Monitoring of Fatigue Crack Stages in a High Carbon Steel Rotating Shaft Using Vibration

This paper describes experimental tests to examine the vibration characteristics in two different types of cracks in a long rotor shaft, a notch cut to varying depths and actual crack growth from a pre-crack. The approach was to set up experimental apparatus, develop a vibration detection system, and maximize the dynamic range. Fatigue crack initiation and propagation in a pre-cracked high carbon steel shaft was experimentally evaluated and monitored using a vibration based condition health monitoring method. The identification of the difference between three different pre-crack depths and five different runs after the crack's growth show shifts in the frequency spectrum and modifications to the peaks’ amplitudes. The results of the tests and analysis clearly demonstrate the feasibility of using vibration to detect the change in frequency of a shaft due to a change in stiffness such as those associated with a shaft crack. Finally, investigation was carried out on the fracture surface in order to show the fatigue crack depth inside the shaft. The results show the fatigue crack fracture surface along the notch depth. A section of the crack surface is presented in close up showing the fatigue crack beach marks, and evidence that the fatigue propagated along the artificial notch internal edges.