Using Roving Disc and Natural Frequency Curves for Crack Detection in Rotors

In this work, a simple crack identification and localisation approach, which is based on a finite element model of a cracked rotor, is proposed. The model simulates the behaviour of a cracked rotor to provide data to be used to identify the location and depth ratios of a crack in stationary rotor systems. The cracked region, which is an open crack type, is modelled as time-varying stiffness. The rotor is considered as a simply-supported beam carrying a roving disc as an auxiliary mass which transverses along the shaft from one end to the other. A new technique, which is called frequency curve product (FCP) method, is presented to identify and locate a crack clearly in rotor systems. The proposed technique is based on the normalised natural frequency curves (NNFCs) of cracked and intact rotors using the principle of roving masses and natural frequency curves. This technique is developed in order to firstly, solve the disappearance of a crack effect from NNFCs when the crack is close to or exactly at a node of a mode shape. Secondly, FCP curves combine the first four NNFCs in a single plot to identify the crack location clearly. Different pairs of the normalised natural frequencies of different modes of vibration are multiplied together in order to enhance the identification and location of cracks. It is shown that this technique identifies the exact crack location through unifying all the first four natural frequency curves at the maximum positive value in the plot.