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 0.49; the stress necessary to maintain this constant strain was measured as a function of time. determine er(6) for this material if the initial stress level was 3.1 mpa (440 psi), which dropped to 0.41 mpa (59 psi) after 32 s.