Enhancing mechanical properties of auxetic structures through optimization and experimental testing

Abstract

Auxetic materials and structures have been attracting attention due to their mechanical properties, also the notably their high capacity to absorb energy. Some types of auxetic tubular structures have been studied and designed for application in diverse engineering fields such as mechanical, aerospace, and medical engineering. In the present study, inspired by the dragonfly wing shape, a novel auxetic unit cell was developed and applied in a tubular structure with the goal of proposing a new structure with lower stress concentration and consequently increased energy absorption. The dragonfly wing (DFW) shaped unit cells were integrated into a tubular structure, and experimental samples were produced using an additive manufacturing process. To validate the energy absorption capability of the novel unit cell, a comparison was made with the classical reentrant auxetic tubular structure using two different parameters: weight and the number of unit cells, which were developed in two different DFW structures. The results from the compression tests showed that the bio-inspired dragonfly wing shape, in both proposed configurations, demonstrated excellent energy absorption compared to the classical reentrant structure. Specifically, the structure with the same quantity of unit cells and the structure with the same weight absorbed 163% and 79% more energy, respectively. Subsequently, an optimization process was conducted to enhance the mechanical properties of the structure. An optimization framework was implemented to simultaneously minimize three critical structural objectives: Poisson’s ratio, mass, and stress. Numerical simulations facilitated metamodeling via the response surface method, creating surrogate models that accurately represent each response variable. A metaheuristic optimization technique, the Nondominated Sorting Genetic Algorithm (NSGA-II), was then employed to optimize these responses for compression performance. Experimental validation supported the numerical findings, with two optimized designs proposed. The first design (TOPSIS 1) showed reductions in Poisson’s ratio by up to 3% and stress by 45%, while the second design (TOPSIS 2) demonstrated a stress reduction of 537%. Additionally, experimental validation revealed significant improvements in energy absorption capabilities, with TOPSIS 1 and TOPSIS 2 increasing energy absorption by 58% and 545%, respectively, compared to the baseline. The present study present the significant potential of bio-inspired auxetic structures for high complexity applications requiring high energy absorption capacity.