Notch Sensitivity
Is notch sensitivity a geometry property or a material property?
Notch sensitivity is a material property. The geometry sets the theoretical stress concentration factor Kt, but the material response determines q and therefore the fatigue stress concentration factor Kf = 1 + q(Kt - 1). A ductile material may show little sensitivity to the same notch that severely affects a hardened material.
What happens when q=0 versus q=1?
When q=0, the material is effectively insensitive to the notch and Kf equals 1, so no fatigue strength reduction is applied. When q=1, the material has full notch sensitivity and Kf equals Kt, meaning the full geometric stress concentration is realized in fatigue.
Why can a notched test piece sometimes show higher static tensile strength than a smooth one?
In certain test conditions, local plasticity, constraint, and the specific test definition can make a notched specimen show higher apparent tensile strength than a smooth one. This does not mean the part is less notch-sensitive in fatigue service; fatigue crack initiation at the notch still controls life under cyclic loading.
Notch sensitivity is a material's tendency to lose fatigue strength when a notch, hole, groove, sharp corner, keyway, or other geometric discontinuity raises local stress. It is commonly quantified by q, where q = (Kf - 1)/(Kt - 1); q=0 means no notch sensitivity and q=1 means full notch sensitivity. The fatigue stress concentration factor is Kf = 1 + q(Kt - 1), linking theoretical geometry Kt to actual material response.
On the CNC shop floor, notch sensitivity matters most when a machined feature that looks perfectly to print sees repeated loading. A shoulder with the minimum allowable fillet radius, a snap-ring groove, a drilled cross-hole, or a sharp internal corner in a pocket all raise local stress above nominal calculations. The engineer's job is to decide whether that geometry is safe for fatigue or whether the part needs a larger radius, a smoother toolpath, a deburring pass, or shot peening. Material response is key: a ductile material may absorb some of the theoretical stress concentration, while a hardened or high-strength material can be strongly notch-sensitive and crack at the same geometry. Fatigue cracks typically start at the surface discontinuity, so tool marks, burrs, and torn finish are not cosmetic—they are initiation sites that can shorten service life. In practice, using Kf instead of Kt gives a realistic fatigue design stress.
Fatigue crack at a shaft shoulder: A shaft meets dimensional tolerances with a minimum fillet radius, but the sharp transition concentrates stress and initiates a crack under rotating bending. The failure occurs long before nominal stress calculations predicted because the material's notch sensitivity was ignored.
Sharp internal corner in a milled pocket: A pocket corner is left nearly square because of cutter diameter limits, creating a sharp geometric notch. Under cyclic clamping or vibration, that corner becomes the crack initiation point, especially in a highly notch-sensitive material.
Burrs and tool marks at a cross-hole: A cross-hole or keyway keeps its dimensional accuracy but carries burrs, tool marks, or a torn finish. These surface defects amplify local stress, and fatigue cracks initiate there even though the part passes dimensional inspection.