基于应变指标的微弱损伤识别的实验方法

针对位移指标对微弱损伤不敏感的特点,研究了基于应变模态的损伤识别方法。该指标从应变模态参数识别着手,获得低阶应变模态振型,构造应变模态差,达到了结构微弱损伤的识别目的。通过与位移指标对比,实验表明该指标能通过应变模态振型和应变模态差准确识别微弱损伤的位置和损伤的程度。     

An experimental study on damage detection of structures using a timber beam

Using vibration methods for the damage detection and structural health monitoring in bridge structures is rapidly developing. However, very little work has so far been reported on timber bridges. This paper intends to address such shortcomings by experimental investigation on a timber beam using a vibration based method to detect damage. A promising damage detection algorithm based on modal strain energy was adopted and modified to locate/evaluate damage. A laboratory investigation was conducted on a timber beam inflicted with various damage scenarios using modal tests. The modal parameters obtained from the undamaged and damaged state of the test beam were used in the computation of damage index, were then applied using a damage detection algorithm utilising modal strain energy and a statistical approach to detect location of damage. A mode shape reconstruction technique was used to enhance the capability of the damage detection algorithm with limited number of sensors. The test results and analysis show that location of damage can be accurately identified with limited sensors. The modified method is less dependent on the number of modes selected and can detect damage with a higher degree of confidence.     

基于逆有限元的应变模态损伤检测方法

基于光纤光栅传感器(fiber Bragg grating,简称FBG)逆有限元方法(inverse finite element method,简称iFEM),仅利用有限测点的应变数据进行全域应变场重构,得到近似完全测量应变模态,提高了直接采用实测应变数据来构建应变模态损伤指标的实用性。利用基于损伤应变模态差分原理的损伤指标法,只需用损伤后应变模态数据即能定位损伤,并给出了损伤指标数学模型。计算结果表明,基于光纤光栅传感器和逆有限元方法可以快速进行全域应变场重构,为基于应变模态的损伤检测提供数据保障,而应变模态差分曲线只在损伤处发生剧烈变化,损伤程度不同,曲线突变程度略有不同但规律一致。最后以某板的实验结果证明了该方法的有效性。     

基于曲率模态的木梁损伤定量识别

为对木结构构件局部缺陷进行 有效检测,结合试验模态分析和有限元模拟方法,对含不同位置、大小和数量孔洞缺陷的木梁 进行研究,计算其第1阶位移模态振型和曲率模态,分析木梁损伤前后的曲率模态变化,探讨 模态分析方法在木材缺陷检测中的适用性。研究结果表明:曲率模态是一个对木梁损伤比较 敏感的参数,可用于对孔洞位置、大小及数量进行定量的估计;通过降低有限元模型局部单 元的弹性模量能够较好地模拟木梁损伤,有限元模态分析和试验模态分析得到的位移模态振 型及曲率模态吻合均较好,验证了模态分析对木梁无损检测是一种有效的方法。     

损伤结构的实验曲率模态研究

为探讨基于振动模态分析的结构无损检测技术，把曲率模态应用于单向板这样的承弯结构，从理论上验证了该指标的变化是结构损伤程度和位置的函数。通过有限元数值模拟和单向板破坏实验，证明了利用该指标对判断承弯结构的损伤效果显著。该方法的进一步完善使利用振动诊断技术对大型结构进行无损检测成为可能。     

基于摄动方法的受损薄板模态分析

将结构的损伤用δ函数进行表达,建立损伤条件下薄板的自由振动方程。利用一阶摄动方法给出摄动项的一般表达式,并结合δ函数的性质,求出受损薄板模态参数的解析表达式。以一个四边简支的受损薄板为数值算例,计算损伤条件下其模态参数的变化。最后分析损伤情况对模态参数的影响,为利用动力学特性对板的损伤监控和检测提供理论依据。     

振型曲率在板类结构动力检测中的应用

以四边简支方形弹性薄板为研究对象，通过数值计算得到板损伤前后的多阶模态参数，进而得到板面内两个方向的位移振型曲率并用于板的损伤检测研究。结果表明：当布置有足够数量的振型测点时，振型曲率及板损伤前后的振型曲率差均可用于板损伤的探测与定位，并能大致判断损伤的程度；当振型测点间距过大，或测点偏离损伤区域时，均可能导致检测的失败。     

Novel Laplacian scheme and multiresolution modal curvatures for structural damage identification

