Mechanical metamaterials harnessing buckling instabilities

A range of instabilities can occur in soft bodies and structured materials that undergo large deformation. The goal of this work is to design soft reconfigurable porous structures, that use nonlinear mechanical instabilities to dramatically tune their shape, and with that their mechanical (or other) functionalities. For example, when applying a compressive load to a porous structures with circular holes, a sudden pattern transformation occurs changing the effective Poisson’s ratio of the structure from positive to negative. Moreover, by changing the pore shape of this structure we can not only tune the non-linear stiffness and Poisson’s ratio, but also the type of instability that occurs, i.e. microscopic or macroscopic instability. Both numerical and experimental work has been done to characterize these porous structures under uniaxial compression.

Coulais, C., Overvelde, J. T. B., Lubbers, L. A., Bertoldi, K., van Hecke, M., (2015). Discontinuous Buckling of Wide Beams and Metabeams. Physical Review Letters, 115, 044301. [pdf]

Overvelde, J. T. B., Bertoldi, K., (2014). Relating Pore Shape to the Non-linear Response of Periodic Elastomeric Structures. Journal of the Mechanics and Physics of Solids, 64, 351-366. [pdf]

Overvelde, J. T. B., Shan, S., Bertoldi, K., (2012). Compaction Through Buckling in 2D Periodic, Soft and Porous Structures: Effect of Pore Shape. Advanced Materials, 24(17)2337-2342[pdf]

Tensile instabilities in mechanical metamaterials

While most of them arise under compressive forces, it has previously been shown analytically that a tensile instability can occur in an elastic block subjected to equitriaxial tension. Guided by this result, we conducted centimeter-scale experiments on thick elastomeric samples under generalized plane strain conditions and observed for the first time this elastic tensile instability. We found that equibiaxial stretching leads to the formation of a wavy pattern, as regions of the sample alternatively flatten and extend in the out-of-plane direction. Our work uncovers a new type of instability that can be triggered in elastic bodies, enlarging the design space for smart structures that harness instabilities to enhance their functionality.

Overvelde, J. T. B., Dykstra, D. M. J., de Rooij, R., Weaver, J., Bertoldi, K., (2016). Tensile Instability in a Thick Elastic Body. Physical Review Letters, 117, 094301. [Harvard News][web][pdf]