New Discoveries on Electromagnetic Action and Signal Presentation in Magnetic Flux Leakage Testing

Abstract

Based on the authors’ previous work of magnetic flux leakage (MFL) quantitative description and the research plan especially for the magnetic component of eddy current effect factor $$ B_{other} $$, further explorations for electromagnetic actions, magnetic and signal presentations in the defect in electromagnetic nondestructive testing (ENDT, such as direct current field measurement (DCFM), MFL and eddy current testing (ECT)) are continued here. Primarily, new discoveries indicate that the magnetic component of $$ B_{other} $$ is actually composed of the secondary magnetic field component of $$ \Delta B_{mfl} $$ which manifests as raised signal waves and volume current perturbation component of $$ \Delta B_{\Delta I} $$ which features as double peaked signals due to the departure or bypassing of the “volume current” in the defect. What’s more, the concrete expressions of magnetic components are deduced and given through Ampere’s (or the Ampere–Maxwell) law. Afterwards, the theory is well demonstrated and verified by means of finite element analysis for current density and magnetic flux density or MFL signals in typical quasi-static testing by DC magnetization for different specimens with defects. Additionally, suggestions for installation site of sensor elements in ENDT are also given. Finally, a unified evaluation standard for magnetic flux density is formed, which provides the basic theory or analysis principles for the ENDT techniques as well as research directions for precision defect detection and related applications.