Modal curvature is more sensitive to structural damage than directly measured mode shape, and the standard Laplace operator is commonly used to acquire the modal curvatures from the mode shapes. However, the standard Laplace operator is very prone to noise, which often leads to the degraded modal curvatures incapable of identifying damage. To overcome this problem, a novel Laplacian scheme is proposed, from which an improved damage identification algorithm is developed. The proposed step-by-step procedures in the algorithm include: (1) By progressively upsampling the standard Laplace operator, a new Laplace operator is constructed, from which a Laplace operator array is formed; (2) by applying the Laplace operator array to the retrieved mode shape of a damaged structure, the multiresolution curvature mode shapes are produced, on which the damage trait, previously shadowed under the standard Laplace operator, can be revealed by a ridge of multiresolution modal curvatures; (3) a Gaussian filter is then incorporated into the new Laplace operator to produce a more versatile Laplace operator with properties of both the smoothness and differential capabilities, in which the damage feature is effectively strengthened; and (4) a smoothened nonlinear energy operator is introduced to further enhance the damage feature by eliminating the trend interference of the multiresolution modal curvatures, and it results in a significantly improved damage trait. The proposed algorithm is tested using the data generated by an analytical crack beam model, and its applicability is validated with an experimental program of a delaminated composite beam using scanning laser vibrometer (SLV) to acquire mode shapes. The results are compared in each step, showing increasing degree of improvement for damage effect. Numerical and experimental results demonstrate that the proposed novel Laplacian scheme provides a promising damage identification algorithm, which exhibits apparent advantages (e.g., high-noise insusceptibility, insightful in damage revealment, and visualized damage presentation) over the standard Laplace operator.     

运用曲率模态技术的木梁损伤定量识别

为对木结构构件局部缺陷进行有效检测,结合试验模态分析和有限元模拟方法,对含不同位置、大小和数量孔洞缺陷的木梁进行研究,计算其第1阶位移模态振型和曲率模态,分析木梁损伤前后的曲率模态变化,探讨模态分析方法在木材缺陷检测中的适用性.研究结果表明:曲率模态是一个对木梁损伤比较敏感的参数,可用于对孔洞位置、大小及数量进行定量的估计;通过降低有限元模型局部单元的弹性模量能够较好地模拟木梁损伤,有限元模态分析和试验模态分析得到的位移模态振型及曲率模态吻合均较好,验证了模态分析对木梁无损检测是一种有效的方法.     

基于曲率模态与柔度曲率的损伤识别

以含两条裂纹的两端固定梁为例,采用曲率模态差和模态柔度曲率差来检测结构的损伤。首先将梁的裂纹模拟为无质量的等效扭转弹簧,推导了裂纹梁的特征微分方程,利用边界条件和裂纹位置的连续性条件推导得到该裂纹梁的振形函数解析表达式。然后用中心差分法分别求解裂纹梁损伤前后的曲率模态值和模态柔度曲率值,利用其差值确定梁的损伤位置,进而确定损伤程度。最后讨论了曲率模态和柔度曲率对结构损伤识别的敏感性。     

Structural crack detection without updated baseline model by single and multiobjective optimization

Identification, localization and quantification of structural damage can be performed through a model-updating procedure. Model-updating methods require a baseline finite element (FE) model of the undamaged structure, which imposes a restriction on their applicability and can become very problematic especially for large and complex civil structures. Modeling errors in the baseline model whose effects exceed the modal sensitivity to damage are critical and make an accurate estimation of damage impossible. This paper presents an identification algorithm using modal data for assessing structural damage that is based on FE-updating procedures and takes modeling error into account. To overcome its influence, differences of mode shapes and frequencies before and after damage for both numerical model and experimental measurements are used instead of the mode shapes and frequencies themselves. To formulate the objective function, two different approaches have been considered taking into account how these differences are grouped: a single-objective approach and a multiobjective approach. The effectiveness of both approaches is verified against numerical and experimental results.     

基于应变模态的模态应变能损伤识别方法

传统模态应变能计算需要完备模态振型信息,而模态振型信息中存在转角自由度难以准确获取的问题,为解决该问题,开展基于应变模态的模态应变能损伤识别研究,实现了结构损伤的定量识别。首先,通过基于应变与位移之间的联系,推导出应变模态与位移模态之间的转换矩阵;其次,利用应变模态代替位移模态计算单元模态应变能,建立基于灵敏度分析的损伤识别方程组;最后,根据奇异值截断法求解该方程组识别结构损伤。以一两端固支梁结构为对象,开展数值仿真和实验研究。结果表明,该方法可以有效识别出结构的损伤位置和损伤程度,相对于基于振型扩充的模态应变能损伤识别方法,具有更好精度和抗噪性能。     

Frequency response function shape-based methods for structural damage localisation

The ultilisation of structural shape signals for damage localisation has shown some promise, especially in the applications where an accurate finite element model of the structure is not available. For this purpose, traditional shape signals, like mode shapes, flexibility matrices, uniform load surface (ULS) and operational deflection shapes (ODS) have been widely used. Using frequency response function (FRF) shapes for structural damage localisation is however, a relatively new but promising technique. Unlike mode shapes, ULS and ODS, FRF shapes are defined on broadband data and so have potential to reveal damage location more clearly. Another advantage of using FRF shapes is that the test data can be directly used without the necessity of conducting modal identification. Nevertheless, some problems associated with this approach still remain to be solved. No solid foundation or deduction about the use of FRF shapes for damage localisation has been given in any literature so far. In addition, it has been observed that this method only works for a low-frequency range. This limitation of FRF shapes has not been explained or well treated so far. In this study, a scheme of using FRF shapes for structural damage localisation is proposed. Methods within this scheme include some important modifications like using the imaginary parts of FRF shapes and normalising FRF shapes before comparison. The theoretical explanation of using FRF shapes for damage localisation is presented and the limitations of the previous FRF shape methods have been overcome. The proposed methods have shown great potential in structural damage localisation.     

