Use of defect field harmonics for inspection of surfaces of ferromagnetic products

Controlling of crack-type surface defects by magnetizing of articles by longitudinal sinusoidal field under stationary conditions is considered. Minor surface defects of ferromagnetic articles are best detected by higher harmonics of the field. Block-diagrams of the experimental device are given, and the obtained dependences of harmonic amplitude on the distance to the defect are reported. Smi-industrial tests of the Foerster-probe using the harmonic method described, have shown that the method can reveal small surface cracks up to 2 mm long and 0.2 mm wide