A Methodology for Identifying Defects in the Magnetic Flux Leakage Method and Suggestions for Standard Specimens

In magnetic flux leakage (MFL) testing technology, the MFL signals are thought to result from all defects and are used in their evaluation. The tested defects include two types of defects, concave and bump-shaped features, and recently described mechanisms in the MFL method indicate that the former defects produce positive MFL because of magnetic refraction and the latter ones produce negative magnetic fields because of self-magnetization regulation; consequently, these defects result in raised test signal waves and sunken test signal waves, respectively. Thereby, a new methodology for accurately identifying the defect type based on the mapping relation between the signal features and defect types is proposed. Both simulations and experiments with three representative defects (i.e., notch, protrusion and combination) were conducted to confirm their identification using this new methodology. Combined with MFL standards such as American Society for Testing and Materials (ASTM) E570-09 (Standard Practice for Flux Leakage Examination of Ferromagnetic Steel Tubular Products, 2009) and British Standards (BS) EN 10246-4 (Non-destructive Testing of Steel Tubes—Part 4: Automatic Full Peripheral Magnetic Transducer/Flux Leakage Testing of Seamless Ferromagnetic Steel Tubes for the Detection of Transverse Imperfections, 2007), suggestions for standard specimens with reference defects that consist of both types of defects are provided.