EMS-2000+, a smart metal magnetic memory instrument, is a new generation non-destructive testing (NDT) instrument with advanced microelectronic, computer and magnetic memory techniques Experiment proves that under the action of the alternative loading, the normalization phenomenon of the magnetic domain occurs at the places containing defects and inclusions in in-service ferromagnetic components, and leakage magnetic field arises on it At the places of defects or internal stress relative concentration the metal magnetic conductivity is in possession of minimum value, and the tangent component of its magnetic field is in possession of maximum value, and therefore the sign of the normal component is changed and its value is zero The stress concentration areas can be located by detecting the normal component of leakage magnetic field on the surface In this way, whether the work piece has defects or not can be indirectly determined