ANELASTIC STUDY ON LOW AMPLITUDE FATIGUE OF COPPER

Abstract
In situ observation of changes in vibrational period (P), internal friction (IF) and driving force (DF) was made for as-drawn and as-annealed copper polycrystals during the strain-controlled fatigue test at around room temperatures using resonant flexural vibration at about 200Hz. The maximum surface strain used was an order of 10^-4. IF first increased, then stayed almost constant and finally increased. P increased continuously. DF was not parallel with IF, but rather decreased in the middle stage where IF stayed almost constant. Overlapped on the above general changes, almost periodic saw-toothed changes were observed in P and DF, in the initial and middle stages of the fatigue life. For a typical case, the amplitude of this sawtooth in P amounted to an order of 10^-4 in a relative value of P and stayed almost constant at least in the initial and middle stages. The period or the width of tooth was about 3min which corresponded with an order of 10⁴ cycles of deformation. This period decreased as the fatigue proceeded. No change in IF corresponding with this change in P was detected probably because IF was less accurate. It was observed that the general feature was not sensitive to the strain amplitude and the pre-treatment of specimens, but the characteristic periodic changes were strongly dependent on the strain-amplitude and condition of the specimens before the fatigue test. Comparable fatigue tests made for as-drawn vanadium polycrystals showed analogous results with those observed for as-drawn copper polycrystals. Therefore, these observed phenomena were considered to be associated with the fundamental fatigue processes. It is considered that the above saw-toothed type change in P is possively associated with dislocation avalanche or strain burst typically observed in fatigued specimens, but further study is needed to correlate them.