Design and mechanical performance of novel 3D enhanced isomorphic secondary hierarchical re-entrant auxetic structures fabricated by additive manufacturing

In order to overcome the drawbacks of difficulty in synchronously improving structural strength and auxetic effect, new 3D enhanced isomorphic secondary/triple hierarchical re-entrant auxetic (ISHRA/ITHRA) structures were constructed by embedding self-similar structures to the regular re-entrant (RA) structures. The structures were fabricated by the selective laser melting (SLM) method with the application of Fe-21Mn-0.6C obtaining high strength and plastic accumulation as the base material. The mechanical response, Poisson's ratio, deformation mode, and energy absorption behavior of the ISHRA structures were analyzed experimentally and numerically. Experimental and numerical analyses reveal that ISHRA structures present the typical stress-strain relationship, which is similar to cellular materials. The new structures outperform the RA structures in auxetic behavior, strength, and specific energy absorption (SEA). The enhancement in strength and SEA of the new structures may stem from the self-contact interaction and more plastic hinges formed during the compression. The adding of the isomorphic structures significantly improves the stability of deformation to some extent, leading to the more stable auxetic effect and stress-strain behavior. Compared to the original RA structures, the optimized ISHRA1 and ISHRA2 structures showcase 19.8% and 18.4% enhancement in specific plateau stresses, 75.1% and 50.2% increase in SEA under compression. Furthermore, the effect of the prominent structural parameters on the comprehensive performance of ISHRA structures was systematically analyzed by numerical method verified by the experimental results. Findings indicate that the geometric parameters dramatically influence the compressive strength, Poisson's ratio, and energy absorption behavior of the structures. As the length of re-entrant strut and re-entrant angle decrease, the ISHRA structures demonstrate more excellent mechanical property and energy absorption behavior, while the auxetic effect exhibits declined trend. A comparison between RA, ISHRA, and ITHRA structures indicated that the ISHRA and ITHRA structures exhibited superior specific strength and SEA. Furthermore, the ISHRA and ITHRA structures also possess superior SEA, outperforming the SEA of other auxetic structures proposed in prior studies. These findings illustrate that the auxetic structures can achieve satisfactory performance by adding the isomorphic structures into the original structures, which provides an effective structural optimization strategy.