Keywords: crack, shaft, breathing mechanism, eccentricity, rotative machinery.

The machine failures frequently occur as a result of the presence and propagation of fatigue cracks arising from the loads and forces they carry. Those failures sometimes are catastrophic and produce personal injuries or economic problems. The increasing importance of safety and costs derived from failure in machinery has pushed researchers in the field of damage detection to analyse the behaviour of mechanical cracked components.

Shafts, that are one of the main components of machines, perform in rotation and in bending and torsion. All of them, together, can produce the shaft failure by generation and propagation of fatigue cracks. Sometimes the shafts perform with an additional problem which is misalignment or theunbalance. This unbalance can be caused by the presence of eccentric masses.

When a cracked shaft rotates the crack opens and closes. The opening and closing of the crack has been modelled in different ways. The simplest one is to consider that the crack is open or closed but the most feasible behaviour of the crack is the breathing mechanism. In this case, the crack is closed when it is situated in the compression zone and it is open when situated in the traction zone. The transition between both produces partial opening and closing of the crack. The partial opening and closing of the crack has been studied, numerically or analytically always considering an aligned shaft.

In this paper a finite element study of the influence of the eccentricity of the position of a rotating cracked shaft is studied. The model chosen for this study is the classical Jeffcott rotor. This simple but useful model consists of a massless shaft with a concentrated mass (a disc) with flexible or rigid supports. The crack is situated at the mid span of the shaft having a straight front for the sake of simplicity. The eccentric mass has been placed on the disc of the Jeffcott rotor.

To simulate the rotation of the shaft eight different angular positions have been considered. The influence of the position of the mass eccentricity on the opening of the crack has been studied for each angle of rotation. The mass eccentricity has been considered to be placed on the disc and situated at different angles measured from the position of the crack.

A comparison, for each angle of revolution, of the partially opening and closing of the crack for the different positions of the eccentricity is presented. The paper shows the influence of the unbalance of rotating shafts in the crack breathing mechanism and allows the prediction of the influence of this behaviour on the values of the stress intensity factor and in the propagation of the cracks.

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