# Homework Help: How to find Poisson's ratio?

1. Jun 8, 2015

### theone

1. The problem statement, all variables and given/known data
I have some strain readings using a rosette gauge, but I want to know how to calculate the poissons ratio for the steel pressure vessel

2. Relevant equations

3. The attempt at a solution
I tried Strain A/Strain B and got an average value of 0.4.
But the provided value for steel is around 0.29

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2. Jun 10, 2015

### DEvens

http://en.wikipedia.org/wiki/Poisson's_ratio

It is not clear that your strain gauges are measuring the right thing. Indeed, from the diagram it is not clear what they are measuring at all.

But basically, you want to be measuring the thickness of the wall as a function of the wall being stretched. The diagram seems to be showing that you are measuring changes of the surface, and not measuring changes in the thickness.

The tank may be stretching circumferentially much more than axially. If it is much longer than its radius this is likely to be true. A little bit of geometry should give you a fair estimate of the relative forces and the relative amount of material that is stretching.

3. Jun 11, 2015

### andrevdh

The measurements indicates that the material is stretching much more along the axis
of B, which is to be expected since the internal pressure on the container is pushing
the walls outwards. The hemispherical structures at the ends also accomodates the
expansion of the tank along its circumference.