Study On Air-coupled Lamb Waves Imaging Detection For CFRP Plate Damages

Large thin plates are widely used in various important fields due to its light weigh,strong mechanical properties and other advantages.For example,the aluminum plate and CFRP laminate occupy a major position in the field of aerospace,which reducing the weight of aircraft and fuel consumption greatly,improving transportation efficiency and saving energy significantly.However,although the tensile and compressive properties of the thin plates in the horizontal direction are great,the impact resistance in the vertical direction is poor.Under the manufacturing process and service conditions,varying degrees damage defects from internal and external can be suffered,which poses a great threat to the health and safety of the material.Air-coupled ultrasonic Lamb wave testing has become the focus of research on thin plate components nondestructive testing in aerospace field because of the advantages of non-contact,fast and high efficiency testing.Firstly,based on the basic theory of Lamb wave propagation in the thin plates,the propagation characteristics of Lamb wave in isotropic and anisotropic laminates were analyzed in this paper,the influence of the wave structure of symmetric and antisymmetric Lamb wave and the receiving position of probe on the echo signal were discussed.A0 mode Lamb wave was selected as the detection wave in the experimental because of the large off-plane displacement of the anti-symmetric Lamb wave.Compared with the receiving signals method of pitch-catch,the defect echo signals received by the pulse-echo receiving method has time information.In this paper,the A0 mode Lamb wave received by the pulse-echo excitation receiving method was used for imaging detection of the thin plates.In terms of experimental measurements,two pair of Air-coupled ultrasonic probes with central frequency of 0.2 MHz and 0.4 MHz were adopted to receive Lamb wave signals by means of pulse-echo testing method.In order to identify the echo signals effectively,the time-domain echo signals were analyzed,the timefrequency analysis method of Short-Time Fourier Transform and Hilbert Huang Translate were adopted.The results show that the time-frequency spectrogram obtained by the Short-Time Fourier Transform can be used for two-dimensional imaging of the defect,so that the defect can be presented more intuitively.The Hilbert spectrum obtained by the Hilbert Huang Translate has great time-frequency analysis ability which can provides accurate positioning on time and frequency,expressing the energy distribution and trend of the signals effectively.In order to further identify the excited Lamb wave modes,the time-domain wave velocity method and the twodimensional Fourier Transform method were used to identify the modes of the signals respectively.Confirmed that the Lamb wave signals received by the excitation of two frequency were A0 mode.In terms of research on imaging methods,based on the basic theory of Synthetic Aperture Focusing Technique(SAFT)imaging,combined with ipsilateral excitation receiving pulse-echo detection method,time-domain SAFT imaging based on the Delay and Sum and frequency-domain SAFT imaging methods based on the Phase shift were designed respectively.SAFT imaging analysis was performed on aluminum plates with different shapes and sizes defects.The results of aluminum plates imaging show that SAFT imaging method can improve the imaging resolution and imaging quality greatly,the imaging speed of frequency-domain SAFT imaging method is much faster than that of time-domain SAFT imaging method.On this basis,frequency-domain phase shift SAFT imaging method was used to detect CFRP laminates with impact damage and then the imaging results were compared with the Air-coupled penetrating C-scan imaging results.Experimental results show that SAFT imaging can effectively improve the imaging quality of CFRP laminates with impact damages.For laminates with impact damage energies of 10.5 J and 15.3 J,SAFT imaging quality of Air-coupled Lamb wave were not very good due to the weak echo signals of damages,For laminates with impact damage energies of 19.5 J and 24.7 J,the SAFT imaging quality of Air-coupled Lamb wave were in good agreement with the penetrating C-scan imaging results.The study of this paper provides a useful reference for the imaging detection of large thin plate components using Air-coupled Lamb wave.