Answer
$σ = +58.2 MPa$, tensile
Work Step by Step
Given:
As in Part (a), but now $T_{f} = -15 °C$
Required:
type and magnitude of stress
Solution:
$σ = Eα_{l}(T_{0} - T_{f}) = (97 \times 10^{3} MPa)(20.0 \times 10^{-6} (°C)^{-1})(15 °C - (-15°C)) \\ = +58.2 MPa$
Since the computed value is positive, the stress is tensile.