Ohara Yoshikazu, Nakajima Hiromichi, Haupert Sylvain, Tsuji Toshihiro, Mihara Tsuyoshi
Department of Materials Processing, Tohoku University, 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
Sorbonne Université, CNRS UMR 7371, INSERM UMR S 1146, Laboratoire d'Imagerie Biomédicale, Paris, France.
J Acoust Soc Am. 2019 Jul;146(1):266. doi: 10.1121/1.5116017.
The nondestructive evaluation of closed cracks is a challenging subject in ultrasonic testing. Recently, nonlinear ultrasonic phased array with fixed-voltage fundamental wave amplitude difference (fixed-voltage FAD) has been proposed as a practical approach. In this study, the maximum incident wave amplitude, which is one of the most critical parameters in closed-crack imaging, was investigated. First, a theoretical model was formulated to explicitly show the essence of the fundamental principle of FAD and the advantage of fixed-voltage FAD over different-voltage FAD. In experiments, the authors imaged a closed fatigue crack using a nonlinear ultrasonic phased array with fixed-voltage FAD while varying the incident wave amplitude. It was found that when the incident wave amplitude was sufficiently high, the nonlinear image visualized the closed crack tip, which could not be visualized in linear images. In addition, the incident-wave-amplitude dependence of the nonlinear responses was quantified. It was found that different parts within a single fatigue crack showed different nonlinear behaviors. This suggests that fixed-voltage FAD is useful not only for practical application of closed crack imaging but also for examining the nonlinear dynamics at various parts of closed cracks with a high spatial resolution.
封闭裂纹的无损评估是超声检测中的一个具有挑战性的课题。最近，提出了具有固定电压基波幅度差（固定电压FAD）的非线性超声相控阵作为一种实用方法。在本研究中，对封闭裂纹成像中最关键的参数之一——最大入射波幅度进行了研究。首先，建立了一个理论模型，以明确展示FAD基本原理的本质以及固定电压FAD相对于不同电压FAD的优势。在实验中，作者使用具有固定电压FAD的非线性超声相控阵对封闭疲劳裂纹进行成像，同时改变入射波幅度。结果发现，当入射波幅度足够高时，非线性图像能够显示出封闭裂纹尖端，而这在线性图像中是无法显示的。此外，还对非线性响应的入射波幅度依赖性进行了量化。结果发现，单个疲劳裂纹内的不同部位表现出不同的非线性行为。这表明固定电压FAD不仅对封闭裂纹成像的实际应用有用，而且对于以高空间分辨率研究封闭裂纹各部位的非线性动力学也很有用。
