Impedance-based fault detection methodology for rotating machines

Abstract

Visual examination, ultrasonic tests, and dye penetrant inspection are some examples of nondestructive techniques widely used for crack detection in rotors. These methods have proved to be costly, since satisfactory results rely on detailed and periodic inspections. Significant research effort has been directed in recent years to online monitoring techniques, that is, based on vibration signals measured during rotor operation. However, most of them are able to only detect deep cracks. The uniqueness of this article relies on the possibility of detection of incipient transverse cracks in rotating shafts using the so-called, electromechanical impedance method. This method has become a promising tool for structural health monitoring of systems due to its sensitivity to small local damage. Basically, the method monitors changes in the electric impedance of piezoelectric transducers, bonded to (or embedded into) the host structure, through specific mathematic functions, the so-called damage metrics, to detect damage. This is possible because the electrical impedance of the transducer is directly related to the mechanical impedance of the structure. In this context, successful experimental tests were performed in a horizontal rotor supported by roller bearings. Lead zirconate titanate (PZT) patches were bonded along the shaft of the rotor in which saw cuts approximating a breathing transverse crack were machined. The technique was validated under different rotating speeds and unbalance conditions.