The Rotary Bending Fatigue of a Eutectoid Steel (1st Report, Effects of Surface Finish and Defects on Fatigue Limit)

To examine the effects of the surface finish and defects on fatigue limit, specimens differently finished, i.e., sand paper(Grade #600, #80) finish and electrolytic polish, were prepared. Moreover, to simulate surface defects, a small artificial hole of which diameter is varied from 40 to 200 μm is introduced on the specimen surface. Both as-received and annealed specimens were applied so as to extract the effect of the work-hardened surface layer on the fatigue limit. Through examination of the fatigue limit and the non-propagating crack the following results are obtained: (1) The surface roughness slightly influences the fatigue limit, while the work-hardened surface layer has a primary effect on the fatigue limit. (2) An increase in the diameter of the small hole decreases the fatigue limit and results in the prolongation of the non-propagating crack. (3) For the non-propagating crack, the threshold stress intensity factor range ΔK_th decreases with a decrease in the crack length.