映了问题的一个方面。本文根据金属系中显微结构稳定性原理,提出并论证了对于循环硬化材料,在高-低两级载荷作用下,当第一级高载荷作用次数较少时必然有累积损伤的结果,进而推出在按高-低顺序排列的多级载荷作用下也有累积损伤的情况。而对于循环软化材料则相反。并就循环硬化材料的光滑试样进行了高-低两级加载的试验难证。在试验的基础上对累积损伤的计算提出了一些设想。     

Severe wear of rolling/sliding contacts

The wear of wheel flanges against the sides of rails is shown to be caused by rolling/sliding contacts sustaining high cyclic stresses at low slide/roll ratios. When these conditions are reproduced in the laboratory severe metallic wear is initiated when the resultant shear stress, which is dependent on both normal (pressure) and tangential (friction) forces, exceeds a critical value. The results are discussed in terms of Johnson's shakedown limit and the material's stress/strain characteristics. It is concluded that the severe wear of contacts at low slide/roll ratios is caused by high resolved cyclic stresses that result in continual plastic deformation of the surface layers.