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 LabTM. 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.
Highlights
The appearance of transverse cracks in overhanging shafts having propellers carries with it a greater risk of sudden collapse
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; early identification of crack existence becomes essential to prevent sudden failures in rotating shafts
From the physical morphology of a cracked rotor, cracks can be classified based on their geometries as follows: (i) transverse cracks that are perpendicular to the shaft axis; (ii) cracks parallel to the shaft axis known as longitudinal cracks; (iii) slants cracks that are at an angle to the shaft axis; (iv) open and close cracks when the affected part of the material is subjected to tensile stresses and stress www.ccsenet.org/mer
Summary
The appearance of transverse cracks in overhanging shafts having propellers carries with it a greater risk of sudden collapse. In the last four decades, many numerical and experimental studies have been carried out to identify the effects of different type of cracks, such as transverse, longitudinal, slant, breathing cracks and notches. In these studies the researchers have used different methods to identify crack presence in structures, viz., (i) Traditional vibration-based methods using modal testing and numerical analysis; (ii) Non-traditional methods based on ultrasonic guided waves, magnetic induction, radiofrequency identification tag, acoustic intensity and acoustic Laser-Doppler vibrometry (Sabnavis, Gordon, Kasarda, & Quinn, 2004); and (iii) Numerical procedures using FEM in conjunction with modal analysis, wavelet transforms, neural net works, genetic algorithms and fuzzy set theory. A number of studies have been carried out on the above crack types (Sabnavis, Gordon, Kasarda, & Quinn, 2004)
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