Variable Stiffness Structures Utilizing Fluidic Flexible Matrix Composites

In this research, the capability of utilizing fluidic flexible matrix composites ((FMC)-M-2) for autonomous structural tailoring is investigated. By taking advantage of the high anisotropy of flexible matrix composite (FMC) tubes and the high bulk modulus of the pressurizing fluid, significant changes in the effective modulus of elasticity can be achieved by controlling the inlet valve to the fluid-filled (FMC)-M-2 structure. The variable modulus (FMC)-M-2 structure has the flexibility to easily deform when desired (open-valve), possesses the high modulus required during loading conditions when deformation is not desired (closed-valve locked state), and has the adaptability to vary the modulus between the flexible/stiff states through control of the valve. In the current study, a 3D analytical model is developed to characterize the axial stiffness behavior of a single (FMC)-M-2 tube. Experiments are conducted to validate the proposed model, and the test results show good agreement with the model predictions. A closed/open modulus ratio as high as 56 times is achieved experimentally. With the validated model, an (FMC)-M-2 design space study is performed. It is found that by tailoring the properties of the FMC tube and inner liner, a wide range of moduli and modulus ratios can be attained. By embedding multiple (FMC)-M-2 tubes side by side in a soft matrix, a multi-cellular (FMC)-M-2 sheet with a variable stiffness in one direction is constructed. The stiffness ratio of the multi-cellular (FMC)-M-2 sheet obtained experimentally shows good agreement with a model developed for this type of structure. A case study has been conducted to investigate the behavior of laminated [+60/0/-60](s) multi-cellular (FMC)-M-2 sheets. It is shown that the laminate can achieve tunable, steerable, anisotropy by selective valve control.