BS 7034-4:1988

National foreword
Committees responsible
Method
0 Introduction
1 Scope and field of application
2 Reference
3 Principle
4 Apparatus
5 Preparation of tet specimens
6 Procedure
7 Expression of results
8 Test report
Annex
   Bibliography
Tables
1 Preliminary values for the estimation of the time
   interval delta t1 and for a midpoint deflection
   delta f = 0,1 mm, valid for the medium viscosity
   range according to 6.2.1
2 Preliminary values for the estimation of the time
   interval delta t2 and for a midpoint deflection
   delta f = 0,2 mm, valid for the lower viscosity
   range according to 6.2.2
3 Preliminary values for a measuring device for
   viscosities in the upper viscosity range according
   to 6.2.3
Figures
1 Principle of viscosity determination by beam bending
2 Example of a testing device for the beam bending
   method
3 Correction factor fl for the estimation of the time
   interval delta t'1 according to equation (9), taking
   into account spans ls other than that given in table
   1
4 Correction factor fI for the estimation of the
   measuring time delta t'1 according to equation (9),
   taking into account cross-sectional moments of
   inertia Ic other than that given in table 1

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).