For some viscoelastic polymers that are

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

where s(t) and s(0) represent 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 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.