Instabilities in the internal friction of some specimens of copper

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Abstract
Occasionally a curious phenomenon has been observed when exciting copper beams in flexural modes of oscillation. At a certain value of the exciting force, the amplitude of the resonant oscillations varies rapidly by a factor of order 2 with a period of the order of a second. This behaviour appears similar to that previously reported by Takahashi (1952) and Kessler (1957). The instability is shown to be associated with a large discontinuous change in the amplitude of the internal friction when measured as a function of strain amplitude. A consideration of the present results indicates the existence of time-dependent effects in the pinning of dislocations by point defects, not hitherto reported.