Thickness Measurement Method Research Of Double-layer Material Based On Eddy Current Technology

Multi-layer coating materials are widely used in aerospace,nuclear industry and other fields.Their quality is the vital factor that affecting the corrosion resistance and wear resistance of key components.Among them,the thickness of each layer is an important indicator to measure the quality to protect itself.Therefore,the thickness detection of multi-layer materials is significant to ensure the safe operation of equipment.Eddy current testing technology is used for the thickness measurement of multi-layer materials at the advantages of low cost and fast response.The traditional single-frequency eddy current technology is mainly suitable for the thickness measurement of single-layer materials which has large measurement error.Multi-frequency eddy current technology is mainly applied to the thickness measurement of conductive materials,and the frequency selection has a great impact on the measurement accuracy.Therefore,based on single-frequency eddy current and multi-frequency eddy current technology,this thesis establishes a finite element model for the stacked structure which is composed of nonconductive layer,conductive coating and conductive substrate.Combined with the impedance change law of the coil above the laminated structure,the thickness detection research of the double layer material including non-conductive layer and conductive layer is carried out.The main work of this thesis includes:(1)By analyzing the variation of coil impedance and eddy current loss,as well as the finite element simulation and numerical analysis of the stacked structure,proposed a double-layer material thickness detection method based on single-frequency eddy current technology,which can simultaneously measure the thickness of non-conductive layer and conductive coating materials.The frequency is selected based on impedance sensitivity and skin depth,and fitted the thickness of non-conductive layer.The maximum relative error is less than 3.0%;The maximum relative error of fitting the conductive coating thickness is less than 6.0%.(2)Combined with multi-frequency eddy current technology and electromagnetic field theory,the thickness detection model of stacked structure based on impedance is established,and proposed the thickness measurement method of double-layer material based on multi-frequency eddy current technology.By optimizing the selection of multifrequency eddy current frequency,the thickness of double-layer material obtained by combining the three excitation frequencies in pairs is weighted,and fitted the thickness of non-conductive layer and conductive coating double-layer material,with the maximum relative error less than 3.0%.Moreover,this thesis verified the influence of conductive substrate conductivity and coil geometric parameters on the detection effect.(3)Based on the finite element model,the design and construction of the experimental system are completed,this thesis verified the change law of the coil impedance and the thickness of the tested piece,and test the detection method has proposed.Aimed at the thickness of double-layer material containing non-conductive layers,the experiment shows that the maximum relative error of single-frequency eddy current testing method is less than 10%,and the maximum relative error of multifrequency eddy current testing method is less than 8.0%.This thesis explores the relationship between the coil impedance and the thickness change of the double-layer material,and proposes a single-frequency eddy current detection method based on lift-off effect and skin effect,which realized the simultaneous measurement of the thickness of the double-layer material.The thickness measurement method of double-layer material based on multi-frequency eddy current technology is proposed to achieve higher accuracy measurement of double-layer material thickness.The detection ability of single frequency eddy current and multi-frequency eddy current detection technology is further improved,and provides technical support for the thickness detection of key parts of complex equipment such as aerospace.