For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to

where ?(t) and ?(0) represent the time-dependent and initial (i.e., time = 0) stresses, respectively, and t and ? denote elapsed time and the relaxation time; ? is a time-independent constant characteristic of the material. A specimen of some viscoelastic polymer the stress relaxation of which obeys Equation 15.10 was suddenly pulled in tension to a measured strain of 0.6; the stress necessary to maintain this constant strain was measured as a function of time. Determine Er(10) for this material if the initial stress level was 2.76MPa (400psi), which dropped to 1.72MPa (250psi) after 60 s.

(15.10) O(t)=0(0) exp