Health monitoring of rotating structures: identification of cracks
Nicolò Bachschmid1; Aldemir Ap. Cavalini Jr2; Valder Steffen Jr2
1 Politecnico di Milano; 2 LMEst, Federal University of Uberlandia, School of Mechanical Engineering
doi:10.20906/CPS/CILAMCE2015-0835
Resumo
Aim of the present paper is to combine the crack propagation laws stated by fracture mechanics with the model of crack induced vibrations in rotating shafts, for simulating the evolution of the vibrations of a cracked shaft. The model of the shaft of an industrial turbo-generator unit is used for simulating numerically the evolution of the vibrations, which could be measured in correspondence of the bearings of the machine. Different scenarios are presented, depending on the position the crack and on the operating conditions of the unit. The crack is assumed to develop in the high pressure steam turbine, where grooves and thermal stresses may help the development and propagation of cracks. Once the presence of a crack has been detected, it is possible to define its position and depth, with a model based identification procedure, and to forecast both the crack propagation speed and the associated vibration evolution. Residual life can be estimated and propagation can be controlled by vibration monitoring. It is well known from field experience that vibration changes in cracked rotors develop very slowly as long as crack is not very deep, during its first stage of propagation. When the depth or extension of the propagating crack become large and reduce the resisting section to 50% or less, then the vibrations induced by the crack start to increase rapidly, with a trend which is almost exponential. This has allowed in many cases to recognize the presence of the crack in rotating industrial machinery and to stop the machine avoiding catastrophic failures. The fracture mechanics approach allows modelling the propagation velocities in radial and circumferential direction. Once a crack with a certain shape has developed in a shaft, the crack excited vibrations can be calculated using the model of the breathing mechanism, the model of the cracked beam element and the model of the cracked shaft dynamical behaviour. These models are described in detail. Moreover with these models, the vibrations in correspondence of t
Palavras-chave: transverse cracks; rotating structure; identification; vibrations; residual life estimation