This paper focuses on using the modal analysis technique as a non-destructive test to detect for fatigue cracks in simple shaft-bearing systems. The method used is based upon the fact that the dynamic characteristics of a structure (such as the modal frequencies) change with the introduction of a crack. In the paper, the analytical development and computer implementation were considered as the first phase of the project on nondestructive testing technique using modal analysis. The second phase will concentrate on the experimental verification of the analytical model. The inverse problem of estimating the crack size, location, and possibly geometry from the knowledge of the changes in the modal frequencies is discussed. The crack in the shaft is modeled as a torsional spring element because the crack introduces a discontinuity in the slope curve of the shaft. An analytical method and a finite element method were used to determine the modal frequencies variations for various crack sizes and locations on the shaft.
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