Evaluation of metal fatigue characteristics considering the effect of defects

When the fatigue strength is evaluated for machine designs, fatigue evaluation methods and fatigue data which match physical phenomena and the design conditions need to be chosen properly. Generally, fatigue tests are performed under stress controlled conditions and controlled conditions. Because the load condition is closely associated with crack closure phenomena, the differences of the load conditions cause the differences of the crack propagation and non-propagation behavior. Then, the fatigue crack is divided into a small and long one depending on a plastic zone size at a crack tip. The propagation and nonpropagation characteristics of the small crack are different from those of the long one. Therefore, it should be noted that the fatigue data under the stress controlled conditions are different from those under the controlled conditions, and the propagation and nonpropagation characteristics of the small crack are different from those of the long one. The authors proposed the evaluation methods for the fatigue strength of specimens with the various dimensional cracks and notches and applied the methods to the predictions of the fatigue life reliability and the fatigue limit reliability of the inhomogeneous materials. In this paper, it is mentioned that the propagation and non-propagation characteristics of the fatigue crack depends on the relative plastic zone size and the load conditions, and the simple forms which are obtained by formulating these characteristics under a constant stress controlled condition are introduced. Then, their applications to predictions of fatigue life reliability and fatigue strength reliability at N = 10₇ of aluminum cast alloy JIS AC4B-T6 which contains various stress concentration parts are roughly introduced.