Effects of surface finish on the fatigue life of "cold-rolled" mild steel specimens
Abstract
In the first part of this investigation, the Krouse rotating beam fatigue machine and the R. R. Moore rotating beam fatigue machine were compared. On the basis of determining the fatigue life of cold-rolled steel at one stress level, it was found that there was no significant difference between the machines at the 1% significance level. Hand-polished and electro-polished fatigue specimens were tested on both machines. It was found that there was a significant difference between the effects on fatigue life due to the different finishes at the 1% significance level and that the electro-polished specimens affected a greater degree of scatter in the data than the electro-polished specimens. Also, fatigue cracks could be observed sooner on the electro-polished specimens than on the hand-polished specimens. The ratio of cycles to crack to total life ranged from 0.69 for electro-polished specimens to 0.83 for hand-polished specimens.
In the second part of this investigation, the study of the initiation of fatigue cracks on electro-polished specimens revealed that evidence of plastic deformation leading to the formation of a fatigue crack was visible as early as 1/4% of the total life of the specimen. Slip-lines which developed into micro-cracks were observed first. These micro-cracks later joined to form fatigue cracks.