New Class of Multifunctional Bioinspired Microlattice with Excellent Sound Absorption, Damage Tolerance, and High Specific Strength

Dynamic Compressive Behavior of a Novel Bioinspired Gradient Negative Poisson's Ratio Sign-Switching Metamaterial Made of Thermoplastic Polyurethane

Inspired by Scylla serrata, a novel thermoplastic polyurethane (TPU) negative Poisson's ratio sign-switching metamaterial is proposed, and the corresponding original and gradient structures (i.e., OPSM and GPSM) are created. Numerical simulation is utilized to simulate the quasi-static and dynamic compression behavior of the proposed structures considering the rate-dependent properties, elastoplastic response, and nonlinear contact. The neo-Hookean hyperelastic constitutive model and the Prony series are adopted to model the target structures. Finite element results are validated through experimental results. Parametric studies are conducted to study the effects of gradient characteristics and loading velocities on the mechanical behavior and Poisson's ratio of the structures. Testing results indicate that the proposed novel bioinspired structure patterns exhibit fascinating mechanical behavior and interesting negative Poisson's ratio sign-switching characteristics, which would provide the design guidance for the development and application of bioinspired structural materials.

Multi-Physical Lattice Metamaterials Enabled by Additive Manufacturing: Design Principles, Interaction Mechanisms, and Multifunctional Applications

Lattice metamaterials emerge as advanced architected materials with superior physical properties and significant potential for lightweight applications. Recent developments in additive manufacturing (AM) techniques facilitate the manufacturing of lattice metamaterials with intricate microarchitectures and promote their applications in multi-physical scenarios. Previous reviews on lattice metamaterials have largely focused on a specific/single physical field, with limited discussion on their multi-physical properties, interaction mechanisms, and multifunctional applications. Accordingly, this article critically reviews the design principles, structure-mechanism-property relationships, interaction mechanisms, and multifunctional applications of multi-physical lattice metamaterials enabled by AM techniques. First, lattice metamaterials are categorized into homogeneous lattices, inhomogeneous lattices, and other forms, whose design principles and AM processes are critically discussed, including the benefits and drawbacks of different AM techniques for fabricating different types of lattices. Subsequently, the structure-mechanism-property relationships and interaction mechanisms of lattice metamaterials in a range of physical fields, including mechanical, acoustic, electromagnetic/optical, and thermal disciplines, are summarized to reveal critical design principles. Moreover, the multifunctional applications of lattice metamaterials, such as sound absorbers, insulators, and manipulators, sensors, actuators, and soft robots, thermal management, invisible cloaks, and biomedical implants, are enumerated. These design principles and structure-mechanism-property relationships provide effective design guidelines for lattice metamaterials in multifunctional applications.

Broadband Sound Absorption and High Damage Resistance in a Turtle Shell-Inspired Multifunctional Lattice: Neural Network-Driven Design and Optimization

Incorporating acoustic and mechanical properties into a single multifunctional structure has attracted considerable attention in engineering. However, effectively integrating these sound absorption properties and damage resistance to achieve multifunctional structural designs remains a great challenge due to imperfect design methods. In this study, the inherent mechanical properties of turtle shells by introducing dissipative pores are leveraged to present a lattice structure that possesses both excellent sound-absorbing and high damage-resistant characteristics. To achieve acoustic optimization design, a universal high-fidelity neural network correction model is proposed to address the impedance calculation challenge in complex structures. Building upon this foundation, a multi-cell combination design enables to achieve high absorption through optimization with a low thickness of 50 mm, resulting in average sound absorption coefficients reaching 0.88 and 0.93 within the frequency ranges of 300-600 Hz and 500-1000 Hz, respectively. It is also found that the optimized structures exhibit exceptional damage resistance under varying relative densities via the coupling effect of the shell thickness on the acoustic and mechanical properties. Overall, this work introduces a novel paradigm for designing intricate multifunctional structures with acoustic and mechanical properties while providing valuable inspiration for future research on multifunctional structure design.

Superior Strength, Toughness, and Damage-Tolerance Observed in Microlattices of Aperiodic Unit Cells

Characterized by periodic cellular unit cells, microlattices offer exceptional potential as lightweight and robust materials. However, their inherent periodicity poses the risk of catastrophic global failure. To address this limitation, a novel approach, that is to introduce microlattices composed of aperiodic unit cells inspired by Einstein's tile, where the orientation of cells never repeats in the same orientation is proposed. Experiments and simulations are conducted to validate the concept by comparing compressive responses of the aperiodic microlattices with those of common periodic microlattices. Indeed, the microlattices exhibit stable and progressive compressive deformation, contrasting with catastrophic fracture of periodic structures. At the same relative density, the microlattices outperform the periodic ones, exhibiting fracture strain, energy absorption, crushing stress efficiency, and smoothness coefficients at least 830%, 300%, 130%, and 160% higher, respectively. These improvements can be attributed to aperiodicity, where diverse failure thresholds exist locally due to varying strut angles and contact modes during compression. This effectively prevents both global fracture and abrupt stress drops. Furthermore, the aperiodic microlattice exhibits good damage tolerance with excellent deformation recoverability, retaining 76% ultimate stress post-recovery at 30% compressive strain. Overall, a novel concept of adopting aperiodic cell arrangements to achieve damage-tolerant microlattice metamaterials is presented.