Matrix of the covariance of covariance of acceleration responses for damage detection from ambient vibration measurements

A new matrix on the covariance of covariance is formed from the auto/cross-correlation function of acceleration responses of a structure under white noise ambient excitation. The components of the covariance matrix are proved to be function of the modal parameters (modal frequency, mode shape, and damping parameter) of the structure. Information from all the vibration modes of the structure limited by the sampling frequency contributes to these components. The formulated covariance matrix contains more information on the vibration modes of the structure that cannot be obtained by the general methods for extracting modal parameters. When the component of the covariance matrix is used for damage detection, it is found more sensitive to local stiffness reduction than the first few modal frequencies and mode shapes obtained from ambient excitation. A simply supported 31 bar plane truss structure is studied numerically where a multiple damage scenario with different noise levels is identified with satisfactory results.     

The free vibration analysis and optimal design of an adhesively bonded functionally graded single lap joint

In this study, three-dimensional free vibration and stress analyses of an adhesively bonded functionally graded single lap joint were carried. The effects of the adhesive material properties, such as modulus of elasticity, Poisson's ratio and density were found to be negligible on the first ten natural frequencies and mode shapes of the adhesive joint. Both the finite element method and the back-propagation artificial neural network (ANN) method were used to investigate the effects of the geometrical parameters, such as overlap length, plate thickness and adhesive thickness; and the material composition variation through the plate thickness on the natural frequencies, mode shapes and modal strain energy of the adhesive joint. The suitable ANN models were trained successfully using a series of free vibration and stress analyses for various random geometrical parameters and compositional gradient exponents. The ANN models showed that the support length, the plate thickness and the compositional gradient exponent played important role on the natural frequencies, mode shapes and modal strain energies of the adhesive joint whereas the adhesive thickness had a minor effect. In addition, the optimal joint dimensions and compositional gradient exponent were determined using genetic algorithm and ANN models so that the maximum natural frequency and the minimum modal strain energy conditions are satisfied for each natural frequency of the adhesively bonded functionally graded single lap joint.     

模态应变能在复合材料机翼结构损伤检测中的应用

以复合材料机翼结构为研究对象，采用单元模态应变能改变率作为结构损伤标识量，对损伤结构进行损伤识别仿真。应用NASTRAN有限元程序对模型进行模态分析，模态分析的结果表明，单元模态应变能改变率对不同位置和不同程度损伤都具有较强的敏感性。应用神经网络成功识别了损伤的位置和程度，指出其可行性。     

On-line fan blade damage detection using neural networks

This paper presents a methodology for monitoring the on-line condition of axial-flow fan blades with the use of neural networks. In developing this methodology, the first stage was to utilise neural networks trained on features extracted from on-line blade vibration signals measured on an experimental test structure. Results from a stationary experimental modal analysis of the structure were used for identifying global blade mode shapes and their corresponding frequencies. These in turn were used to assist in identifying vibration-related features suitable for neural network training. The features were extracted from on-line blade vibration and strain signals which were measured using a number of sensors.The second stage in the development of the methodology entails utilising neural networks trained on numerical Frequency Response Function (FRF) features obtained from a Finite Element Model (FEM) of the test structure. Frequency domain features obtained from on-line experimental measurements were used to normalise the numerical FRF features prior to neural network training. Following training, the networks were tested using experimental frequency domain features. This approach makes it unnecessary to damage the structure in order to train the neural networks.The paper shows that it is possible to classify damage for several fan blades by using neural networks with on-line vibration measurements from sensors not necessarily installed on the damaged blades themselves. The significance of this is that it proves the possibility to perform on-line fan blade damage classification using less than one sensor per blade. Even more significant is the demonstration that an on-line damage detection system for a fan can be developed without having to damage the actual structure.     

利用单元模态应变能法的地下杠架结构损伤诊断

基于地下框架结构损伤前后动力特性的分析,证明了模态应变能变化对单元损伤的敏感性.选择单元模态应变能变化率作为人防结构损伤诊断的标识量,证明了多位置损伤状态下产生的单元模态应变能变化率是单位置损伤状态的叠加,并提出了应用组合优化求解结构多位置损伤诊断的方法.以一典型的人防工程为算例,通过对其在不同损伤情况下计算结果的分析和讨论,说明所提出方法是有效可行的,它不仅可以准确完成人防结构的损伤诊断,并且简化了损伤诊断过程.     

弹性梁损伤识别模态应变能法

针对飞行器弹性梁结构的振动疲劳损伤问题,基于单元模态应变能的变化提出了一种新的结构损伤识别方法。该方法以结构损伤会导致其模态性能变化为原则,通过对比结构单元损伤前后模态应变能的变化率来构建损伤指标。在构建损伤指标之前,假定弹性梁的刚度是由单元刚度组成的,在定义单元刚度灵敏度公式的基础上,建立了弹性梁损伤前后模态应变能变化的联系。最后采用数值计算和实验测试方法,得到了弹性梁损伤状态的识别结果,验证了该方法对损伤识别的正确性和有效性。