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, B_z, is almost uniform over the no-crack area (denoted as B_{Z, BASE}), while the magnetic flux density is smaller in the surroundings of the crack (denoted as B_{Z, CRACK}) : If B_{Z, BASE} is a bit larger than the saturated magnetic flux density of the MO sensor (B_S), then small magnetic domains occur over the crack area and a large domain over the non-crack area because B_{Z, CRACK} is smaller than B_s.