Greek Key Inspired Fractal Metamaterials with Superior Stretchability for Tunable Wave Propagation

Stretchable materials that can sustain a large deformation are in high demand, because they find broad applications ranging from stretchable energy storage devices to tunable noise and vibration devices. One main challenge is creating strain‐releasing mechanisms from inherently brittle materials. This work explores a new approach to designing stretchable metamaterials, using a "kerfing" pattern inspired by the ancient Greek Key configuration. The kerfing architecture allows for substantial in‐plane elongation. In‐plane tensile experiments show an ≈8‐times increase in stretchability when the kerfing width is enlarged four times. With higher‐order fractal patterns, the fractal lattice exhibits a stretchability of up to ≈520%, far beyond the inherent deformability of the brittle constituent. Moreover, this design also enables the tunability of various mechanical properties, including stiffness, strength, toughness, and Poisson's ratio. Ashby‐type plots are presented, revealing the relationships between stretchability and other mechanical properties to aid in the design and fabrication of advanced engineering materials. To demonstrate a vital application of the achieved stretchability, elastic wave propagation in the proposed kerfing metamaterials is studied. Simulations indicate that multiple broad phononic bandgaps arise in these structures as the fractal order increases. These bandgaps prove to be adjustable not only through the fractal lattice geometry but also by means of applied mechanical loading. This investigation highlights the potential of fractal‐based layouts as a promising avenue for designing cutting‐edge stretchable metamaterials with customizable mechanical properties and functionalities.