Effect of Grain Size on Cyclic Stress-Strain Relation and Fatigue Strength of Copper and α-Brass

Abstract

Many studies have been carried out in regard to the grain size dependence of fatigue strength of metals. And it has been made clear that fatigue strength of f. c. c. metals greatly depends on the stacking fault energy γ, and the effect of γ on the crack propagation process is different from that on the crack initiation process. In this work, the grain size dependence of the fatigue strength was mainly examined under the rotating bending fatigue test, and the cyclic work hardening behavior under the push-pull fatigue test. Furthermore, the surface structure due to fatigue deformation was observed by means of optical and electron microscopes. From the viewpoints of both microscopic structure and dynamical behavior of the materials, the grain size dependence of fatigue strength was investigated in copper and α-brass.
The results obtained are summarized as follows:
(1) The grain size dependence in the crack initiation process in α-brass is very remarkable at 200°C as well as at room temperature. This result suggests that the effect of grain size on fatigue strength must be considered from both viewpoints of the configuration of slip line and ageing effect.
(2) The grain size dependence in crack propagation process in α-brass is very small at R.T, but it increases at 200°C.
(3) The cyclic work hardening in α-brass is larger in fine grains than in coarse grains. On the other hand, in copper almost opposite result was obtained. From these results the difference in grain size dependence under constant stress fatigue test can be explained.