基于波数分析的激光Lamb波缺陷检测试验研究

Experimental Research on Defect Detection of Laser-induced Lamb Waves Based on Wavenumber Analysis

Lamb波的频散和多模态特性,使得利用Lamb波信号的时域或频域特征实现缺陷的定量检测具有一定的困难。基于全光学型激光超声检测系统,采用波数分析方法对铝板中缺陷开展定量检测研究。脉冲激光在固定位置激励,连续激光一维线扫描接收,获得时间-空间波场信号,Lamb波信号的传播特征以及Lamb波与缺陷之间的作用规律被直观的展现。采用二维傅里叶变换将波场信号从时间-空间域转换到频率-波数域,信号中包含的各模态可很好的识别出来。为保留空间信息,借鉴短时傅里叶变换的思想,采用短空间二维傅里叶变换得到沿扫描路径上波数的分布,从中可直观看出缺陷的位置和尺寸。进一步根据波数和频厚积之间的关系,可计算得到缺陷处铝板的厚度。试验结果表明:该方法有效实现了缺陷位置、大小以及深度的评估。

It is difficult to realize the quantitative detection of defects by using the time domain or frequency domain features of Lamb wave signals due to their dispersion and multi-modal characteristics. The study of defect detection quantitatively in aluminum plates is carried out by means of wavenumber analysis based on the full-optical laser ultrasonic testing system. The time-space wavefield signals are obtained by pulse laser in the fixed location excitation and continuous laser one-dimensional scanning acquisition, where the propagation characteristics of Lamb wave signal and the regular pattern of interaction between Lamb wave signal and defect can be visually displayed. The two-dimensional Fourier transform is used to convert the wavefield signal from the time-space domain to the frequency-wavenumber domain where individual modes included in the signals are well discernible. Based on the idea of short-time Fourier transform, for retaining the space information, a short-space two-dimensional Fourier transform is adopted to obtain the distribution of wavenumber along the scanning path, where the location and size of defect can be visually displayed. Furthermore, the thickness of the aluminum plates in the defect area can be calculated based on the relationship between the wavenumber and the frequency-thickness product. Experimental results show that the method can effectively evaluate the location, size and depth of the defects.