Fatigue of copper single crystals in vacuum and in air II: Fatigue crack propagation

Abstract

Copper single crystals oriented for single slip were fatigued in symmetrical push-pull at room temperature in high vacuum and in air. The characteristic features of fatigue crack growth in its early stage and, during programmed loading, at larger crack lengths were investigated by scanning electron microscopy. The observations showed clearly that early crack growth occurs in stage I along persistent slip bands both in air and in vacuum. Further, it was shown that, irrespective of the environment, transitions from stage I to stage II crack growth and vice versa could be induced at arbitary crack lengths by appropriate increases or reductions in the amplitude. The observed crack growth rates were significantly lower in high vacuum than in air, in particular in stage I. It is suggested that the prolonged fatigue life in vacuum is largely due to the strongly reduced rate of early crack growth in stage I and possibly also in early stage II.