It is challenging but meaningful to detect and image the micro-damage in the early stages of engineering structure failure. The nonlinear ultrasonic technique has gained considerable attention for micro-damage detection, meanwhile, the Lamb wave phased array technique has become a practical tool for macro-damage imaging. The combination of these two techniques can pave a promising way for micro-damage imaging in plate-like structures. However, the research on nonlinear Lamb wave phased array imaging is complicated and not reported. The weak nature and hard extraction of the nonlinear features, such as the second harmonic generation, limits the application of the nonlinear Lamb wave phased array. A framework of nonlinear Lamb wave phased array for micro-damage imaging in the plate-like structure is developed. To maximize the second harmonic response of the recorded Lamb waves, two kinds of lead zirconate titanate (PZT) wafers are selected to compose the nonlinear phased array. Furthermore, focusing on the nonlinear feature extraction, the sparse representation approach is introduced for reconstructing the time-domain second harmonic generated by the micro-damage, from which a wealth of waveform information corresponding to the micro-damage can be extracted. The total focusing method (TFM) is used for micro-damage imaging based on the reconstructed second harmonic. Numerical simulation and experimental validation demonstrate the capability of the proposed framework to image the local contact acoustic nonlinearity represented by the barely visible closed fatigue crack with the nonlinear Lamb wave phased array.

中文翻译：

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基于二次谐波重建的非线性兰姆波相控阵揭示微损伤


在工程结构失效的早期阶段对微损伤进行检测和成像具有挑战性但有意义。非线性超声技术在微观损伤检测方面受到了广泛关注，同时兰姆波相控阵技术已成为宏观损伤成像的实用工具。这两种技术的结合可以为板状结构的微损伤成像开辟一条有前途的道路。然而，非线性兰姆波相控阵成像的研究较为复杂且尚未见报道。二次谐波等非线性特征的弱性质和难以提取限制了非线性兰姆波相控阵的应用。开发了一种用于板状结构微损伤成像的非线性兰姆波相控阵框架。为了最大化记录的兰姆波的二次谐波响应，选择两种锆钛酸铅（PZT）晶片来组成非线性相控阵。此外，针对非线性特征提取，引入稀疏表示方法对微损伤产生的时域二次谐波进行重构，从中提取出与微损伤对应的丰富波形信息。全聚焦法（TFM）用于基于重构二次谐波的微损伤成像。数值模拟和实验验证证明了所提出的框架能够利用非线性兰姆波相控阵对几乎不可见的闭合疲劳裂纹所代表的局部接触声学非线性进行成像。