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Experimental investigations of the internal friction of optical and technical glasses with a flexure pendulum equipment
Α special flexure pendulum equipment for investigation of the mechanical relaxation behaviour (internal friction) of glasses, glassceramics and other materials is presented. With the help of this apparatus it is possible to investigate the internal friction in the temperature ränge from room temperature until beyond Tg with a frequency of about 1 Hz. For analyzing the experimental results the shear behaviour of the investigated material is described in terms of a Maxwell body characterized by the rheological parameters Young's modulus and relaxation time. From the theoretical treatment of the bending problem for the experimental equipment calculation, formulae are obtained which connect the rheological parameters with the measured quantities period of oscillation and logarithmic decrement. Investigations of the internal friction of optical and technical glasses show weak effects in the ränge from room temperature until Tg and very strong damping peaks with different peak widths and maxima temperatures in the ränge beyond Tg. The syntheses and glass transformation ranges of the investigated glasses are very different. The activation energies of the hightemperature relaxation processes are obtained from the measured temperature dependences of relaxation times. These activation energies of the high-temperature relaxation process beyond Tg are smaller than the activation energies of viscosity determined by means of a bar elongation method
Internal friction spectroscopy in Li20-2SiO2 partially crystallised glasses.
Oscillatory torsion deformation experiments were performed on partially crystallised Li2O–2SiO2 glasses in the temperature range 350–480 °C and with frequencies between 20 and 0.002 Hz. The experiments were carried out in a torsion deformation apparatus exerting a small strain on cylindrical samples. Data obtained at varying temperatures and frequency were reduced to master plots using a normalised frequency. The frequency shift factor has been taken as a function of temperature in an Arrhenian form, yielding an activation energy of a background Q−1 close to the activation energy of oxygen defect diffusion (=120 kJ/mol). The master curves of real and imaginary components of shear modulus and internal friction indicate a stretched exponential shear stress relaxation with a an exponent of ≈0.45, characteristic of a broadened relaxation spectrum. The dynamic viscosity was estimated at temperatures of 470 and 480 °C. The extrapolation of dynamic viscosity to zero frequency allowed estimation of the relaxed shear viscosity. The presence of crystals increases the relaxed shear viscosity by ≈0.2log(Pa s)/10 vol.% of crystallinity. Dependence of the relative shear viscosity of partially crystallised lithium disilicate melts on crystal content is discussed