Decoupling particle-impact dissipation mechanisms in 3D architected materials.

Ultralight architected materials enabled by advanced manufacturing processes have achieved density-normalized strength and stiffness properties that are inaccessible to bulk materials. However, the majority of this work has focused on static loading and elastic-wave propagation. Fundamental understanding of the mechanical behavior of architected materials under large-deformation dynamic conditions remains limited, due to the complexity of mechanical responses and shortcomings of characterization methods. Here, we present a microscale suspended-plate impact testing framework for three-dimensional micro-architected materials, where supersonic microparticles to velocities of up to 850 m/s are accelerated against a substrate-decoupled architected material to quantify its energy dissipation characteristics. Using ultra-high-speed imaging, we perform in situ quantification of the impact energetics on two types of architected materials as well as their constituent nonarchitected monolithic polymer, indicating a 47% or greater increase in mass-normalized energy dissipation under a given impact condition through use of architecture. Post-mortem characterization, supported by a series of quasi-static experiments and high-fidelity simulations, shed light on two coupled mechanisms of energy dissipation: material compaction and particle-induced fracture. Together, experiments and simulations indicate that architecture-specific resistance to compaction and fracture can explain a difference in dynamic impact response across architectures. We complement our experimental and numerical efforts with dimensional analysis which provides a predictive framework for kinetic-energy absorption as a function of material parameters and impact conditions. We envision that enhanced understanding of energy dissipation mechanisms in architected materials will serve to define design considerations toward the creation of lightweight impact-mitigating materials for protective applications.

Elastically Isotropic Truss-Plate-Hybrid Hierarchical Microlattices with Enhanced Modulus and Strength.

Bioinspired hierarchical design principles have been employed to create advanced architected materials. Here, a new type of truss-plate-hybrid two-level hierarchical architecture is created, referred to as the ISO-COP hierarchical lattice (isotropic truss at the first level and cubic+octet plate at the second level), in which truss-based unit cells are arranged according to the topology of the plate-based unit cell. Finite element analyses reveal that the ISO-COP hierarchical lattice outperforms the best existing octet-truss hierarchical lattices based on fractal geometries in achieving elastic isotropy and enhanced moduli. According to the designed architecture, ISO-COP and several other comparison hierarchical microlattices are fabricated via projection microstereolithography. In situ compression tests demonstrate that the fabricated ISO-COP microlattices exhibit elastic isotropy and enhanced moduli, as predicted from finite element simulations, and superior strength compared with existing fractal octet-truss hierarchical lattices. Theoretical models are further developed to predict the dependence of modulus and failure modes on two design parameters of the hierarchical lattices, with results in good agreement with those from experiments. This study relates mechanical properties of ISO-COP hierarchical lattices to their architectures at each level of hierarchy and exemplifies a route to harnessing hierarchical design principles to create architected materials with desired mechanical properties.

Less Is More: Hollow-Truss Microlattice Metamaterials with Dual Sound Dissipation Mechanisms and Enhanced Broadband Sound Absorption.

Being a lightweight material with high design freedoms, there are increasing research interests in microlattice metamaterials as sound absorbers. However, thus far, microlattices are limited to one sound dissipation mechanism, and this inhibits their broadband absorption capabilities. Herein, as opposed to improving performances via the addition of features, a dissipation mechanism is subtractively introduced by hollowing out the struts of the microlattice. Then, a class of hollow-truss metamaterial (HTM) that is capable of harnessing dual concurrent dissipation mechanisms from its complex truss interconnectivity and its hollow interior is presented. Experimental sound absorption measurements reveal superior and/or customizable absorption properties in the HTMs as compared to their constitutive solid-trusses. An optimal HTM displays a high average broadband coefficient of 0.72 at a low thickness of 24 mm. Numerically derived, a dissipation theorem based on the superimposed acoustic impedance of the critically coupled resistance and reactance of the outer-solid and inner-hollow phases, across different frequency bands, is proposed in the HTM. Complementary mechanical property studies also reveal improved compressive toughness in the HTMs. This work demonstrates the potential of hollow-trusses, where they gain the dissipation mechanism through the subtraction of the material and display excellent acoustic properties.

Microlattice Metamaterials with Simultaneous Superior Acoustic and Mechanical Energy Absorption.

The advent of 3D printing brought about the possibilities of microlattice metamaterials as advanced materials with the potentials to surpass the functionalities of traditional materials. Sound absorbing materials which are also tough and lightweight are of particular importance as practical engineering materials. There are however a lack of attempts on the study of metamaterials multifunctional for both purposes. Herein, we present four types of face-centered cubic based plate and truss microlattices as novel metamaterials with simultaneous excellent sound and mechanical energy absorption performance. High sound absorption coefficients nearing 1 and high specific energy absorption of 50.3 J g-1 have been measured. Sound absorption mechanisms of microlattices are proposed to be based on a "cascading resonant cells theory", an extension of the Helmholtz resonance principle that we have conceptualized herein. Characteristics of absorption coefficients are found to be essentially geometry limited by the pore and cavity morphologies. The excellent mechanical properties in turn derive from both the approximate membrane stress state of the plate architecture and the excellent ductility and strength of the base material. Overall, this work presents a new concept on the specific structural design and materials selection for architectured metamaterials with dual sound and mechanical energy absorption capabilities.

Survey of Microstructures and Dimensional Accuracy of Various Microlattice Designs Using Additively Manufactured 718 Superalloy.

