Separation Of Multimode Lamb Waves Based On Fractional Differentiation

In the actual use of ultrasonic lamb wave testing process,due to the long-term use of the plate formed by the fracture,deformation,crack and other damage changed the characteristics of the plate parameters.At the same time,the side lobe effect of the piezoelectric transducer produces the harmonic effect,which leads to the mode change of the detection signal and the overlapping of the harmonic signal,resulting in the phenomenon of multimode signal aliasing of lamb wave,which greatly increases the difficulty of relevant application analysis.Therefore,in order to obtain more accurate detection information and achieve more accurate detection results,the research on ultrasonic lamb multi-mode signal separation is of great significance.The research work of this paper mainly focuses on the separation algorithm of multi-mode ultrasonic lamb wave signals,and the results obtained are as follows:1,Multi-mode separation of ultrasonic Lamb wave based on fractional differential and phase reconstruction: A method of time-frequency overlapping multi-mode Lamb wave mode separation using fractional differential theory is proposed.Firstly,the formula for calculating the characteristic parameters of the amplitude spectrum is derived from the fractional differential of the signal amplitude spectrum,and the amplitude spectrum is reconstructed according to the signal model.Then the corresponding phase spectrum is deduced according to the dispersion characteristic curve of Lamb wave,and the time-domain signals of each mode are reconstructed by the amplitude spectrum and phase spectrum,thus the time-frequency overlapping multi-mode Lamb wave signals are separated.In this method,fractional differential is applied to the signal analysis and processing of multi-mode ultrasonic Lamb wave,which solves the problem that it is difficult to extract the mode characteristic parameters of multi-mode mixed signal due to time-frequency aliasing.When the frequency domain of mixed signal is not completely overlapped and there are independent components,it can be applied to the separation of time-frequency overlapped multi-mode Lamb wave signals.2,Multi-mode separation of ultrasonic Lamb wave based on dispersion compensation and fractional order differential: This method combines dispersion compensation and fractional order differential,establishes time-delay function model,and transforms the separation of complex mixed signals into the separation of partial mixed signals depending on the difference of dispersion of different modes.Then,the fractional derivative is applied to extract the characteristic parameters of the maximum fitting polynomial of the fractional derivative of the signal amplitude spectrum,and the non-dispersive modes of the partialaliasing signal are separated.Then,according to the dispersion characteristics and detection distance,the time domain waveform is restored to extract the mode signals and realize multi-mode separation.The results of simulation and experiment show that this method can not only achieve multi-mode separation,but also ensure the separation accuracy,which is helpful to the separation and processing of complex multi-mode dispersion signals.Compared with the existing multi-mode separation algorithms,the main advantage is the combination of dispersion compensation and fractional differential,which solves the problem that the single-mode dispersion compensation still overlaps with other modes,and improves the extraction accuracy of the single-mode non-dispersive signal.