Bistable Auxetic Mechanical Metamaterials Inspired by Ancient Geometric Motifs

A. Rafsanjani; D. Pasini

Bistable Auxetic Mechanical Metamaterials Inspired by Ancient Geometric Motifs

Citation:

A. Rafsanjani and D. Pasini. 12/2016. “Bistable Auxetic Mechanical Metamaterials Inspired by Ancient Geometric Motifs.” Extreme Mechanics Letters, 9, Pp. 291-296. Publisher's Version

Abstract:

Auxetic materials become thicker rather than thinner when stretched, exhibiting an unusual negative Poisson’s ratio well suited for designing shape transforming metamaterials. Current auxetic designs, however, are often monostable and cannot maintain the transformed shape upon load removal. Here, inspired by ancient geometric motifs arranged in square and triangular grids, we introduce a class of switchable architected materials exhibiting simultaneous auxeticity and structural bistability. The material concept is experimentally realized by perforating various cut motifs into a sheet of rubber, thus creating a network of rotating units connected with compliant hinges. The metamaterial performance is assessed through mechanical testing and accurately predicted by a coherent set of finite element simulations. A discussion on a rich set of mechanical phenomena follows to shed light on the main design principles governing bistable auxetics.

Notes:

[available on arXiv]

Watch a video about this work on Vimeo!

A US Patent (US2017/0362414A1) is filed for Bistable Auxetics and we are actively seeking commercialization opportunities.

Featured in PhysicsWorld Magazine, APS News, BBC News, Ahmad's interview with BBC Newshour, BBC Newsroom report, NewScientist, PhysicsWorld, Creators, Science, McGill Tribune, Smithsonian Magazine, PNAS

Last updated on 03/15/2018

Recent Publications

A. Rafsanjani, Y. Zhang, B. Liu, S. M. Rubinstein, and K. Bertoldi. 2/21/2018. “Kirigami skins make a simple soft actuator crawl.” Science Robotics, 3, 15, Pp. eaar7555. Publisher's Version Abstract

Bioinspired soft machines made of highly deformable materials are enabling a variety of innovative applications, yet their locomotion typically requires several actuators that are independently activated. We harnessed kirigami principles to significantly enhance the crawling capability of a soft actuator. We designed highly stretchable kirigami surfaces in which mechanical instabilities induce a transformation from flat sheets to 3D-textured surfaces akin to the scaled skin of snakes. First, we showed that this transformation was accompanied by a dramatic change in the frictional properties of the surfaces. Then, we demonstrated that, when wrapped around an extending soft actuator, the buckling-induced directional frictional properties of these surfaces enabled the system to efficiently crawl.

Featured on Science Robotics Cover, Harvard SEAS News, Scientific American, Wired, The Verge, IEEE Spectrum, Digital Trends, NewsWeek, LA Times, DailyMail, BBC Click

X. Shang, L. Liu, A. Rafsanjani, and D. Pasini. 2/14/2018. “Durable Bistable Auxetics Made of Rigid Solids.” Journal of Materials Research, Focus Issue: Architected Materials, 33, 3, Pp. 300-308. Publisher's Version Abstract

Bistable Auxetic Metamaterials (BAMs) are a class of monolithic perforated periodic structures with negative Poisson’s ratio. Under tension, a BAM can expand and reach a second state of equilibrium through a globally large shape transformation that is ensured by the flexibility of its elastomeric base material. However, if made from a rigid polymer, or metal, BAM ceases to function due to the inevitable rupture of its ligaments. The goal of this work is to extend the unique functionality of the original kirigami architecture of BAM to a rigid solid base material. We use experiments and numerical simulations to assess performance, bistability and durability of rigid BAMs at 10,000 cycles. Geometric maps are presented to elucidate the role of the main descriptors of BAM architecture. The proposed design enables the realization of BAM from a large palette of materials, including elastic-perfectly plastic materials and potentially brittle materials.

[available on arXiv]
Featured in MRS Bulletin

F. Javid, J. Liu, A. Rafsanjani, M. Schaenzer, M. Q. Pham, D. Backman, S. Yandt, M. C. Innes, C. Booth-Morrison, M. Gerendas, T. Scarinci, A. Shanian, and K. Bertoldi. 10/2017. “On the design of porous structures with enhanced fatigue life.” Extreme Mechanics Letters, 16, Pp. 13-17. Publisher's Version Abstract

Many components of gas turbines, including the combustion liners, ducts, casings and sealing structures, comprise metallic sheets perforated with arrays of circular cooling holes. These parts are highly prone to fatigue failure due to the stresses induced by temperature variations during operation. Here, we demonstrate both experimentally and numerically that the fatigue life of these porous components can be greatly enhanced by carefully designing the pores’ shape. In particular, we show that while the fatigue life of a metallic sheet with a square array of conventional circular cooling holes is <100k cycles, by replacing the pores with novel orthogonal S-shaped holes the life of the structure increases up to more than one million cycles. This is because the S-shaped pores introduce a soft mode of deformation based on rotation of the domains between neighboring holes that significantly affect the stress distribution and crack propagation in the structure.

Ahmad Rafsanjani

Harvard John A. Paulson School of Engineering and Applied Sciences

Bistable Auxetic Mechanical Metamaterials Inspired by Ancient Geometric Motifs

Citation:

Abstract:

Notes:

Recent Publications