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

Abstract

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-Jine 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 asymmetiy 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 atigular response orbits and 2X harmonic amplitudes of the rotor tilt are provided for shafis 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.

Rotordynamic Crack Diagnosis: Distinguishing Crack Depth and Location

“…The results presented herein are based on the following parameters, obtained from the test rig discussed by Vamey and Green [2]. The 10.2 mm diameter shaft of length 88.9 mm is composed of AISI 4140 steel.…”

“…Fig. In general, the linear defiection M, caused by the internal bending moment induced by load F i is [2]. The rotating frame always maintains its orientation relative to the notch/crack edge.…”

“…Vamey and Green [2] developed a transfer matrix for a finitewidth rectangular notch that emulates an experimentally manufactured crack. The steady-state 2X angular response of the overhung rotor was subsequently obtained.…”

“…An overhung FMR noncontacting mechanical face seal test rig was subsequently constructed to experimentally investigate the face seal dynamics [3] (the rotordynamic model used in this work emulates this overhung test rig). [2] advanced the aforementioned research by studying the feasibility of using the overhung test rig to detect a transverse shaft crack. The test rig parameters and data acquisition system are discussed extensively by Lee and Green [8].…”

“…Three eddy-current proximity probes mounted to the stator directly extracted the rotor's angular response. A gaping crack is introduced, and the rotor's response is obtained analytically [1,2] and experimentally [2] (the particular models employed will be discussed shortly). During testing, a unique vibratory phenomenon was observed: higher harmonic oscillations in the steady-state rotor response.…”