Discussion
If the above sample is stressed to 2 MPa initially, then the time required to relax the stress to 1 MPa will be
Answer: B
The stress halves in about 41.6 days.
From the relaxation law, the time to fall from 2 MPa to 1 MPa is
\[ \begin{aligned} \frac{\sigma}{\sigma_{0}} &= e^{-t/\tau} = \frac{1}{2} \\ t &= \tau\ln 2 = 60 \times 0.693 = 41.6\ \text{days} \end{aligned} \]
This is the half-life of the stress, the same for any starting stress. After one relaxation time, 60 days, the stress would be down to 1/e, about 37 percent, which is already below 1 MPa, so 61.6 days is too long; 31.6 and 51.6 do not follow from the exponential law.
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