Fig. 1. Schematic diagram of Lamb wave propagation model in an infinite plate.无限大薄板中兰姆波传播模型示意图

Fig. 2. Numerical simulation model of modal analysis of received signals.接收信号模态分析数值仿真模型

Fig. 3. Dereference signals in the z direction received at different positions. 不同位置接收到的z方向的去参考信号

Fig. 4. Dereference signal in the x direction received at different positions. 不同位置接收到x方向的去参考信号

Fig. 5. 2D-FFT of the reference signal in the z direction. z方向去参考信号的2D-FFT

Fig. 6. 2D-FFT of the reference signal in the x direction. x方向去参考信号的2D-FFT

Fig. 7. Numerical simulation model of influencing factor analysis: (a) A model of two-factor comprehensive impact; (b) model of distance difference influencing factor.影响因素分析数值仿真模型　(a) 双因素综合影响模型; (b) 距离差影响因素模型

Fig. 8. Received signal and reverse path delta signal when l = 0. 当l = 0时接收信号及反转路径差信号

Fig. 9. Received signal and reverse path delta signal when l ≠ 0. 当l ≠ 0时接收信号及反转路径差信号

Fig. 10. Maximum amplitude of the delta signals under different propagation distance differences.不同传播距离差下的差信号最大幅值

Fig. 11. Calculation results of under different propagation distance differences. 不同传播距离差下的 计算结果

Fig. 12. Amplitude of delta signals at different defect locations.不同缺陷位置下的差信号幅值

Fig. 13. Scattering coefficient of the circular through hole defect圆形通孔缺陷的散射系数

Fig. 14. Scattering coefficients at different defect locations.不同缺陷位置下的散射系数

Fig. 15. Values of at different defect locations. 不同缺陷位置下的 值

Fig. 16. Simulation model of lamb waves detection for sparse array of plate structure.板结构稀疏阵列兰姆波检测仿真模型

Fig. 17. Imaging results of circular through hole defects: (a) Number 1 (based on reference signal); (b) number 2; (c) number 3; (d) number 4.圆形通孔缺陷成像结果　(a) 序号1 (基于参考信号); (b) 序号2; (c) 序号3; (d) 序号4

Fig. 18. Imaging results of rectangular defect: (a) Number 1; (b) number 2.矩形缺陷成像结果　(a) 序号; (b) 序号2

Fig. 19. Inverted path delta signal of a typical sensor pairs and its contribution to imaging: (a) Inverted path delta signal of sensor pairs of number1 and number 3; (b) contribution to imaging of inverted path delta signal of sensor pairs of number1 and number 3; (c) inverted path delta signal of sensor pairs of number 2 and number 4; (d)contribution to imaging of inverted path delta signal of sensor pairs of number 2 and number 4.典型传感器对的反转路径差信号及其对成像的贡献　(a) 1号和3号传感器对的反转路径差信号; (b) 1号和3号传感器对的反转路径差信号对成像的贡献; (c) 2号和4号传感器对的反转路径差信号; (d) 2号和4号传感器对的反转路径差信号对成像的贡献

Fig. 20. Imaging results based on the inverted path delta signal.基于反转路径差信号的成像结果

Fig. 21. Schematic diagram of the experimental system.实验系统示意图

Fig. 22. Receiving signals and delta signal for sensor pairs 2 and 4.2号和4号传感器对的接收信号及差信号

Fig. 23. Imaging results of the experiment: (a) Four sensors; (b) six sensors.实验成像结果　(a) 4个传感器; (b) 6个传感器