Static and fatigue strengths of pre-cracked silicon nitride balls under pressure load

Balls rotate during the rolling contact fatigue so that they are not affected by contact stress alone which occurs around the contact area. In the present work, we found that the tensile stress occurring around the equator of the ball causes fatigue of the ball itself. Four groups of pre-cracks, whose surface lengths were 140, 220, 550 and 700 μm, were initiated on ball surfaces and the strength of these pre-cracked balls was measured. The static strengths and fatigue limits of all pre-cracked balls decrease with crack sizes. We concluded that the fatigue caused by a pre-crack located on the equator of the ball surface is important for understanding fatigue of silicon nitride balls. Comparing the features of fatigue and static strength of the four cracks, we noticed that they fell into two groups: one for the small pre-cracks (140 and 220 μm cracks), and the other for large pre-cracks (700 and 550 μm cracks). In particular, the fatigue limits of the balls with small pre-cracks ranging from 140 μm (59%) to 220 μm (48%) are found to be predicted from their static strengths. However, the pre-crack size was less important for the fatigue limit than for the static strength.