Extension at Constant Force

Extension at force F = const corresponds to conditions with intensively increasing stress. The stationary flow, in this case, is obviously not attained, and deformation, 

In extension under conditions F — const, dependencies of total strain e and elastic strain a upon time t are measured as usual. In Refs. 11-13 15-35 38 these experiments were arranged with polyisobutylene samples with different molecular weights and with low-density polyethylene (see below). 

Dependencies of total strain b = 1/10 upon time t, in extension of polyisobutylene 11-20 (USSR) at constant force F for different initial stresses ct0 = F/S0 (here S0 is the cross-sectional area of the sample at t = 0) are given in Fig. 4. 

In the region of t under investigation these dependencies are strictly increasing and practically independent of ct at low values of t. The higher ct0, the faster they grow with time. 

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Fig. 14. Alteration of total strain (Ins) with time (t) in extension at constant force (F = const, ct0 = 2.1 x 104 Pa) in case of polystyrene of two different brands manufactured in the USSR and in the FRG...

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