Flux leakage testing

Magnetic flux leakage testing (MFL) is a nondestructive electromagnetic testing method for detecting defects in ferromagnetic materials. By magnetizing the test object and measuring local field disturbances, the method reliably identifies imperfections before the material is further processed or installed.

Magnetic flux leakage testing is based on the controlled magnetization of the material being tested. This generates a magnetic flux within the ferromagnetic material. In homogeneous, defect-free areas, the magnetic flux can flow largely undisturbed through the material. Discontinuities such as cracks, laps, corrosion, or material loss disrupt the magnetic circuit and alter the path of the magnetic flux.

At such discontinuities, part of the magnetic flux is forced out of the material, creating a local magnetic leakage field above the surface. Sensitive magnetic field sensors detect these local field changes and convert them into electrical test signals. The characteristic signal response can then be evaluated to detect and locate relevant indications. The basic principle is simple: Where a defect disturbs the magnetic flux, a measurable leakage field is generated.

Which defects can be detected with particularly high sensitivity depends largely on the type and direction of magnetization as well as the penetration depth of the magnetic field. The leakage field is particularly pronounced when the direction of magnetization is perpendicular to the orientation of a defect. By selecting the appropriate magnetization direction, longitudinal or transverse defects can therefore be detected selectively.

With alternating-field magnetization (AC), the frequency-dependent skin effect concentrates the magnetic field increasingly near the surface of the material. The resulting high magnetic flux density in the surface region provides particularly high sensitivity to small surface defects. With DC magnetization, by contrast, the magnetic field penetrates the entire material cross-section, enabling the detection of deeper and internal defects.

Magnetic flux leakage testing combines high testing sensitivity with the advantages of a nondestructive and highly automated method. It can be integrated into continuous production processes, enabling reliable quality control before semi-finished products are further processed or components are put into service.