The Propagation of Fatigue Cracks in Pre-strained Carbon-Steel Round Bars with a Transverse Circular Hole −under the Rotating Bending Fatigue Test−

The fatigue crack propagation was examined and evaluated by the net stress amplitude after the fatigue test of the S 35 C steel specimen in the heading, the stress amplitude tentatively named being the macroscopic stress amplitude at the spot of the crack tip, varying with the distance along the cyclindrical surface between the spot and the center of the hole, though hitherto the nominal stress amplitude for specimens of such a configuration has been either the surface stress of the corresponding plain bar or the mean maximum surface stress of the minimum cross section at the brim of the hole. The conclusion reached is abbreviated as follows : The fatigue crack propagation rate at the early stage of the stress cycle was almost constant and that at the middle stage proportional to the crack length at the then cycle, the proportional constant. i.e. the rate coefficient being expressible as an exponential function of the net stress amplitude. Even in the early stage the proportionality is considered to exist, for the rate appears constant as the multiplicative effect by the short crack length and the large rate coefficient due to the high net stress amplitude near the hole