Correlation of the Alternating Stresses required to initiate and propagate a Fatigue Crack in Mild Steel

Abstract

EXPERIMENTAL, results obtained from mild steel plates containing small edge cracks subjected to reversed direct stress loading (that is, zero mean load) have suggested that whether or not a crack grows is governed by the parameter σ³ l (σ = nominal alternating stress, tons/sq. in., l = length of edge crack, inches)¹. If the value of σ³ l is greater than 5.5, a crack will grow ; if less, it will remain dormant. It has been shown^1–4 that with certain notched specimens, non-propagating cracks may be present at the root of the notch of those tested at stresses equal to and below the conventional notched fatigue limit (that is, for specimens either remaining unbroken or being completely broken). When the geometry of the specimens is such that non-propagating cracks are present, the conventional notched fatigue limit is independent of the radius of the notch root and is equal to the alternating stress required to propagate the crack. This depends on the depth of the notch, for it has been shown¹ that the relationship σ³ l = 5.5 is applicable if l is taken as the depth of notch plus the length of non-propagating crack formed.