Sensing inaccessible damage in a structure using frequency response functions

The use of Frequency Response Functions (FRFs) measured at some points in the accessible area of a structure to detect damage in the inaccessible area is studied, based on the difference in the normalized FRFs from a previous observed state. Numerical simulations and experiments were carried out to investigate the effectiveness of the proposed damage index. Results showed that from the indices at the first flexural resonance mode and in the vicinity of the first resonant frequency, the damage can be accurately detected and located if there is damage in the accessible area. If the damage is in the inaccessible area, the existence of the damage can be successfully ascertained although its location may not be identified accurately. For multi-damage conditions, with damage in both the accessible and inaccessible areas, the proposed method can accurately detect and locate the damage in the measurable area and detect the damage in the inaccessible area. The magnitude of the indices provides useful information with regards to the damage severity.