$1.$ explain in brief the following :

a$)$ poisson's ratio

b$)$ corrosion allowance

c$)$ joint efficiency

d$)$ vaccum

e$)$ stress and strain

$2).$ A torque of $2\mathrm{.}5$kN is transmitted by a hollow shaft of thickness $2\mathrm{.}0$ mm$.$ design the shaft if the elastic limit of the material under simple tension is $350$ kN$/$m$^2$ and the factor of safety is $2\mathrm{.}5.$ use the following theories of faliure :

a$)$ maximum shearing stress

b$)$ maximum strain energy theroy