Changes in strain-dependent internal friction during fatigue (of Al-alloys)

Abstract An investigation on the fatigue of an Al-3.5% Mg alloy in the annealed state has been performed by means of internal friction measurements evaluating, at different fatigue stresses and numbers of cycles, the curves expressing the dependence of the internal friction on the amplitude of vibration. By using the parameters of the Granato-Lucke segment of these curves, the total length of movable dislocation line per unit volume Λ and the mean free-to-vibrate dislocation length L c have been determined. Plots of Λ versus number of cycles may be divided into three stages: the first is characterized by an increase in the dislocation density up to a maximum whose magnitude depends on the applied fatigue stress; during the second stage the dislocation density decreases at a rate independent of the stress. The third stage, which progresses to fracture, is due to a second increase in dislocation density. These trends have been interpreted in terms of evolution of the dislocations structure due to the fatigue stress.