STUDY OF THE BREATHING MECHANISM OF AN ELLIPTICAL CRACK IN A ROTATING SHAFT WITH AN ECCENTRIC MASS

Abstract

Rotative machinery (blades, rotors, compressors,..) are used quite often in several industries such as aerospace, nuclear, oil, transportation and industry processes in general. Operation failures of this kind of machinery are related with the presence and growing of fatigue cracks in one of the main components: the shafts. The shafts, that perform in bend- ing and torsion, present additionally misalignments or/and unbalances that alter the normal function of the components. The unbalance in a rotating system can be produce by a eccen- tric mass. When a cracked shaft rotates, the crack opens and closes in what is called the breathing mechanism in which the crack passes from an open state to a close state with a transition between both situations producing a partial opening or closing of the crack. On the other hand it is very frequent to consider cracks with straight fronts, but the experience shows that most of the cracks that appear in rotating shafts are elliptical. In this paper we present the numerical model (Finite Element Method) of the quasi-static behaviour of an unbalance cracked shaft (with elliptical front) considering an eccentric mass. The model chosen for this study is the classical Jeffcott rotor. To simulate the rotation of the shaft, different angular positions have been considered. The influence of the mass eccentricity in the opening of the crack has been studied considering different angles of eccentricity measured from the position of the crack. In this paper we present the comparison of the partially opening/closing of the crack considering different angles of rotation and different positions of the eccentricity. The work allows to know the influence of the unbalance of rotating shafts in the crack breathing mechanism and will allow to predict the influence of this behavior on the values of the Stress Intensity Factor and in the propagation of cracks.