where s(t) and s(0) represent the time-dependent and initial (i.e. time = 0) st

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.