Session: 06-03-02: Advances in Aerospace Structures and Materials-2

Paper Number: 172602

Binary Stiffness Adaptive Elements in Reprogrammable Metamaterials for Response to Unknown Environments 

Adaptive structures that can change their mechanical behavior are crucial in fields such as aerospace engineering, civil engineering, and robotics, leading to efficiency gains enabled by the adaptations. For example, aircraft that encounter adverse and unexpected flow conditions during flight could adapt their wings into a new shape optimized for these unknown environmental conditions, increasing fuel efficiency and improving stability. However, a challenge in the field of adaptive structures is to realize response to these arbitrary environmental stimuli rather than preprogramming adaptations during design and manufacturing. Mechanically, this is a technically challenging endeavor, as it introduces the need for a structure with “infinite” adaptation capabilities, all while ensuring feasible physical implementation for real systems. This also poses an ill-posed, non-linear inverse problem in which the correct adaptation of the structure must be determined in real time, depending on the loading scenario encountered.

To overcome these challenges, this research proposes a reprogrammable structure comprised of binary stiffness adaptive unit cells. These unit cell elements adapt their axial stiffness by altering the load path through the element via magnetic stimuli, achieving two operational stiffness states. When the elements are in their “stiff” state, load is carried through a thick center column running through the center of the elements. When the elements are in their “soft” state, load is instead carried through an outer, thin compliant section of the element. The stiffness of the unit cell in the “soft” state can be tuned depending on the geometry of the compliant section, allowing for targeted stiffness ratios to be realized. Once these unit cells are assembled into a metamaterial, locally adapting the stiffness of one unit cell globally alters the properties of the structure. Therefore, structural dimensionality increases, allowing the structure to achieve arbitrary mechanical response. Through in situ strain sensing from each unit cell, the structure can iteratively interact with its environment and perform structural state updates, converging to the target mechanical response given unknown environmental stimuli in real time.

In this work, we show a baseline configuration of these binary stiffness unit cells with the use of a compliant, multi-wavelength, circular arch section through geometrically non-linear finite element models in Abaqus. With parametric studies that include changing the radius, thickness, and number of wavelengths in the circular compliant section, we show that the achievable stiffness ratio can be tuned according to the chosen design parameters. We then show experimental versions of these binary stiffness elements that are tested in tensile and compressive loading on the Instron load frame and that validate the Abaqus models, in which a seven times stiffness ratio is realized between the “soft” state and the “stiff” state. Finally, we implement these unit cells into an axially dominated metamaterial and demonstrate real time convergence of the metamaterial response towards a target deformation. This convergence is made possible through feedback from in situ strain sensors that allow for iterative state updates, in which a simple comparative strain algorithm is employed to determine which element must actuate at each iteration to drive the structure towards its target shape. This convergence is achieved in under 7 iterations with small errors (3-5%), displaying real time structural adaptation to arbitrary loading scenarios.

Presenting Author: Catherine Catrambone Stanford University

Presenting Author Biography: Catherine Catrambone is a PhD student in the Aeronautics and Astronautics department at Stanford University. She received her bachelor's degree in Aerospace Engineering from the University of Maryland in 2023, with a research focus on rotorcraft vehicles for the Martian environment. She is now a member of the Reconfigurable and Active Structures Lab with a research focus on adaptive structures for operation in unknown environments.

Authors:

Catherine Catrambone Stanford University
Kai Jun Chen Stanford University
Christopher Sowinski Fort Rucker Army Base
Jacob Mukobi Stanford University
Enzo Andreacchio Stanford University
Enquan Chew Stanford University
Maria Sakovsky Stanford University