Pressure Vessels: Radial & Hoop Stresses at Inner & Outer Surfaces

[tone999]

Homework Statement

A cylinder has an internal diameter of 30mm and an external diameter of 50mm and is subject to an internal pressure which is 60MPa above that in the air surrounding the cylinder. Determine the radial and hoop stresses at the inner and outer surfaces.

Homework Equations

Basically i know:
internal radius in know as r1
external radius is r2

radial stress: pr1 squared/r2squared-r1squared X (r squared - r2 squared/ r squared)

hoop stresses : pr1 squared/r2squared-r1squared X (r squared + r2 squared/ r squared)

I don't know if these two equations are correct hopefully somebody could clarify this for me because the working out should be simple when this is known.

The Attempt at a Solution

I don't know what "r" is?
If i knew that i could take a stab at the equations hopefully somebody could give me two answers and if the equations are correct and two answers and i will do it myself to check this. Thanks

 

[ShawnD]

I don't know what "r" is?

Radius is half the diameter, which would be 15mm and 25mm. I don't know if your formula is correct but make sure you keep the units consistent; I think MPa requires that you work in meters (m), not milimeters (mm).

 

[tone999]

Yeah I've got that. I mean "r" on its own in the equation. I know the values for r1 - 0.015m
r2 - 0.025m
Internal pressure - 60x10 to the power of 6 Nm2