Research And Applications Of Characteristics Of Eddy Current Attenuation In Eddy Current Testing Of Metallic Plates

Metal materials are widely used in various fields of production due to their excellent properties.The metallic plates commonly used in industrial production are mainly monolayer plates,compound plates and multi-layer plates.It is inevitable that errors and damages occur during the manufacturing and service processes.Therefore,it is very important to carry out nondestructive testing(NDT)on metal plates.As one of the five conventional NDT methods,eddy current testing(ECT)has unique advantages in testing conductive materials.In conductive half space under the excitation of uniform plane field,the eddy current(EC)density decreases exponentially in the depth direction.However,in conductor with finite thickness,characteristics of attenuation of EC are more complicated.The speed of attenuation of EC in the depth direction is very important for detecting deeply hidden defects and measuring thicknesses of metallic plates;so fully understanding the characteristics of attenuation of EC is significant for the applications of ECT.Currently,there are only a few studies on the characteristics of attenuation of EC at home and abroad.Based on the finite element method,this thesis studies the characteristics of attenuation of EC in monolayer plate,compound plate and double-layer plate,all of finite thickness.After that,detailed analyses and physical explanations based on the discovered phenomena and laws are performed.Simulation results show that the attenuation of EC in monolayer plate of finite thickness is slower than that in conductive half space.In compound plate,compared with the case that the conductivities of the two layers are identical,when the conductivity of the bottom layer is larger than that of the top layer,the EC density in the bottom layer is increased and the speed of attenuation of EC in the top layer becomes faster.When the conductivityof the bottom layer is smaller than that of the top layer,the EC density in the bottomlayer is reduced and the speed of attenuation of EC in the top layer becomes slower.Indouble-layer plate,the air gap between the layers reduces the amplitude of EC density in the bottom layer;in the meantime,it also decreases the speed of attenuation of EC in the top layer.Considering the fact that the skin depth in monolayer plate of finite thickness cannot be calculated if the plate thickness is small,this thesis uses the exponential function to fit the curve of EC attenuation to find the equivalent attenuation coefficient.This coefficient is then used as a substitute for skin depth to quantify the speed of attenuation of EC in the plate.In this thesis,the thickness of monolayer plate,the thickness of coating layer on metal base and the thickness of air gap in double-layer plate are measured respectively using transmitting-receiving(T-R)probe and differential probe.The selection of testing parameters is based on the analysis of the characteristics of EC attenuation.The experimental results show that within the range of thickness of test sample in each case,there is a monotonous relationship between output voltage and thicknesses.Then,polynomials are used to perform function fitting for each monotonic relationship.Substituting output voltage value of probe into functional formula,the thickness of test sample can be calculated.Finally,apply error analysis on the calculated values and the thickness values measured using the thread micrometer.The results of error analysis show that the maximum relative errors of the two probes are all small,which means that the thickness of monolayer plate,the thickness of coating layer on metal base and the thickness of air gap in double-layer plate can be effectively measured.