Specimens of annealed and cold-rolled 99.99% pure aluminium were fatigued under completely reversed plane bending. The effect of stress amplitude on the crystal deformation during the fatigue process was investigated by using the X-ray microbeam diffraction technique and optical miroscopy. The results are summarized as follows:
(1) Fatigue strength increases as the reduction of thickness increases.
(2) Configuration of slip bands in the cold-rolled specimens, at high stress amplitudes, is similar to that in the annealed specimens. At low stress amplitudes, however, slip bands become coarse in the specimens of low reduction, and discontinuous or island-like in those of high reduction.
(3) Both in the annealed and the cold-rolled specimens, substructures are developed during the fatigue process. The amount of crystal deformation is dependent on stress amplitude. In the annealed specimens, the total misorientation β and the excess dislocation densities (Db)max and (Db)min increase while the subgrain size t decreases with increasing stress amplitude. On the other hand, in the cold-rolled specimens, β, (Db)max and (Db)min increase and the subgrain size lessens only when stress amplitude exceeds a certain critical value, which augments with decreasing thickness.
(4) In the annealed specimens the excess dislocation density in the grain (Db)min is correlated to stress amplitude by the following equation.
σ=0.49+1.15×10-4√(Db)min
In the cold-rolled specimens, when stress amplitude is higher than the value calculated by substituting the excess dislocation density in the grain (Db)min due to cold-rolling into the above equation, the dislocations in the subgrain being rearranged, “fatigue-induced recovery”takes place and furthermore the substructure is well developed. On the contrary, when stress amplitude is lower than that, only the rearrangement of the pre-existing dislocations in the subgrain comes to be the principal crystallographic deformation during the fatigue process and the fatigue-recovery is not so remarkable as at high stress amplitudes.
