3. Dimension or scale effect

Mechanical cause: when a mechanical part is subjected to an inhomogeneous stress field (torsion, bending, traction, compression on a notched part, etc.), a stress gradient is observed in the surface layers of the part. If the dimensions of the part increase, the surface stress gradient will vary. The existence of different stress gradients for an identical nominal stress is one of the main causes of the fatigue scaling effect.

The scale effect can then be intuitively explained by the following reasoning:

• on a thin...