Design And Test Of The AC Field Detection System

The alternating current field measurement (ACFM) is an emerging electromagneticnondestructive testing. The technology is based on the amount of induced magneticfield intensity of disturbance to detect the surface of the metal member and near surfacecracks, and can measure the length of the crack, and calculating the depth of the crack.Has a very broad application prospects in the petrochemical, offshore platforms,underwater equipment in service inspection, shipping and aviation manufacturing.This paper describes the study of the status of the alternating current fieldmeasurement technology at home and abroad, on the basis of the principle ofelectromagnetic induction, given ACFM detection principle and mathematical model."Incentive" type three-dimensional finite element model of the rectangular coil andrectangular slot defects, giving the relationship between defect distribution andmagnetic field perturbations.Software system based on DSP and Labview is designed in this paper. With theadvantage of efficient data processing capability, DSP greatly improved the real-timedata processing and extended the function of system combined with Labview.Long-distance data transmission between upper computer and lower machine can beachieved by the method of serial data communication. Data from two channelstransmitted by coding data frame, which improves the accuracy and reliability of datatransmission.Adopting ACFM detector detect the pressure vessel and steel plate withArtificial defects.It has been verified by experiment that this system can completelysatisfy the requirements of testing process in real-time.The alternating current stress measurement (ACSM) which is developed fromalternating current field measurement(ACFM) is a new and effective electromagneticnon-destructive testing method with greatly application potential for stressmeasurement. In this paper, a mathematical model of the relationship between stressand induced magnetic field was builded under the external magnetic field according tothe principle of ACFM. In the present paper, the relationship of stress and inducedmagnetic field was studied by tensile experiments of20#low carbon steel. Experimentresults which are extremely valuable show when the tensile force is less than the yieldstrength of the specimen, the percentage change of testing signal in the verticaldirection of stress can be up to8%and higher than that in the parallel direction. Whenthe tensile force exceed the yield strength, the percentage change of testing signal in thevertical direction increases dramatically and can be up to30%, while that in the paralleldirection can be up to20%too, which can effectively detect the degree of stressconcentration of the ferromagnetic specimen. Meanwhile, The mathematical model ofthe relationship between stress and induced magnetic field under the external magneticfield can be verified through experiments.