What Is the Correct Way to Calculate Poisson's Ratio?

[tomtomtom1]

Hello all

I was hoping someone could help clarify Poissons ratio.

I understand that it is Lateral Strain / Axial Strain but I wanted to understand it in the context of an example.

The question is:-
At 5kN compression load
Original Depth = 30mm
Change in Depth = 2mm
Original Height = 100mm
Change in height= 5mm

I would calculate Poissons ratio by doing the following:-

((2/30) / (5/100))*-1 = -1.4

However I have been told that this is wrong and the correct answer should be:-

((2/30) / (-5/100))*-1 = 1.4

Why is it -5?

I am assuming that the height was reduced by 5mm due to the compression then this is what makes it a negative is this correct?

Thank you.

 

[JBA]

The standard convention is that tension loads are positive and compressive loads are negative.