Measurement of Internal Friction in the Kilocycle Frequency Range

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Abstract
An instrument for measuring the internal friction of rod shaped specimens in the kilocycle frequency range is described. The specimens are excited into longitudinal resonance electrostatically and a frequency modulation technique is used to detect the resultant vibration. The linearity of the system is established by an absolute optical calibration technique and the accuracy of measurement is shown to be as follows. The internal friction is measured to an accuracy of ±1% up to a value of δ (logarithmic decrement) of 6×10−3, with a background damping less than 5×10−6. The dynamic strain amplitude can be measured with an accuracy of ±2% over the working range of 5×10−8 to 10−5.