When the temperature is at $30^{\circ} \mathrm{C}$, the $\mathrm{A}-36$ steel pipe fits snugly between the two fuel tanks. When fuel flows through the pipe, the temperatures at ends $A$ and $B$ rise to $130^{\circ} \mathrm{C}$ and $80^{\circ} \mathrm{C},$ respectively. If the temperature drop along the pipe is linear, determine the average normal stress developed in the pipe. Assume the walls of each tank act as a spring, each having a stiffness of $k=900 \mathrm{MN} / \mathrm{m}$