Normal Stress transformation equation:
Below is the derivation of normal stress transformation equation when the axis is inclined at some angle.
For normal stresses:
We will show tension with positive sign and compression with negative sign.
For shear stresses:
If vertical couple is Counter clockwise then (+) sign and if vertical couple is Clockwise (-) sign will use.
If horizontal couple is Counter clockwise then (-) sign and if horizontal couple is clockwise then + sign.