Active Tuning of Variable Stiffness Structures utilizing Fluidic Flexible Matrix Composites

A variable stiffness adaptive structure idea that has the potential to achieve orders of magnitude change in stiffness has been proposed. Such components could have potential applications in systems such as soft robotics, isolation mounts, and morphing aircraft. The new adaptive system is a multicellular structure composing many small-diameter fluidic flexible matrix composite (F²MC) tubes integrated into supporting matrix materials.

The F²MC tubes are fluid-filled composite tubes where the tubes consist of multiple layers of oriented, high performance fibers such as carbon in a flexible matrix. Due to the internal fluid having a high bulk modulus (e.g. water, oil), significant changes in stiffness can be obtained by simply opening or closing an inlet valve to the F²MC cells. With an open valve, the new adaptive structure can be very flexible.

On the other hand, when the valve is closed, the fluid resists volume change due to its high bulk modulus, and because of the fiber reinforcement, the constrained F²MC structures will develop very high stiffness. In other words, the variable stiffness adaptive structure has the flexibility to easily deform when desired (open valve) and possesses the high stiffness required under loading conditions when deformation is not desired (closed valve – locked state). By actively controlling the flow and pressure in the F²MC cells by use of a variable orifice/valve and a feedback control system, the adaptive structure can achieve a desired force-displacement (stress-strain) trajectory as seen in the figure, thus possessing the unique ability to change stiffness online.

Some example publications:

• Shan, Y., Philen, M., Lotfi, A., Li, S., Bakis, Rahn, C. D, Wang, K. W., “Variable Stiffness Structures Utilizing Fluidic Flexible Matrix Composites,” Journal of Intelligent Material Systems and Structures,  Vol. 20, No. 4, pp. 443-456, 2009.
• Philen, M., Shan, Y., Prakash, P., Wang, K. W, Rahn, C. D., Zydney, A. L., and Bakis, C. E., “Fibrillar Network Adaptive Structure With Ion Transport Actuation for High Strain and High Blockstress Applications,” Journal of Intelligent Material Systems and Structures.  Vol. 18, No. 4, pp. 323-334, 2007.
• Shan, Y., Philen, M., Bakis, C. E., Wang, K. W, and Rahn, C. D., “Nonlinear-Elastic Finite Axisymmetric Deformation of Flexible Matrix Composite Membranes under Internal Pressure and Axial Force,” Composites Science and Technology, Vol. 66, pp. 3053-3063, 2006.
• Philen, M., “Force Tracking Control of Fluidic Flexible Matrix Composite Variable Stiffness Structures”, Journal of Intelligent Material Systems and Structures,” Journal of Intelligent Material Systems and Structures, Vol. 22, pp. 31-43, 2011.
• Zhang, Z., Philen, M., "Modeling, analysis, and experiments of inter yarn compaction effects in braided composite actuators," Journal of Composite Materials, Vol. 47, No. 25, 2012.
• Philen, M., "Fluidic Flexible Matrix Composite Semi-Active Vibration Isolation Mounts," Journal of Intelligent Material Systems and Structures, Vol. 23, No. 3, pp. 353-363, 2012.