Τ Problem 5: For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to o(1) = o(0) exp(--) where a(t) and o(0) represents the time-dependent and initial (i.e., time =0) stresses, respectively, and t and * denote elapsed time and the relaxation time; t is a time-independent constant characteristic of the material. A specimen of some viscoelastic polymer was suddenly pulled in a tension to certain strain/length and kept at that strain/length for some time. Calculate initial stress (0) and relaxation time constant t if measured stress was 2.5 and 1.9 MPa at 10 and 60 sec respectively (15 points).