含椭圆表面裂纹橡胶-钢球支座在扭转载荷下的断裂分析

通过非线性有限元方法对橡胶-钢球支座在扭转载荷作用下存在的椭圆表面裂纹情况进行了数值模拟,给出撕裂能与裂纹尺寸、载荷和橡胶层厚度的关系曲线．结果表明,对界面椭圆表面裂纹,同一裂纹深度,撕裂能随载荷的增加而增加,随橡胶层厚度的增加而减小．对橡胶层中间含椭圆表面裂纹,撕裂能随裂纹深度的增加而增加．最后,利用非线性回归工具得到撕裂能的经验计算式．     

两种材料的性质对双材料界面裂纹能量释放率及应力强度因子的影响

在实际工程中,结构往往是由多种材料构成,而裂纹经常在两种材料的界面上扩展.针对双材料界面裂纹,建立相应的计算模型.用能量释放率来描述裂纹的扩展情况,并根据界面裂纹能量释放率解析计算方法对不同的材料性质、外荷载、构件厚度,以及裂纹两侧不同的材料宽度对裂纹扩展时的能量释放率和应力强度因子进行仿真分析,得到了能量释放率和两种材料的弹性模量、外荷载及构件厚度之间的关系变化的曲线,以及裂纹两侧不同的材料宽度和应力强度因子之间的关系.并对界面裂纹和单一材料裂纹的情况进行分析、比较.     

裂纹故障对轮齿时变啮合刚度的影响分析

针对裂纹引起齿轮时变啮合刚度(TVMS)减小这一现象,研究了裂纹故障对TVMS的影响规律。首先,构建了完整的轮齿齿廓曲线,基于传统势能法分析了相邻齿耦合效应对TVMS的影响,对TVMS计算公式进行修正。其次,采用有限元法确定了裂纹萌生点所在位置,提出了一种沿深度拓展的裂纹曲线,分析了裂纹深度对TVMS和负载分担比的影响,研究了裂纹同时沿深度与长度方向拓展的中早期故障模型。最后,构建了不同故障齿轮副模型,采用有限元法对裂纹沿深度结果进行验证,结果表明势能法与有限元法相吻合。     

压电柔性梁裂缝损伤识别实验

针对压电柔性悬臂梁裂缝损伤检测与损伤程度识别问题,采用小波包分析和小波神经网络相结合的方法进行裂缝深度识别实验研究.利用小波包频带能量谱构造柔性悬臂梁裂缝损伤指标,即能量比相对变化量的H2范数,并建立压电柔性梁裂缝损伤实验装置.激励柔性梁的振动,记录两路压电传感器采集的振动信号,进行小波包分解并计算损伤指标.将这些损伤指标进行组合,作为小波神经网络的输入特征参数,进行裂缝深度即损伤程度的识别.实验结果表明:能量比相对变化量的H2范数对柔性梁的裂缝损伤敏感,对测试噪声不敏感;采用的小波神经网络可以精确识别柔性梁的裂缝深度.     

双盘双呼吸型裂纹转子的非线性动力学特性

采用有限元方法建立了双盘双呼吸型裂纹转子系统的动力学方程,利用应变能释放率方法得到了裂纹单元的刚度矩阵,采用应力强度因子为零法模拟裂纹的呼吸效应,研究了双裂纹转子振动响应的特点,以及在一个稳态旋转周期内裂纹轴刚度时变规律,分析了不同裂纹深度和裂纹夹角对转子振动响应的影响。     

钢在多次冲击载荷下裂纹发展过程的研究

应用在破断面上显微镜直接观察的方法研究了回火碳结构钢在多次冲击载荷下的裂纹发生与发展过程。发现裂纹深度t与冲击次数N之间存在t＝t0e（N/b）的指数关系,裂纹发展速度v与裂纹深度t之间存在v＝1/b*t的线性关系（相应于普通疲劳德v＝Ktn关系式中n＝1时的情况）。并得出多次冲击抗力（冲击破断寿命）既决定于裂纹发展速度v又决定于极限裂纹深度tK,它们对冲击破断寿命的影响随钢回火温度或钢中碳含量的变化而互为矛盾的变化着。表明裂纹发展速度主要与强度因素有关,而极限裂纹深度主要与塑性因素有关,二者的最好配合使钢在某一回火温度或某一碳含量出现多冲抗力极值。这就使以前发表的文[3][4][5]中所得多冲抗力基本规律得到了有力的支持,同时为充分发挥结构钢的强度潜力提供了新的依据。     

