The notch effect describes the local increase in stresses in a component at a point where the geometry changes abruptly, for example at a notch, a radius, a thread root, or a sharp edge. Mechanical stresses concentrate more strongly there than in the rest of the material, reducing component strength and increasing the risk of cracks or fractures.
Why does a notch effect occur?
When a component is loaded, stresses normally flow evenly through the material. However, if this stress distribution encounters a geometric discontinuity, i.e., a notch, the stresses “pile up” and increase locally.
Typical notch locations include:
- Thread notches and thread roots
- Grooves and undercuts
- Bores and cross-section reductions
- Sharp edges or transitions without a radius
Influencing factors
- Notch geometry: Sharp, deep notches generate significantly higher stress peaks.
- Material behavior: Brittle materials react more sensitively; more ductile materials can dissipate stress peaks better.
- Type of loading: Alternating and pulsating loads intensify the effect, especially under fatigue loading.
- Surface quality: Rough or damaged surfaces further increase the notch effect.
Importance in mechanical engineering
The notch effect is particularly important for:
- Thread spindles: The core diameter determines the permissible tensile force.
- Shafts: Grooves and shoulders must be smoothly radiused.
- Bolts: Fractures almost always occur at the thread root.
- Manufactured parts under dynamic loading: e.g., crankshafts, axles, levers, KGT spindles.
Engineers take the notch effect into account in strength calculations using the notch factor (α or β), which indicates how severe the stress concentration is.