Abstract

In this study, experimental and analytical investigations are carried out on an overhanging cylindrical shaft carrying a propeller at the cantilever end, in order to identify the crack existence in shafts using the mechanical impedance approach. Also the experimental study uses the modal analysis software, LMS Test Lab^TM, for measuring and analyzing the response results from un-cracked and cracked shafts. In the numerical study, both the un-cracked and the cracked shafts (with varying crack depths) are modeled by finite element procedure. 3-D iso-parametric elements (element types 186 and 187), available in the ANSYS FEM program, are utilized to model the system. The impedance and velocity frequency response functions are used to identify the crack depth in the shaft system. Impedance and mobility were measured and simulated in the vertical direction for the resonant frequencies and anti-resonant frequencies. The experimental results are used to validate the numerical results. A better crack detection procedure was obtained by the plot of the slope of the non-dimensional frequency ratio (in the resonant and anti-resonant regimes) vs. the non-dimensional crack depth ratio. By this method crack presence could be definite detected when the non-dimensional crack ratio is greater than 0.20 to 0.25. In addition monitoring of the lower torsional frequency indicated the crack presence even from the beginning stages.

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