Research On C-scan Imaging Detection Based On Electromagnetic Ultrasound

Conventional ultrasonic testing usually uses piezoelectric ultrasonic probe,which is difficult to be applied in many cases due to its high requirements on the surface roughness of the workpiece,the need for coupling agent,and the inability to work in high and low temperature environment.As a new type of ultrasonic testing method,electromagnetic ultrasonic is generated in the metal workpiece,which requires low surface smoothness,no couplant,and large working temperature range.It can make up for some disadvantages of traditional piezoelectric ultrasonic.Based on electromagnetic ultrasonic testing technology,this paper studies the overall architecture of electromagnetic ultrasonic Cscan imaging testing system.Firstly,according to the target of C-scan imaging detection to be realized in this paper,the required body wave EMAT is determined to be in the form of spiral coil and cylindrical permanent magnet.In order to achieve enough coil density,the spiral coil is made by hand winding.In order to achieve strong magnetic induction on the surface of the workpiece,N52 Nd Fe B permanent magnet is selected to provide bias magnetic field.According to the magnetic field distribution of the cylindrical permanent magnet and the actual EMAT volume demand,the diameter and thickness parameters of the permanent magnet are determined.Secondly,the excitation frequency,wire diameter,coil diameter and layer number of the spiral coil are optimized.The finite element simulation model including excitation circuit is established for EMAT detection process,and the influence of the first three parameters on the ultrasonic longitudinal wave component generated by EMAT is obtained by orthogonal experiment through simulation,and the influence of wire diameter and coil diameter on the receiving ability of EMAT signal is obtained by simulation of the receiving process.The parameters of EMAT single coil with signal strength and low longitudinal mode component are determined by further experimental research and simulation results.Then the series parallel experiment of multi-layer coil is carried out with this parameter,and the structure of two-layer series spiral coil with the highest signal-to-noise ratio is obtained.Thirdly,the signal processing of EMAT ultrasonic testing signal is studied.First,the main shock will cause the signal baseline drift,so it is necessary to remove the signal trend;Second,the C-scan electromechanical system will cause electromagnetic interference to EMAT,so it needs to remove the noise;The third is the need for signal detection and feature extraction.Considering the processing effect and operation speed of various methods,the signal processing route of removing signal trend by central moving average method and extracting useful echo signal features from noisy signal by phase sensitive detection method is determined.The imaging effect of the system was tested,and the flat bottom hole defect with diameter of Φ3mm and depth of 7mm and the flat bottom hole defect with diameter ofΦ4mm and depth of 5mm under 40 mm aluminum alloy plate were successfully detected,which verified the effectiveness of the system.