# Calculating Poisson's ratio

1. Oct 28, 2005

### bono141

Hi, im new to this so bear with me,

I'm stuck on a question, basically, i need to work out the poissons ratio for a solid cylinder 140mm dia by 4.0m long to find out if it's steel or aluminium. I know the formula for poisson ratio is lateral strain / axial strain. The question states that there is a force acting on the cylinder but gives no values, how would I go about working out the poisson's ratio? Any help would be much appreciated.

Many Thanks
Scott.

2. Oct 28, 2005

### Staff: Mentor

Does the problem provide stresses or strains, and the elastic or bulk modulus?

One would need to know the axial and lateral strains, or a force/stress.

Please write the problem as given.

3. Nov 1, 2005

### bono141

the whole question

A circular cross section tie rod located in a structure has a diameter of 140mm and a length of 4.0m. If under load the length increases by 1.2mm and the diameter decreses by 0.11mm, identify the relationship between lateral and transverse strain and determine the value of poisson's ratio.

I have no idea how to go about completing this one. Any help would be greatly appreciated.

4. Nov 1, 2005

### FredGarvin

What is the definition for strain? You have two things you need to calculate in order to calculate Poisson's Ratio, the axial strain and the lateral strain. You are given all of the pieces of the two parts. What two numbers will make up the axial strain? What two numbers will make up the lateral strain?

5. Nov 1, 2005

### bono141

continued...

the strain formula is: change divided by original

the 4.0m & 1.2mm increase is for the lateral strain and the 140mm and 0.11mm decrease is for the axial strain i beleive.

6. Nov 1, 2005

### FredGarvin

Right. Now just make sure you are dealing with the same units for each number. Either m or mm, it doesn't matter, just make sure you use only one.

7. Aug 29, 2011

### Kent B

Okay I know this thread is so old now, but I'm working a similar problem and its also asking for me to find the modulus of elasticity.

I know the basic formula for it is normal stress divided by the normal strain.

So I'm thinking the normal strain is the same as the axial strain?
Also when it says the normal stress does that mean I calculate the Initial stress with the undeformed diamter or do i calculate both of them and just use the difference of the two?