Study On The Method Of Aluminum Plate Structure Health Monitoring Based On Low Frequency Ultrasonic Guided Wave

As an efficient,advanced and low-budget detection method,ultrasonic guided waves has been applied to the structural health monitoring of plate structure in recent years.Ultrasonic Lamb wave is the most commonly used for plate detection.Low-frequency ultrasonic Lamb wave has lower attenuation and longer propagation distance.Point-point detection can carry the structural information on the whole propagation path.Based on the application of giant magnetostrictive materials in the field of magnetic-mechanical coupling,this project developed a kind of transducer suitable for stimulating low-frequency guided wave signals,combined with advanced and efficient detection methods in structural health monitoring of aluminum plate structure.The main work content are as follows:(1)Based on the basic theory of elasticity and stress wave,the propagation mechanism of Lamb wave and SH wave in free and isotropic plate was deeply analyzed,and the two basic forms(symmetric mode and anti-symmetric mode)are decomposed.The dispersion curves of Lamb wave and SH wave in aluminum plate were plotted by solving the frequency equations,the relationship between group velocity,phase velocity and frequency-thickness products of different modes groups was described.(2)The magnetostrictive effect and the parameters of the giant magnetostrictive material Galfenol performance were introduced in detail.According to the material excellent magnetic properties,using the magnetic-mechanical coupling principle,developed a guided wave transducer that suitable for exciting low-frequency guided waves,utilized finite element software simulate working process of transducer,verified the feasibility of the transducer is used for exciting low-frequency guided wave.The structure and excitation mode of the transducer were optimized in theory from the aspect of improving signal strength and reducing guided wave mode,the optimization mode was verified by finite element simulation.(3)A method was presented that combined the magneto-mechanical impedance with ultrasonic guided wave for measuring aluminum plate.In the premise of Lamb wave location,based on the magneto-magnetic equation of magnetostrictive material,the theoretical formula of magneto-impedance was derived.Then,combining with the properties of the transducer,the equivalent conversion of the theoretical formula was carried out,and the practical measurement formula of the magneto-mechanical impedance was derived.A damage index suitable for both ultrasonic guided wave and magneto-mechanical impedance was proposed,and the data fusion of the two test signals was realized through a fusion standard,finally a comprehensive damage index was obtained,which can be used to determine the damage degree.(4)According to the completed theory and method have been analyzed,the real object of the low-frequency guided wave transducer was made,and the characteristics of the transducer are evaluated through experiments to determine the optimal working frequency.A method of detecting defect with different modes of ultrasonic Lamb wave was proposed to determine the location of defect.