Identification of multiple cracks in beams under bending

The identification of a transverse crack on a beam is the subject of many investigators. Identifying the crack means to find its position and depth. In many cases there are more than one cracks on a beam. Then the solutions, or the combinations of parameters characterising the cracks are more and the problem becomes more complicated particularly when the crack must be identified using one more parameter, the relative each other angular position.In the present paper the dynamic behaviour of a cracked beam with two transverse surface cracks is studied. Each crack is characterised by its depth, position and relative angle. Both cracks are considered to lie in arbitrary angular positions with respect to the longitudinal axis of the beam and at any distance from the left end. A local compliance matrix of two degrees of freedom, bending in the horizontal and the vertical planes is used to model the rotating transverse crack in the shaft and is calculated based on the available expressions of the stress intensity factors and the associated expressions for the strain energy release rates. The compliance matrix is calculated for the first time at any angle of rotation. Thus, the compliance is given as a function of both the crack depth and the angular location. These expressions are usable, due to the stress intensity function limitations, only for limited regions around the zero angular position of the crack and not for every crack angle. For these cases, B-spline curves are used to interpolate the known points and a function in analytical form is given for every crack depth and angle. It is well known that when a crack exists in a structure, such as a beam, then the natural frequency of vibration decreases. This reduction is studied here for six independent parameters namely the depth, the location, and the rotational angle of each crack. By keeping these six parameters constant, the first three flexural eigenmodes can be computed and plotted.Due to its sensitivity in slope or displacement changes the theory of wavelets is used here to identify the locations of the cracks reducing thus the number of independent parameters. As it is well known the existence of a crack on a beam in bending, creates in the elastic line of the beam a slope discontinuity analog generally to the crack depth and additionally here to the angular position. The wavelet transformation of a vibration mode or of the vibration response of the structure under some circumstances could be used to locate the cracks. If the positions are known, then the depths and the respective angles can be determined. Here the diagrams of the first three eigenvalues versus both the crack depth and the rotational angle, are used to identify the remaining unknown parameters for both cracks.     

裂纹对齿轮轮齿结构振动的影响

建立具有裂纹的齿轮轮齿的动力学模型，分析齿轮轮齿发生裂纹后轮齿结构的动力响应及动力特性，并对裂纹出现位置和裂纹尺寸对齿轮结构动力特性的影响进行深入探讨；通过分析计算和有限元数值模拟验证表明，裂纹发生位置对齿轮轮齿振型影响较大，在裂纹发生处振型发生突变；而裂纹大小对其振型和固有频率影响都较大，当裂纹出现后齿轮固有频率发生下降，振型也发生变化，随着裂纹深度的增加，固有频率更加下降，低阶下降显著，而高阶下降缓慢，振型也与无裂纹的情形完全不同。这显示出裂纹对齿轮轮齿结构振动的影响随裂纹尺寸的增加而强烈。     

裂纹齿轮动力特性分析与模拟

建立了齿轮的动力学模型 ,分析了齿轮轮齿发生裂纹后的动力特性 (固有频率、振型等 ) ,并对裂纹出现位置和裂纹尺寸等对齿轮动力特性的影响进行了深入探讨和计算机模拟 ,指出裂纹发生位置对齿轮轮齿振型影响较大 ,在裂纹发生处振型发生突变 ;而裂纹尺寸对其振型和固有频率影响都较大 ,当出现裂纹后固有频率发生下降 ,振型也发生变化 ,随着裂纹尺寸增加 ,固有频率下降更加显著 ,各阶幅值下降大小不一 ,振型也与无裂纹的情况完全不同。这对齿轮的损伤监测和诊断具有重要价值     

Numerical simulation of kink angle prediction for non-lubricated surface crack subjected to Hertzian pressure

Analysis of determining linear fracture mechanics parameters for a surface crack loaded with Hertzian pressure without the presence of a lubricant in the crack is presented. For this purpose, the elements of Williams equation for describing stresses near the crack tip are determined: Stress intensity factors K_I and K_II, and T-stress, which were taken into account when determing the direction of crack propagation by means of Strain energy density (SED) criterion. The elements are determined based on the case of a gear tooth flank, with various friction coefficients on the free surface and friction in the crack being taken into account. The results show that friction on the free surface and, consequently, tangential forces have an impact on whether the crack will propagate along the free surface or into the depth.     

Crack identification in a rotor with an open crack

A finite element (FE) model, which is based on a transfer matrix analysis and local flexibility theorem, is introduced for crack identification of a static (non-rotating) rotor with an open crack. Through numerical simulation, the effects of crack location and crack depth on the mode shapes and the changes in the eigenfrequencies of the cracked rotor are investigated. A crack identification algorithm that makes use of the translations of the first mode at two symmetric points and the contour diagram of crack location versus crack depth for the first two given normalized eigenfrequencies is proposed to estimate the crack location and depth in the rotor. Two illustrative examples are demonstrated and compared for availability and validity of the proposed algorithm.     

