Equal forces_ _
applied at each
node around -
circumference
Figure 4.8 Representative application of forces to nodes around circumference
4.1.3 Element Selection and Properties
The geometry is created in ANSYS by extruding a circle along a linear path. This
area is then meshed with rectangular shell elements. The material to be modeled is a thin
fabric with orthotropic material properties. Since fabrics have tensile stiffness, but very
little compressive stiffness, a thin shell element with optional compressive stiffness is the
best choice. The Shell41 element with Keyopt(1) = 2 meets these requirements. It was
determined that the zero compressive stiffness caused a numerical instability in the
solution of a pressurized beam in drag which resulted in spikes in the deformed shape.
Initially, a small elastic foundation stiffness (EFS) was added to the Shell41 elements to
attempt to fix the instability. Successive ANSYS simulations varying the EFS showed a
large change in mean deflection, shown in Figure 4.9. This large deflection makes
choosing a small EFS value unacceptable and eliminates the use of the EFS in all models.
In another attempt to remedy the instability, equivalent-geometry Shell63
elements with small elastic modulus were merged to the Shell41 elements to add a small
compressive stiffness to the elements. Successive ANSYS simulations varying the added