including both the longitudinal and hoop oriented samples. Sample shape C is used for
all future testing for convenience and predictability.
Table 3.1 Experimental elastic modulus results for three test sample shapes
Sample Shape and Description Orientation Elastic Modulus (psi)
A Folded Continuous Tube Longitudinal 49009
B Folded Cut Tube Longitudinal 38148
C Strip from Template Longitudinal 36098
3.3 Data Analysis and Results
This section describes the analysis of the data collected and its application
towards the orthotropic material properties. Figures 3.10 and 3.11 show plots of axial
stress and axial length-strain for VF-600 material in hoop and longitudinal orientations,
respectively. Five samples per orientation were tested and are plotted with distinct linear
trends. In order to interpret one linear trend from all five tests, all the tests are grouped
together with one trend as shown in Figures 3.12 and 3.13. Combining five distinct
trends is justified by looking at the slope and y-intercepts of the trends. The trend from
each test has a distinct slope, but they are all close enough to consider within
experimental error. The y-intercepts between tests, however, are largely different. The
y-intercept of each trend line indicates an initial load present on the sample before any
length-strain is recorded. This is explained by the observed behavior of the test grips
during the test. The self-cinching grips initially sink in and tighten around the soft
material for a brief period of time before a length-strain is present in the entire sample.
This settling period is interpreted as a pre-loading on the sample by the load cell,
resulting in the y-intercept in the data trend. The presence of the y-intercept is a result of