Experiments for Determining Viscous Properties

We first determine the creep function /(f) based on the results of a creep test If the creep stress is given by (2.260), the response is given by (2.261), therefore e(f) = /(f)CTo. Let us assume that the relaxation spectrum is represented by the first approximation of (2.275), i.e., 

We can then plot the values of (2.278) calculated by the creep curve shown, for example, in Fig.2.16. In this figure we specify discrete values ti, T2,. Since the discrete representation of (2.264) is given by 

We now need a procedure to determine tjo- From (2.280) the slope at the final elapsed time t is given by 

For an isotropic material, the creep functions B t) and C(r) are determined using the same procedure. For an axisymmetric triaxial stress state, let cti be the axial stress, CT3( o) (with 2 = 0 ) the confining pressure, i the measured axial strain and 3 the measured lateral strain. Then setting 

EHie to (2.277), the first approximations of the relaxation spectra of each component are given by 

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