Magnetization Changes Induced by Stress Noncoaxial with the Magnetic Field in a Low-Carbon Steel

Ferromagnetic materials are widely used in the manufacturing of key parts of energy equipment, due to their good mechanical properties, such as in nuclear power and pipes. Mechanical stress exists inside of these key parts during operation. Stress can be estimated indirectly by nondestructive testing methods that measure the magnetic flux leakage signals on the surface of the structure, which is of great importance for ensuring the safety of the equipment. However, the physical mechanism of the stress and magnetic field in the magnetization of ferromagnetic materials is still unclear, leading to limited applications of the technique in practice. In this paper, magnetization tests were carried out to investigate magnetization changes under the coupling effect of stress and a noncoaxial magnetic field. Two identical Q195 low-carbon steel specimens were tested. Specimen 1 was subjected to magnetic field values successively increasing from 0 A/m to 6000 A/m under constant uniaxial tension at different angles theta between the field and stress axis. Specimen 2 was subjected to the same magnetic field under different levels of stress at an angle of 0 degrees. The surface magnetic induction B of the specimens was measured and analyzed at each angle of stress-field orientation and at different levels of stress. It was found that there was a difference in the direction between the B and the magnetic field H at different angles theta. The magnetization curves correlated to the angle theta and the stress levels. The behavior of the derived maximum differential permeability and maximum magnetic induction could be used for the nondestructive evaluation of stress magnitude and direction in materials already in service.