width and thickness is assumed to be constant in Samples A and B. Only the thickness is
assumed constant in Sample C since the varying width was recorded. The instantaneous
axial strain, F, is calculated at each point using Equation 3.1.
s-s
c5- 5 (3.1)
80
The variable, 6, represents the recorded distance between marks and 60 represents the
original distance between marks. This references strain calculations to the original
unstressed length.
An attempt was made to experimentally determine the material Poisson's ratio by
using measurements of the strain in the width and the strain in the length during the
uniaxial pull tests. It was discovered, however, that since the cut samples do not have
stiffness in compression, and the width decreases with the sample under tension, the
width strain is not a valid measure to be used in determining Poisson's ratio. To
accurately describe Poisson's ratio, the material must be loaded in two directions at once.
Two possible solutions are to use a pull tester capable of pulling in two directions, or to
internally pressurize a uniaxial test specimen. Given the complexity of these tests,
Poisson's ratio is instead determined in chapter 4, using a parameter study in an ANSYS
simulation.