Validation of a Finite Element Modeling Process for Auxetic Structures under Impact

Auxetic materials behave unconventionally under deformation, which enhances material properties such as resistance to indentation and energy absorption. Auxetics, therefore, have the potential to enhance sporting protective equipment. Herein, finite element modeling, additive manufacturing and impact testing of three auxetic lattices, and a conventional equivalent, with a view to advance auxetic implementation within sports equipment, are explored. The lattices are modeled and impacts are simulated between 1 and 5 J, for flat and hemispherical drop hammers. Simulation outputs including peak impact force, impact duration, maximum axial strain, and Poisson's ratio are compared with experimental results from equivalent impact energies on additively manufactured lattices, using an instrumented drop tower and a high-speed camera. The simulation and experimental results show broad agreement for all lattices and scenarios, demonstrated by comparative force versus time plots and maximum compression images. The benefits of developing and validating finite element models of three auxetic lattices (and the conventional honeycomb lattice) under various impact scenarios, as a process, are discussed, including material characterization of an exemplar thermoplastic polyurethane. Future work can use the models to further investigate auxetic lattices, selecting and tailoring candidates to further explore their potential application to specific personal protective equipment in sport.