Quantitative Research Of Defect In Eddy Current Nondestructive Testing

Eddy current nondestructive testing technology(ECNDT)is one of the most commonly used nondestructive testing techniques.With the further improvement of China' s comprehensive national strength,the ECNDT will be increasingly applied in the machinery industry.At present,most of the eddy current detecting instruments using impedance analysis can detect whether there are defects inside the material,but it is difficult to achieve the quantitative identification and analysis of the defects.The investigation of the new theories,methods of ECNDT has become a hotspot for domestic and abroad scholars.From the qualitative evaluation to quantitative analysis of the defects is an inevitable trend of ECNDT development,which is also of great significance to further accelerating the development of Chinese industry.Therefore,in view of the difficulty for the traditional ECNDT to quantitatively measure the defects,the effects of the eddy current distribution within the conductor on the response of the probe coils are discussed in this paper.Firstly,the concept of eddy current effect intensity(ECEI)is put forward and defined,then some novel ideas and methods of defect quantitative research are further derived based on the concept.Finally,the correctness of the above mentioned ideas and methods are verified by simulation and experiment results.Hence,this paper provides a novel train of thought and clew for the implementation of defect quantitative evaluation of ECNDT.The main research contents and conclusions are as follows:(1)A cylindrical probe consisting of double drive coils and a single pick-up coil is fabricated,in which the double drive coils are winded in the opposite direction using the same enameled wire to eliminate the influence of exciting source on the response of the pick-up coil.The system basic model of this paper is established by using this cylindrical probe.On the basis of summarizing the related literature on the eddy current testing(ECT)theory domestic and abroad,the integral analytical model of the eddy current density(ECD)and the pick-up coil response under the double drive coils are deduced.Furthermore,the determination method of the actual structural parameters of the probe based on MATLAB numerical simulation is summarized.(2)The response of the pick-up coil generated by ECD per unit volume at a certain position in the conductor is defined as the ECEI at this position.The integral analytical model of the ECEI inside the plane conductor is deduced based on its definition.Comparing the response of pick-up coil obtained by simulating calculation through the methods of magnetic vector potential and ECEI respectively,the relative error is found less than 5%,which verifies the correctness of ECEI integral analytical model.(3)The method of defect quantitative research based on the concept of ECEI is proposed,and the shape factor determined by the shape and orientation of the defect is introduced.Using this method,the shape factors corresponding to micro spherical defect,micro cylindrical defect whose axis is parallel or perpendicular to the direction of the ECD are deduced respectively.The shape factors of the micro defects in the three different cases are 1,which shows that when the defect is very small,the changes in the pick-up coil response are proportional to the volume of the defect.(4)Through the simulation of the integral analysis model of ECD and ECEI by MATLAB,it is found that the maximum ECD occurs in the conductor at a position about twice the radius of the coil.Besides,the maximum ECEI is not at the position where the eddy current density is largest,but where it is closer to the axis of the probe coil.(5)The finite element simulations of spherical defects,cylindrical defects which are axial parallel or perpendicular to the direction of ECD are carried out by COMSOL software.The simulation results show that,for the defects of three different cases,the relationships between the changes in response of the pick-up coil and the defect volume all satisfy the linear combination of two exponential functions.When the defect volume is small,the changes in the response coil are proportional to the defect volume,and the slope of the proportional relationship is approximately equal to the ECEI magnitude at the location of the defect.This result also proves the correctness of the defect quantitative research based on the concept of ECEI.(6)The ECT system is set up and used to investigate the relationship between the changes in probe response and the volume of cylindrical defect perpendicular to the direction of ECD.The experimental results demonstrate that when the volume of the defect in the conductor is less than 50 mm3,the changes in the pick-up coil response is proportional to the volume of the defect.That confirms the reliability of the above theory and simulation.