Detecção e identificação de trincas transversais incipientes em eixos horizontais flexíveis de máquinas rotativas

Abstract

This Thesis proposes two Structural Health Monitoring (SHM) techniques concerning the detection and identification of incipient transverse cracks in rotating horizontal flexible shafts. Numerical and experimental applications are presented, dedicated to a specially designed test rig. The first SHM technique encompasses both numerical and experimental investigations. This technique is based on the nonlinear behavior of the cracked rotor. Diagnostic forces are used in conjunction with an evolutionary optimization method (Differential Evolution) in order to characterize the crack signatures in the spectral responses of the rotor (the so-called diagnostic peaks). The method of Multiple Scales determines the conditions required to induce the system to a combination resonance. This nonlinear SHM technique requires accurately mathematical models for the rotor and the crack. Therefore, a reliable Finite Element model to represent the dynamic behavior of the rotating machine is required. This model is obtained from various subsystems (shaft, couplings, discs, bearings and the gyroscopic effect). Once the defined subsystems are assembled, unknown parameters can be identified as based on experimental results. Concerning the crack model, three methods available in the literature to represent the breathing mechanism (typical characteristic of crack behavior found in horizontal flexible rotors) are analyzed, namely the Gasch, Mayes, and FLEX models. Additionally, the Linear Fracture Mechanics concepts used to determine the relationship between the shaft flexibility and the crack depth are described. The formulation used to adapt the models of Gasch and Mayes to the Finite Element method is also presented. Finally, the dynamic behavior of the cracked rotor under the three breathing models is analyzed for comparison purposes. The second SHM technique is a novel contribution, regarding rotordynamic applications. The detection of incipient transverse cracks is performed by the Electromechanical Impedance method, which is an eminent experimental approach (it does not require mathematical models). This type of application is accompanied by additional difficulties as characterizes by the dynamic excitation of the machine. Also, the socalled damage metrics are presented aiming at quantifying the severity of damage. The impedance- based technique is applied to the rotor system for three damage conditions, as follows: rotor at rest, rotor at a given rotating speed, and rotor under different unbalance excitation. Considering that the detection of incipient cracks through SHM techniques is a challenging problem these days in the context of rotating machinery, the obtained satisfactory results represent an important step to the state-of-the-art.