The multilayer structure of fiber-reinforced polymers can be extracted by several ultrasonic techniques, each operating in a different frequency range. Generally, it is assumed that ultrasound at higher frequency provides better depth resolution, but at the expense of the dynamic depth range due to excessive attenuation. Hence, a lower frequency is usually necessary to fit the depth setting to the probing requirement. And lower frequency approaches coupled to deconvolution techniques have been developed to improve the depth resolution and signal-to-noise ratio. However, when the frequency becomes as low as the ply-resonance frequency, the conventional metrics such as instantaneous amplitude envelope is completely not valid for distinguishing resin-layer reflections. To add more valid metrics in this situation, recent studies proposed the use of ultrasound at the ply-resonance frequency and subsequent analytic-signal analysis. This study compares the performances of various ultrasonic approaches, in different frequency ranges, for extracting the multilayer structure of composites. The following techniques are studied: (i) 50 MHz ultrasound coupled to instantaneous amplitude envelope, (ii) 15 MHz ultrasound coupled to Wiener deconvolution (with spectral extrapolation), (iii) 5 MHz ultrasound coupled to analytic-signal analysis (with log-Gabor filter). The performances of the various techniques are first investigated and discussed on synthetic data representative for a 24-ply composite. The robustness of the techniques is tested for different signal-to-noise ratios. An experimental study is performed on a 24-ply carbon fiber reinforced polymer to further validate the performances of different techniques. The extracted multilayer structure is presented in the form of both B-scan and C-scan images and the thickness of each ply is estimated. The obtained results indicate the high performance of the 5 MHz frequency ultrasound coupled to analytic-signal analysis combined with a log-Gabor filter.

中文翻译：

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基于解析信号技术的多层复合材料层间跟踪参数研究

纤维增强聚合物的多层结构可以通过几种超声波技术提取，每种技术都在不同的频率范围内工作。通常，假设更高频率的超声提供更好的深度分辨率，但由于过度衰减而以动态深度范围为代价。因此，通常需要较低的频率来使深度设置适合探测要求。已经开发出与去卷积技术相结合的低频方法，以提高深度分辨率和信噪比。然而，当频率变得与层共振频率一样低时，瞬时幅度包络等传统指标对于区分树脂层反射完全无效。要在这种情况下添加更多有效指标，最近的研究建议在层共振频率和随后的分析信号分析中使用超声波。本研究比较了各种超声波方法在不同频率范围内提取复合材料多层结构的性能。研究了以下技术：(i) 50 MHz 超声耦合到瞬时振幅包络，(ii) 15 MHz 超声耦合到 Wiener 解卷积（具有频谱外推），(iii) 5 MHz 超声耦合到分析信号分析（对数- Gabor 滤波器）。首先在代表 24 层复合材料的合成数据上研究和讨论各种技术的性能。针对不同的信噪比测试了这些技术的稳健性。对 24 层碳纤维增强聚合物进行了实验研究，以进一步验证不同技术的性能。提取的多层结构以 B 扫描和 C 扫描图像的形式呈现，并估计每层的厚度。获得的结果表明 5 MHz 频率超声耦合到分析信号分析与对数 Gabor 滤波器的高性能。