Papers by Author: Zhong Yi Cai

Abstract: The breathing mechanism of a transverse crack in an elastic shaft has been extensively researched. Although many different crack models have been presented, little published research has explored experimentally how the shaft crack breathing mechanism interacts with the change of stiffness during each revolution. This study addressed static analysis of the stiffness of elastic shafts with experimentally-induced fatigue, welding or wire cut transverse cracks. Bending stiffness was measured from different angles under a three point bending fixture, to efficiently define the correct periodical stiffness variation for different crack types. The opening and closing of each crack (breathing effect) were investigated for their association with shaft stiffness. It was found that width of the crack opening gap is a crucial factor which influences the crack breathing behaviour. Experimental results are expected to assist with crack diagnosis.

Abstract: Shaft fatigue crack is one of the most common defects in rotating equipment, due to its extensive operation with continuous heavy loads. Finding an efficient way to evaluate the true stiffness variation due to the crack rotation is the key step to develop both on-line and off-line crack diagnostic techniques. This study analyzed time-variant bending stiffness of elastic shafts with experimentally-induced fatigue, welding and wire cut transverse cracks. It was found that crack gap has a significant effect on the opening and closing behaviour of the transverse crack. As in the case of a cut crack, large crack gap could completely prevent the crack from closing during rotation. A fatigue crack without a clear gap shows a typical opening and closing behavior. Further, it remains fully closed within a small angular range and most of time it is partially closed. It was also observed that both switch and harmonic models cannot describe periodic stiffness variation well enough to represent the actual breathing function of the fatigue crack.