Adaptive stiffness structures via additively manufactured fluid accumulators

Lightweight mechanical structures often have low stiffness that prevents their use in structural applications. The demand for lightweight mechanical structures that operate under wide-ranging loading conditions motivates the development of adaptive stiffness structures. The ability to control the stiffness of a mechanical structure allows for tailored static and dynamic properties, including resonant frequencies. However, adaptive stiffness structures that are low cost, offer design flexibility, and can be additively manufactured still remain a challenge. To this end, we introduce adaptive stiffness devices called pressure-actuated adaptive structural cells (PASCells) with controllable axial stiffness. The proposed PASCells consist of four, flat arches that seal at the edges to contain a working fluid. The axial stiffness of the PASCell increases when the enclosed working fluid is compressed due to volume reduction under an axial load. Axial compression of a PASCell creates large internal volume change and internal pressure that resists this compression, increasing stiffness when the fluid volume is constrained by, for example, closing an outlet valve. Designed for additive manufacturing, PASCells can be integrated with mechanical structures to enable adaptive stiffness. In this paper, we derive the governing equations that describe the static deformation of PASCells under an axial load and internal pressurization and experimentally evaluate the stiffness of the PASCells in empty (or open valve) and filled (or closed valve) configurations. Single, series-connected, and parallel-connected PASCells are additively manufactured and experimentally tested, verifying the model predictions, and experimentally demonstrating a 70% stiffness increase.