Design of phononic crystals for self-collimation of elastic waves using topology optimization method

Self‐collimating phononic crystals (PCs) are periodic structures that enable self‐collimation of waves. While various design parameters such as material property, period, lattice symmetry, and material distribution in a unit cell affect wave scattering inside a PC, this work aims to find an optimal material distribution in a unit cell that exhibits the desired self‐collimation properties. While earlier studies were mainly focused on the arrangement of self‐collimating PCs or shape changes of inclusions in a unit cell having a specific topological layout, we present a topology optimization formulation to find a desired material distribution. Specifically, a finite element based formulation is set up to find the matrix and inclusion material distribution that can make elastic shear‐horizontal bulk waves propagate along a desired target direction. The proposed topology optimization formulation newly employs the geometric properties of equi‐frequency contours (EFCs) in the wave vector space as essential elements in forming objective and constraint functions. The sensitivities of these functions with respect to design variables are explicitly derived to utilize a gradient‐based optimizer. To show the effectiveness of the formulation, several case studies are considered.