| Ferromagnetic materials such as high temperature and high pressure oil and gas pipelines and high-speed train tracks often produce stress corrosion defects or fatigue cracks due to working conditions and external loads,which cause huge economic losses and sometimes even threaten the life safety of relevant personnel.It is always an important direction to promote the development of machinery industry to detect the defects of metal components,then detect and monitor the health status of materials and predict the service life of metal materials.Eddy current testing for metal defects nondestructive testing method is one of the important ways,it has the characteristics of safety,green,convenient,economic,has been praised by the attention of researchers,nondestructive testing field from the fifties and sixties of the last century has been in the shuttle,ships,automobiles,pipeline in such fields as defects detection plays an important role.This thesis mainly conducts theoretical research on eddy current detection of corrosion defects,subsurface defects and natural defects with seam shape,deduces the expression of magnetic vector potential,and studies the process that geometrical parameters such as depth,width,volume and burial depth of defects affect magnetic field distribution.The following research contents are carried out:Firstly,the approximate calculation is made for the corrosion defect of multi-layer pipeline wall which depends on high temperature and high pressure in petroleum,coal and natural gas.The specific approximation method is to divide the common inner wall corrosion defects of multi-layer pipeline into a large number of blind hole defects,and superposition the magnetic field of blind hole defects as the approximation of the magnetic field detection of conical defects of multi-layer pipeline wall.It is found that the efficiency of approximate calculation is 40 times that of numerical simulation calculation.Then,the subsurface dark damage and subcutaneous corrosion defects encountered in the field of high-speed railway track detection are theoretically analyzed,and a mathematical analytical model containing subsurface defects is established.The eigenvalues of defect regions are calculated by using truncated region（TREE）method,and the coefficient expression of magnetic vector potential of each layer is solved by using boundary conditions between each layer.It is found that the relative error of analytical calculation is less than 3.5%,and the calculation efficiency is 100 times that of numerical simulation.Finally,real defects in engineering for the approximation analytical,contains the natural defects of transition area as close to real defects,and using the elliptic nature defects to seam shape natural defect for approximate calculation,and found that the proportion of approximate method can make the approximate calculation and numerical simulation calculation of the relative error controlled within 1.16%.Through the experiment of four approximation methods of slit defect,the final experimental results prove that the equal proportion approximation method is the best.The above research contents are helpful for understanding the process of eddy current testing in solving practical engineering problems where defects affect eddy current field distribution.On the other hand,it can quickly calculate the electromagnetic field of eddy current NDT when there are different types of defects,so as to optimize the coil with different types of defects and select the defect characteristic signal reasonably. |
