Theoretical And Experimental Research On Total Magnetic Flux Stress Testing Technique For Steel Structure Based On The Magnetomechannical Effect

Steel is a kind of ferromagnetic material, which is extensively applied in such fields as buildings, bridges, railways, machines and lifeline engineering etc. Those engineering structures built of construction steel will unavoidably experience some damages during their service lifetime, thus which will influence the distribution regularity of internal forces in structures, result in over-limit stresses, cause the local failure of structures, and even lead to collapse of the whole structure. Therefore, it is a pressing topic to study how to directly evaluate the real-time stressed states of structural members, damages and steel characteristics in present structural health monitoring and diagnosing fields. And the achievements of this research will be of theoretical significance and of application value of engineering.Based on the above background, through the theoretical analysis and model experiments, the fundamental theory and application methods of total magnetic flux stress testing technique for steel structure are systematically investigated. The main contents of this dissertation are as follows:(1) According to involved theories of ferromagnetics, the microscopic mechanism of domain structure is analyzed systematically; through the movement variation of domain structure, that is the movement of domain wall and rotation of magnetic moment, the magnetization process is illustrated in an applied magnetic field. Meanwhile, by the use of energy methods, a study is performed on influencing factors of domain movement including stress, dope, scattering field, etc. The influence of various energies is dealt with on the magnetic behavior and parameters of ferromagnetic material; the corresponding theories of the spontaneous magnetization and technical magnetization are also discussed.(2) The magnetostrictive effect, counter-magnetostrictive effect and magnetostrictive effect producing mechanism is discussed; the influence of mechanical stress on magnetization of positive and negative magnetostrictive materials is analyzed. Based on counter-magnetostrictive effect and principle of energy minimum, combined with domain theory, the stress sensitivity of magnetization is explained in detail, which forms a fundamental basis for stress testing technique based on magnetomechanical effect. (3) In view of the microscopic mechanism of ferromagnetic material, the influence of internal stresses on magnetic domains and domain wall is studied, and the influence of internal stresses on initial susceptibility is also explored for different variation states of domain movement. Through the analysis of energies in ferromagnetic material, the dependence relationship between susceptibility and internal stresses is derived, which theoretically demonstrates the practicability of magnetic nondestructive internal stress testing based on magnetomechanical effects.(4) Varieties of magnetomechanical effects of ferromagnetic material are discussed, such as Barrett effect, Wiedenmann effect and magnetobendig effect etc. According to the inverse magnetostrictive effect, the magnetomechanical behavior of ferromagnetic material is researched. Both magnetic hysteretic models are modified, that is the modification of rate-independent magnetomechanical hysteresis and Jiles Atherton model. Through software Matlab, the numerical analysis program is compiled to solve the magnetic hysteretic curves, and then various characteristic parametes are obtained. In the meanwhile, two major magnetic hysteretic curves are researched when considering and neglecting the magnetomechanical effect, and the influence of stress on this curve is also discussed; besides, the hysteretic curve of Q235 steel model testpiece is simulated, and then is compared with experimental results.(5) The 6 kinds of Q235 stee lmodel testpieces with the number of 66 are designed. The diameters of these testpieces are respectively 4mm and 16mm, their cross-sections are solid round steel and steel tube, and their stressed states are axially tensile and compressive. The magnetomechanical coupling tests are performed on these testpieces in NIM-200HF magnetomechanical coupling equipment. Then, the hysteretic curves, residual magnetization, coercivity, etc. are obtained. The experimental results are analyzed with domain theory. The influence of applied stresses on the magnetic hysteretic curves and the influence of applied magnetic field on the coercivity are investigated. The dependence relation of magnetization versus applied stresses is established, and the optimum magnetic field is determined in the stress sensitive range of magnetization. Additionally, the size effect of magnetization is discussed, that is the outer diameters and wall thicknesses of testpieces will have impact on magnetization as well.(6) In accordance with the characteristics of stress sensitivity of magnetization for ferromagnetic material, the magnetomechanical constitutive model of magnetic flux or magnetic induction versus applied stresses is established, and this model can well reflect the main feature of constitutive relations for ferromagnetic material. In the meanwhile, according to thermodynamics, the magnetomechanical coupling equation is also established, and the corresponding constitutive relation between the incremental magnetic flux and incremental stress is attained. By utilizing the self-compiling program, the numerical simulated analysis is conducted on the magnetomechanical constitutive equation presented here, and the analysis results are compared to experimental results, which exhibits a good agreement.The theoretical and experimental results indicate that: the domain structure of ferromagentic material determines their magnetomechanical behavior; both the applied magnetic field and applied stresses will contribute to the movement of domain wall, but the internal stresses will impede this kind movement. Under the concurrent action of applied magnetic field and stresses, the magnetic characteristic of ferromagnetic material is closely related to magnitostrictive coefficients; moreover, in demagnetization the applied stresses can just make domain wall move. When the applied magnetic field is given, the applied stresses will change the magnetization of ferromagnetic material, and thus the variation of magnetization will lead to variation of magnetic flux, so the actual stressed state of ferromagnetic material can be evaluated by testing their change in magnetic flux. Furthermore, the research results also indicate that: the magnetic hysteretic curve is comparatively sensitive to the applied stresses; each model testpiece possess its own stress sensitive range, which is characterized by the magnetization and magnetic flux will increase with increasing tensile stresses, but decrease with increasing compressive stresses during this sensitive range. Meanwhile, the bigger the diameter of model testpiece is, the wider the stress sensitive range of magnetization and in the optimum magnetic field the testpiece is most sensitive to applied stresses. As result, the optimum magnetic field can be chosen as applied magnetic field used to realize total magnetic flux stress testing technique for steel structure, and thus the actual stressed states of members can be accurately detected.