How do stress concentration factors affect shaft design?

Stress concentration factors (Kt for static, Kf for fatigue) multiply nominal stresses at geometric discontinuities like keyways, shoulders, and holes. For fatigue, Kf = 1 + q(Kt-1), where q is notch sensitivity (material dependent, 0 to 1). A keyway might have Kt=2.0, reducing fatigue strength significantly. Design strategies include generous fillet radii (r/d > 0.1), gradual diameter transitions, avoiding sharp corners, and surface treatment like shot peening at critical locations.