Fatigue Limit of Blunt‐notched Components

Abstract

The position and effective resistance of microstructural barriers and their relation to the fatigue strength of blunt‐notched specimens are analysed and modelled for three low‐carbon steel microstructures. A relationship for the notch size effect on the basis of the experimental evidence that the fatigue limit (both plain and notched) represents the threshold stress for the propagation of the nucleated microstructurally short cracks, was derived. The derived relationship characterizes the fatigue notch sensitivity by means of the parameter ktd defined as the stress concentration introduced by the notch at a distance d from the notch root surface equal to the distance between microstructural barriers, and was experimentally verified for two notch geometries in three microstructures: ferrite, ferrite–bainite and bainite–martensite.

Effect of small scale notches on the very high cycle fatigue of AISI 310 stainless steel

“…The effect of small scale damage and notches, having depth and width of the order of grain size of the material, on the fatigue life has been studied previously . Khan et al .…”

“…studied the notch size effect for the same stress concentration factor ( K t ) and developed a threshold stress relation. Chapetti et al . studied blunt notches in different steel alloys and found that fatigue limit depends on the microstructure barriers.…”

Effect of surface stress concentrators and microstructure on the fatigue limit of the material

“…In [6,7], a concept is proposed of the sensitivity to the stress concentration for blunt concentrators that characterizes the fatigue sensitivity to a notch in terms of the parameter K td defined as the stress concentration due to a notch at the distance d i from the notch tip to the ith microstructural barrier in the form 377 a c b …”

Fatigue notch sensitivity of steel blunt‐notched specimens

“…In this microstructure the cracks initiate in the ferritic laths of the bundles, thus the size and the relative orientation of these bundles define the position and resistance of the first microstructural barrier ( d 1 = 50 μm and Δσ e0 d 1 = 580 MPa, approximately) 11 . The position and resistance of the second and third barriers is given by the austenitic grain size: d 2 = 120 μm with Δσ e0 d 2 = 540 MPa, and d 3 = 240 μm with Δσ e0 d 3 = 470 MPa.…”

“…In a previous work, 11 a model for the notch size effect on the basis of the experimental evidence that both, the plain‐ and the blunt‐notched fatigue limit represents the threshold stress for the propagation of the nucleated microstructurally short cracks, was derived. The derived relationship characterises the fatigue notch sensitivity by means of the parameter k t d defined as the stress concentration introduced by the notch at a distance d from the notch root surface equal to the distance between microstructural barriers, as follows:…”