Unprecedented mechanical wave energy absorption observed in multifunctional bioinspired architected metamaterials

The research addresses the challenge of designing materials capable of simultaneously absorbing sound and stress wave energy, a crucial need in various engineering applications like aerospace, where sound-absorbing liners also require structural resilience. Traditional methods use separate materials, leading to increased size and weight. This research aims to create lightweight, multifunctional materials that are strong, energy-absorbing, and effective in sound attenuation. The approach focuses on biomimicry, specifically mimicking the structure of cuttlebone, known for its strength, toughness, and adaptable cellular features. Previous cuttlebone-inspired materials primarily focused on mechanical strength using sinusoidal undulating walls, overlooking the multilayered structure and asymmetric cambering of cell walls, which offer significant potential for sound absorption and enhanced mechanical properties. This study explores these overlooked features to design a metamaterial with broadband sound absorption, high strength, and damage tolerance. Additive manufacturing (selective laser melting using Ti6Al4V alloy) is employed to create these bioinspired architected metamaterials (MBAMs). The choice of Ti6Al4V is motivated by its high strength, corrosion resistance, and high-temperature performance.

The paper reviews existing research on sound-absorbing materials, including perforated panels, foams, fabrics, aerogels, and graphene foams. It highlights the challenge of balancing high strength with broad-spectrum sound absorption in existing microlattice designs. The authors note that previous cuttlebone-inspired metamaterials primarily focused on mechanical strength, neglecting the multilayered structure and asymmetric cambering crucial for sound absorption. This gap in research motivates the current study's exploration of cuttlebone's multifaceted structural features for designing multifunctional materials.

The study employs a bioinspired design principle, mimicking the multilayered "wall-septa" microstructure and the asymmetric cambering pattern of cuttlebone's cell walls. The design incorporates three cascaded panels with sound-dissipative pores for enhanced resonance. A heterogeneous arrangement, dividing each panel into two parts, creates a multimodal hybrid resonance system. The air phase geometry is determined by the cell walls, which follow a nonlinear relationship mimicking the cuttlebone's natural curvature. The pore diameter is optimized using a genetic algorithm to achieve optimal resonance. The sound absorption coefficient is calculated using an impedance model. Four camber levels are investigated: straight-wall and A₀ = 0.5, 1.0, and 1.5. The metamaterials were fabricated using selective laser melting (SLM) with Ti6Al4V alloy. Sound absorption was measured using a two-microphone setup, following ISO 10534-2 standards. Mechanical properties were evaluated via compression experiments using a Shimadzu AG25-TB universal testing machine. A finite element (FE) model in COMSOL Multiphysics was developed to simulate sound absorption and compression behavior. The model uses the Thermo-viscous Acoustics Module and incorporates built-in boundary layer theory for acoustics and a large-strain plastic deformation model for compression.

Experimental results demonstrate that the MBAMs exhibit unprecedented sound absorption capabilities with an average absorption coefficient (α) of 0.80 from 1.0 to 6.0 kHz. 77% of the data points surpassed the 0.75 threshold, despite the compact 21 mm thickness. This significantly outperforms existing sound-absorbing materials. The MBAMs also showed excellent mechanical properties: an average modulus of 4.93 GPa, strength of 211 MPa, and a specific energy absorption of 50.7 J/g at a density of 1.53 g/cm³. The specific energy absorption represents a 558.4% increase compared to a straight-wall design. The high-fidelity model confirmed the air friction damping mechanism as the primary contributor to broadband sound absorption, while the cambered design was responsible for the enhanced energy absorption. The weakly-coupled design successfully integrated high sound and stress wave energy absorption capabilities.

The findings demonstrate the successful creation of a lightweight, multifunctional metamaterial with superior sound absorption and mechanical properties. The weakly-coupled design strategy offers a novel approach for designing multifunctional materials, allowing independent optimization of acoustic and mechanical performance. The results suggest that biomimicry, coupled with advanced manufacturing techniques, can lead to innovative materials with significant advantages over conventional designs. The superior performance compared to existing materials highlights the potential of this approach for various engineering applications.

This study introduces a novel design paradigm for multifunctional wave energy absorbing materials using bioinspired architected metamaterials. The MBAMs demonstrated exceptional sound absorption and mechanical properties, surpassing existing materials in both areas. Future work could explore the optimization of the design parameters to further enhance performance and expand the application range. Investigating different materials and manufacturing processes is also warranted.

The study focused on a specific material (Ti6Al4V) and geometry. Further research is needed to explore the applicability of the design principles to other materials and geometries. The sound absorption measurements were conducted under specific conditions, and the performance under different environmental conditions requires further investigation.