Simple extension at constant strain rate

At small deformations, viscoelastic information can in principle be obtained from stress-strain measurements at a constant strain rate, as shown for shear deformations in equations S6 to S9 of Chapter 3. Such experiments are often made in simple extension, but the deformations can become rather large so there are marked deviations from linear viscoelastic behavior. The most commonly used instrument is the Instron tester other carefully designed devices have been described. - The sample is usually a dumbbell or a ring. In the former case, the strain in the narrow section as checked by separations of several fiducial marks can be calculated from the separation between the clamps by a suitable multiplication factor. In 

If the practical tensile strain is defined as e = L/Lo 1. where L and Lq are the stretched and unstretched lengths respectively, the practical tensile strain rate is (1 /Lo)dL/dt (cf. equation 60 of Chapter 3), and a constant e can be achieved by pulling the clamps apart at a constant rate. However, if the elongational strain rate is defined as the ratio of the velocity of a material point to its displacement, this quantity, denoted ci, is ( /L)dL/dt and it will remain constant only if the clamps are pulled apart at a rate which increases exponentially with time. Several instruments which accomplish this have been described. oo-io2a Qf 

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