Nonlinear dynamic behavior of rotating blade with breathing crack

This study aims at investigating the nonlinear dynamic behavior of rotating blade with transverse crack. A novel nonlinear rotating cracked blade model (NRCBM), which contains the spinning softening, centrifugal stiffening, Coriolis force, and crack closing effects, is developed based on continuous beam theory and strain energy release rate method. The rotating blade is considered as a cantilever beam fixed on the rigid hub with high rotating speed, and the crack is deemed to be open and close continuously in a trigonometric function way with the blade vibration. It is verified by the comparison with a finite element-based contact crack model and bilinear model that the proposed NRCBM can well capture the dynamic characteristics of the rotating blade with breathing crack. The dynamic behavior of rotating cracked blade is then investigated with NRCBM, and the nonlinear damage indicator (NDI) is introduced to characterize the nonlinearity caused by blade crack. The results show that NDI is a distinguishable indicator for the severity level estimation of the crack in rotating blade. It is found that severe crack (i.e., a closer crack position to blade root as well as larger crack depth) is expected to heavily reduce the stiffness of rotating blade and apparently result in a lower resonant frequency. Meanwhile, the super-harmonic resonances are verified to be distinguishable indicators for diagnosing the crack existence, and the third-order super-harmonic resonances can serve as an indicator for the presence of severe crack since it only distinctly appears when the crack is severe.     

Crack localisation and sizing in a beam based on the free and forced response measurements

In the present paper, a method of the crack localisation and sizing in a beam from the free and forced response measurements is developed. The method gives crack flexibility coefficients as a by-product. Timoshenko beam theory is used in the beam modelling for transverse vibrations. The finite element method (FEM) is used for the cracked beam free and forced vibration analysis. An open transverse surface crack is considered for the crack model. The effect of the proportionate damping has been included. A harmonic imbalance force of known amplitude and frequency is used to dynamically excite the beam with the help of an independent exiting unit. The crack localisation and sizing algorithm is iterative in nature. The iteration starts with an initial guess for the crack depth ratio and iteratively estimates the crack location and the crack depth until getting the desired convergence for both the crack location and the crack depth. For estimation of bounded flexibility coefficients, a regularisation technique has been adopted. The method has been illustrated through numerical examples. The prediction of the crack location and size are in good agreement even in the presence of the measurement error and noise.     

Three-steps-meshing based multiple crack identification for structures and its experimental studies

Multiple crack identification plays an important role in vibration-based crack identification of structures. Traditional crack detection method of single crack is difficult to be used in multiple crack diagnosis. A three-step-meshing method for the multiple cracks identification in structures is presented. Firstly, the changes in natural frequency of a structure with various crack locations and depth are accurately obtained by means of wavelet finite element method, and then the damage coefficient method is used to determine the number and the region of cracks. Secondly, different regions in the cracked structure are divided into meshes with different scales, and then the small unit containing cracks in the damaged area is gradually located by iterative computation. Lastly, by finding the points of intersection of three frequency contour lines in the small unit, the crack location and depth are identified. In order to verify the effectiveness of the presented method, a multiple cracks identification experiment is carried out. The diagnostic tests on a cantilever beam under two working conditions show the accuracy of the proposed method: with a maximum error of crack location identification 2.7% and of depth identification 5.2%. The method is able to detect multiple crack of beam with less subdivision and higher precision, and can be developed as a multiple crack detection approach for complicated structures.     

Monte-Carlo法计算含缺陷油气输送管线的失效概率

利用Monte-Carlo方法全面分析了裂纹深度、裂纹深长比、断裂韧度、屈服强度和输送压力等主要随机参数对含裂纹长输油气管线结构的失效概率的影响。利用故障树分析中的参数敏感性方法确定了随机参数的敏感性。计算结果表明,断裂韧度和裂纹深度对结构的失效概率影响较大,裂纹深长比对结构的失效概率也有重要影响。此外,还分析了焊接接头在不同的裂纹深度下,强度匹配对失效概率的影响,并还给出了管线在不同的失效模式下的年失效概率。     

