Recently, flexible matrix composite (FMC) actuators were demonstrated in a robotic fish for swimming. When actuated at a specific frequency in the experiments, the sinusoidal component of the thrust was eliminated, leaving only a constant thrust. This elimination of the sinusoidal component of the thrust is due to the hydroelastic tailoring of the tail stiffness with the actuation frequency. The FMC actuators are pressure-driven muscle-like actuators capable of large displacements as well as large blocking forces. The FMC actuators can also exhibit a large change in stiffness through simple valve control when the working fluid has a high bulk modulus. Several analytical models have been developed that capture the geometrical and material nonlinearities, the compliance of the inner liner, and entrapped air in the fluid. This paper focuses on the inter fiber compaction in the composite laminate, which is shown to reduce the effective closed-valve stiffness. In this paper, a new analytical model considering the inter fiber compaction effect as well as the material and geometric nonlinearities is developed. Analysis and experimental results demonstrate that the new compaction model can improve the prediction of the response behavior of the actuator.

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