A study of the fatigue limit of copper

Abstract

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 1010 cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 109 cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10-4) of the fatigue limit. The slip bands continued to develop up to about 9 × 109 cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 1010 cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 1010 cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.