中文摘要：

针对结构中微裂纹检测难题，发展了一种闭合裂纹非共线混频超声检测方法。在对非共线混频超声检测机理分析基础上，进行了结构中疲劳裂纹混频非线性超声检测实验。对有无裂纹试件中检测信号进行了滤波和时频分析，结果表明，可根据信号滤波后时域波形中是否存在明显的混频波包或时频分析中是否存在明显的和频分量，实现有无闭合裂纹的判识；通过移动激励探头的位置，控制两列入射声波在试件中的交汇位置，实现试件中不同深度位置的}昆频非线性检测。并根据测得的混频非线性系数沿试件深度方向上分布，实现了闭合裂纹沿深度方向上长度的测量。研究工作为结构中微裂纹定量评价做了有益探索。

英文摘要：

Aiming at the detection of micro crack in structures, a non-collinear mixed frequency ultrasonic method for detection of closed cracks was developed. Based on theoretical analysis, non-collinear mixed frequency ultrasonic testing experiment was conducted on closed cracks in structure. Testing signals in both intact and cracked specimens were filtered and anMyzed by time-frequency method. Results showed that the closed crack can be recognized by the emergence of obvious wave form in time-domain filtered signal, or the sum-frequency component in frequency domain. By changing locations of excitation probes, the confluence position of two incident waves can be controlled and non-collinear mixed frequency ultrasonic testing at different depths was achieved. Moreover, according to the distribution of mixed frequency nonlinear parameter the length of closed crack along depth was measured. The research makes some useful efforts on quantitative evaluation of micro-crack.