In this paper the requisite foundational numerical and experimental investigations that are carried out, to model the ââ?¬Å?uncracked\r\nand crackedââ?¬Â shaft and to identify its bending and torsional vibration responses, are reported. The cylindrical shaft used in this experimental\r\nstudy is continuous over two spans (with a cantilever span carrying a propeller) with ball-bearing supports. During\r\nmodal tests the backward end of shaft (away from the propeller end and connecting it to an electric motor, required for online\r\nmonitoring) is fixed to one of the test frame supports; later on this backward end will be connected to an electric motor to carry out\r\nonline modal monitoring for crack identification. In the numerical study, beam elements are used for modeling the bending and\r\ntorsional vibrations of the rotating shaft. The paper describes in detail the numerical ââ?¬Å?linear springââ?¬Â models developed for representing\r\nthe effects of ââ?¬Å?ball bearings and the (experimental test) frame supportsââ?¬Â on the vibration frequencies. Shaft response\r\nparameters are obtained using modal analysis software, LMS Test Lab, for bending vibrations monitored using accelerometers, and\r\nthree ââ?¬Å?setsââ?¬Â of shear strain gages fixed at three different shaft locations measure the torsional vibrations. Effects of different crack\r\ndepths on bending and torsional frequencies and mode shapes are investigated experimentally and numerically, and the results interpreted\r\nto give better comprehension of its vibratory behavior.
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