Effects of Size on the Fatigue Limit and the Branch Point in Rotary Bending Tests of Carbon Steel Specimens

Abstract

The effects of size on the fatigue limits and the branch points of electropolished notched specimens were investigated. To clarify the effect of work hardened layers due to machining, the specimens having work hardened layers were also investigated. The main results obtained are as follows. 1) The necessary condition of non-propagating cracks is to take the root radius of a notch under a critical value ρ₀ which is a material constant. ρ₀ is about 0.5 mm for the 0.23% carbon steel. 2) The repeated maximum stress at the root of a notch at the fatigue limit based on crack initiation is determined only by the stress gradient x=(1/σ) (dσ/dx) at the root of the notch. 3) The size effect of the fatigue limits based on fracture in the range of existing of nonpropagating cracks is explained by the fact that the fatigue limits are determined from the stress concentration factor at the branch point α|_ρ=ρ₀.