Fatigue Notch Factor
Is Kf the same as Kt?
No. Kt is the purely elastic geometric stress concentration factor, while Kf is the fatigue notch factor reflecting geometry and material notch sensitivity.
Why is Kf usually smaller than Kt?
Real materials do not fully transmit the theoretical elastic concentration into fatigue damage because local plasticity, microstructure, and crack-initiation behavior reduce the notch effect. This is captured by q in Kf = 1 + q(Kt - 1).
What does q = 0 or q = 1 mean?
q = 0 means the material is notch-insensitive and Kf = 1; q = 1 means full notch sensitivity and Kf = Kt.
The fatigue notch factor (Kf) is the ratio of the smooth-specimen fatigue strength to the notched-specimen fatigue strength at the same life or load ratio. It quantifies how a real notch reduces cyclic load capacity compared with a smooth part. Kf is related to the elastic stress concentration factor Kt by Kf = 1 + q(Kt - 1), where q is material notch sensitivity, so Kf lies between 1 and Kt.
On the shop floor, fatigue notch factor governs how a machined feature behaves under repeated loading. A CNC-milled shoulder, keyway, thread root, or sharp pocket corner can look acceptable in a static check yet still initiate cracks when vibration or cyclic bending concentrates stress. Choosing larger fillet radii, avoiding CAM simplification that leaves sharp internal corners, and controlling deburring and surface finish at part transitions directly lowers Kf and extends service life. In welded assemblies, weld toes and roots behave as pre-existing notches, so endurance limits run well below smooth base material. The same logic applies to routed millwork and fabricated panels: cutouts, drilled holes, hinge pockets, and sharp MDF edges concentrate stress under repeated clamp or fastener loads. Thus Kf is the practical bridge between CAD geometry and real fatigue life, reminding machinists that nominal loads do not govern crack initiation; local geometry and surface condition do.
Sharp internal corners left by tooling: A pocket corner without a proper fillet creates a high Kt, and a notch-sensitive material pushes Kf close to it. Early cracks then start under vibration or repeated bending.
Burrs and tool marks after machining: A tiny burr or torn edge behaves as a micro-notch, amplifying local tensile stress. In cyclic service, the crack initiates at that surface defect rather than from the bulk material, shortening fatigue life.
Static strength assumptions: Passing a one-time load check does not guarantee fatigue life, because alternating stress and surface condition control crack initiation. Ignoring Kf at holes, threads, weld toes, and machined steps causes unexpected field failures.