考虑结构裂纹扩展的振动疲劳寿命计算方法

工程实践中任何结构都存在不同程度的裂纹损伤,振动激励下动响应与疲劳裂纹扩展之间互相耦合,直接影响结构振动疲劳寿命.为了考虑结构振动疲劳耦合效应对疲劳寿命的影响,提出了一种考虑结构裂纹扩展的振动疲劳寿命计算方法.分析时,通过建立若干个含不同长度裂纹的结构有限元模型模拟结构裂纹扩展,采用Paris方程分段计算结构振动疲劳裂纹扩展寿命,通过试验确定的固有频率降变化规律反推结构裂纹萌生寿命,最后累计得到结构疲劳总寿命.结论表明,仿真计算结果与试验结果比较吻合.     

电磁外力激励下裂纹转子非线性振动响应实验研究

介绍了转子裂纹故障预测和诊断常用方法、发展过程及意义,重点对振动检测诊断技术的相关理论和应用研究成果做了阐述,然后就裂纹转子在周期电磁外加载荷激励下的非线性振动响应及实验结果进行研究.目的是提供一种预测和诊断裂纹转子的新思想.     

裂纹梁振动分析和裂纹识别方法研究进展

作为大型复杂结构重要的组成单元,梁类结构在土木工程中有着极其广泛的应用。研究裂纹梁结构的振动理论和裂纹识别方法具有重要意义。对裂纹梁结构振动分析和裂纹识别方法的研究进行了综述。详细阐述了基于模态分析和基于智能方法的裂纹识别方法的研究进展。在此基础上提出了结构裂纹识别存在的问题和研究发展方向。     

基于小波有限元的悬臂梁裂纹识别

研究了悬臂梁裂纹识别中的正反问题．即通过裂纹位置和尺寸求解梁的固有频率以及利用梁的固有频率．识别裂纹位置和尺寸。以矩形截面裂纹悬臂梁为例，利用小波有限元方法建立了梁自由振动的有限元模型．其中裂纹被看作为一刚度已知的扭转线弹簧，求解出了系统的固有频率；通过行列式变换，将反问题求解简化为只含线弹簧刚度一个未知数的一元二次方程求根问题，分别做出了以不同固有频率作为输入值时裂纹位置与弹簧刚度之间的解曲线，曲线交点预测出裂纹的位置与尺寸。数值算例证实了算法的有效性，为工程结构裂纹故障预示与诊断提供了新的方法。     

基于结构振动损伤识别技术的研究现状及进展

对目前关于结构振动损伤识别技术的基本方法、研究现状及进展进行了回顾与总结。对基于结构振动响应和系统动态特性参数进行损伤识别方法的特点进行了简单的评述，介绍了智能诊断方法在损伤识别中的应用，并提出了结构损伤识别的发展趋势和研究方向。     

含裂纹功能梯度材料梁结构的振动功率流特性分析

功能梯度材料(FGM)梁在工程中应用日益广泛,而梁中裂纹的存在改变了局部刚度等特性,使得功能梯度材料梁的振动和波传播特性发生改变。以含有张开型裂纹的功能梯度梁为对象分析其波传播和振动功率流特性。利用转动弹簧模型模拟裂纹,给出由裂纹引起的局部柔度表达式。建立无限长FGM欧拉梁结构的动力学方程,采用波动法结合梁的连续条件计算得到FGM欧拉梁的振动特性,对无缺陷梁和裂纹梁的输入功率流和传播功率流进行分析。讨论了材料梯度指数、激励频率、裂纹深度和裂纹位置等信息与输入功率流、传播功率流之间的关系,为基于振动功率流的裂纹FGM梁的损伤识别提供理论基础。     

转轴横向裂纹的振动分析诊断

从金属材料疲劳过程的特征出发，对具有横向裂纹转轴的运动情况进行了分析，研究了具有横向裂纹转轴在断裂过程中的振动频谱分布情况及其变化规律，给转轴横向裂纹的诊断提供了进一步的信息。     

