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与常规超声波无损检测相比，兰姆波检测具有快速高效的特点，非常适合于板形结构的大面积无损检测。尽管金属薄板的无损检测是兰姆波技术的最早应用领域，但是由于兰姆波理论及检测机理的复杂性，此项技术至今仍未取得重大突破，还存在许多不一致的观点和未解决的问题，大大地限制了它在工业生产中的应用。
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毕业论文采用实验系统，将宽频信号激励到薄铝板以获得多模态的兰姆波，对响应信号进行时频分析的处理，并与Lamb波的频散特性进行比较研究。本文主要采用短时傅立叶变换(STFT)和小波变换对金属板中的兰姆波检测信号进行时频分析。4979
通过实验分析和研究，结果表明，短时傅立叶变换采用高斯窗函数的时频分析取得了较好的效果，较之文章里采用几种不同母小波的小波变换，STFT的时频分辨率更高。
关键词：无损检测；Lamb波；金属薄板；短时傅立叶变换；小波变换；时频分析
Abstract
Compared with the conventional ultrasonic nondestructive testing techniques, Lamb Waves method is rapid and efficient and especially suitable for nondestructive testing of large plate-shaped structure. The nondestructive test in metal sheet is the most early application domain of Lamb wave technique, however, because of the complication of the theory and mechanism of Lamb waves, this technique has not yet achieved significant breakthrough. There are also a lot of inconsistent viewpoints and unresolved problems which have greatly limited its application in the industry.
In this paper, experimental system, the broadband signal excitation to the thin aluminum to obtain a multi-modal Lamb, the response signal processing of time-frequency analysis, and with the Lamb wave dispersion characteristics were compared. This paper mainly uses the short-time Fourier transform (STFT) and wavelet transform in the metal plate Lamb wave detection signal time-frequency analysis.
Through experimental analysis and research, the results show that the short-time Fourier transform of the Gaussian window function time-frequency analysis achieved good results, compared with the wavelet transform by several different mother wavelet which the article used，the time-frequency resolution of STFT is higher.
KeyWords: Nondestructive testing; Lamb wave; Metal sheet; Short-time Fourier Transform; Wavelet Transform；Time-frequency analysis
目录
1绪论 1
1.1 课题研究的背景和意义 1
1.2 金属薄板Lamb波无损检测概况及国内外研究现状分析 3
1.3 Lamb波激励信号 4
1.4 时频分析信号处理方式 6
2时频分析原理及研究方式 8
2.1 短时傅立叶变换 8
2.1.1 传统傅立叶变换与短时傅立叶变换 8
2.1.2 短时傅立叶变换原理特点 10
2.2 小波变换 11
2.2.1 小波分析原理 11
2.2.2 母小波 12
2.2.3 小波变换的特点和作用： 13
2.2.4 傅立叶变换和小波变换的比较： 13
3激励信号确定 16
3.1 激励信号选取 16
3.2 实验方案 18
4兰姆波信号处理 20
4.1 Lamb波的频散特性 20
4.2 实验信号时频分析 21
4.2.1 实验信号短时傅立叶变换时频分析 24
4.2.2 实验信号小波变换的时频分析 26
5结论 29
参考文献 30
致谢 34