U75V 60kg/m钢轨接触疲劳裂纹伤损分析

针对U75V 60 kg/m钢轨在重载小半径线路中出现的探伤报警,对报警钢轨进行了宏观形貌、非金属夹杂物检验、低倍和显微组织检验等分析。分析结果表明,该钢轨在重载小半径曲线上,通过量达到4亿吨,使用状况良好。钢轨试样为接触疲劳裂纹伤损,与钢轨使用条件和线路运营情况有关,与钢轨材质无关。

地铁钢轨闪光焊缝裂纹伤损成因分析与对策——以杭州地铁某线为例

以杭州地铁某线发生的闪光焊缝裂纹伤损作为研究对象,分析裂纹伤损成因,通过统计裂纹伤损在不同列车线型、运行方向、运行频次及钢轨部位下的发生频率,明确裂纹伤损的易发场景,开展对策研究。分析发现:1)在圆曲线区段最易出现闪光焊缝裂纹伤损,从焊缝裂纹伤损数的每公里占比角度,以及导致需加固或更换大修的线型占比角度分析可知,圆曲线区段出现的频率均高于缓和曲线区段和直线区段;2)线路上行区段相比下行区段更易出现裂纹伤损;3)在单位时间内,列车作用频次越多,越容易导致裂纹伤损的出现。为降低闪光焊缝裂纹伤损的发生率,提出预防与控制对策:在焊接技术应用初期,应优化焊接参数与焊接过程;后期,严密监控裂纹伤损的发展状况,根据伤损程度制定缩短探伤周期、打磨车打磨、钻眼加固、伤轨更换等轻重缓急有别的检查、养护与修理(检养修)方案。相关分析研究思路可极大地提升闪光焊缝质量,降低裂缝伤损发生率,节约"检养修"成本,有力保障地铁行车安全,为国内其他地铁单位的运维管理提供值得参考借鉴的经验与做法。

Fatigue behavior and cumulative damage rule of concrete under cycli ccompression with constant confined stress

The effects of different lateral confinement stress on the fatigue behavior and cumulative damage of plain concrete were investigated experimentally. Eighty 100mm×100mm×100mm specimens of ordinary strength concrete were tested with constant-or variable-amplitude cyclic compression and lateral confinement pressure in two orthogonal directions. A fatigue equation was gained by modifying the classical Aas-Jakobsen S-N equation and used for taking into account the effect of the confined stress on fatigue strength of plain concrete. The present study indicates that the fatigue failure is greatly influenced by the sequence of applied variable-amplitude fatigue loading, and Miner’s rule is inapplicable to predict the residual fatigue life, especially in the sequence of low to high. The present research also shows that the exponent d of the Corten-Dolan’s damage formula is a constant depending on the materials and the levels of load spectrum, and d can be determined through the two-stage fatigue tests. The residual fatigue lives predicted by Corten-Dolan’s damage formula are found to be in good agreement with the results of the experiments.

Prediction of low-cycle crack initiation life of powder superalloy FGH96 with inclusions based on damage mechanics

The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 ℃, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.

Effects of loading waveforms on rock damage using particle simulation method

The particle simulation method is used to study the effects of loading waveforms （i.e. square, sinusoidal and triangle waveforms） on rock damage at mesoscopic scale. Then some influencing factors on rock damage at the mesoscopic scale, such as loading frequency, stress amplitude, mean stress, confining pressure and loading sequence, are also investigated with sinusoidal waveform in detail. The related numerical results have demonstrated that： 1） the loading waveform has a certain effect on rock failure processes. The square waveform has the most damage within these waveforms, while the triangle waveform has less damage than sinusoidal waveform. In each cycle, the number of microscopic cracks increases in the loading stage, while it keeps nearly constant in the unloading stage. 2） The loading frequency, stress amplitude, mean stress, confining pressure and loading sequence have considerable effects on rock damage subjected to cyclic loading. The higher the loading frequency, stress amplitude and mean stress, the greater the damage the rock accumulated; in contrast, the lower the confining pressure, the greater the damage the rock has accumulated. 3） There is a threshold value of mean stress and stress amplitude, below which no further damage accumulated after the first few cycle loadings. 4） The high-to-low loading sequence has more damage than the low-to-high loading sequence, suggesting that the rock damage is loading-path dependent.

