Research On Mode Superposition Damage Imaging Method In F-k Domain For Phased Array

Plate structure,especially composite plate structure,is widely used in aerospace,high-speed rail,civil engineering and many other fields.In the long-term service process,this structure is vulnerable suffer from the external load impact,chemical corrosion and other effects,which resulting in cracks,spalling,fatigue and other damage.Ultrasonic Lamb wave nondestructive testing technology is widely used in the damage detection of plate structure because of its advantages of high detection efficiency,which is suitable for large-scale detection and small damage identification.In the process of ultrasonic Lamb wave propagation in plate structure,multi-mode and dispersion effects,and the coexistence of multiple modes will lead to the time-domain signal packet aliasing and the damage information is difficult to identify.It is difficult to effectively extract the damage scattering information without reference of healthy structure reference signal.To solve the above two problems,this paper uses frequency wavenumber(F-K)domain filtering method.Based on the following methods,the corresponding relationship between the propagation direction of guided waves and frequency wavenumber matrix,combined with the difference of wavenumbers of different modes of guided waves at the same frequency,constructs a three-dimensional window function to extract different modes and their corresponding damage reflection signals.Common source method(CSM)and f-k domain imaging were used to realize damage imaging.Finally,combined with the data fusion imaging method,the damage imaging results of different modes are weighted to further improve the accuracy of damage imaging.The main contents of this paper are as follows:(1)The three-dimensional Fourier transform for two-dimensional wave conduction is studied,and the three-dimensional window functions for different modes separation of Lamb waves in isotropic and anisotropic plate structures in f-k domain are constructed.In this paper,a method of extracting wavenumber curve without reference in f-k domain of anisotropic materials is presented,and the construction of three-dimensional window function is realized on the basis of no reference of theoretical wavenumber curve.Three data fusion methods for multi modal damage images are studied.(2)The finite element numerical model of Lamb wave propagation in isotropic aluminum plate is driven by sinusoidal modulated five peak wave and chirp signal is established.The fusion of damage imaging results of the two modes is realized by data fusion imaging method.The first mock exam results show that the two modes of the Chirp signal have strong damage to the reflection energy,and all the frequency components in the frequency domain are involved in the f-k domain CSM damage imaging compared with the single mode damage imaging results.The superposition of the two modes can improve the imaging accuracy of the damage and verify the feasibility of the algorithm.(3)An experimental platform for aluminum plate damage detection is built,which is driven by sinusoidal modulated five peak wave signal and linear chirp signal,it used for phased array mode superposition damage imaging method.The image fusion of the two modes is realized by the data fusion imaging algorithm.The experimental results show that modal superposition can improve the imaging accuracy of damage,which is in good agreement with the numerical simulation results.Finally,the f-k domain mode division method is applied to the orthotropic composite material,and the three-dimensional window function is constructed to realize the f-k domain CSM damage imaging algorithm without reference dispersion curve.