Keywords: structural health monitoring, damage detection, localized damage, frequency based method, beam, modal analysis, finite element method.

Detecting damage in real structures, as early as possible, before it reaches a critical phase in its growth constitutes an essential task. Spectacular failures, such as those that occur during in-flight, have focused attention on the need of structural health monitoring to ensure safety. A number of important applications in this field comprise the essential facilities of the offshore oil industry and the high technology space shuttles.

Structural damage detection can be performed by various non-destructive testing techniques including optical, acoustic emission, ultrasonic, radiography, eddy-current, thermal or electromagnetic wave based methods. All of these require that the damage location is already known and that the part of the structure where it is located is accessible. In addition, these techniques are not effective all the time to perform regular serviceability tasks especially for complex structures where some parts are inaccessible, furthermore they are reputed to be time-consuming and costly.

Vibration based methods seem to be more adequate as they are low cost and capable of monitoring damage detection where the conventional techniques are expected to not perform well. Vibration based approaches are initiated from the changes that could occur on the commonly measured modal parameters consisting of frequencies, mode shapes and damping. Detectable changes in these modal properties can be produced when significant changes of the physical properties of the structure happen such as reduction in stiffness, arising from the appearance of cracks or as a reduction in mass such as those resulting from oxidation.

In this paper, the focus is mainly to determine if damage has occurred in the structure and the determination of its geometric location. The methodology used is based on modal frequency changes. The main questions for which answers are investigated concern the number of frequencies to be measured and the detectability of damage that was not included as a possible scenario. A highly accurate finite element model of the structure is built, initially in order to compute the frequencies of the first modes for both the undamaged structure and all the postulated damage scenarios. For the candidate structure, the frequency shifts are calculated and compared to those of the damage scenarios. The likelihood of damage is based on an error damage index.

The results obtained have shown that the proposed procedure works well if the actual damage magnitude and extent were assumed as possible damage scenarios during the derivation of a frequency shift database. Attention should be however given to the correct number of frequencies to be measured. The method presented continues to work approximately for some damage characteristics that are close to the considered damage patterns indicating only their overall location. The proposed method can be useful if one is able to enrich the damage scenarios by including those which are regularly diagnosed in the beam systems considered.

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