柔性光声换能器研制及其超声表面波调制与应用

       金属材料在服役过程中的载荷和环境因素作用下，容易产生各种微观缺陷，进而演化出宏观缺陷，最终导致材料失效。非线性超声检测技术具有精度高、对微观组织敏感的特点，是预防金属材料失效的一种重要检测技术。然而目前接触式非线性超声检测技术存在重复性差、无法适应恶劣工况等问题；非接触激光超声非线性检测技术存在低信噪比和材料烧蚀的问题，在应用中仍存在局限性。光声换能器可以在非烧蚀条件下产生高信噪比的超声信号，有望用于非线性超声检测；但其存在声阻抗失配、反射信号干扰的问题，因此在应用中必须对其结构和信号进行合理调制。

       为了解决以上问题，本文将光声换能器与阵列式激光结合，开展了基于光声换能器结构优化的窄带表面波信号调制研究，并对其非线性超声检测应用进行了探索。本文使用烛灰纳米颗粒和聚二甲基硅氧烷（Poly-dimethylsiloxane, PDMS）作为原材料制备了具有三层结构的光声换能器，并通过调整PDMS旋涂参数的方式实现了光声换能器结构的调制。本文将光声换能器与狭缝阵列掩膜调制的阵列式激光结合实现了窄带表面波的激励，并通过理论推导、有限元仿真、实验研究的方法，提出了光声换能器结构与表面波谐波幅值的理论模型。基于以上理论模型，本文通过优化光声换能器结构，成功将表面波的谐波成分抑制至噪声水平，并将信号幅值提高至超过10倍。利用优化后的光声换能器产生的窄带超声表面波与材料的非线性作用，本文实现了对铝板试样表面疲劳裂纹的检测。

       Under the influence of in-service load and environment conditions, metal materials are prone to produce various minute defects which may turn into macro defects, leading to material failure eventually. With the merits of high precision and sensitivity to microstructure, nonlinear ultrasonics is an important testing technique for preventing the failure of metal materials. However, current contact-based nonlinear ultrasonic techniques have drawbacks such as poor repeatability and inability to work in hostile environments, and non-contact nonlinear laser ultrasonic techniques have the problems of low signal-to-noise ratio and material ablation, which limit their application. Photoacoustic transducer can produce ultrasonic signals with high signal-to-noise under non-ablative condition and therefore has promising potential for nonlinear ultrasonic testing. But photo-acoustic transducer faces the problems of acoustic impedance mismatch and reflected signal interference, so its structure and signal must be modulated properly in application.

       To overcome these drawbacks, photoacoustic transducer and arrayed laser are combined in this work to study the narrowband surface wave modulation based on structural optimization of photoacoustic transducer and to explore their application on nonlinear ultrasonic testing. Three-layered photoacoustic transducers are fabricated with candle soot nanoparticles and polydimethyl-siloxane (PDMS). By adjusting the spin-coating parameters of PDMS layers, structural modulation of photoacoustic transducers is achieved. Narrowband surface waves are excited with the combination of photoacoustic transducers and arrayed laser modulated by arrayed slit masks. A theoretical model between the structure of photoacoustic transducer and the harmonic amplitudes of generated surface wave is proposed through theoretical analysis, finite element simulation and experimental investigation. Based on the proposed theoretical model, the harmonic components of the surface wave are successfully minimized to noise level by optimizing the structure of photoacoustic transducers, and the signal amplitude is increased to over 10 times. Utilizing the nonlinear interaction between the material and the narrowband surface waves generated by the optimized photoacoustic transducers, the detection of surface fatigue crack on an aluminum plate is realized.

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