20.4 A viscoelastic polymer that can be assumed to obey the Boltzmann superposition principle is subjected to the following deformation cycle. At a time, t = 0, a tensile stress of 20 MPa is applied instantaneously and maintained for 100 s. The stress is then removed at a rate of 0.2 MPa s-1 until the polymer is unloaded. If the creep compliance of the material is given by J(t) Jo1-exp {}) where Jo=3 m²/GPa To = 200 s determine (a) The strain after 100s (before the stress is reversed) (b) The residual strain when the stress falls to zero.