Theoretical And Methodological Study Of Stress Measurement Based On Magnetic Anisotropy

The stress can be measured conveniently, quickly, nondestructively and contactlessly by using magnetic methods, and the magnetic methods are notable in nondestructive test in recent years. Based on the basic theory of Ferromagnetism, we studied the relative topics systematically, and made some instructive efforts for the theory research and application of magnetic stress measurement. Our theoretic work includes: magnetization mechanism of ferromagnetic materials, The Effect of stress (strain) on magnetization status, the calculation of magnetostrictive coefficient and The Effect of stress (strain) on magnetic susceptibility. Based on theory studies, the relationship between the output signal and stress in magnetic stress measurement was derived, the magnetic measurement method was also improved, and novel magnetic core materials were prepared as well. The main achievements and innovation are listed as below:(1) The Effect of stress on the output signal induced in detection coils was studied by application of magnetic circuit theory, and an equation between electromotive force and principal stress difference was derived. Thus, the theoretical foundation of stress measurement based on magnetic anisotropy was developed further.(2) The application a Fourier transform was proposed to determine the maximum output signal and thus to determine the difference of the principal stresses and the direction of the principal stress, thus the rotation of the magnetic probe becomes redundant, and the measurement efficiency and presion were improved as a result; The Effects of demagnetization and excitation frequency on the measurement results were investigated as well, and the measurement error was reduced as a result.(3) The Effect of grinding, tensile or compressive stress, temperature, and the distance between the specimen and the sensor were also studied by using a Four point bending experiment and the variation rule of output signal under different measurement conditions was obtained. Our investigation provides some valuable reference for stress measurement under different conditions.(4) Based on the newly appeared Finemet type alloy, by the substitution of Nb by Al, novel nanocrystalline magnetic core materials for stress measurement sensors were studied and prepared. Results show that soft magnetic properties of nanocrystalline alloys are superior to amorphous alloys, the permeability and magnetic loss of the samples are improved by the substitution of Nb, the annealing condition exerts drastic effects on permeability of the alloys, the applied magnetic field exerts obvious effects on permeability spectra, another relaxation peak appears under high field amplitude, and the relaxation frequency in low frequency region increases with the increase of the magnetic field amplitude. The new fabricated alloys possess more excellent soft magnetic properties with less cost, and they are thought to accelerate the process of miniaturization of magnetic sensor and the saving of energy sources.The research achievements were applied successfully in laboratory experiment, which proves that the stress measurement method based on magnetic anisotropy is useful and effective.