Effects of small defects on fatigue strength of metals

Abstract

The effect of a small artificial defect (ie a drilled hole, the diameter of which is from 40 to 200 μm) on the fatigue strength of low carbon steel and medium carbon steel was investigated. The fatigue strength of the specimen containing the very small hole was found to be of almost the same order of magnitude as the fatigue limit of the plain specimen - that is, the existence of the very small hole did not weaken the fatigue strength of the holed specimen. This phenomenon had previously been predicted, and explained quantitatively, from a comparison of the hole dimensions with the size of non-propagating cracks which appear in the plain specimen at the stress level of the fatigue limit. The results of the present investigation can be applied to the evaluation of fatigue notch sensitivity of materials with high tensile strength or high hardness, and, from the metallurgical aspect, to the quantitative control of defects or inclusions in metals.