Microlattices hold significant potential for developing lightweight structures for the aeronautics and astronautics industries. Laser Powder Bed Fusion (LPBF) is an attractive method for producing these structures due to its capacity for achieving high-resolution, intricately designed architectures. However, defects, such as cracks, in the as-printed alloys degrade mechanical properties, particularly tensile strength, and thereby limit their applications. This study examines the effects of microlattice architecture and relative density on crack formation in the as-printed 718 superalloy. Complex microlattice design and higher relative density are more prone to large-scale crack formation. The mechanisms behind these phenomena are discussed. This study reveals that microlattice type and relative density are crucial factors in defect formation in LPBF metallic alloys. The transmission electron microscopy observations show roughly round γ″ precipitates with an average size of 10 nm in the as-printed 718 without heat treatment. This work demonstrates the feasibility of the additive manufacturing of complex microlattices using 718 superalloys towards architectured lightweight structures.

Additively Manufactured Deformation-Recoverable and Broadband Sound-Absorbing Microlattice Inspired by the Concept of Traditional Perforated Panels.

Noise pollution is a highly detrimental daily health hazard. Sound absorbers, such as the traditionally used perforated panels, find widespread applications. Nonetheless, modern product designs call for material novelties with enhanced performance and multifunctionality. The advent of additive manufacturing has brought about the possibilities of functional materials design to be based on structures rather than chemistry. With this in mind, herein, the traditional concept of perforated panels is revisited and is incorporated with additive manufacturing for the development of a novel microlattice-based sound absorber with additional impact resistance multifunctionality. The structurally optimized microlattice presents excellent broadband absorption with an averaged experimental absorption coefficient of 0.77 across a broad frequency range from 1000 to 6300 Hz. Extensive simulation and experiments reveal absorption mechanisms to be based on viscous flow, thermal and structural damping dissipations while broadband capabilities to be on multiple resonance modes working in tandem. High deformation recovery up to 30% strain is also possible from the strut-based design and viscoelasticity of the base material. Overall, the excellent properties of the microlattice overcome tradeoffs commonly found in conventional absorbers. Additionally, this work aims to present a new paradigm: revisiting old concepts for the developments of novel materials using contemporary methods.

Carbon fiber/microlattice 3D hybrid architecture as multi-scale scaffold for tissue engineering.

Multiscale 3D carbon architectures are of particular interest in tissue engineering applications, as these structures may allow for three-dimensional cell colonization essential for tissue growth. In this work, carbon fiber/microlattice hybrid architectures are introduced as innovative multi-scale scaffolds for tissue engineering. The microlattice provides the design freedom and structural integrity, whereas the fibrous component creates a cellular microenvironment for cell colonization. The hybrid structures are fabricated by carbonization of stereolithographically 3D printed epoxy microlattice architectures which are pre-filled with cotton fibers within the empty space of the architectures. The cotton filling result in less shrinkage of the architecture during carbonization, as the tight confinement of the fibrous material prevents the free-shrinkage of the microlattices. The hybrid architecture exhibits a compressive strength of 156.9±25.6 kPa, which is significantly higher than an empty carbon microlattice architecture. Furthermore, the hybrid architecture exhibits a flexible behavior up to 30% compressive strain, which is also promising towards soft-tissue regeneration. Osteoblast-like murine MC3T3-E1 cells are cultured within the 3D hybrid structures. Results show that the cells are able to not only proliferate on the carbon microlattice elements as well as along the carbon fibers, but also make connections with each other across the inner pores created by the fibers, leading to a three-dimensional cell colonization. These carbon fiber/microlattice hybrid structures are promising for future fabrication of functionally graded scaffolds for tissue repair applications.

3D Printed Compressible Quasi-Solid-State Nickel-Iron Battery.

The design of a compressible battery with stable electrochemical performance is extremely important in compression-tolerant and flexible electronics. While this remains challenging with the current battery manufacturing method, the field of 3D printing offers the possibility of producing free-standing 3D-printed electrodes with various structural configurations. Through the simple and scalable strategy, various structural configurations can be produced. Herein, we demonstrate a 3D-printed quasi-solid-state Ni-Fe battery (QSS-NFB) that shows excellent compressibility, ultrahigh energy density, and superior long-term cycling durability. Through a rational design and adjustment of chemical components, two electrodes consisting of ultrathin Ni(OH)2 nanosheet array cathode and holey α-Fe2O3 nanorod array anode are achieved with a ultrahigh active material loading over 130 mg cm-3 and excellent compressibility up to 60%. It is noteworthy that the compressible QSS-NFB demonstrated an excellent cycling stability (∼91.3% capacity retentions after 10000 cycles) and ultrahigh energy density (28.1 mWh cm-3 at a power of 10.6 mW cm-3). This work provides a simple method for producing compression-tolerant energy-storage devices, which are expected to have promising applications in next generation stretchable/wearable electronics.

Tailored Buckling Microlattices as Reusable Light-Weight Shock Absorbers.

Structures and materials absorbing mechanical (shock) energy commonly exploit either viscoelasticity or destructive modifications. Based on a class of uniaxial light-weight geometrically nonlinear mechanical microlattices and using buckling of inner elements, either a sequence of snap-ins followed by irreversible hysteretic - yet repeatable - self-recovery or multistability is achieved, enabling programmable behavior. Proof-of-principle experiments on three-dimensional polymer microstructures are presented.