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 above was suddenly pulled in a tension to a measured strain of 0.42; the stress necessary to maintain this constant strain was measured as a function of time. Determine Er(7) for this material if the initial stress level was 3.1 MPa (440 psi), which dropped to 0.30 MPa (43 psi) after 52 s.