The behaviour of a polymer is found to follow the Maxwell model consisting of a spring

element of modulus $109$ Pa and a dashpot of viscosity $1011$ Pa s at $25$oC$.$ A sample of

this polymer is subjected to the following time sequence of strain $(\backslash$epsi $)$ at $25$oC:

$\backslash$epsi $=0$ for t $<0$;

$\backslash$epsi $=0\mathrm{.}6\%$ for $0<=$ t $<20$ s;

$\backslash$epsi $=1\%$ for $20$ s $<=$ t $<30$ s;

$\backslash$epsi $=2\%$ for t $>=30$ s$.$

Calculate the tensile stress at $35$ s$.$ Assume that, under these conditions, the polymer is

linearly viscoelastic and obeys the Boltzmann superposition principle