Contact Fatigue under Oscillatory Normal Load

Experimental studies were made wherein contact fatigue was brought about by a cyclic variation of normal load applied to the contact between a ball and a flat specimen. Curves of flat-specimen temperature versus time showed that a rapid temperature rise occurred in the initial stage of crack formation rather than after the appearance of fully developed spalls; thus the temperature criterion appeared to provide an early indication of fatigue. Fatigue failure was observed in unlubricated as well as lubricated contact, but with much longer life when unlubricated.

Toroidal rings of hardened and softened material were noted both on and below the contact surface. These hardness variations bore symmetrical relationship to the contact geometry and were apparently related to the stress pattern.

It was found that the fatigue cracks did not initiate in the most severely hardened material but in the material adjacent to it. Further, the slope of the cracks tended to follow the boundary of the hardened region in general direction. Propagation of microcracks in this general direction appeared to be influenced by the prior austenitic grain boundaries. These microcracks at times appeared to propagate along carbide stringers.