Research On Oblique Incidence And Multipath SAFT Imaging Method Of Electromagnetic Ultrasonic For Material Damage

As metal components such as pressure vessels gradually tend to high-temperature,highpressure,and highly corrosive environments,their internal damage is becoming increasingly complex.Conventional ultrasonic flaw detection technology cannot be applied to hightemperature environments above 600 degrees Celsius,and electromagnetic acoustic transducer(EMAT)can meet the online detection requirements for pressure vessel defects.However,the efficiency of electromagnetic ultrasound energy transfer is generally low,the EMAT model for oblique incidence imaging testing in low carbon steel for pressure vessels is not complete,the propagation characteristics of electromagnetic ultrasound single probe body wave effective detection of vertically arranged defects and near-surface defects in metal equipment components are not clear,and the time domain synthetic aperture imaging(SAFT)algorithm for full area testing under single probe is not mature enough,This limits the development of electromagnetic ultrasound single probes in the field of pressure vessel defect detection.Aiming at the problem of blind spots in defect detection using a single probe synthetic aperture scanning model for butterfly coil EMAT,a finite element model for runway coil and zigzag coil EMAT synthetic aperture scanning was introduced.Simulation analysis shows that butterfly shaped and runway coil EMAT have more advantages in low-frequency sound field detection.The runway coil EMAT can identify vertical defects,but cannot accurately locate and quantify them;The zigzag coil EMAT utilizes the oblique incidence synthetic aperture imaging algorithm to characterize vertically arranged defects in the entire region,and can achieve precise positioning and quantification of defects.Aiming at the problem of energy conversion efficiency of bidirectional oblique incidence shear vertical wave(SV wave)of zigzag coil EMAT,by analyzing the influence of oblique incidence direction angle and coil turns on the distribution of sound field intensity,a finite element model for sensitivity detection and spatial resolution detection of vertically arranged defects under optimized parameters is established.The simulation and experimental results show that the detection performance of the zigzag coil EMAT is better at an oblique incidence angle around 30 degrees.When the coil turns are 16,the advantages of the synthetic aperture imaging algorithm can be utilized to the maximum extent.The algorithm can distinguish vertically arranged hole defects with a diameter of 1mm,inclined cracks with a 15 degree angle,and thinning areas with a depth of 2mm.This verifies the applicability of the algorithm in detecting vertically arranged defect blind areas,and the feasibility of detecting cracks and thinning areas.An oblique incidence secondary reflection SAFT imaging algorithm(FS-SAFT)is proposed to solve the problem of blind spots in the detection of near-surface defects in conventional synthetic aperture imaging algorithms using volume wave primary waves.By establishing a finite element model for secondary reflection acoustic wave data acquisition of the zigzag coil EMAT,the fixed mode acoustic waves that need to be collected for secondary reflection are determined.Combined with experiments,the detection performance of the secondary reflection SAFT algorithm for near-surface defects is analyzed.The results show that the algorithm can fully achieve defect detection at a depth of 6.5 mm near the surface using full shear wave mode in simulation and experiments.The zigzag coil EMAT and two types of multipath SAFT imaging algorithms studied in this paper lay the foundation for the detection and imaging of body wave defects in the entire region of metal material equipment components under high temperature environments.