Flux leakage testing is a non-destructive evaluation (NDE) method used to detect surface and internal flaws in ferromagnetic materials like steel tubes. A powerful magnetic field is applied to the material. If the metal is sound, the magnetic flux stays within the material. However, if there is a crack, pit, or fracture, the magnetic flux "leaks" out of the surface. Sensors detect this leakage to identify defects. This is a primary quality control tool for inspecting the longitudinal weld seams of structural tubing.