A Method Of Mechanical Structure Local Damage Diagnosis Based On 3D Simulation And Modal Parameter Minitoring

With the progress of science and technology, modern industrial development, as well as the growing demand for human life, modern machinery and equipment and the structure is developing towards large scale, complication and automation.Mechanical structure will be partially damaged during working process, which is caused by long-term load operation or other unexpected external factors. When the damage accumulates to a certain extent, the overall or local structure will lead to a sudden failure, which will cause emergent stop of equipments. This will bring large economic losses and even cause disastrous accidents. There are a lot of instances in the world. Structural damage detection and early warning is helpful to eliminating hidden dangers and avoiding catastrophic incidents. Therefore, research on structural damage monitoring and diagnosis is of great significance.Structural damage detection and diagnosis technique based on vibration can be used to conduct online damage (especially the early damage) and quantitative detection and diagnosis. With this technique, safe operation of in-service equipment and structure can be ensured, and reliable information can be provided for predictive maintenance. Thus, this technology is developing rapidly in recent years. Among them, model-based approach can be used to find the intrinsic changes of the structural dynamic characteristics real-timely and sensitively. Using this approach, it is possible to achieve quantitative assessment of damage. Structural damage detection and diagnosis technique based on model-based has attracted widespread attention by domestic and foreign researchers.The main technology of structural the damage detection is the diagnosis method based on structural dynamic characteristics parameters, including modal frequency, modal shape, modal curvature, strain mode, transfer function, MAC(Modal Assurance Criterion), COMAC(Coordinate Modal Assurance Criterion) and so on.In this paper, quantitative damage detection and diagnosis method based on modal parameters identification is presented, and problems aroused by the key techniques to apply this method on complicated mechanical structure are solved. The proposed method has following characteristics:prediction of the easily damaged local sections and key positions, analysis on relationship between structure damage and dynamic response features and less use of sensors. With these characteristics, the feasibility to apply this method on complex engineering structures is improved.A constrained Bayesian estimation method is proposed to update finite element model. Using this method, the finite element model of the multi-segment gantry is established. According to this model, the dynamic responses excited by work load and structure damage sensitive parameters are analyzed. On this basis, the easily damaged positions are defined, which provides foundation for dynamic analysis of the damaged structure.The modal frequencies and modal shapes changes of the damaged structure are theoretically studied and feasibility to monitor structure damage according to modal frequency changes is discussed. For a gantry, with finite element method, modal frequency changes are calculated under the condition of the fatigue crack and the flange bolts loose. To verify the calculated results, modal experiments are carried out. By comparison, the experimental results and the calculated results are consistent.The substructure's FRFs(Frequency Response Functions) are deduced. And for several composite structures with special connection ways, the substructure's FRFs are discussed. The results show that the impact on the structural dynamics characteristics or FRFs by substructure damage or stiffness change is complex and depends on connection condition of structures. For rigid connection structure, the poles of FRFs of composite structure are the same as the substructures'. The poles of the substructure FRFs can reflect the modal frequencies of composite structure.The inconsistent identification results using two modal parameter identification methods are analyzed. On this basis, the structure damage identification algorithm is improved. A new method to build the Fisher information matrix is developed, which use cross-power spectrum amplitudes of the modal of damage characteristics as its elements. The sensor layout is optimized through gradual elimination of sensor. Thus the number of sensor can be eliminated without effecting the damage detection of critical part. At last, the experiments are conducted, and the results show that the proposed technology is correct.