BS 7034-4:1988

Gives a method of determining the dynamic viscosity of glass on a rod-shaped test specimen supported at its ends. The viscous deflection rate of the beam is measured under a given load at the midpoint between the supports. The viscosity-temperature relationship and the dependence of the viscosity on the thermal history of the sample can be determined. Limited to small deflections and to small deflection rates.

This part of ISO 7884 specifies a method of determining the dynamic viscosity of glass on a rod-shaped test specimen (called a beam) supported at its ends. The viscous deflection rate of the beam is measured under a given load at the midpoint between the supports. In addition the viscosity-temperature relationship and the dependence of the viscosity on the thermal history of the sample can be determined.

The viscosity range covered by this method extends from 10⁹ to 10¹⁵ dPa s²⁾, corresponding to measuring temperatures between about 900 and 400 °C for all glasses of normal bulk-production compositions.

The procedures are limited to small deflections and to small deflection rates (see 3.6).

NOTE During beam bending, elongation flows of both signs occur (zero passage within the neutral plane). The determination of shear viscosity is possible only with Newtonian or linear-viscoelastic behaviour of the glass. The procedures are sensitive to interference by devitrification of the sample. With viscosities above 10¹² dPa s the adjustment of the structure equilibrium within the glass is perceptibly delayed with respect to the temperature setting. For tests within this range it should be agreed whether it is necessary to wait for the final equilibrium viscosity at a given temperature or to take the viscosity value corresponding to a conventional temperature-time programme (see 6.3).