Stress concentration factor
The stress concentration factor is the ratio of the highest stress at a notch to the nominal stress in the undisturbed cross-section. It shows how strongly a geometric irregularity increases the stress. A high stress concentration factor means high stress peaks and thus increased crack risk.
Physical-Chemical Working Principle
At a notch, the force lines crowd together. The stress locally rises sharply. The stress concentration factor (αK) = maximum notch stress / nominal stress. It is a pure geometry number. It depends only on notch radius, notch depth, and part shape, not on the material. Example: a sharply notched shaft has a stress concentration factor of 3 to 5, a smooth shaft has a factor of 1.
Metallurgical Relevance & Welding Risks
Welds create natural notches: weld toe, root notch, undercut, porosity, or slag inclusions. A high stress concentration factor increases local stress. It can exceed the yield strength. Then microcracks form. In tough material, stress relaxes by yielding. In brittle structure (martensite), the part cracks immediately. Especially dangerous: fatigue and cold cracking.
Workshop Practice & Instructions
Reduce the stress concentration factor at every notch. Grind weld toes smooth and notch-free. Avoid undercuts and abrupt cross-section changes. Test critical areas with penetrant testing or magnetic particle testing. Under cyclic loading: grind the weld toe with a radius of at least 2 mm. Goal: keep the stress concentration factor below 1.5. That can increase service life by a factor of 3 to 10.