For many viscous materials, the viscosity η may be defined in terms of the expression where Ï and dε/dt are, respectively, the tensile stress and

For many viscous materials, the viscosity η may be defined in terms of the expression

where σ and dε/dt are, respectively, the tensile stress and the strain rate. A cylindrical specimen of a soda-lime glass of diameter 5 mm (0.2 in.) and length 100 mm (4 in.) is subjected to a tensile force of 1 N (0.224 lbf) along its axis. If its deformation is to be less than 1 mm (0.04 in.) over a week's time, using Figure 13.7, determine the maximum temperature to which the specimen may be heated.

de/dt