A method of characterizing axial stress in ferromagnetic members using superficial magnetic flux density obtained from static magnetization by permanent magnet

It is of great significance to obtain the information about the stress of load-bearing ferromagnetic members quickly in order to maintain the safety of the infrastructure. The key point is to accurately and quickly determine the characterization parameters which change sensitively and linearly with the stress. Among the existing electromagnetic methods of determining axial stress in ferromagnetic members, exciting coils are usually adopted to exert a time-varying magnetic field on the ferromagnetic members, which will induce the problems of winding coils, coil heating, and eddy current that influences the test results. What is worse is that it is inevitable to compare the experimental data point by point to determine the adequate magnetic parameter characterizing the stress, which influences the fast determining of the axial stress in ferromagnetic members. In order to break through these limitations, in this paper we propose a method of determining the axial stress in ferromagnetic members by using superficial magnetic flux density obtained from static magnetization in permanent magnets. In this method, permanent magnetizers are adopted to excite the overall damping and local uniform spatially-varying constant magnetic field on ferromagnetic members. A testing probe including Hall chip array is adopted to measure the superficial axial and radial magnetic flux density to determine the axial stress of the ferromagnetic member. The principle is elaborated to choose the adequate superficial magnetic flux density fast and precisely for characterizing the axial stress in ferromagnetic members. According to the theory of demagnetizing field, the continuity of the tangential magnetic field strength and Gauss's law for magnetism, the relational equation between the derivative of superficial axial magnetic flux density with the stress and the derivative of superficial radial magnetic flux density with the stress is established. Then, an experiment is conducted to verify the proposed method. The experimental results show that according to this relational equation, the superficial magnetic flux density with the highest stress sensitivity can be determined quickly and accurately. What is more, the linearity of the superficial magnetic flux density varying with the stress is good, and the goodness of the corresponding linear fitting R2 is greater than 0.98. It means that the determined superficial magnetic flux density can be used as a feature parameter to characterize the stress in ferromagnetic members. The proposed method of determining the axial stress in this paper can provide a new way of on-line detecting the working stress in ferromagnetic components.