3-D FEM Simulation of Velocity Effects on Magnetic Flux Leakage Testing Signals

The development of computer-based defect characterization scheme for magnetic flux leakage (MFL) method require adequate mathematical models to describe the complicated interactions of sources, fields, velocity and defects in materials. In this paper, a 3-D transient finite element model based on T-Omega formulation was used to analyse MFL system at high velocity, with regard to eddy currents induced in a steel tube because of the relative movement between the exciters and specimen. The eddy currents change the profile of electromagnetic field, which increases difficulty in MFL signals interpretation and defect identification. Typical 3-D defects were modeled and MFL signals caused by the defects of different sizes were simulated respectively. Eddy currents induced by velocity and their characterization in high velocity MFL testing system were investigated.

I. Introduction

The magnetic flux leakage (MFL) method is used very widely in industry for the nondestructive evaluation of ferromagnetic materials. In this technique, testing equipment usually involves a magnetizer together with the necessary Hall sensors and instrumentation required to measure the leakage fields [1], [2]. In a general MFL testing system of a steel tube, the wall of the steel tube is magnetized axially to near saturation by Helmholtz coils. When the wall thickness is reduced by any defect or irregularities, a higher fraction of the magnetic flux will “leak” from the wall into the adjacent air. A circumferential array of Hall sensors is installed to detect the flux signals. The MFL signals contain lots of information about the tube conditions. Either the axial or the radial components of the leakage flux is measured and analysed to characterize the defects in the tube [3]–[5].