Crack detection by modal analysis in 3D beams based on FEM

During the past decade, the development of Structural Health Monitoring (SHM) systems has gained considerable interest in several engineering fields. The possibility of monitoring structure status under various operational conditions has resulted in a drastic change in the maintenance schemes. The introduction of SHM systems would allow a transition of the maintenance strategy from scheduled basis (i.e. Time Based) to a condition basis. The development of SHM systems relies on the availability of reliable techniques to acquire the characteristic features of damage through experimental measurements. In this paper, the flexural vibration of a cantilever beam, fixed-roller beam, and portal frame with a transverse surface crack is considered. The natural frequencies and mode shapes are computed by the Finite Element Method (FEM) using SAP2000 Software for various crack locations and depths. The computed mode shapes are animated to give the designers a good picture on the failure modes and shapes. The modal frequencies are experimentally measured and compared with the computed values and showed good compatibility. The sensitivity of the modal frequencies to a crack increases when the crack is near the fixed end, and decreases as the crack moves towards the free end of the beam or frame. The present investigations have revealed the following generalized results. The presence of crack in structure leads to an appreciable difference in the dynamic response. The findings based on condition monitoring technique can be utilized in various industrial applications, particularly for fault detection on structures. The proposed technique represents actually a non-destructive procedure having great benefits for health monitoring of structures.