Small non-propagating cracks (NPCs) are usually found at notch roots when the nominal stress range is below certain limits that depend on the notch sensitivity. Analysis is performed on geometrically similar three-point bend beams of three different sizes and subjected to fatigue loading. The important factors that affect the notch sensitivity and the minimum stress range required for the initiation and propagation of short cracks are determined. A criterion for crack initiation and propagation is proposed. Using this criterion, the maximum length of NPC that can be formed from fatigue loading alone and the minimum stress range required to propagate a crack without arrest are computed.

Keywords: Fatigue, Short crack, Threshold & probability.