METHOD OF MAGNETIC FLAW DETECTION IN BODIES OF NON-CIRCULAR CROSS SECTION USING UNIDIRECTIONAL MAGNETIZATION AND DEMAGNETIZATION PULSES TO ELIMINATE EDGE DISTORTION OF THE MAGNETIC FIELD

摘要

Method of detecting the location and extent of superficial faults, especially cracks, and for measuring the depths of such cracks in bars, billets, blooms, ingots, rods and tubes, especially of noncircular cross section. An electric current is passed axially through the elongated metallic body in one direction, and the magnetic flux of field (crossflux) generated by the passage of the electric current through the body is detected along the surface thereof. To avoid or reduce the effects of the edges of the noncircular body upon the detected magnetic field strength or flux and, therefore, to reduce the possibility that longitudinal cracks in the region of these edges will be undetected or poorly evaluated, the electric current passed axially through the bar is pulsed with the pulse shape being selected such that, in relation to the geometry of the bar, cracks in the surface in the region of the edges are detected. The pulsed current may derive from, say, a 50 or 60 Hz line source subjected to half-wave rectification; preferably pulse frequencies of 20 to 100 Hz are used. Periodically, or prior to each pulse, an erasing pulse is passed through the bar in the opposite direction to eliminate the residual magnetism from the previous scanning pulse or pulses. Hence the magnetization pulse is applied only to a nonmagnetized or previously demagnetized body.