$($c$)$ A polymer material described by the Voigt model has a spring modulus of

$600$ MPa $.$ Under constant stress, the strain reaches $0\mathrm{.}111$ after $30$ minutes.

After another hour, the strain becomes $0\mathrm{.}264.$ Given that the creep response is

described by

$=\frac{{}_0}{E}[1-$exp$(-\frac{Et}{})]$

calculate the strain in the material three hours after the initial loading and

estimate the time it takes for the strain to reach $0\mathrm{.}001.$

$(6$ marks$)$