Simulation And Optimization Of EMAT For Rail Inspection Based On Magnetostriction Mechanism

With the increase of high-speed railway mileage and train running time,the phenomena of rail overload and extended service occurred.For those rails that were already mildly damaged during production period,long-term squeeze and friction will easily lead to potential safety hazard.The critical cracks resulted from occluded foreign substance will terribly threaten people's lives and properties.Electromagnetic ultrasonic detection can be used for rapid detection of rail internal defect,and it can be implemented in various environment because coupling agent is not necessary.It is a challenging problem to implement rapid detection on critical cracks of rails with Electromagnetic Acoustic Transducer(EMAT).Based on high-speed railway damage detection,the research on EMAT and its application in rail nondestructive examination is carried out.Here are the main content:(1)Based on the theoretical analysis of EMAT rail inspection,the EMAT system is divided into several parts.The transformation law of electromagnetic field is described by magnetic vector potential and the constitutive equations of static magnetic field and alternating magnetic field is provided.The non-linear magnetization characteristic,non-linear magnetostrictive property,magnetostrictive constitutive equations and particle displacement control equations of ferro magnetic materials are studied,meanwhile the theoretical support for EMAT system research is provided.(2)The multiple field coupling problem is solved based on transducer level,and the EMAT system problem is transferred to solving partial differential equations by analyzing the physical model and building mathematical model.The finite element method is used to solve non-linear problem and the magnetostrictive physical field is simulated by COMSOL Multiphysics.Firstly,the relationship of variables in the model is declared,then the field properties and edge properties of the physical field are given,lastly,the weak form equation and constraint condition of EMAT multiple field coupling is presented.The transmission of physical quantities in the positive and inverse coupling of magnetostrictive field is realized by equations,at the same time the simulation and calculation of strong coupling physical field is carried out.(3)The non-linear magnetic materials data is defined.The EMAT rail inspection simulation system using ultrasonic body wave which can simulate the motivation,transmission,reflection and reception of ultrasonic in rails is designed.It is superior to the rail inspection standard stipulated by Ministry of Railways,it also verifies the feasibility of critical cracks detection using EMAT.(4)The energy conversion efficiency is optimized by parameters' design.The increase of Static magnetic field strength and coil spacing and the decrease of lift distance is shown to contribute to the enhancement of energy conversion efficiency.This discovery make up the imperfection of the parameter design of EMAT which was usually set according to experiences.The results of EMAT simulation show that under the action of magnetostriction force,the stress will concentrate on the defects of steel plate and different defect position have different effects on received signals.The system optimization scheme is proposed in this thesis on the base of electromagnetic ultrasonic transduction and EMAT signal collection experiment is designed,which has guidance for engineering application.