Research Of Remote Field Eddy Current Testing Technique Based On Multi-layer Electromagnetic Shielding

Ferromagnetic materials have been widely used in key components of high-speed trains,oil and gas pipelines,aerospace and nuclear industry.During in-service process,damage or failure may occur on these critical components thanks to the impact of longterm exterior stress from load,shock,temperature,humidity,corrosion and acerbic environment.The damage of these units undoubtedly leads to catastrophic outcome such as derailment of rail trains,explosion of pipelines,aircraft crash and nuclear leak,causing huge loss to lives and property.Therefore,regular inspection without any destruction to these ferromagnetic materials on industry are urgently required,from principles and experiments to evaluate the degree of failure and make relevant strategies in order to guarantee safety and sustainable development.Electromagnetic eddy current testing has been extensively used because of fast speed of inspection,high efficiency,non-contact.In addition,with the changes of detection modes,traditional eddy current testing has gradually developed multifrequency eddy current,pulsed eddy current,flexible eddy current array and remote field eddy current(RFEC).In this paper,we mainly study the physical phenemonon and features of receiving signals with the support of RFEC technique.The primairy contents are divied into the several parts:(1)On the fully understanding of the precious research,based on electromagnetic principle and electromagnetic shielding theory,how RFEC existing in pipeline can be extended to plate specimens for improving the ability to detect deep defects;(2)According to the principle of RFEC on plates and the typical feature,twice pentration of indirect coupling energy,COMSOL frequency-domain analysis is expoited to simulate the proposed RFEC structure and optimize parameters including the setting of material property,coils' s height and turns,shielding structures and corresponding materials,and defects simulation.The confirmation of simulation parameters aim to realize RFEC characteristics which comprise the twice penetration of indirect coupling energy,the logarithmic amplitude knot and phase changes.Then time-domain information consisting of single-frequency and pulse excitation;(3)On the basis of the proposed RFEC probe structure,practical machining operation such as excitation coils,pick-up coils,shielding structures and specimen defects and experimental platform are performed.The hardware system includes excitation module,signal conditioning module and data acquisition module.After the experimental platform has been finished,different types of defects,for example,the changes of angle,depth and width,are detected.Moreover,their signal features are also summarized.In this paper,we finished the design of the RFEC sensing structure on ferromagnetic plates,and the simulation and experimental verfifacation were conducted to confirm the detectability of 8mm below the surface with a minimum defect of 20*2*2mm.