Lamé's equations - wrong notes given? Please help, exam on Monday

[ironlight]

Homework Statement

This relates to using Lamé equations to find the stresses on a thick cylinder, but there are problems with our notes and the exam is on Monday.

For an exam question, the formula I've been given is that sigma r (radial stress) = a + b/r^2, but in the notes (http://www.tobynorris.com/work/prog/cpp/mfc/concyl_hlp/algorithmderivation.htm) it appears that sigma r = a - b/r^2. I can't find an instance where it shows the same equation. Which one is correct? I understand that a is Pi a^2...etc and that b is (Pi-Po)...etc, but which way around is it?

Homework Equations

According to formula sheet in past exam paper:

sigma r = a + b/r^2
sigma c = a - b/r^2

According to notes:

sigma r = a - b/r^2
sigma c = a + b/r^2

The Attempt at a Solution

I can do the equations easily enough, but knowing which way around the plus and minus goes in relation to the radial or hoop stress is making it hard to say which is which in the exam. The question is worth around 35 out of 100 marks, quite considerable in a final year exam, so every point counts.

 

[nilesh_pat]

I'm not sure which one is correct, but it seems likely that the formula on the exam paper is correct. The notes appear to be incorrect, as they are a few years old and may not have been updated in line with current best practice. The notes also seem to be missing some information, such as the values of a and b. It is possible that the notes are incomplete or out of date, so it is best to use the formula on the paper.