Crack Detection in a Rotor Dynamic System by Vibration Monitoring—Part II: Extended Analysis and Experimental Results

An increase in the power-to-weight ratio demand on rotordynamic systems causes increased susceptibility to transverse fatigue cracking of the shaft. The ability to detect cracks at an early stage of progression is imperative for minimizing off-line repair time and cost. The vibration monitoring system initially proposed in Part I is employed herein, using the 2X harmonic response component of the rotor tilt as a signature indicating a transverse shaft crack. In addition, the analytic work presented in Part I is expanded to include a new notch crack model to better approximate experimental results. To effectively capture the 2X response, the crack model must include the local nature of the crack, the depth of the crack, and the stiffness asymmetry inducing the gravity-forced 2X harmonic response. The transfer matrix technique is well suited to incorporate these crack attributes due to its modular nature. Two transfer matrix models are proposed to predict the 2X harmonic response. The first model applies local crack flexibility coefficients determined using the strain energy release rate, while the second incorporates the crack as a rectangular notch to emulate a manufactured crack used in the experiments. Analytic results are compared to experimental measurement of the rotor tilt gleaned from an overhung rotor test rig originally designed to monitor seal face dynamics. The test rig is discussed, and experimental angular response orbits and 2X harmonic amplitudes of the rotor tilt are provided for shafts containing manufactured cracks of depths between 0% and 40%. Feasibility of simultaneous multiple-fault detection of transverse shaft cracks and seal face contact is discussed.