Some effects of vibration on the internal friction of sodium chloride

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Abstract
The room temperature-internal friction of single crystals of NaCl vibrating at a frequency of 90 kc/s has been measured at strain amplitudes up to 2.7 × 10−4. The damping of plastically deformed crystals increased with increasing strain amplitude, and can be attributed to the motion of the dislocations introduced by the deformation. Vibration of specimens at high amplitudes changed their internal friction in three distinct ways. (a) When deformed specimens were annealod at 100 to 200[ddot]c the internal friction was reduced, but subsequent vibration at high amplitudes caused the damping to rise towards its value before annealing. (b) While lightly deformed crystals were being driven at very high amplitudes the damping was observed to decrease; the value at lower amplitudes was afterwards found to be lees than that determined before this vibration. (c) At the highest amplitudes attained in these experiments new dislocations were produced in the crystals, and their presence has been shown by etching experiments; an increase in the damping was associated with the introduction of these dislocations. These observations are discussed in terms of current theories.

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