Li Haiyang, Pan Qianghua, Zhang Xiaotong, An Zhiwu
Key Laboratory of Information Detection and Processing, North University of China, Taiyuan 030051, China.
China Special Equipment Inspection and Research Institute, Beijing 100029, China.
Sensors (Basel). 2020 Sep 7;20(18):5077. doi: 10.3390/s20185077.
In this paper, the interaction of a broadband Rayleigh wave generated by a laser and an artificial rectangular notch is analyzed theoretically and experimentally. For the theoretical analysis, a Gaussian function is adopted to analyze the modulation of notch depth on the frequency spectrum via reflection and transmission coefficients. By the finite element method, the Rayleigh wave generated by pulsed laser beam irradiation and its scattering waves at cracks are calculated. A curve with a slope close to 4 fitted by crack depth and critical wavelength of the threshold phenomenon is obtained by the wavelet transform and Parseval's theorem according to simulated and experimental results. Based on this relationship, the critical frequency at which the threshold phenomenon happens due to energy transformation of transmission/reflection Rayleigh waves is adopted to determine the size of sub-wavelength surface crack. The experimental results of artificial notch depth estimation on aluminum alloy specimens consistent with theoretical analysis validates the usefulness of the critical frequency method based on a broadband Rayleigh wave generated by laser ultrasonic.
本文对激光产生的宽带瑞利波与人工矩形缺口的相互作用进行了理论和实验分析。在理论分析中，采用高斯函数通过反射和透射系数来分析缺口深度对频谱的调制。利用有限元方法计算了脉冲激光束辐照产生的瑞利波及其在裂纹处的散射波。根据模拟和实验结果，通过小波变换和帕塞瓦尔定理得到了一条由裂纹深度和阈值现象的临界波长拟合的斜率接近4的曲线。基于这种关系，采用由于透射/反射瑞利波能量转换而出现阈值现象的临界频率来确定亚波长表面裂纹的尺寸。铝合金试件人工缺口深度估计的实验结果与理论分析一致，验证了基于激光超声产生的宽带瑞利波的临界频率法的有效性。
