A MODEL FOR THE UNLOADING IN THE SATURATION FATIGUE STAGE OF PURE POLYCRYSTALLINE ALUMINUM

Stiffness measurements are frequently used for determining the fatigue damage of materials. Other mechanisms, such as interactions between dislocations and structural defects, can also affect the stiffness value. The aim of this study is to evaluate their contribution to the stiffness change. This study is concerned with pure polycrystalline aluminium cycled in push-pull mode at room temperature with a plastic strain amplitude between 10-4 and 1.5 × 10-3. Stiffness is determined all along the unloading curve. Effects of both temperature and strain rate on stiffness evolutions are investigated. A model for the unloading phenomena is derived from the theory of the thermally activated deformation. A contribution due to changes in the curvature of dislocations is also included in the model. Moreover a long range internal stress profile is introduced by taking into account the cellular nature of dislocations in the cycled aluminium. The analysis of experimental results from the present approach shows that the proposed model is very convenient to explain the main phenomena involved in the unloading. © 1990.