Stress Concentration Factor

The stress concentration factor (Kt) is a dimensionless ratio of the maximum local stress at a geometric discontinuity to the nominal stress in the surrounding section, written Kt = σmax / σnom. It quantifies how holes, notches, shoulders, grooves, or sharp corners amplify stress compared with the average load path in a component.

On the shop floor, stress concentration factor matters whenever the toolpath creates an abrupt change in section. Shaft shoulders, keyways, thread roots, counterbores, relief grooves, sharp inside corners, and routed wood profiles with small internal radii all become candidates for crack initiation. Machinists and programmers use the Kt value mentally when choosing fillet radii, blending transitions, or deciding whether to rough the corner with a larger tool and finish with a smaller one. The nominal stress is the average over the net section; the peak stress appears right at the discontinuity and can be several times higher. That is why a part can pass a hand calc on gross area and still fail at a fillet, tool mark, sharp sawcut, or undercut. In millwork, fastener holes, notches, dado ends, and cutouts interrupt the load path, and the same geometric principle applies although the failure mechanics differ.