Contact-aided Cellular Compliant Mechanisms (C³M)
Compliant Mechanisms (C3M) are cellular structures with novel integrated contact mechanisms that provide stress relief. C3M are capable of very large strains compared to their bulk material constituents, and, due to the stress relief, are capable of even greater strains than their non-contact cellular counterparts.
We are working to develop integrated design and fabrication methods for high-strength high-strain ceramic C3m. Ceramic materials are of interest because of their high strength particularly at the mesoscale, with over 2 GPa bend strength, and potential high temperature capability. Bulk ceramic materials also have high strength, but low strain at failure, perhaps 0.2 – 1 percent, depending on the size. In contrast, ceramic C3M are capable of ultimate strains of 11 to 13 percent, an order of magnitude higher than the ultimate strain of the bulk material. The result is a material that has the potential to be used in large strain applications. The fabrication methods for ceramic C3M are based on the novel mesoscale ceramic fabrication known as the lost mold, rapid infultration forming (LM-RIF) pioneered by Dr. Adair and Dr. Frecker. THus, the combination of high strength by fabrication and high strain to failure by design will be used to create novel ceramic C3M materials and structures suitable for applications ranging from morphing aircraft to damage tolerant composite armor. This project brings together expertise in materials, fabrication, modeling, and design. The design and manufacturing requirements are closely linked, where the design requirements from the top down direct the manufacturing capabilities from the bottom up, as shown in Figure 1.
Figure 1: Hierarchical manufacture demonstrated for aerospace structures. A. Nanoparticulates microfabricated into B. a mesoscale cellular contact-aided compliant mechanisms capable of lateral strains of 13%, integrated into C. Prototype tape appliqué for assembly into D. Large integrated structural systems.
Cellular structures can provide a high overall elastic strain, which may be as high as 25 times the core material elastic strain, They exist in various shapes, materials, etc. Some examples of these structures may be found in Figure 2.
Contact between two surfaces reduces the maximum stress. Reduction of failure causing stress allows more elastic deformation and, therefore, more strain as seen in Figure 3.
A combination of contact and cellular structures could be a promising solution to creating these high-strength high-strain ceramic C3M parts.
Figure 2: Various cellular structures , 
Figure 3: Illustration of contact induced stress relief
Proposed Cellular Structure
The proposed cellular structure incorporates both a cellular structure and contact for stress relief. The cellular structure is an auxetic cellular structure as seen in Figure 4. The mechanism seen inside each of the unit cells in Figure 4 is a contact mechanism. As the cellular structure is stretched, the two separate components of the contact mechanism come in contact with each other, providing stress relief in the structure.
Figure 4: A 5-cell model of the contact-aided cellular structure and its stress strain plot
|Material Strain||Cellular structure without contact||Contact-aided cellular structure|
Figure 5: Application of cellular structures to morphing skin. As the number of cells per unit area increases the structural mass decreases. In general, contact-aided designs have lower mass. Also there are more feasible designs using contact as compared to non-contact designs.
The processing of micron scale ceramic and metal parts for contact aided mechanisms (C3M), has been investigated. In this work we have demonstrated that it is possible to manufacture both ceramic (3YSZ) and metal (316L SS) parts.
a) SU8 photoresist molds are fabricated on refractory substrates using a modified lithography technique.
b) Molds are filled with a high vol% particulate slurry consisting of either yttria stabilized zirconia or 316L stainless steel via a screen printing technique.
c) The entire assembly is fired to remove the mold and sinter the part; leaving free standing parts on the substrate.
d) Final parts show feasibility of creating both ceramic and metal C3M structures at this size scale.
Process results may be found in Figure 6. Future work includes fabrication of arrayed structures and mechanical testing.
Figure 6: Process results
Future work includes further optimization of the current cellular structure. As the C3M parts are manufactured, cracking tends to occur in the parts as they dry due to capillary forces as water leaves the parts. It has been shown that rounded parts tend to result in less cracking due to these forces. We are working to modify the existing cellular structure to include parts that have more rounded edges. Throughout this design process, we are also working to achieve an optimal shape that will be able to withstand high strains comparable to or greater than the current designs.
 Shaw, J. A., Grummon, D.S., and Foltz, J., “Superelastic NiTi Honeycombs: Fabrication and Experiments,” Smart Mater. Struct., 16, pp. 170 – 178, 2007
 Bornengo, D., Scarpa, F., Remillat, C., “Evaluation of Hexagonal ChiralStructure for Morphing Airfoil Concept,” Inst. Mech. Enging. Part G, 219(3), pp. 185 – 192, 2005
|Dr. Mary Frecker||The Pennsylvania State University||National
|Dr. James Adair|
|Dr. George Lesieutre|
|Jennifer Hyland||The Pennsylvania State University|