裂纹对弧齿锥齿轮传动系统振动特性影响研究

为全面掌握裂纹对弧齿锥齿轮传动系统振动特性影响,用Pro/E软件建立了无裂纹和含裂纹弧齿锥齿轮完整的三维接触模型,并基于有限元分析软件ANSYS对模型进行了仿真模拟与数值计算,得到了啮合刚度的变化规律。借助置于转子上弧齿锥齿轮传动系统8自由度动力学模型,建立了非线性振动方程并使用MATLAB软件的ode45()函数对其求解,得到了无裂纹和有裂纹两种状态下的系统振动响应。发现裂纹不仅对弧齿锥齿轮传动系统振动大小有影响,而且对系统振动的形式也有影响,甚至会导致系统产生拟周期振动和混沌振动,使系统的有序性降低,工作性能变差。从而得出可以根据振动特性的变化来对弧齿锥齿轮结构的损伤进行故障诊断的结论。     

具有横向裂纹的转子系统耦合振动研究

利用数值方法研究了裂纹引起的转子耦合振动。采用有限元方法建立裂纹转子的模型 ,每个单元结点考虑 6个自由度 ,采用 Dimarogonas法推导了 12× 12阶裂纹轴段有限元刚度矩阵。采用数值积分方法和谱分析方法研究了开裂纹、开闭裂纹引起的转子横向、纵向、扭转耦合振动。得到了一些对转轴横向裂纹诊断有意义的结论     

工程结构裂纹损伤诊断神经网络方法应用

阐述了神经网络方法在结构裂纹损伤诊断中的应用.回顾了结构损伤领域中神经网络方法应用的发展历程,总结了主要研究成果,提出了有待进一步研究的重点和方向.     

基于MSVAR模型的非线性损伤识别方法

实际工程中出现的损伤往往具有非线性特征,比如裂缝开合导致结构刚度呈现明显的双线性特性。采用简单的线性模型会遗漏与损伤相关的非线性信息,进而影响到损伤识别结果的可靠性。为解决上述问题,该文提出基于马尔科夫状态转移向量自回归模型(MSVAR)的非线性损伤识别方法。该方法假定损伤会导致结构振动响应出现非线性特征,利用MSVAR模型隐状态平滑概率能够反映数据非线性变化的特点,构造信息熵作为损伤预警指标监测结构损伤状态。MSVAR模型的自回归系数包含损伤位置信息,可据此构造损伤定位向量确定裂缝位置,以裂缝位置处的层间位移(平滑概率由0突变到100%时)表征裂缝宽度。最后通过数值算例和模型试验验证该文所提方法在裂缝类型损伤识别的有效性。     

Effects of non-proportional loading paths on the orientation of fatigue crack path

Fatigue crack path prediction and crack arrest are very important for structural safety. In real engineering structures, there are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path, etc. In this paper, both experimental and numerical methods are applied to study the effects of loading path on crack orientations. Experiments were conducted on a biaxial testing machine, using specimens made of two steels: 42CrMo4 and CK45 (equivalent to AISI 1045), with six different biaxial loading paths. Fractographical analyses of the plane of the stage I crack propagation were carried out and the crack orientations were measured using optical microscopy. The multiaxial fatigue models, such as the critical plane models and also the energy‐based critical plane models, were applied for predicting the orientation of the critical plane. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the shear‐based multiaxial fatigue models provide good predictions for stage I crack growth for the ductile materials studied in this paper.     

Dynamic fracture research based on XFEM and its application on discharge valve guard of hydrogen compressor

The structural safety analysis under complex environment and load conditions is at the leading edge of engineering researches. The dynamic fracture research based on Extended Finite Element Method (XFEM) is probed into in the paper, and the modeling principle of strong discontinuities based on XFEM is applied to dynamic fracture mechanics to overcome the difficulty of defining meshes around crack tip without remeshing. XFEM-based dynamic interaction integral method is developed to reduce the calculation error caused by the non-matching meshes in the crack-tip area. Typical examples, the edge crack plate and central inclined crack plate under Heaviside function type impact loads, are taken to validate effectiveness of the method. Furthermore, as an engineering application, a valve guard subjected to impact load is simulated using the method, and numerical simulation results suggest that the valve guard is relatively safe under static load without hydrogen environment, but the real operating conditions with both hydrogen environment and dynamic load, will lead to higher brittle fracture risk for the valve guard.     

