A Fractographic Study of Stage 1 Cracking with Particular Reference to Endurance Limit

Abstract

A fatigue test has been performed using a rotating bending testing machine on the smoothed specimens of two kinds of low carbon steels, CH10A and CH10R, which show the endurance limits with well-defined“knee”. Then, the mechanism of endurance limits has been investigated by means of electron microfractography and of fracture mechanics.
The main results obtained are as follows:
(1) The crystallographic facets of Stage 1 cracking are found in all specimens both broken and unbroken under stresses near the endurance limits.
(2) The endurance limit is not the critical stress for cracking, but for transition from Stage 1 to Stage 2 cracking.
(3) The criterion of transition from Stage 1 to Stage 2 cracking can not be expressed only by the stress intensity factor. A model for the mechanism of transition has been devised and the effect of active slip planes which cross the Stage 1 crack plane has been discussed.