Overview of fatigue behavior in copper single crystals—II. Population, size distribution and growth kinetics of Stage I cracks for tests at constant strain amplitude

Abstract

In copper single crystals cycled at constant plastic strain amplitudes, Stage I cracks are formed in persistent slip bands (PSB) soon after the crystals reach saturation. Thus Stage I cracks play a key role in determining fatigue damage during the whole fatigue process. In the present work, the behaviors of short Stage I cracks, such as crack population, crack size distribution, crack depth and crack growth rate, have been studied systematically by means of the newly developed “sharp corner section” technique. Crack propagation behavior has been investigated for applied strain amplitudes ranging from 2 × 10 −4 to 8 × 10 −3, which covers almost the whole of the plateau of the cyclic stress-strain curve. It is found that the crack population is linearly proportional to applied cycles and applied strains and the distribution of crack sizes corresponds qualitatively to the distribution of localized strain measured in PSB's. The Stage I crack growth behavior is divided into three phases, a rapid initial growth for depths less than 10 μm which is amplitude independent, a slower second phase whose duration is amplitude dependent, and a third accelerating stage. The amplitude dependence of these stages causes the Coffin-Manson dependence of life on strain amplitude. The mechanism of Stage I crack growth appears to be a ratchetting mechanism, as shown by new evidence gathered by interferometry and SEM.