Examination of Inspection Method for Outer-side Defect on Ferromagnetic Steel Tube by Velocity Effect Using Insertion-type Static Magnetic Field Sensor

When an insertion-type magnetic sensor using only a static magnetic field is moved in a steel tube at a high speed, an eddy current is generated in the steel. When there is a defect in the steel tube, it is possible to detect the defect from the change in the distribution of the eddy current in the tube. Generally, ferromagnetic heat transfer tubes in power plants or oil plants are inspected for defects by moving an internally inserted electromagnetic sensor at a high speed of about 1 m/s. In this study, an inspection method is proposed in which an insertion-type electromagnetic sensor using only a static magnetic field is moved at a high speed in a steel tube to detect an outer-side defect in the tube. The eddy current density and alternating flux density inside the steel tube generated by constant velocity movement of the static magnetic field are determined by 3D nonlinear finite element method (FEM) analysis of the eddy current by a step-by-step method. In addition, a verification experiment is also conducted.