DISLOCATION-STRUCTURES OF FATIGUED POLYCRYSTALLINE COPPER UNDER VARIABLE AMPLITUDES

Cyclic behaviors of polycrystalline copper are studied under variable strain amplitudes. It is shown that the specimens tested at incrementa l strain amplitudes have the same stress as at conventional constant t rain amplitudes. However, the specimens tested at decremental strain a mplitudes have a history effect on the stress as compared with those a t constant strain amplitudes. TEM observations show that the increment al loading produces a dislocation wall structure similar to that at co nstant amplitudes. Under the decremental loading, the cellular disloca tion structure formed at higher strain amplitude recovers to labyrinth structure. This degeneration process is found to be incomplete and th e dislocation structure remains as dense dipolar walls.