Stress Monitoring For In-service Steel Structures Based On Elasto-magnetic Effect And Magneto-electric Laminated Composites

With the rapid development of social economy and continuous progress of science and technology, the research and application of space structures have become increasingly extensive. Among them, the space steel structures have been widely used in infrastructures, such as all kinds of industry factory buildings, large stadiums, and airport terminals, some of which even are public utility buildings or symbolic projects related to the national economy and people’s livelihood. These structures not only are with complicated stress behavior and resistant mechanism, but also are inevitable to suffer from the influence of kinds of natural and artificial factors in the service period. The possibly excessive deformation and internal force would result in the deterioration of structural health status, threatening the normal operation and safety of the entire structures. The member/component stress is one of the most important factors determining the structure safety. To realize the real-time on-line nondestructive monitoring of their actual stress is an urgent and important scientific and engineering task, the study of which is therefore of important social and practical significance.In this thesis, the current situation and methods of the stress monitoring for the in-service steel structures are firstly summarized. Aiming at the shortages in the current methods, a new elasto-magneto-electric (EME) sensor is proposed, which is based on the elasto-magnetic (EM) effect of common steel materials and uses the smart magneto-electric (ME) sensing unit as the signal detector. The nondestructive detecting (NDT) technology using the EME sensor is studied in depth from the perspectives of theoretical analysis, model simulation, system design, laboratory experiment, and engineering application. Specific research contents and the results obtained are as follows:(1) According to relative basic theories of ferromagnetics, the influence of the stress on the magnetization and the magnetic domain structure is explored. The basic theoretical model of EM method is given based on the relationship between magnetic characteristic and stress. After introducing the working principle and application of the ME laminated composites, the ME sensing unit made of them is designed for the stress sensor. And the testing and verification of the performance of the ME sensing unit are carried out. On the basis of the EM effect and the ME laminated composites, the idea and design theory of the EME sensor are proposed. Preliminary design is conducted based on the theoretic calculation, including the design of the excitation source, the exciting current, and other parts.(2) The simulation model of the EME sensor is established, taking into account the EM effect of the steel materials and the ME effect of the laminated composites, with the spatial magnetic distribution topography characterized by the Finite Element Analysis (FEA) method. Thus, the model simulating the elasto-magneto-electric conversion of the system is completed. Based on the improved Jiles-Atherton (J-A) model, the differential equations describing the EM effect is put forward, and the solutions are derived. The hysteresis loops under different stresses are deduced, as well as the relationships between stress and magnetization under constant magnetic field. The features of the detected signals under pulse excitation are studied by the field-circuit coupling method. Based on the features, the sensor design is optimized. Based on the equivalent circuit method, the ME effect of the ME laminated composites is discussed, and the ME coefficients of longitudinally magnetized and transversely polarized longitudinal-transverse (L-T) type ME laminated composites is calculated. Then, the process of establishing the EME model is presented via a numerical example.(3) The stress monitoring system is developed based on the LabVIEW platform, in which the test process is operating in a computer-controlled mode through the virtual instrument technology. The steel components of circular section are widely used in practical engineering. Laboratory experiments are conducted on the stress/force monitoring of steel components with different materials and diameters using the designed EME sensors. The results show that EME sensor is able to test the steel components in various sizes, and that it is also promising to test the composite-material structures containing the material beyond the pure steel. The collected signals are analyzed and the physical mechanism is discussed. The superiorities of the EME sensor and the monitoring technique are concluded, such as nondestructive monitoring, high measurement accuracy, fast response, actual-stress measurement without knowing the stress history, ease of installation, strong resistant ability and long prospected service-life.(4) The developed EME sensory system has been applied to the intelligent monitoring of cable forces in the2nd Jiaojiang Bridge of Zhejiang Province. The factory calibration and in-situ verification of the cable forces for engineering applications are successively carried out. (5) After generalizing the factors influencing the magnetoelasticity of the steel materials, the microstructure of the steel materials and the temperature are respectively studied in detail by means of a series of experiments, to conduct stress monitoring in more types of materials using the EME sensors. Optical microscope and magnetic force microscope (MFM) are employed to investigate the microstructure and magnetic domain structure of the steel materials. The magnetic curves are measured by the vibrating sample magnetometer (VSM). The magnetic parameters are obtained according to the law of the technical magnetization. The impact of the temperature on the measuring precision is also analyzed in detail. On the basis of the molecular field theory, the influence mechanism of the temperature on the magnetoelasticity of the steel materials is induced. The variation of the magnetic characteristic with the temperature is tested. The magnetization curves and the hysteresis loops of the Galvanized high strength steel wire are obtained at different temperatures, as well as the temperature sensitivity. Practical methods to improve the electromagnetic compatibility (EMC) of the EME sensory system are suggested in case of the possible unfavorable environment.Finally, the conclusions drawn from this study are summarized, and areas in need of further research are highlighted.