A simple approximate expression for finite life fatigue behaviour in the presence of 'crack-like' or 'blunt' notches

In this note, we explore the possibility of simple extensions of the heuristic El Haddad formula for finite life, as an approximate expression valid for crack-like notches, and of the Lukas and Klesnil equation for blunt notches. The key starting point is to assume, in analogy to the Basquin power-law SN curve for the fatigue life of the uncracked (plain) specimen, a power law for the finite lifeintrinsic El Haddad crack size. The approach has similarities with what recently proposed by Susmel and Taylor as a Critical Distance Method for Medium-Cycle Fatigue regime. Reasonable agreement is found with the fatigue data of Susmel and Taylor for notches, and in particular the error seems smaller in finite life than for infinite life, where these equations are already used. In these respects, the present proposal can be considered as a simple empirical unified approach for rapid assessment of the notch effect under finite life.