Theoretical consideration of nondestructive testing by use of vertical magnetization and magneto-optical sensor

This paper describes a new magnetization method for non-destructive testing with magneto-optical sensor (denoted as MO sensor) which have the following characteristic; high observation sensitivity, independence of the crack orientation, and precise imaging of a complex crack geometry such as multiple cracks. When a magnetic field is applied normally to the surface of a specimen which is significantly larger than its defects, approximately the same magnetic charge per unit area occurs on the surface of the specimen. If there is a crack in the specimen, magnetic charge per unit area has the same value at the bottom of the crack. The distribution of the vertical component of the magnetic flux density, Bz, is almost uniform over the no-crack area (denoted as BZ, BASE), while the magnetic flux density is smaller in the surroundings of the crack (denoted as BZ, CRACK) : If BZ, BASE is a bit larger than the saturated magnetic flux density of the MO sensor (BS), then small magnetic domains occur over the crack area and a large domain over the non-crack area because BZ, CRACK is smaller than Bs.