Cracked Cross Section Measurement in Rotating Machinery

Abstract

The assessment of the operating condition of machines and complicated structures is often based on the analysis of the vibration behaviour of the structure. Refering to the trouble free system, differences between the structural answers and the known reference signature are used as an indicator for a defect in the system. In rotor dynamics information from vibration monitoring can usually be used but must be analysed with respect to the type of trouble. The case of a cracked rotor has been reported by Mayes and Davies [1], Grabowski [2], Mahrenholtz [3] and other authors Mahrenholtz [3] has given a comprehensive survey in the field of rotor dynamics and especially on the calculation of cracked rotors. It is the common aim of all the investigations to establish a significant and — if possible — unique relation of the crack to the vibration behaviour so that one can conclude the crack configuration from measurements and vice versa. In case of rotors in operation it is known from experience where cracks appear, for instance at the root of the blade discs in turbo machinery or at the notches of generator rotors in electric power plants etc. The published investigations often deal with modelling a cracked rotor for calculation purposes in order to find out the rate of change of the transverse crack growth. Mayes and Davies [4] showed this for real rotors in a power plant, Grabowski and Poppel [5] used a laboratory experiment to measure the crack growth. Fig. 1 shows, for example, measuring results and calculations of the Grabowski rotor, that are well known from literature [5].