Internal Friction of Zinc Single Crystals

Abstract

Measurements were made of the internal friction of single crystal rods when vibrating longitudinally at 45 kilocycles per second. The decrement, or ratio of energy dissipated per cycle to twice the total vibrational energy, was used as a measure of the internal friction. At strain amplitudes below about 10−5 or 10−6 the decrement was independent of strain amplitude. The value of the constant decrement at low amplitudes, and the changes which occurred as the amplitude was increased, were both greatly dependent on the history of the specimen. The constant decrement for different crystals ranged from 7×10−5 to values higher than could be measured (about 200×10−5). For some of the initially high-decrement crystals, values decreased with time (room temperature anneal). In general, for annealed crystals the decrement, measured as a function of increasing strain amplitude, remained almost constant up to and beyond the static elastic limit and then rose rapidly. It returned to the initial value when the strain amplitude was again reduced. On a second run, however, the rise in decrement occurred at much lower amplitude than on the initial one. It is concluded that the decrement is the result of two separate processes. The changing decrement at higher strain amplitudes is qualitatively explained with the use of dislocation theory.