For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to
σ(t) = σ(0) exp (-t/t)
where σ(t) and σ(0) represents the time-dependent and initial (i. e., time =0) stresses, respectively, and t and t denote elapsed time and the relaxation time; t is a time-independent constant characteristic of the material. a specimen of some viscoelastic polymer, the stress relaxation of which obeys this equation, was suddenly pulled in tension to a measured strain of 0.69; the stress necessary to maintain this constant strain was measured as function of time.
determine er (9) for this material, in mpa ,if the initial stress level was 3.9 mpa (560 psi) which dropped to 0.65 mpa (93 psi) after 46 s.