Internal stress measurments during the saturation fatigue of polycrystalline aluminum

Abstract

The internal stress and the effective stress were measured using stress relaxation techniques for 99.988% purity polycrystalline aluminum containing a well-defined cellular dislocation microstructure characteristic of large amplitude low cycle fatigue. Room temperature stress relaxation measurements were carried out after cyclic saturation at constant plastic strain amplitude and constant total strain rate as a function of location within a particular hysteresis loop. Measurements of stress relaxation at the tensile and compressive hysteresis loop tips show the dependence of internal and effective stress on plastic strain amplitude and strain rate. Both the internal stress and the effective stress were found to exhibit cyclic behavior representable by a hysteresis loop on each strain reversal. The internal stress was insensitive to strain rate while the saturation stress and the effective stress showed significant rate dependences. Both the internal stress and the effective stress increased with strain amplitude. The stress sensitivity exponent of the strain rate was determined from the variation in the effective stress with rate both during a given hysteresis loop and during strain rate changes and was found to decrease with increasing strain amplitude. The findings are discussed in the context of dislocation velocity and mobile dislocation density contributions to the strain rate sensitivity from which it is suggested that the mobile dislocation density decreases with increasing strain amplitude and strain rate. Plastic strain changes during stress relaxation suggest that the mobile density during saturation fatigue is equivalent to one mobile dislocation segment per dislocation cell.