Life Extending Control of Helicopter Based on Dynamic Inversion

The purpose of using life extending control for Black Hawk UH-60 helicopter is to make a trade-off between the handling qualities and the service life of critical components. An increase in service life span results in enhanced safety and the reduction in maintenance costs. This paper presents a design methodology of life extending control for structural durability and high performance of mechanical system, which is based on an explicit dynamic inversion control scheme. A real-time nonlinear fatigue crack growth model is built to predict fatigue damage resulting from the impact of cyclic bending stress on rotor shaft, which serves as an indicator of service life. The 4-axis gainscheduled flight controller, whose gains are adjusted as a function of damage and flight velocity, is designed to regulate roll attitude, pitch attitude, vertical velocity and yaw rate. The nonlinear system simulation results show that the responses can meet the requirements on ADS-33 Level 1 handling qualities and that the 4-axis decoupling control is realized. As the damage increases, the tracking performance is slightly degraded, which results in smaller transients in bending moment response.

A Novel Method for Obtaining a Crack Tip Appropriate Multiaxiality Constant for Damage Mechanics

Many engineering components, such as power-plant steam pipes, aero-engine turbine discs, etc., operate under severe loading/temperature conditions. As a result, cracks can initiate and subsequently propagate over time due to creep. The Liu and Murakami＇s model has proven to be a useful tool for the prediction of creep crack growth under such conditions. Previously, experimental conditions used in obtaining the constant ofmultiaxiality, a, have not reflected the multiaxial severity of the stress state ahead of a crack tip. Therefore, the present study presents a novel method for interpolating crack growth data to obtain a.

From monoscale to multiscale modeling of fatigue crack growth:Stress and energy density factor

The formalism of the earlier fatigue crack growth models is retained to account for multiscaling of the fatigue process that involves the creation of macrocracks from the accumulation of micro damage.The effects of at least two scales,say micro to macro,must be accounted for.The same data can thus be reinterpreted by the invariancy of the transitional stress intensity factors such that the microcracking and macrocracking data would lie on a straight line.The threshold associated with the sigmoid curve disappears.Scale segmentation is shown to be a necessity for addressing multiscale energy dissipative processes such as fatigue and creep.Path independency and energy release rate are monoscale criteria that can lead to unphysical results,violating the first principles.Application of monoscale failure or fracture criteria to nanomaterials is taking toll at the expense of manufacturing super strength and light materials and structural components.This brief view is offered in the spirit of much needed additional research for the reinforcement of materials by creating nanoscale interfaces with sustainable time in service.The step by step consideraton at the different scales may offer a better understanding of the test data and their limitations with reference to space and time.

Study on prediction model of cavitation erosion incubation period of duplex stainless steel surfacing layer in a benign medium

In order to estimate the cavitation incubation period of duplex stainless steel(DSS)under ultrasonic cavitation erosion(CE),this paper analyzed the loading process of material during ultrasonic CE.Local strain theory,cumulative damage theory,and Bernoulli probability model were used to derive the minimum number of cavitation impacts required for the initiation of fatigue cracks,and the predictive equation of the material cavitation incubation period was given.DSS was obtained by tungsten inert gas welding(TIG)powder surfacing method.Tensile test and ultrasonic CE test were performed,and the tensile test data were used to calculate the cavitation incubation period by using the predictive equation.Due to the possible presence of precipitates and micro-cracks in the sample,there was an error between the calculated and the test results.The results showed that the max error between the calculated results and the test results was 8.1%,and the errors of the remaining two groups were within 3.5%.