Compressive deformation modes and properties of 3D printed continuous fiber reinforced composite auxetic honeycomb structures regulated by structural parameters

In this study, continuous fiber reinforced composite (CFRC) auxetic honeycomb structures with different struts length ratios ($h/l$) and cell angles ($\theta$) were designed and fabricated by 3D printing process. The compressive behaviors were investigated by in-plane compression tests in two directions (1- and 2-directions). The results showed that the structural deformation modes were influenced by structural parameters, thereby affecting compressive properties. $h/l$ determined the strut contact forms, affecting the modes of structural deformation and stress evolution during compression. The structural deformation modes were divided into: strut-end no-contact collapse ($h/l$ ≥ 2.0) and strut-end contact collapse ($h/l$ < 2.0) in 1- direction; hinge-like collapse ($h/l$ > 2.0), self-locking collapse ($h/l$ = 2.0) and strut-end contact collapse ($h/l$ < 2.0) in 2- direction. And the triggering strains of these deformation modes increased with $\theta$. Besides, $\theta$ also determined the capability of inclined struts to resist deformation, influencing structural load-bearing capacity. The compressive modulus and energy absorption showed a consistent dependence on $\theta$, with opposite trends in different compressive directions. Moreover, the negative Poisson's ratio characteristics were also affected by structural parameters. Large $\theta$ (e.g. 75°) in 1-direction and small $\theta$ (e.g. 30°) in 2-direction should be avoided, as they could not maintain negative Poisson's ratio characteristics at large deformation. Finally, a comprehensive assessment method was proposed, and 2.0–60° specimen exhibited the best comprehensive performance among 60° specimens. This work provided references for the structural design and performance customization of 3D printed CFRC auxetic honeycomb structures.