固有非线性结构中闭合型裂纹损伤的亚谐波识别方法研究

During the past decade, the development of Structural Health Monitoring (SHM) systems has attracted considerable interest from several engineering fields. It is hard to determine the source of superharmonics coming from the structural damage or the intrinsic nonlinear boundary or measurement system when utilize the nonlinear ultrasonic method to the structural initial damage detection. In this proposal, the mechanism of subharmonic of closed crack on the complex plate is investigated, and the different threshold behavior of nonlinear boundary condition and closed crack damage is established in order to distinguish the source of nonlinear subharmonic signal features. The driving frequency and amplitude to produce subharmonic signal component for the closed crack is determined through theory model and laboratory experiments. After separating subharmonics signal by scale subtraction method using low and high voltage excitation respectively, the nonlinear DORT (decomposition of time reversal operator) method is used to localize and discern nonclassical nonlinear closed cracks and initial nonlinear effects generated by contacts between mating parts and boundary interfaces. To experimentally evaluate the performance of the proposed method, a linear array of small piezoelectric transducers attached to an aluminum plate and repaired composite plate with closed cracks and delamination with nondamaged nonlinear interface features is used to obtain interelement responses. The array is used for the transmission of high frequency wave and receiving of subharmonic time domain signals calculated with the nonlinear DORT algorithm to get subharmonic accurate images for the closed cracks on the plate with nonlinear boundary conditions. Experimental and numerical results prove the theoretical approach and provide guidelines for the structural damage detection under real environment and load condition for future engineering applications.

由于难以确定非线性超声检测方法中超谐波成分来源于结构损伤还是结构或测量环节中的固有非线性源，本项目针对具有边界非线性的复杂板类结构，研究闭合型裂纹亚谐波损伤识别与定位方法。通过建立固有边界非线性和迟滞型闭合裂纹的非线性动力学模型，分析产生亚谐波的机理和不同条件，确定迟滞型闭合裂纹亚谐波共振激励频率和幅值门槛值条件，以区分固有非损伤非线性结构特征和闭合型裂纹损伤。具体采用亚谐波激励频率附近的高频主动扫频信号，基于闭合裂纹应变阈值分别以小电压和大电压激励结构，利用比例相减方法分离得到亚谐波等非线性成份。研究非线性时间反转分解方法，对具有固有非线性特征的实际板类结构中的闭合型裂纹进行准确识别与定位。以压电作动/传感阵列为物理实现手段，以金属铝板连接结构中的疲劳裂纹及复合材料板修补后的层脱为检测目标，建立一套与理论分析相应并在实际振动工况下可行可靠的结构损伤识别方法，为工程应用提供理论与技术支撑。

本项目研究闭合型裂纹亚谐波损伤识别与定位方法，分析产生亚谐波的机理和不同条件，确定迟滞型闭合裂纹亚谐波共振激励频率和幅值门槛值条件，以区分固有非损伤非线性结构特征和闭合型裂纹损伤。由于亚谐波在闭合型裂纹中的产生表现出阈值特性，利用亚谐波方法识别闭合型裂纹，确定了亚谐波的产生所需激励条件。采用多尺度法对单自由度非线性系统进行动力学求解，分析了其响应信号中的亚谐波成分并研究了亚谐波的产生所需的激励条件，同时扩展至双自由度非线性系统，用多尺度法求得模型一阶近似解，分析了系统亚谐波共振所需的阈值条件，实验和数值模拟证实了理论部分析。考虑裂纹张开-闭合的阈值特性，研究了亚谐波的产生所需激励频率和激励幅值条件，并讨论了模型参数对所需激励条件的影响。以疲劳裂纹铝梁为对象进行了亚谐波损伤识别实验研究，进一步验证了闭合型疲劳裂纹的亚谐波损伤识别方法的有效性，并得到该疲劳裂纹铝梁中产生亚谐波所需激励条件的阈值范围。对模型在非线性超声过程中表现出的调制现象和亚谐波共振现象进行分析。针对结构中存在的多处具有不同疲劳程度的裂纹损伤，结合Lamb波的非线性特性，提出了在二阶谐波频率处分解时间反转算子的方法，并基于全矩阵聚焦的后处理方法，进行了选择性识别与定位铝板中的多处疲劳裂纹的实验研究。通过实验验证了波经过裂纹界面散射后，响应信号中产生了二阶谐波成分。针对铝板中存在的两处不同疲劳周数裂纹，提出了在二阶谐波频率处分解时间反转算子的方法，开选择性识别与定位疲劳裂纹的实验研究，结果表明方法的有效性。提出了一种通过环境噪声激励重构格林函数的疲劳裂纹检测方法，并验证了不均匀噪声源分布下被动检测方法的有效性。使用噪声激励激发裂纹非线性效应，提取不同被动传感器位置间的格林函数，根据从裂纹初始闭合状态和裂纹张开闭合状态之间的格林函数变化程度构建损伤指数，通过从环境噪声重构格林函数识别板状金属中的疲劳裂纹损伤。