Recovery or Elastic Recoil

An alternative to the preceding experiment is, after steady shear flow of a viscoelastic liquid under stress r, to remove the stress at time t2 and observe the decreasing strain as a function of time (Fig. 1-12). It has already been pointed out in Chapter 1 (equation 41) that the ensuing creep recovery is simply given by the creep function. The strain at 2 is 7(t2) = + tV Jo) the strain during the re- 

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There are other more complicated experimental situations where viscoelastic behavior can also be predicted in terms of the relaxation and retardation spectra or other functions. These include deformations at constant rate of strain and constant rate of stress increase, stress relaxation after cessation of steady-state flow, and creep recovery or elastic recoil, all of which were mentioned in Chapter 1, as well as nonsinusoidal periodic deformations. In referring to stress a, strain y, and rate of strain 7, the subscript 21 will be omitted here although it is understood that the discussion applies to shear unless otherwise specified. 

Modifications of the devices described in Chapter 14 can also be used to obtain information on the material s elastic response. For example, if the stress is suddenly removed from a rotational viscometer, the creep recovery or elastic recoil of the material can be followed. This provides a value of Xc for the material. 

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