Mythbusters: A small thread pitch reduces fatigue life due to the notch effect
The Myth: A small thread pitch reduces fatigue life due to the notch effect.
The Truth: A general statement is, as so often, not possible in this case since various parameters interact. In the literature there are partially contradictory statements. Heinrich Wiegand et al have conducted comprehensive studies on the influence of thread pitches. Diagram (a) shows the influence of thread pitches on bolt fatigue life.
Different property classes result in different characteristics. The fatigue life of high-strength 12.9-class bolts decreases with increased thread fineness, caused by the increasing notch effect. With decreasing strength and increasing ductility this effect is not visible anymore.
For bolts of property class 8.8, a decrease in fatigue life can also be seen. A larger cross-section of the fine thread will compensate for the decreasing fatigue life. The tolerable load amplitude (diagram b) is practically continuous.
There are more factors that influence the thread pitch than those mentioned. These include, for example, the displacement of the force application point, the root radius, the notch depth, the distribution of load force at the thread pitch and the notch sensitivity of the bolt material.