Determination of Poisson's ratio from axial/transverse strain 2

[shimo1989]

I just did a tensile test on a piece of cold-rolled 1018 steel specimen to determine the properties of the batch of steel. With all the data in Excel, I have a column for axial strain, and a column for transverse strain.

To find the Poisson's ratio of the material, would it be better to find the value of the Poisson's ratio at each point, plot against axial stress, and use the most frequently appearing value (i.e. the value of the Poison's ratio at the flattest region of the Poisson's ratio vs. stress graph), or to plot transverse stress against axial stress, and use the negative of the slope?

Interestingly enough, the first method yields a value of 0.26, while the second method, yields 0.44. This is mostly because the second method produces a graph that has 2 distinct linear regions with different slopes.

 

[CoryPad]

Did you load the material beyond the yield point? It sounds like you did. Poisson's ratio is only valid in the elastic regime. During plastic deformation, Poisson's ratio is replaced with a plasticity factor and happens to be exactly equal to 0.5. Use the 0.26 value.

 

[shimo1989]

Yay! Turns out I was using data points from after plastic deformation. So of the two linear regions, the first one (closer to the origin of the graph) actually has a slope of -0.26.

Thanks for the help!