Glossary term
Notch Sensitivity
The degree to which a material's fatigue strength is reduced by notches or geometric discontinuities.
Definition
metricThe degree to which a material's fatigue strength is reduced by notches or geometric discontinuities.
Notch sensitivity describes how closely a real material under fatigue loading responds to the theoretical elastic stress concentration at a notch, fillet, groove, hole, keyway, thread root, or surface defect. It links geometry, material microstructure, strength, notch radius, and fatigue design.
Notch sensitivity measures how strongly fatigue strength is reduced by geometric discontinuities. A notch raises local elastic stress through the theoretical stress concentration factor K_t, but the fatigue penalty is usually represented by the fatigue notch factor K_f. Notch sensitivity q connects them:
If q=0, the material behaves as if it is insensitive to the notch for the fatigue condition considered, so K_f approaches 1. If q=1, the fatigue penalty approaches the full elastic stress concentration, so K_f approaches K_t. Most real cases lie between these limits.
Material and geometry effects
Notch sensitivity is not a universal material constant. It depends on material strength, ductility, microstructure, surface condition, notch radius, stress gradient, load ratio, environment, temperature, and fatigue life regime. High-strength steels and brittle materials often show greater notch sensitivity than more ductile materials, but details matter.
A sharp thread root, keyway, shoulder fillet, weld toe, scratch, corrosion pit, or drilled hole may control fatigue life even when nominal stress is modest. Designers reduce risk by increasing radii, improving surface finish, removing tool marks, avoiding abrupt section changes, using compressive residual stress treatments, or relocating discontinuities away from high alternating stress.
Fatigue assessment
Fatigue calculations often use K_f rather than K_t when estimating alternating stress for S-N or Goodman-type checks. For crack-like flaws, fracture mechanics may be more appropriate than notch sensitivity because the controlling physics shifts from initiation to crack growth. For welded structures, design codes often classify details directly rather than requiring a separate notch sensitivity calculation.
Common mistakes
A common mistake is to apply a static stress concentration factor directly to fatigue life without considering notch sensitivity, mean stress, surface finish, residual stress, and size effect. Another is to use a tabulated q value outside its material and notch-radius range. A good fatigue review identifies the physical notch, the stress gradient, the chosen K_t and K_f, and whether initiation or crack propagation is the limiting failure mode.