A MODEL FOR THE UNLOADING STIFFNESS CHANGE DURING THE FIRST STAGES OF PURE POLYCRYSTALLINE ALUMINIUM

ABSTRACT Stiffness measurements are frequently used to determine the cyclic damage of materials. Other mechanisms such as interactions between dislocations and structural defects, can affect the stiffness value. The aim of this study is to evaluate their stiffness change contribution. 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 10 -2 . The unloading stiffness expression has been deduced from the theory of thermally activated deformation.Moreover this expression includes a non linear elastic strain rate due to the curvature changes of dislocations. There is a good agreement between the experimental results and the expected one obtained by the present model. The model suggests an internal stress evolution during unloading. From these results quantitative detection and estimation of the threshold damage by stiffness change can be envisaged.