3D-Printed Lattice Structures for Sound Absorption: Current Progress, Mechanisms and Models, Structural-Property Relationships, and Future Outlook

The reduction of noises, achieved through absorption, is of paramount importance to the well-being of both humans and machines. Lattice structures, defined as architectured porous solids arranged in repeating patterns, are emerging as advanced sound-absorbing materials. Their immense design freedom allows for customizable pore morphology and interconnectivity, enabling the design of specific absorption properties. Thus far, the sound absorption performance of various types of lattice structures are studied and they demonstrated favorable properties compared to conventional materials. Herein, this review gives a thorough overview on the current research status, and characterizations for lattice structures in terms of acoustics is proposed. Till date, there are four main sound absorption mechanisms associated with lattice structures. Despite their complexity, lattice structures can be accurately modelled using acoustical impedance models that focus on critical acoustical geometries. Four defining features: morphology, relative density, cell size, and number of cells, have significant influences on the acoustical geometries and hence sound wave dissipation within the lattice. Drawing upon their structural-property relationships, a classification of lattice structures into three distinct types in terms of acoustics is proposed. It is proposed that future attentions can be placed on new design concepts, advanced materials selections, and multifunctionalities.

3D Chiral Energy-Absorbing Structures with a High Deformation Recovery Ratio Fabricated via Selective Laser Melting of the NiTi Alloy

Excellent energy-absorbing structures have been highly sought after in engineering applications to improve devices and personal safety. The ideal energy absorption mechanism should exhibit characteristics such as lightweight, high energy absorption capacity, and efficient reusability. To address this demand, a novel three-dimensional (3D) chiral lattice structure with compression-twist coupling deformation is fabricated by combining the left and right chiral units. The proposed structure was fabricated in NiTi shape memory alloys (SMAs) by using laser powder bed fusion technology. The compression experiment result indicates that the shape recovery ratio is as high as 94% even when the compression strain is over 80%. Additionally, the platform strain reaches as high as 66%, offering high-level specific energy absorption, i.e., 213.02 J/g. The obtained results are of great significance for basic research and engineering applications of energy-absorbing structures with high deformation recovery ratios.

Kookaburra Optimization Algorithm: A New Bio-Inspired Metaheuristic Algorithm for Solving Optimization Problems

In this paper, a new bio-inspired metaheuristic algorithm named the Kookaburra Optimization Algorithm (KOA) is introduced, which imitates the natural behavior of kookaburras in nature. The fundamental inspiration of KOA is the strategy of kookaburras when hunting and killing prey. The KOA theory is stated, and its mathematical modeling is presented in the following two phases: (i) exploration based on the simulation of prey hunting and (ii) exploitation based on the simulation of kookaburras' behavior in ensuring that their prey is killed. The performance of KOA has been evaluated on 29 standard benchmark functions from the CEC 2017 test suite for the different problem dimensions of 10, 30, 50, and 100. The optimization results show that the proposed KOA approach, by establishing a balance between exploration and exploitation, has good efficiency in managing the effective search process and providing suitable solutions for optimization problems. The results obtained using KOA have been compared with the performance of 12 well-known metaheuristic algorithms. The analysis of the simulation results shows that KOA, by providing better results in most of the benchmark functions, has provided superior performance in competition with the compared algorithms. In addition, the implementation of KOA on 22 constrained optimization problems from the CEC 2011 test suite, as well as 4 engineering design problems, shows that the proposed approach has acceptable and superior performance compared to competitor algorithms in handling real-world applications.

Harnessing cavity dissipation for enhanced sound absorption in Helmholtz resonance metamaterials

Helmholtz resonance, based on resonance through a pore-and-cavity structure, constitutes the primary sound absorption mechanism in majority of sound-absorbing metamaterials. Typically, enhancing sound absorption in such absorbers necessitates substantial geometrical redesign or the addition of dissipative materials, which is non-ideal considering the volume and mass constraints. Herein, we introduce a new approach - that is to simply reshape the cavity, without alterations to its overall mass and volume - to drastically enhance sound absorption. This is achieved by bringing the cavity walls close to the pores where additional thermoviscous dissipation along these boundaries can occur. Experimentally validated, with three sides of the cuboid cavity close to the pore and at a particular pore-cavity geometry, a 44% gain in maximum absorption is achieved compared to the original structure. Through numerical simulations, we fully elucidate structure-property relationships and their mechanisms, and propose analytical models for design and optimization. Ultimately, utilizing this concept, we demonstrate a heterogeneously porous broadband (1500 to 6000 Hz) absorber that exhibits an excellent average absorption coefficient of 0.74 at a very low thickness of 18 mm. Overall, we introduce a new and universal concept that could revolutionize the design principles of Helmholtz resonators, and demonstrate its potential for designing advanced sound-absorbing metamaterials.

Koch Hierarchical Honeycomb: A Fractal-Based Design for Enhanced Mechanical Performance and Energy Absorption

A novel energy-absorbing structure, the Koch hierarchical honeycomb, which combines the Koch geometry with a conventional honeycomb structure, is proposed in this work. Adopting a hierarchical design concept using Koch has improved the novel structure more than the honeycomb. The mechanical properties of this novel structure under impact loading are studied by finite element simulation and compared with the conventional honeycomb structure. To effectively verify the reliability of the simulation analysis, quasi-static compression experiments were conducted on 3D-printed specimens. The results of the study showed that the first-order Koch hierarchical honeycomb structure increased the specific energy absorption by 27.52% compared to the conventional honeycomb structure. Furthermore, the highest specific energy absorption can be obtained by increasing the hierarchical order to 2. Moreover, the energy absorption of triangular and square hierarchies can be significantly increased. All achievements in this study provide significant guidelines in the reinforcement design of lightweight structures.