中文摘要：

给出了一种基于混频效应的非线性超声微裂纹检测方法。首先，对结构损伤混频检测机理及信号特征提取方法进行了理论分析，之后，根据试件中差频分量及和频分量幅值分布随激励信号频率变化关系，优化确定出混频检测参数。最后，进行了异侧混频激励下无损检测试验研究，并分析了激励信号频率变化对混频检测效果的影响。结果表明，异侧激励混频检测模式不仅可以实现结构中疲劳微裂纹检测，而且可以实现缺陷的定位。且检测信号频率选择对混频检测信噪比有一定的影响。当检测信号中的混频分量幅值最大时，混频检测效果最佳。因此，在优化检测参数基础上，异侧混频激励检测模式可以很好实现结构微裂纹的检测与定位。

英文摘要：

Toward invalidity of the linear ultrasonic technique for nondestructive detection of micro-crack in structures, a frequency-mixing nonlinear ultrasonic technique was developed for micro-crack detecting. Firstly, the mechanism of frequency-mixing and signal processing method for feature extraction were investigated; secondly, By experimental mea- surement of frequency response of mixing frequencies, the optimum parameters for frequency-fixing detection frequency- mixing were determined; experiments for micro-crack detection were conducted in mode of heterolateral excitation, and research focused on the influence of frequencies of excitations on the detection results. It was shown that in mode of heterolateral excitation, not only fatigue micro-cracks in the structure can be detected, but also the position of the defect can be located. Meanwhile, the frequencies of excitations have great influence on signal-to-noise ratio. The perfor- mance of damage detection experiments was best when the amplitudes of mixing frequencies are maximum. Therefore, frequency-mixing nonlinear ultrasonic technique is a valid approach for crack detection and location.