Research On Mechanical-structure Health Monitoring Mechanism Basing On Ultrasonic Guided Waves

The security problem of large-scale structures attracts more and more attention because the early damage of their key components can cause sudden failure if they cannot be found and the fatigue propagates continuous. Thus, real-time monitoring the health status of structures and accurate identification of the initial micro cracks, have important significance and practical value in engineering.Recently, the relative technologies of structure health-monitoring have developed continuously into a hot topic in academic and engineering domains, and have been used for status monitoring of structures. Among many detection technologies, the structure-health detection basing on the guided wave is one of effective tools for real-time monitoring and identification of structure damage. The technology is a multidisciplinary integrated technology which involves multiple disciplines, such as mathematics, mechanics and electronic information. As for the widely used H-type steel and symmetric steel piping, the difficulty in process of guided-wave damage monitoring is increased because of the following factors: There is geometrical non-linearity at the joints of H-type steel, and guided waves from web plate and flange fuse with each other. There are two kinds of guided wave in axial symmetry steel piping: axially and circumferential guided wave. Theoretical analysis, finite-element simulation and experiments were used to research the transmission mechanism of guided wave. In addition, the damage location was verified by experimental data considering the errors. The major research and innovative conclusions are as follows:1. A novel model for structure damage and transmission of guided wave: Firstly, the dispersion equation and transmission characteristics under different models of guided wave were analyzed for ideal plates, hollow cylinders, including Lamb- and SH- waves, and longitudinal-mode guided wave, torsional-mode guided wave, bending-mode guided wave, circumferential Lamb wave and circumferential SH wave of steel pipe. The dispersion curve was then plotted by using simulation, and the choice criterion of excitation signal was analyzed. Finally, structure damage model and PZT model were proposed with considering the finite element simulation of guided wave transmission, which provided theoretical basis of structure monitoring of H-type steel and pipeline.2. Research on the transmission mechanisms of guided wave in H-type steel and damage location analysis: Basing on the dispersion equation of guided wave for ideal plate structure, guided wave mode, time resolution, amplitude of modes and structure of wave were analyzed to provide theoretical basis for damage identification & location and choice criterion of excitation signal of H-type steel. Three-dimensional finite element model of H-type steel was proposed to analyze transmission mechanism of guided wave, to analyze the influences of guided wave fusion from web plate and flange and boundary reflection, and to compare two-point symmetrical excitation and whole cycle excitation. According to the simulated results of finite element model, a imaging method for damage location basing on relative-distance probability density was proposed by using the whole cycle excitation, and was verified by experiments. Finally, the influence factors of location error were analyzed.3. Research on the transmission mechanisms of guided wave in pipeline and damage location analysis: The frequency of excitation signal was chosen by analyzing transmission characteristics of hollow-cylinder structure under different mode and considering the factors of guided wave mode, cut-off region, structure of wave and group velocity. Three-dimensional finite element model of pipeline was proposed to analyze transmission mechanism of guided wave, to analyze the influences of axial symmetry. Single-point excitation and multi-point reception were adopted to simulate, and the effective of FE model was verified by experiments. Damage trajectory of sensing path was confirmed according to flight distance of response wave. The relative distance of corresponding trajectory was used to construct damage probability density function, then realize the damage location image. Finally, the influence factors of location error were analyzed.