Folding of Hyper-Elastic Fiber Composites
Francisco Lopez Jimenez
Most space structures include elements that are deployed after launch. This has traditionally been achieved with mechanical elements, such as joints, that allow relative motion between parts of the structure. In more recent alternatives, this motion is achieved by elastically straining the structural elements, usually constructed with very thin fiber composites.
In order to further improve the packing ratio of such structures, it is necessary to find materials that can be deformed elastically to very high curvatures. One of the possible candidates are composites in which the fibers are bonded by a very soft and flexible matrix, such as silicones and elastomers. Such materials retain the high stiffness along the direction of the fibers of composites made with traditional stiff matrix, while at the same time can be folded to a much higher curvature. They have already been used to build models showing exceptional folding capabilities. However, the mechanics allowing this behavior are not properly understood yet.
The aim of this project is to characterize the mechanical behavior of fiber composites with silicone matrix, and to create the analytical and numerical tools that are necessary to incorporate this kind of material in a real design. In particular, the research focuses in the micromechanics of the fibers within the matrix, and how elastic microbuckling allows the fibers to reduce their overall strain. This phenomenon also produces a very nonlinear moment-curvature relationship. Another difference with traditional composites is the existence of strain softening, due to the high strains that the matrix needs to undergo to accommodate the fiber deflection.
These features have been measured experimentally in materials produced in the Space Structures Laboratory. Several finite element simulations of the material hve been produced. They are able to capture the micromechanics of the fibers, including microbuckling and the nonlinearity. The introduction of cohesive elements between fibers and matrix allows the modeling of the damage process as an effect of fiber debonding. The models also take into account the fiber arrangement observed in the real material, instead of using a regular matrix.
- Lopez Jimenez, F., Pellegrino, S. (2009). Folding of thin-walled composite structures with a soft matrix. 50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference. 4-7 May 2009, Palm Springs, CA, AIAA-2009-2633. (pdf)