Understanding Hierarchy and Multi-stability in Mechanical Metamaterials for Advanced Energy Absorption

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Energy-absorbing materials are essential in engineering applications; however, conventional materials possess inherent shortcomings that limit their utility. For example, foams are lightweight, but the randomness of their internal structure makes their behavior hard to predict. The behavior of rubber-like materials is easier to predict but they are not lightweight compared to foams. This project will devise a new class of foam-like materials with a regular internal structure to improve the predictability of energy absorption while remaining lightweight. Critically, this award will introduce and investigate the impact of new structural features – hierarchy (as seen in bone) and multi-stability (as seen in pop tubes) – with the aim of enhancing the energy absorption performance, including the capacity, efficiency, and directionality. Increasing capacity and efficiency impacts, e.g., aerospace and automotive capabilities, where space/weight limitations call for a material with maximum energy absorption per unit volume/mass. Controlling the absorption directionality permits tailoring the response to the complex loading environment. Understanding the impact of hierarchy and multi-stability can guide the design of new materials for superior performance and promote their utility in practice. Executing the research will also impact the education of undergraduates (approx. 100) where, as a part of their coursework and to reinforce learned principles, they will contribute to the experimental component of the project. In addition, local high school students will be engaged with lectures and hands-on demonstrations to pique their interest in STEM careers.The goal of this project is to devise a new class of cellular metamaterials characterized by hierarchical, multi-stable internal architecture and to analyze the impact of those attributes on the innate absorption performance – specifically, the capacity, the efficiency, and the directionality thereof. In pursuit of this goal, the project aims to accomplish three main objectives: (i) to enhance the absorption capacity and efficiency toward that of an ideal absorber; (ii) to control the directionality of the absorption beyond that of periodic architectures; (iii) to link the absorption performance of minimal surface/volume architectures to their topology. In general, it is hypothesized that hierarchy and multi-stability may work in tandem to control stiffness, peak load, and deformation range that define the load-displacement hysteresis, affecting the absorption capacity and efficiency, not to mention damage tolerance. The specific use of rotation-based snapping elements will open the cellular architecture to non-periodic designs and, thus, permit the absorption directionality to break free of the response imposed by the rotational symmetry of traditional, periodic lattices. Understanding the combined, potentially synergistic, effects of multi-stable snap-through and hierarchy on the mechanical behavior can guide metamaterial design for specific performance and promote metamaterial adoption and utility in practical settings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.