1. **State the problem:** Calculate the increase in stress on steel when the temperature rises from 12°C to 40°C. 2. **Given data:** - Initial temperature $T_1 = 12^\circ C$ - Final temperature $T_2 = 40^\circ C$ - Coefficient of linear expansion $\alpha = 0.000011$ per $^\circ C$ - Young's modulus $E = 210$ kN/mm² 3. **Formula used:** The thermal stress $\sigma$ induced due to temperature change when the material is constrained is given by: $$\sigma = E \alpha \Delta T$$ where $\Delta T = T_2 - T_1$ is the temperature change. 4. **Calculate temperature change:** $$\Delta T = 40 - 12 = 28^\circ C$$ 5. **Calculate increase in stress:** $$\sigma = 210 \times 0.000011 \times 28$$ $$\sigma = 210 \times 0.000308 = 0.06468 \text{ kN/mm}^2$$ 6. **Interpretation:** The increase in stress on the steel due to the temperature rise is approximately $0.0647$ kN/mm². This means the steel will experience an additional tensile stress of about $0.0647$ kN/mm² if it is constrained and cannot expand freely.