A unified treatment of the fatigue limit of components weakened by stress raisers, subject to multiaxial mode I, II, and III loading

Abstract

This paper discusses the dependency of the fatigue limit (∆σg,th) of components on the size of stress raisers, such as cracks, defects, and U- or V-notches, under mode I, II, and III loading conditions in Constant Amplitude (CA) fatigue.As summarized in the Atzori-Lazzarin-Meneghetti (ALM) diagram, depending on the size and acuity of the stress raiser and the material fatigue properties, three different fatigue limit regimes can be identified: (i) for short cracks, ∆σg,th approaches the plain fatigue limit ∆σ0, (ii) for long cracks or sharp notches, ∆σg,th is governed by the long crack fatigue propagation threshold ∆Kth,LC, and (iii) for blunt notches, ∆σg,th is governed by the theoretical stress concentration factor Kt.In this work, the ALM diagram is extended to include mode II and III loading conditions by proposing a single design curve for any stress raiser subjected to any loading condition by employing the averaged Strain Energy Density criterion. The averaged SED is a fatigue model based on Neuber's concept of elementary structural volume that considers the strain energy density averaged over a material-dependent characteristic volume as the fatigue driving force. The obtained diagram is validated against experimental data obtained from sharp V-notched steel specimens reported in the literature, showing satisfactory results.