Research On Damage Imaging Technique In Plates Based On Om- Nidirectional SH Waves Electro- Magnetic Acoustic Transducer Array

Plates have a strong industrial background that is widely used in aerospace,au-tomobile manufacturing,ship transportation and other fields.During the process of production,transportation and application,plates can easily appear damages which influence the performance of plates and cause potential safety hazard.In order to avoid economic losses and casualties caused by damage aggravation on service of plates,electromagnetic ultrasonic guided waves detection technique can be applied without jeopardizing plates.MPT is a new type of EMAT and the development of MPT broadens the application range of EMAT.Transducer array imaging technique lays the foundation for damage visualization.The corresponding damage information can be displayed through imaging intuitively and clearly.MPT array can be combined with imaging technique for achieving damage detection and structural health moni-toring of plate-like structures.Aluminum plates and composite plates which are pop-ular in engineering applications are taken as research objects.Double-turn coil OSH-MPTs which can generate SH₀ mode omnidirectionally are arranged to trans-ducer array,and then combined with different imaging algorithms to realize damage imaging of plates.The main research work includes:?1?EMATs based on two transduction mechanisms are developed.One is dou-ble-turn coil OSH-MPT based on the magnetostriction mechanism.Double-turn coil OSH-MPT bonded on aluminum plates and composite plates can generate the lowest mode of SH waves,i.e.,SH₀ mode.The other one is OSH-EMAT based on the Lo-rentz force mechanism.OSH-EMAT can be moved to target position without couplant for generating SH₀ mode in aluminum plates.Related structural parameters optimiza-tion of transducers,the excitation and reception of SH₀ mode,frequency characteris-tics and omni-directivity of transducers are studied experimentally.?2?Damage imaging technique of plates based on RAPID and double-turn coil OSH-MPT array is studied.RAPID is a kind of imaging algorithm performed by em-ploying damage information from direct arrival wave of ultrasonic guided waves.From the combination of RAPID,double-turn coil OSH-MPT array and different data fusion methods,damage imaging is applied in composite plates.Meanwhile,imaging coefficient,variation coefficient and the uneven probability distribution of the array are analyzed for modifying RAPID.Imaging contrast is improved by setting RAPID imaging threshold based on ROC curve.?3?Damage imaging technique of plates based on ellipse algorithm and dou-ble-turn coil OSH-MPT array is studied.Ellipse algorithm is used on the basis of wave packets scattering from damage.The detectability of double-turn coil OSH-MPT array is studied through detecting simulated damages with different sizes and different positions in the aluminum plate.Furthermore,common threshold of de-tection is established for improving the damage imaging quality.Based on this,con-sidering the anisotropy of composite plates,ellipse algorithm is modified and there-fore damage imaging in composite plates by using double-turn coil OSH-MPT array is achieved.