A New Method For Two-Stage Structural Damage Identification

Structural damage detection is a promising field with widespread application in civil engineering. Damage in structures causes deviations of geometric or material properties from nominal or baseline values. As a result, some changes in natural frequencies and mode shapes can be detected using experimental modal analysis, which has recently enjoyed intense research for application in structural damage detection. For this purpose, a new method of two-stage structural damage identification is put forward in this paper, which detects structural damage by analyzing the modal information and mainly consists of two stages. Because the structural mode before damage usually remains unknown, in both these stages, a finite element model is set up for the undamaged structure. And it is called the "theory model" which must be updated using the measured information from a real structure in order to acquire correct results. In the first stage, damaged segments of structures can be located and, to some extent, quantified with the help of a parameter named Curvature Mode Index (CMI). CMI is a parameter derived from the eigenvectors, which is sensitive to local damage and dependent upon geometric coordinate of the structures. These characters enable it a perfect index for damage location. But the index, even after some improvements, can merely provide a relative estimation of the damage severity comparing with the theory models. In the second stage, the damage severity of those located segments can be precisely estimated by calculating the Modal Moment Index (MMI) of each element. It is assumed that the modal strain energy differences between damaged and undamaged modes are negligible. Then the equilibrium established, and naturally, the elastic modulus of damage structures can be easily deduced. By calculating the deviation of the elastic modular before and after damage, the damage severity is estimated. Later the numerical analysis of a simple beam is carried out to test the method. It proves that the new method can not only correctly locates damaged segments but also offers practical estimates as to the damage severity. And it can be applied to many types of structures such as beam, frame, truss, and plate.