Detection And Location Of Damage For Plate-like Structures Based On Active And Passive Wave Theory

As bearing carriers, plate-like structures have got their momentum in kinds of engineering. Under the effect of long-term internal and external causes, localized corrosion and fatigue cracks of the material will accelerate the degeneration of the structure’s overall performance, which will lead to the accumulation of defect damage and even engineering accident. Therefore, the detecting, monitoring, location of damage, the evaluation of damage degree, the forecast of residual life and the repairment of damage for plate-like structures are of vital importance in ensuring the safe operation of engineering structure.Making accurate judgement the existence of damage and the location of damage is primary task in structural health monitoring, which will provide evidence for later decision. The paper adopt real-time structural health monitoring technique to monitor the plate-like structure and realize the location image of flaw detection. structural health monitoring techniques are divided into active monitor techniques and passive monitor techniques, passive monitor techniques such as acoustic emission can monitor the evolutionary process of plate-like structures in work, while active monitor techniques that on the basis of Lamb waves can detect the location of static damage utilizing scattered signal. To conduct comprehensive damage assessment for plate-like structures, the thesis combined passive acoustic emission and passive Lamb waves monitor techniques to conduct the location image research of damage for plate-like structures. The thesis first summarized the theory and research progress of the two techniques, and set forth the feasibility analysis of combining the active and pasive monitoring techniques to monitor the plate-like structures, and then present the contents, strategy and outline of the research.All the active and passive monitor techniques are on the basis of wave theory, the damage information in the plate-like structures is spread in the form of Lamb waves, so the analysis of Lamb waves characteristic is the basic of plate-like structure monitoring. The thesis started with the elastic mechanics theory, processed the formula derivation of Lamb waves’spread characteristic in the free slab and drew the frequency dispersion curve of Lamb waves by numerical method, all of which provided theoretical basis for the research. The location of structure’s damage must rely on location algorithm. After analyzing the merits and demerits of the conventional location algorithms, the thesis provided the beamforming algorithm that on the basis of sensor’s network array to realize the location image of damage for plate-like structures, and made optimization and improvement for damage location by Hilbert transform and probability of diagnostic imaging technology.To verify the validity of beamforming algorithm for the active and passive location image of plate-like structures, the thesis combined the finite element simulation and field test to realize the location image for static damage of plate-like structures on the basis of interaction between damage and AE as well as Lamb waves, and discussed the influence of relevant factor to the effect of location image by control variate method. The research indicated that beamforming algorithm realized the accurate location and visual display of damage in the form of array output, and the probability of diagnostic imaging technology could optimize the location result more reasonable. The research got rid of the dependence on the speed of waves by developing wave speed scanner. Also, the thesis expanded the application object to large diameter thin-walled structure and multiple damage location, enlarged the applicability further. In order to get rid of dependence on the health reference signal of structures, this paper attempted to willful intercept characteristic damage signals by the window function, achieved the damage localization imaging of plate-like structures, which improved algorithm application on damage assessment of structures without baseline.