Electromagnetic Acoustic Transducer Mechanism And Micro-damage Detection Of Ferromagnetic Materials

Metal components mainly composed of ferromagnetic materials are widely used in highspeed railway manufacturing,railway track,shipbuilding,aerospace and other important fields.Due to the influence of metal component manufacturing process of metal components and the harsh conditions in the service process,micro-cracks and plastic damage will occur in the plate-like structural steel plate,so it is necessary to detect the micro-damage of the ferromagnetic materials.Electromagnetic acoustic transducer(EMAT)is an important equipment for nondestructive testing of metal component defects.Electromagnetic acoustic transducers have been widely used in non-destructive testing of non-ferromagnetic materials,and the energy conversion mechanism of ferromagnetic materials has not been perfected.This article starts with the analysis of the electromagnetic acoustic transducer’s energy conversion mechanism for different materials.The influence of the electromagnetic properties of the ferromagnetic materials on the electromagnetic ultrasonic energy conversion mechanism is studied,and the foundation for detecting the micro damage of ferromagnetic materials is laid.Therefore,based on electromagnetic ultrasound,the research on micro damage detection of ferromagnetic steel plate is carried out.,the main research work is as follows:1)An accurate simulation model of electromagnetic acoustic transducers for detecting ferromagnetic materials is established.The three energy types of electromagnetic ultrasonic transduction mechanisms of ferromagnetic materials are analyzed theoretically,and the differences between ferromagnetic materials and non-ferromagnetic materials are clearly defined.Ferromagnetic materials have magnetostrictive effect,and the nonlinearity of magnetostrictive causes the complicated mechanism of electromagnetic ultrasonic energy conversion.Based on the constitutive equation of force-magnetic coupling of ferromagnetic materials and considering the combined effects of Lorentz force,magnetostrictive effect and magnetizing force,a finite element model of electromagnetic ultrasonic transducer for steel plates was established.2)The energy conversion mechanism of electromagnetic acoustic transducer(EMAT)is closely related to the electromagnetic characteristics of the tested material.EMAT responds differently to materials with different electromagnetic characteristics.Using the finite element model of electromagnetic ultrasonic wave of steel plate,the influence of the bias magnetic field and the electromagnetic characteristics of the test piece on the electromagnetic ultrasonic energy conversion mechanism was analyzed.By introducing different magnetic permeability and electrical conductivity equivalent simulation plastic deformation test pieces,the relationship between the energy conversion mechanism and electromagnetic characteristics was analyzed.And based on this,a plastic deformation detection system based on electromagnetic ultrasonic surface waves was established.3)Based on the established electromagnetic ultrasonic simulation model of ferromagnetic materials,the response characteristics of surface waves to defects in steel plate are simulated and analyzed.The relationship between the defect depth and the reflection coefficient and transmission coefficient was obtained,which laid the foundation for the quantitative detection of steel plate crack defects.The experimental study of cracks with different depths on standard specimens has verified the correctness and effectiveness of the simulation analysis.4)Correspondences between different plastic damage specimens and changes in relative permeability were obtained by testing the relative permeability of the standard I-plate specimens that were stretched.Plastic deformation will change the electromagnetic characteristics of the steel plate,and then affect the change of the detection signal.Based on the built-in electromagnetic ultrasonic detection system,the electromagnetic ultrasonic nonlinear detection test is performed on different test pieces,and the corresponding relationship between the relative nonlinear coefficient and plastic deformation is obtained.