Frequency error based identification of cracks in beam-like structures

A crack identification method of a single edge cracked beam-like structure by the use of a frequency error function is presented in this paper. First, the dynamic theory of Euler-Bernoulli beams was employed to derive the equation of the natural frequency for a single edge cracked cantilever beam-like structure. Subsequently, the cracked section of the beam was simulated by a torsional spring. The flexibility model of the torsional spring due to the crack was estimated by fracture mechanics and energy theory. Thereafter, a function model was proposed for crack identification by using the error between the measured natural frequencies and the predicted natural frequencies. In this manner, the crack depth and crack position can be determined when the total error reaches a minimum value. Finally, the accuracy of the natural frequency equation and the viabilty of the crack identification method were verified in the case studies by the measured natural frequencies from the literature. Results indicate that the first two predicted natural frequencies are in good agreement with the measured ones. However, the third predicted natural frequency is smaller than the measured natural frequency. In the case of small measured frequency errors, the predicted crack parameters are in good agreement with the measured crack parameters. However, in the case of large measured frequency errors, the predicted crack parameters only give roughly estimated results.     

Crack identification using neuro-fuzzy-evolutionary technique

It has been established that a crack has an important effect on the dynamic behavior of a structure. This effect depends mainly on the location and depth of the crack. To identify the location and depth of a crack in a structure, a method is presented in this paper which uses neurofuzzy-evolutionary technique, that is, Adaptive-Network-based Fuzzy Inference System (ANFIS) solved via hybrid learning algorithm (the back-propagation gradient descent and the least-squares method) and Continuous Evolutionary Algorithms (CEAs) solving single objective optimization problems with a continuous function and continuous search space efficiently are unified. With this ANFIS and CEAs, it is possible to formulate the inverse problem. ANFIS is used to obtain the input (the location and depth of a crack)_— output (the structural Eigenfrequencies) relation of the structural system. CEAs are used to identify the crack location and depth by minimizing the difference from the measured frequencies. We have tried this new idea on beam structures and the results are promising.     

Simulation on the motion of crankshaft with a slant crack in crankpin

A new model for vibration analysis of a crankshaft with a slant crack in crankpin is proposed, and the influence of crack depth on the transient response of a cracked crankshaft is investigated. A slant cracked shaft element is developed by deducing the local flexibility due to a slant crack. The frequently occurred slant crack in crankpin is studied, and a new finite element model of crankshaft including the slant crack in crankpin, which combines the slant cracked shaft element and Timoshenko beam elements, is derived. The support of engine block and the switching behaviour of the crack are considered, and the non-linear equation of motion for cracked crankshaft-bearing system is set up in a rotating coordinate system. The motion of a crankshaft of a four in-line cylinder engine with and without an initial crack is simulated. The influence of the crack depth on the transient response is investigated. The numerical simulation demonstrates that the current model is valid for simulating the motion of cracked crankshaft system. The results show that a useful foundation is laid for crack detection of crankshaft.     

单颗磨粒刻划氧化镓晶体表面的裂纹成核位置及扩展方向研究

为了分析新一代光电子材料氧化镓晶体在超精密磨削、研磨加工过程中的裂纹成核位置及扩展方向,建立了单颗磨粒刻划氧化镓(010)晶面的弹性应力场模型,分析了氧化镓(010)晶面的脆塑性转变临界切削深度。通过MATLAB软件分析预测刻划氧化镓晶体过程中表面径向裂纹的成核位置及扩展方向,分析结果表明：当切削深度小于临界切削深度时,径向裂纹成核位置在磨粒的后方,裂纹扩展方向与切削方向之间的夹角在33°左右;当切削深度超过临界切削深度时,径向裂纹成核位置进一步向磨粒后方移动,裂纹生成方向与刻划方向之间的夹角在51°左右。为验证理论分析结果,对氧化镓晶体进行了纳米刻划试验,对比分析表明,氧化镓应力场的解析结果与试验数据高度一致。在线性加载条件下,Cube金刚石压头在氧化镓晶体(010)晶面上产生的径向裂纹偏转角在33.37°~51.45°之间。     

基于Lamb波的平尾大轴裂纹扩展监测

平尾大轴作为在役飞机的主承力构件,其轴内变厚度截面处存在应力集中现象,是疲劳断裂高发的关键部位。针对平尾大轴变截面处裂纹损伤,研究其基于主动Lamb波的裂纹深度在线监测方法。首先,通过线切割制造真实损伤,对压电传感器采集的监测信号进行Shannon连续复数小波变换,去噪提取Lamb波信号;其次,重点研究了不同模式Lamb波的4种损伤因子对大轴裂纹深度的表征能力,结果表明,基于A_0模式的互相关损伤因子对裂纹深度的表征效果最佳;最后,利用A_0模式的互相关损伤因子实现了平尾大轴裂纹萌生及裂纹尺寸的定量化监测,为平尾大轴的在线监测提供了方法基础。