Some Studies on Rotating Bending Fatigue of Large Mild Steel Specimen : Part 2, Propagation of Crack in Plain Cylindrical Specimens

Abstract

It has been noticed in the rotating bending fatigue test on 125φmm smooth surface specimens, described in part 1, that a shell pattern appeared on the fractured surface when a radical change of the stress was made on the specimen. The author has established a general relationship between stress and propagation speed of fatigue cracks from analytical studies on shapes of the shell pattern, which may be summarized as follows : 1. The shell pattern shows a definite shape, which is drawn as a family of curves depending upon arc length of th crack. 2. Assuming that the stress νσ_n (kg/mm²) on the edge line of the fractured surface was equal to the stress on a beam having a sectional shape same as that of the specimen having a reduced area by the fracture, νσ_n was computed on a number of samples of shell patterns, as a result of which it has been found that the relation between the propagation speed ν_c (μ/c) and νσ_n may be obtained from the following formula : [chemical formula] where : σ₀ is a critical value of νσ_n=11.5kg/mm². When νσ_n exceeds σ₀, the propagation of crack takes place. B is a constant depending upon the size of the specimen ; B=17 for 125φ mm specimen.