For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to where σ(t) and σ(0) represents the time-dependent and initial (i.e., time =0) stresses, respectively, and t and τ denote elapsed time and the relaxation time; τ is a time-independent constant characteristic of the material. A specimen of some viscoelastic polymer with the stress relaxation that obeys Equation 7.40, was suddenly pulled in a tension to a measured strain of 0.5; the stress necessary to maintain this constant strain was measured as a function of time. Determine Er(8) for this material if the initial stress level was 2.62 MPa (380 psi), which dropped to 1.73 MPa (251 psi) after 64 s.