Application of time-frequency analysis to ultrasonic-guided-wave signal interpretation
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摘要: 超声导波信号是一种非平稳信号,存在频散现象,利用传统的时域和频域分析方法不能揭示其频谱随时间变化的特点.为了解决这一问题,研究了导波信号的非平稳特征,介绍了短时傅里叶变换(STFT,Short-Time Fourier Transform)时频分析原理,分别对铝板试样和蜂窝板试样内传播的导波信号进行了STFT和时频谱分析,得出了铝板内导波信号中各频率成分随时间的变化情况,分析出导波信号的模式转换现象,比较精确地计算了导波的群速度;对于蜂窝板内的导波信号,由于能量泄露比较严重,当激励频率为1.5MHz时,A0模式只能近距离传播,这就使得采用STFT不能很好地进行模式识别.研究结果表明,当导波传播距离较远时,利用STFT可以对其信号进行动态的频谱分析和模式识别,并可对实际的群速度进行较精确的测量.Abstract: Ultrasonic guided wave is a kind of non-stationary signal and characterized by disperse. Its frequency spectrum variation can not be traced with traditional methods in time domain and frequency domain respectively. To solve the problem, the non-stationary characteristics of guided waves were studied, the theory of STFT(short-time Fourier transform) was introduced, the STFT and time-frequency spectrum analysis were applied to the signal interpretation of guided waves propagating in an aluminum plate and a honeycomb core structure plate. The frequencies variation of the signal propagated in aluminum plate can be analyzed as well as mode conversion and precise group velocity. Due to energy leakage, A0 mode propagates just in short distance in honeycomb core structure plate when the excitation center frequency is 1.5MHz, which results in that mode conversion can not be done well. The results show that in further distance can the STFT method be applied to not only the frequency spectrum analysis and mode identification of guided waves but also an accurate measurement of their group velocity.
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