The r in Lame' equations is a radius with a value between a and b used to compute the stress at that particular radius. Compute the equation with a series of increasing r values ranging from a to b to get values for a plot of the stress across the wall. The denominator in the second half of the equation should be (b^2-a^2)r^2. Because of the negative sign in the middle, I believe the equation you presented is for radial stress NOT "hoop" or tangential stress. This equation is for thick walled cylinders, only. See this site for a simpler equation when Po=0, equation development, and definitions. Example calculations can be found on related pages at this site: http://www.utm.edu/departments/engin/lemaster/Machine%20Design/Notes%2013.pdf
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