Why Fatigue Limits?

Steel an titanium /seem/ to have a fatigue limit.

Aluminium is fcc and has low melting temperature. Disloscations tend to move easily. Formation of fatigue cracks in metals is due to localized dislocation movement.

Michael Dahms

There are mechanisms in Ti and steel that prevent dislocation movement at very low stresses, which don't operate in most metals (e.g., Cotrell atmosphere). You will also not that there is a "yield point phenomenon" in most materials that exhibit fatigue limits, and less well-defined yield stress in materials that do not exhibit a fatigue limit.

If we look more carefully, the stress-strain-curve before the yield point ist not perfectly straight. Hence, the fatigue limit in steel is an approximation as well.

Michael Dahms