METHOD AND DEVICE FOR FLAW DETECTION

摘要

PURPOSE:To improve the precision of flaw detection by establishing magnetic field in different directions periodically around two coils arranged successively so as to have axial centers on the same line with the axial center of a material to be inspected, and detecting variation of magnetic flux through the sensors. CONSTITUTION:Two exciting coils 31a and 31b are arranged so as to surround the body 11 to be inspected which is conveyed lengthwise and a magnetic field detector 32 is provided in the middle. Then, the coil 31a is applied with an exciting current of frequency (f) and the coil 31b is supplied with an exciting current of frequency 2f having the limit value of its current waveform in phase with the exciting current of frequency (f). Then, a magnetic field along the surface of the steel pipe 11 is formed around both exciting coils 31a and 31b when the exciting currents coincide with each other and a magnetic field perpendicular to the surface of the steel pipe 11 is established when the exciting currents are opposite to each other. Then, leak magnetic flux or the disorder of the magnetic field is detected by the sensor 32.