Internal Friction in Glass. Part II. Flexural and Torsional Vibrations

JSGT 1954 V38 T383-T420

The basic methods which have been used to study internal friction or specific damping capacity are reviewed. Apparatus for studying the internal friction of thin rods of fused silica, a soda–lime–silica and a borosilicate glass undergoing transverse vibration (0·3–50 c/s) and torsional vibration (1–10 c/s) is described. The peculiar difficulties in obtaining an absolute measure of the internal friction in the case of transverse vibration at low frequencies are discussed. It is shown that there are two physical processes which contribute to the internal friction at these frequencies, a relaxation of alkali ions in the glass and the viscoelastic behaviour of the remainder of the vitreous network. This hypothesis is supported by a clear dependence of the internal friction on the thermal history of the specimen. At constant temperature the internal friction decreases with increasing frequency, but there is no evidence of a maximum value at low frequencies. The variation of internal friction with temperature shows a frequency-dependent peak value, indicating a process with an activation energy similar to that governing the movement of alkali ions in the glass. At higher temperatures the dependence of the internal friction on frequency and thermal history is illustrated, and note is made of the use of the results in an estimation of the distribution of relaxation periods among viscoelastic elements in the network. Incidental results are appended giving the variation of elastic moduli with temperature, and .the possibility of thermo-elastic coupling in glass is discussed.

P. L. Kirby