Ferromagnetic materials are widely used for various manufactured products such as cars, trains, and ships. Iron and steel are the most popular materials for frame structures because of their mechanical properties. Nondestructive testing of iron and steel is an extremely practical way of maintaining their mechanical reliability. It is well known that Barkhausen signals are only emitted from ferromagnetic materials having magnetic domain structures. These signals also vary depending on their past mechanical as well as radioactive stress histories.
  We applied a generalized analysis of frequency fluctuations to Barkhausen signals in the present study in order to detect various mechanical stresses. Surprisingly, we succeeded in clarifying that application of our frequency fluctuation analysis to the Barkhausen signals made it possible to detect several kinds of mechanical stresses.