$1.$Explain in brief the following:

$($a$)$ Poisson's Rartio.

$($b$)$ Corrosion Allowance:

$($c$)$ Joint efficiency.

$($d$)$ Vacuum.

$($a$)$ Stres and Strain

$2.$A torque of $25$ kN m is transmitted by a hollow shaft of thickes $2\mathrm{.}0$ mm$.$ Desige the shaft if the elastic limit of the material under simple tension is $350M\frac{N}{m^2}$ and the factor of safery is $25.$ ues the following theories of failure :

$($a$)$ Maximum shearing stress

$($b$)$ Maximum Strain Energy Theory