Novel Functionally Graded Warp-and-Woof Auxetic Lattices with Enhanced Negative Poisson’s Ratio and Superior Energy Absorption

Abstract

Functionally graded lattice metamaterials with optimized density distributions have received growing interest in lightweight structures with improved energy absorption and impact mitigation. Auxetic lattices with negative Poisson’s ratios are among such architectures that exhibit unique deformation mechanisms that enhance mechanical performance under compressive loading. Nevertheless, there is a lack of research on the impact of multidirectional functional grading on the optimization of their crushing behavior. This paper investigates the compressive behavior of functionally graded warp and woof lattice auxetic structures (WWLASs) by proposing a new strategy of density grading that is bidirectional. Three configurations are considered: non-graded WWLAS (NGWWLAS), unidirectionally graded WWLAS (UGWWLAS), and bidirectionally graded WWLAS (BGWWLAS). Quasi-static compression experiments up to 65% strain were conducted and supported by validated finite element simulations, showing very good agreement with the experimental results (error less than 3%). The findings indicate the mechanisms of deformation that are grading-dependent. The BGWWLAS exhibits a stable and symmetrical central collapse mode, with a remarkably large negative Poisson’s ratio with a maximum of ν = −5.56 at strains less than 1%. In addition, the bidirectionally graded design has a high level of energy absorption, resulting in a 15–28% improvement compared with NGWWLAS and UGWWLAS. The results emphasize the effectiveness of bidirectional functional grading as an efficient design methodology for advanced auxetic lattices with much higher energy absorption and impact-mitigation properties.