Abstract

Cracking of cylindrical shafts is an important area for research, since the changes observed in their vibration characteristics even during large-sized cracking are much smaller than those observed for rectangular beams; hence early identification of crack existence becomes essential to prevent sudden failures in rotating shafts. In this paper experimental and numerical investigations are carried out to identify the presence of a crack in a cylindrical overhanging shaft with a propeller at the free end. In the experimental study, cracks of different depths are located at the (un-cracked) maximum bending moment position. Shaft response parameters for lateral (using an accelerometer) and torsional vibrations (using shear strain gages fixed at three different locations) are obtained using the modal analysis software, LMS Test Lab^TM. The experimental results are used to validate the numerical results obtained using the three-dimensional isoperimetric elements available in the ANSYS FEM program; the open crack is embedded in the shaft and the mesh generation is suitably modified to incorporate the stress intensity effects present at the crack tip. The results indicate that the use of the rate of change of frequencies, modal amplitudes (of displacements, velocities and accelerations) as a function of crack depth ratio will indicate the presence of crack in the shaft from a crack depth ratio of 0.2. Also the use of the rate of change of torsional frequency will indicate the presence of a crack in the shaft from the initiation of the crack. The approach indicated in this paper will provide a sound and robust procedure for a first level of damage assessment by using vibration techniques.

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