Rupture above the Glass Transition Temperature

There is a basic difference between rupture above the glass transition temperature where the polymer chain backbones have an opportunity to change their configurations before the sample fails, and below Tg (or somewhat above Tg with very rapid stressing) where the backbone configurations are essentially immobilized within the period of the experiment. 

Above the glass transition temperature, the crack propagation which culminates in rupture can be strongly influenced by relief of stress through viscoelastic relaxation, by crystallization at large extensions (as in natural rubber), by the presence of filler particles, and by mechanical hisotry in periodic stressing. 3 

The values of stress and strain at the moment of rupture—the so-called ultimate 

Tensile strain at break plotted against logarithm of strain rate (in sec ) reduced to 263 K for a cross-linked styrene-butadiene rubber at 14 temperatures as indicated (Smith. ) 

Tensile strength in force per unit initial cross-section area, rr(i)/X, plotted against logarithm of strain rate, both reduced to Ts = 263°K for the material of Fig. 19-3 at the same 14 temperatures. (Smith. ) 

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