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Why can holes, notches, and screw threads create dangerous stress concentrations in structures?

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mechanics of solids Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).

Holes, notches, and screw threads create dangerous stress concentrations because they disrupt the uniform distribution of stress within a material, causing local stresses to rise significantly higher than the nominal stress levels.1

When a structure contains geometric discontinuities—such as circular or elliptical holes, sharp reentrant corners, or screw threads—the material is forced to redistribute the load around these features. Research has shown that the concentration of stress is particularly severe when the radius of curvature at the site is small relative to the overall dimensions of the opening. For example, a simple circular hole can increase local stress by a factor of three under uniaxial tension, while sharper features can lead to far greater concentrations.1

These sites act as critical points where cracks can nucleate. Once a crack forms, it can propagate under service loads, potentially leading to catastrophic structural failure. Understanding these stress concentrations is essential for engineering safety, as it allows designers to identify potential failure points and mitigate risks in critical structures.1