Design, Simulation, and Precision Manufacturing of Flexible-Backing, Morphing Composite Metamaterials that Lock Suddenly in Global Bending

Abstract

Few composite structures, let alone single materials, are known that undergo sudden but reversible stiffening at a pre-engineered strain profile, especially in global bending. This research builds upon this author’s previous disclosure of arrays of rigid tiles adhered to a flexible woven fabric substrate that elastically “lock up” (by increasing their effective bending stiffness by orders of magnitude) at a certain displacement profile, which is reviewed and contrasted with other shell-based metamaterials that stiffen with strain. The research then explores design constraints and the mathematics of tile shape and gaps for initially-planar tiled arrays that bend up to a singly-curved surface with a nearly-arbitrary curvature profile for the final lockup shape; these principles are then employed to create a 3D model of an array in Computer-Aided Design (CAD) software. This CAD geometry is then manufactured with additive manufacturing and tested, verifying that it does indeed lock up at an approximation of the target curvature profile. A simulation of the bending behavior of the initially-planar array is then undertaken in the LS-Dyna Finite Element Analysis (FEA) package to develop a general FEA approach for simulating such arrays. Next, an overview of cylinder-like closed surfaces surrounding a bending joint is undertaken with an eye to understanding what geometric features might enable such surfaces to tolerate large deflections of the joint without severe shell buckling that limits their bending stiffnesses, and what morphing pattern would be desirable for such a surface to enclose the ankle joint. In turn, the concept of using closed and doubly-curved surfaces for fabric-backed tiled arrays that might enclose the ankle joint and protect against excessive excursions is introduced and CAD geometry of such an array created, whereupon various approaches to simulating the array in LS-Dyna are undertaken. Some of these approaches reveal flaws in multiple LS-Dyna material models for woven fabrics, so an alternative approach is implemented. Finally, several closed-surface arrays are manufactured using additive manufacturing with either a fabric or elastomeric backing, and qualitative testing indicates that such arrays can indeed tolerate some internal bending but require further modifications for satisfactory kinematic behavior for an ankle protector.