Two Novel Uses of Cellular Structures
Open Access
- Author:
- Murray, Gabriel Jon
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Farhan Gandhi, Thesis Advisor/Co-Advisor
Farhan Gandhi, Thesis Advisor/Co-Advisor - Keywords:
- bistable system
Cellular Structures
damping - Abstract:
- This thesis contain two studies with two different objectives unified by their common approach. The first study looks at lossy polymeric infills introduced into metallic honeycombs with the goal that the resulting filled honeycombs simultaneously have high stiffness and an appreciable loss factor. This will be of interest in applications where structural damping is important. The second study looks at creating a bistable cellular structure, using the honeycomb as the basis for the unit cell. Morphing structures is the area where a bistable structure could be of most use. For the filled honeycomb a finite element analysis is conducted with the honeycomb walls modeled with beam elements and the polymeric infill modeled with shell elements. Soft (10 MPa) and stiffer (100 MPa) fillers are used, and the filled honeycombs are oriented to deform along the x- and the y-directions. The honeycomb is modeled as being made of aluminum. This thesis presents a discussion on the cell geometries that yield highest stiffness and loss factor, for loading in the different directions. When the honeycomb is loaded, the Poisson's contraction of the cell walls in the transverse direction squeezes and increases the strain energy in the polymer, to levels significantly greater than that in an isotropic polymer sheet subject to the same global strain. From the results in the study, it was observed that filled Aluminum honeycombs can achieve a Young's Modulus in the range of 2-7 GPa while simultaneously having a loss factor in the range of 5-10% for positive cell angles. Auxetic cells of the same material can achieve a Young's Modulus range of 6-10 GPa while simultaneously having a loss factor in the range of 5-10%. An experiment was conducted to validate the finite element model with the stiffness of both unfilled and filled honeycombs. The experimental results compared favorably with the unfilled honeycombs. The filled honeycomb illustrate the trends expected as the cell geometry is changed. The bistable honeycomb is analyzed using a nonlinear finite element model. The walls of the honeycomb are modeled using beam elements. The honeycomb is assumed to be aluminum, however the material is assume linear. The design variations and modifications required to develop the bistable cell are explained. The final result is a modified honeycomb, which is bistable. The inclined walls are replaced with a sinusoidal curve that has zero slope at the midpoint and the endpoints. A horizontal bar has been added across the honeycomb, connecting the midpoints of the vertical walls. This bar and the vertical walls are considerably thicker than the sinusoidal wall to prevent buckling. The analysis of the bistable honeycomb is broken up into two parts. First the sinusoidal wall is isolated from the rest of the honeycomb, with a spring element standing in for the horizontal bar. The geometric properties are varied to determine the effect on important quantities. This allows for direct observation of the parameters that effect whether the system is bistable or not. Then the unit cell's geometric parameters are varied and the effect on the force-displacement curve is examined. From these two studies, conclusions about how the geometric parameter affect the bistable honeycomb are drawn. The concept is extended to multiple cell honeycombs, a sheet made up of unit cells. It is found that the unit cell results can be successfully extended to that of the sheet. A gradient stiffness of the bistable elements along the morphing direction is examined for controlling the load at which the rows snap-through. This arrangement allows a global load control to reliably transition the rows in a predetermined order, at a predetermined load.