Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of stress concentration. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
The stress at a notch tip is higher than the nominal stress by a factor Kt, which depends on the geometry rather than on the material. A small hole in a plate under tension roughly triples the local stress.
For ductile materials under static load, local yielding redistributes the stress and the concentration is usually tolerable. For fatigue, and for any brittle material, it is not — the notch is where the crack starts.
Common misuse
Ignoring stress concentration in a fatigue calculation because the part passes a static check. Fatigue is driven by peak local stress.