stiffness of the shell63 elements showed an acceptable change in deflection and stress as
shown in Figures 4.10 and 4.11. A value of 10,000 Pa is chosen for the added stiffness.
Merging the shell63 elements is a preferable choice to fix the instability because the
added stiffness has a less interfering effect on the desired results. Implementing this
choice eliminates the spikes and smoothes out the deformed shape.
The isotropic material properties needed to solve for stress and deflection are
elastic modulus and Poisson's ratio. Since the material is orthotropic, hoop elastic
modulus, longitudinal elastic modulus, shear modulus, and Poisson's ratio are required.
Values for elastic modulus in the hoop and longitudinal directions are found
experimentally in Chapter 3. Since Poisson's ratio is not experimentally determined, a
value is chosen based on finite element model results. Simulations showed that varying
Poisson's ratio had negligible effect on longitudinal stress and deflection. Figure 4.12
shows maximum deflection plotted for various Poisson's ratios determined in an ANSYS
model of a cantilevered tube loaded with an end force. Figure 4.13 shows a similar plot
for maximum longitudinal stress using the same model. The small variation in stress and
deflection justifies assuming a reasonable value of 0.4 for Poisson's ratio.