$7.($a$)$ Explain creep, stress relaxation and recovery. How can they be measured using Maxwell, Kelvin and Four element models?

$($b$)$ Derive Voigt model for stress relaxation as well as creep condition.

$8.($a$)$ Explain the utility of WLF equation.

$($b$)$ The viscoelastic behaviour of polyethylene is represented by a spring and dashpot element having constant of $2G\frac{N}{m^2}$ and $90G\frac{N}{m^2},$ respectively. If a stress of $12M\frac{N}{m^2}$ is applied for $100$ seconds and then completely removed. Compare the values of strain predicted by Maxwell and Kelvin$-$Voigt models after $50$ seconds and $150$ seconds.