On the application of the damage work density as a new initiation criterion for ductile fracture

In this paper the ‘damage work’ proposed by Chaouadi et al. is used to formulate an energy crack initiation criterion to describe ductile crack initiation. The traditional assessment of structural integrity by the J-integral, a property of elastic-plastic fracture mechanics is compared. Two free-cutting and one structural steel are investigated. The measured values for the critical damage work density at initiation W_di are compared with values for copper and RPV steel. As the fracture mechanical approach is limited to sharp cracks in the material (high-constraint stress state) the present damage mechanics approach is regarded as important as a more general concept closer to reality. While old void growth models of damage mechanics cannot formulate a simple criterion for crack initiation the applied damage work reaches a constant value at initiation W_di which is independent of the stress state during the deformation process. We recommend W_di as a material property of toughness for testing and engineering purposes.     

Material damping in 6061-T6511 aluminium to assess fatigue damage

Studies in fatigue can be summarized into two stages, fatigue crack initiation and crack propagation. Fatigue damage may increase the risk of failure under cyclic load. Energy dissipation, termed damping, occurs in engineering metals and is a function of the cyclic loading history. Damping behaviour of materials has been estimated using many different experimental techniques, and parameters i.e. the loss factor vs. strain amplitude, frequency range, etc. However, micro‐structural changes in the form of fatigue damage are also contributors to damping in engineering materials. In order to measure energy dissipation, a damping monitoring method has been used. Under a constant cyclic load up to the point of fatigue crack initiation, the effects of fatigue on damping factor were studied for 6061‐T6511 aluminium alloy. In the experiments, the stress levels were below yield point, 50% and 70% of ultimate strength. Experimental results showed that the damping factor changes with the number of fatigue cycles. Percentage increase in damping energy was calculated using experimental data.     

Crack initiation and growth path under multiaxial fatigue loading in structural steels

In real engineering components and structures many accidental failures occur due to unexpected or additional loadings, such as additional bending or torsion. There are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path. It is widely believed that fatigue crack nucleation and early crack growth are caused by cyclic plasticity. This paper studies the effects of multiaxial loading paths on the cyclic deformation behaviour, crack initiation and crack path. Three types of structural steels are studied: Ck45, medium carbon steel, 42CrMo4, low alloy steel and the AISI 303 stainless steel. Four biaxial loading paths were applied in the tests to observe the effects of multiaxial loading paths on the additional hardening, fatigue crack initiation and crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It is shown that these materials have different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were conducted using the critical plane approaches, either based on stress analysis or strain analysis (Findley, Smith–Watson–Topper, Fatemi–Socie, Wang–Brown–Miller, etc). Comparisons of the predicted crack orientation based on the critical plane approaches with the experimental observations for the wide range of loading paths and the three structural materials are shown and discussed. Results show the applicability of the critical plane approaches to predict the fatigue life and crack initial orientation in structural steels.     

梁裂缝损伤检测的模态应变能法及试验研究

结构裂缝存在将使其局部刚度减少,而局部刚度下降与其固有频率变化紧密相关。应变能对局部损伤较固有频率更为敏感。把结构遭受裂缝损伤的应变能变化量作为损伤指标,提出一种全新的无损裂缝识别方法。并把影响损伤指标因素与假设检验相结合,利用参数假设检验进行识别结果验证。用简支梁模型进行模态试验,将梁损伤前、后的模态参数提取,并用该方法进行裂缝位置及裂缝深度参数的识别。结果表明,该损伤检测方法识别结果精度较高,将其用于裂缝损伤检测是可行的。     

多视角几何三维重建法识别工程结构缺损与变形

裂缝等工程结构表面缺损与弯曲等构件变形是既有工程结构安全性检测鉴定与监测的重要指标,采用数字图像法可有效改善当前人工检查技术的缺点,但在工程实践中显现出定量检测中图像几何变形修正困难、局部损伤难以在整体结构中定位、无法测量钢结构空间变形等问题。结合多视角几何三维重建法的研究与应用可有效解决上述问题。该文对多视角几何三维重建的方法原理与算法实现进行讨论,列举方法实践的经典算法与高效算法;围绕数字图像法检测在工程实践中存在的问题,提出表面投影法解决成像几何变形与损伤定位、逆向工程建模与特征提取识别几何变形损伤;通过结构表面裂缝识别、大型构筑物表面损伤定位、钢结构构件变形识别等3个应用研究,阐述多视角几何三维重建法在结构表面损伤与变形识别中的具体操作方法与比较优势。多视角几何三维重建法具有设备要求低、操作方便快捷、模型色彩丰富、点云精度较高等特点,与数字图像法结合后在工程结构检测监测领域的研究与应用潜力巨大。