Sequential metamaterials with plasticity

Geometry controls functionality is the dominant paradigm in metamaterial design. This is especially true in mechanics, where many designs exploit a mechanical instability known as buckling. Buckling has primarily been designed by geometry: the aspect ratio of the slender elements making up the metamaterials or the geometric nonlinearities that come from internal rotations, rather than material nonlinearities. Material nonlinearities such as hyperelasticity and viscoelasticity have been used as design tools to create buckling-based metamaterials, yet the use of plasticity—a ubiquitous material nonlinearity displayed by most solids—has so far remained unexplored. In fact, plastic deformations have traditionally been seen as a failure mode and have therefore been carefully avoided. In this thesis, we embrace plasticity instead and discover a delicate balance between plasticity and buckling instability, which we term “yield buckling.” We exploit yield buckling to design metamaterials across a variety of geometric architectures and elastoplastic materials that buckle sequentially in an arbitrarily large sequence of steps. These sequential metamaterials demonstrate superior shock absorption performance, controllable buckling sequences, a tunable number and shape of buckling modes, and multishape self-assembly governed by loading history. Our findings represent a paradigm shift for metamaterial design by embracing plasticity and using it in tandem with geometrical design.