Subsurface propagation of partial ring cracks under rolling contact: Part II. Fracture mechanics analysis

Surface ring crack defects are normally found on the surfaces of ceramic balls. These ring cracks are circular extending for approximately one quarter of circumference of a circle and these cracks can significantly reduce rolling contact fatigue life. In this study, a boundary element analysis was carried out to identify the main factors governing the propagation of sub-surface cracks. The calculations of the main stress intensity factors (SIF), KI and KII, were made to determine which factors were important for crack propagation. Various geometries of the ring crack were selected to investigate their effect on the subsurface propagation of the ring crack and calculated results showed the most important factor of crack geometry was shown to be the subsurface crack length with crack radius. Crack angle and arc length was shown to have relatively little effect. The KII values were mainly affected by load, crack length and crack face friction. Increasing the load or crack length resulted in an increase in absolute KII values, but high crack face friction led to lower KII. Lubricants with less ability to penetrate a crack and thereby increase crack face friction should therefore give a better rolling contact fatigue performance. The numerical results were verified by a comprehensive experimental study [20], which show that present predictions of subsurface crack growth are consistent with the experimental observations.