Fatigue notch factor
The notch sensitivity ratio compares the fatigue strength of a smooth specimen with that of a notched specimen. It shows how much a notch like a weld transition reduces the endurance limit. The higher the value, the greater the strength loss.
Physical-Chemical Mechanism of Action
At a notch, force lines concentrate. The stress peak at the notch root is significantly higher than the nominal stress. Under cyclic loading, tiny plastic deformations occur there. These micro-deformations form the starting point for fatigue cracks.
Metallurgical Relevance & Welding Risks
Welds have natural notches: the weld transition, undercut, root gap, or lack of fusion. These notches greatly increase the notch sensitivity ratio. The fatigue strength of the structure drops to a fraction of the values for unnotched specimens. Fatigue cracks threaten in the weld area, even at low loads.
Shop Practice & Instructions
Reduce every notch in the weld area. Grind the weld transition smooth and crack-free. Avoid undercut, sharp edges, and stop-start points. A smooth transition shape significantly lowers the notch sensitivity ratio. For high fatigue life, the transition radius should not be less than 1 mm. Inspect critical welds with dye penetrant testing for notches and cracks.