Thick Walled Cylinder Question

[PipesAndStuff]

Hello,

I'm determining the change in length of an 8-inch (DIPS) SDR11 HDPE pipe after pressurizing due the Poisson effect. I'm treating the pipe as a thick walled cylinder since the pipes diameter/thickness < 20.
I'm using the attached equations to compute axial, tangential, and radial stresses. Then computing the axial strain and multiplying by the pipe's length.

Here are the parameters:
Poison Ratio = 0.45
Elastic Modulus = 150,000 psi
Inside Radius = 3.7 inches
Outside Radius = 4.5 Inches
Inside Pressure = 180 psi
Pipe Length = 1000 ft

When I compute the stress for the inside wall of the pipe, I get a change in length of 29.1 Inches.
When I compute the stress for the outside wall of the pipe, I get a change in length of 2.9 inches.

I'm not sure which result to use or if I am approaching this problem correctly. I appreciate any insight people have for this problem.

Thanks,
Tony

 

[LittleInch]

Use mid wall?

Expansion from pressure is dwarfed by thermal effects in PE.

 

[prex]

If you look at your equations, you'll find that sigma_theta+sigma_r=const, so the longitudinal elongation (negative, it is a contraction) is, of course, independent of r and is easily found from the strain eq. for eps_z (sigma_z being, of course, null everywhere for a free ends pipe).
Pls post this kind of questions in the students forum.