For some viscoelastic polymers that are subjected to

Chapter 15, Problem 15.6

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For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to s(t) = s(0) expa - t t b (15.10) 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, respectively; t is a time-independent constantcharacteristic of the material. A specimenof a viscoelastic polymer whose stress relaxationobeys Equation 15.10 was suddenly pulled in tensionto a measured strain of 0.5; the stress necessaryto maintain this constant strain was measuredas a function of time. Determine Er(10) for thismaterial if the initial stress level was 3.5 MPa (500psi), which dropped to 0.5 MPa (70 psi) after 30 s.