Two-stage Structural Damage Identification Scheme Based On Complete Modal Space

It has been proved to be more convenient and effective to construct damage indicator to identify structural damage by making use of modal parameters, and many researches and applications have been put forward. Generally, the damage identification of structural and mechanical systems mainly contains three levels: the first level is to detect the presence of the damage; the second level is to indicate the location of the damage; the third level is to determine the severity of the damage (the further level is to predicate the occurrence of the damage). The modal -parameter-based damage detection indices can be classified to be several categories according to the constructive style, and different categories have different identification levels. There are many factors that affect the performance of such damage indices, mainly including modelling errors, modal truncation errors and measurement noise. Also, different damage indices have different sensitivity to these factors, and each damage indicator has its own application condition. From the existing researches involved to the modal -parameter-based damage indices, it is hard to find an indicator capable of detecting damage sensitively under effect of noise.From the point of constructing and applying of the damage indices, the author makes an overall analysis and review on the existing damage indices firstly. And then, some typical damage indices including Modal Flexibility Curvature (MFC), Uniform Load Surface Curvature (ULSC), Elemental Modal Strain Energy Change Ratio (EMSECR), Damage Variable Indicator (D) and Elemental Stiffness Reduction Factor (ESRF) are selected to compare their performances with each other's via a simulated study of simple supported beam. And especially, the effect of modal truncation and measurement noise to these damage indices is studied .The comparative results reveal that some damage indices such as MFC and ULSC are sensitive to the damage extent and need a few lower modes, but they are more sensitive to the noise; other damage indices such as EMSECR and ESRF have a good performance under effect of noise, but they need more modes at the same time, so the effect of modal truncation is more evident. Comparatively, only the improved damage index of D can localize the damage with the first few modes under the effect of noise.In view of the property of ESRF that can localize and quantify the damage in the mean time and the property of the Practical Complete Modal Space Theory (PCMST) that can construct the equivalent higher mode shapes through the lower mode shapes, the author combines the PCMST with the index of ESRF in terms of its own construction characters to reduce the effect of modal truncation. Besides, the author also manages to reduce the scale of the damage equations of ESRF by predetermining the rough location of the damaged elements to improve the stability of solution. The results also prove that the performance of ESRF can be improved significantly by adopting above mentioned measures. Consequently, a two-stage damage identification scheme is proposed by the author. This scheme utilizing the improved index of ESRF together with the improved index of D includes two identification steps: In the first step, the proposed scheme locates damage by the improved index of D; in the second step, a complete modal space is constructed with only a few lower modes and used to quantify damage with the improved index of ESRF. Then a step-by-step procedure of the proposed scheme in practical application is suggested.A few simulations including a simple beam, a continuous beam, a simple supported truss, a cantilever truss and two frame structures with damage scenario are selected respectively as case studies to compare the performance of the ESRF and the proposed scheme. The effect of the structural type, noise level and damage extent to the proposed scheme is investigated. The results show that the proposed scheme is better than the ESRF when only five lower modes are utilized, and it can both localize and quantify damage for various structures at a certain damage extent, noise level and error range. Through all of the investigations, a new thought about the modal-parameter-based damage identification can be obtained from this thesis.