Shearing stress causes change in

Consider a long steel bar under a tensile stress due to force $F$ acting at the edges along the length of the bar (as shown in figure). Consider a plane making an angle $\theta$ with the length. What are the tensile and shearing stresses on this plane?
$(a)$ For what angle is the tensile stress a maximum?
$(b)$ For what angle is the shearing stress a maximum?

$A$ cube of aluminium of sides $0.1 \, m$ is subjected to a shearing force of $100 \, N$. The top face of the cube is displaced through $0.02 \, cm$ with respect to the bottom face. The shearing strain would be

The load versus elongation graph for four wires of same length and the same material is shown in the figure. The thinnest wire is represented by the line:

Which of the following statements is correct?

Why do rail tracks have an '$I$-shaped' girder?