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Dynamic Behavior of Rotor Systems with a Slant Crack on the Shaft Reza Ramezanpour K Reza ramezanpour k@me iut ac ir 7th March 2011 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi M Ghayour Assoc Prof Supervisor ghayour@cc iut ac ir Abstract An ever increasing quest for more power has led to severely stressed rotors particularly in power generation sector With several locations where stresses both mechanical and thermal can increase despite best design practices fatigue cracks are always a possibility in such rotors If undetected early such cracks can pose a potential source of catastrophic failures Modern day rotors are designed and for achieving high rotation speed Due to noticeable mass of such systems they have considerable energy It is obvious that anything that causes sudden releasing of this energy leads to a catastrophic failure in such systems Fatigue cracks are a potential source of catastrophic failures in rotors Researchers have put in considerable effort to develop a foolproof and reliable strategy to detect cracks in rotors In this research the dynamic behavior of a rotor system with a slant crack on the shaft is investigated A Jeffcott rotor with a slant crack under arbitrary crack orientation angles on the midpoint of its shaft is considered This solution relates to three crack model open crack close crack and breathing one Breathing phenomenon is explained in detail some of the existent models that are used in literature are discussed Equations of the motion are obtained in four directions i e two transversal directions one torsional direction and one longitudinal direction The effects of gravity and external torsional excitation are also taken in to account By considering the effects of crack on the stiffness matrix only nonlinear equations of the system are solved using numerical method Also effects of crack depth and the amplitude of external torsional excitation on the spectrum of the responses are investigated In this research the elements of compliance matrix for different crack orientations are calculated The influence of crack orientations on the flexibility coefficients and steady response of the system is also investigated It is shown that some of the flexibility coefficients take their maximum values for approximately 60o crack orientation unexpectedly It is concluded that any increase in the amount of crack angle from 30o to 90o transverse crack does not increase or decrease all of the flexibility coefficients monotonously Spectrums of the responses indicate same characteristics Although the elements of compliance matrix for slant cracks have been already reported in literature some corrections in to the matrix elements were introduced in this thesis For the first time crack compliance matrix for crack orientations more than 90o is calculated Based on the changes in these elements that are in third row and due to symmetry third column of the crack compliance matrix a novel way for crack detection are introduced and developed It is shown that a slant cracked shaft under two directions of rotation has different compliance matrix therefor it can be used as a criteria for crack detection in real rotor systems This simple practical and effective method can be used for any rotating systems especially rotors Improving one of the available crack breathing models CCLP is another innovation that is introduced in this thesis The introduced model ICCLP represents crack breathing behavior more accurately Key Words Rotor Dynamics Slant crack compliance matrix crack detection Breathing Model

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