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Architected materials design across orders of magnitude length scale intrigues exceptional mechanical responses nonexistent in their natural bulk state. However, the so-termed mechanical metamaterials, when scaling bottom down to the atomistic or microparticle level, remain largely unexplored and conventionally fall out of their coarse-resolution, ordered-pattern design space. Here, combining high-throughput molecular dynamics (MD) simulations and machine learning (ML) strategies, some intriguing atomistic families of disordered mechanical metamaterials are discovered, as fabricated by melt quenching and exemplified herein by lightweight-yet-stiff cellular materials featuring a theoretical limit of linear stiffness-density scaling, whose structural disorder-rather than order-is key to reduce the scaling exponent and is simply controlled by the bonding interactions and their directionality that enable flexible tunability experimentally. Importantly, a systematic navigation in the forcefield landscape reveals that, in-between directional and non-directional bonding such as covalent and ionic bonds, modest bond directionality is most likely to promotes disordered packing of polyhedral, stretching-dominated structures responsible for the formation of metamaterials. This work pioneers a bottom-down atomistic scheme to design mechanical metamaterials formatted disorderly, unlocking a largely untapped field in leveraging structural disorder in devising metamaterials atomistically and, potentially, generic to conventional upscaled designs.

In comparison to monolithic materials, cellular solids have superior energy absorption capabilities. Of particular interest within this category are the periodic lattice materials, which offer repeatable and highly customizable behavior, particularly in combination with advances in additive manufacturing technologies. In this paper, the crashworthiness of engineering multi-layer, multi-topology (MLMT) resin lattices is experimentally examined. First, the response of a single- and three-layer single topology cubic and octet lattices, at a relative density of 30%, is investigated. Then, the response of MLMT lattices is characterized and compared to those single-topology lattices. Crashworthiness data were collected for all topology arrangements, finding that while the three-layer cubic and octet lattices were capable of absorbing 9.8 J and 7.8 J, respectively, up to their respective densification points, the unique MLMT lattices were capable of absorbing more: 19.0 J (octet-cube-octet) and 22.4 J (cube-octet-cube). These values are between 94% and 187% greater than the single-topology clusters of the same mass. [ABSTRACT FROM AUTHOR]

In recent years, there has been considerable focus on ensuring that energy is used in the most efficient manner possible. This is due to the fact that globally, over 70% of energy is generated from fossil fuels. Consequently, the matter of designing and utilizing materials that will negate energy losses within the construction industry is of paramount importance. Simultaneously, the necessity for a sustainable approach to the design and production of materials is strongly emphasized. This paper presents an innovative approach to the use of a combination of mineral and plant-based fillers in polyurethane foam technology as a thermal insulation material with the potential to be used in construction to reduce energy consumption. Polyurethane composites containing fly ash from biomass combustion and the addition of rice, sunflower, and buckwheat husks as plant fillers were proposed. The structure of the obtained materials was studied, and the most important physical properties were analyzed. These included apparent density, dimensional stability, water absorption, and the effects of UV radiation and water influence on the carbon, hydrogen, nitrogen, and oxygen content. Moreover, the mechanical properties of the materials were investigated, including compressive strength and brittleness. Additionally, the foams were subjected to flammability tests using a cone calorimeter. Furthermore, additional parameters were determined, including the limiting oxygen index and the vertical and horizontal flammability tests. The results demonstrate the beneficial effects of combining mineral and vegetable fillers in polyurethane foam. [ABSTRACT FROM AUTHOR]

The use of multi-materials, in which a wide variety of materials are used in the appropriate places, is being promoted to reduce carbon dioxide emissions. In the multi-materialization of automobiles, steel-aluminum joints are widely used. Friction stir welding (FSW) is used for joining steel and aluminum. Because of the solid state and short joining time of the FSW, the thickness of the brittle intermetallic compound layer can be minimized and high-strength joining can be achieved. Meanwhile, since recycling aluminum greatly reduces carbon dioxide emissions compared to the production of new ingots, easy disassembly technology is required. In this study, fabrication of thin foaming agent sheet was attempted. Then, we attempted to realize easy disassembly of steel/aluminum joints by FSW, which can realize strong joints, by using foaming agent sheets that can easily introduce foaming agent on joining interface during FSW. It was shown that foaming agent sheet can be prepared by solidifying foaming agent and pore morphology stabilizer powders using spark plasma sintering. It was also shown that even if a foaming agent sheet was introduced at the joining interface and FSW was performed, good jointing can be achieved. In addition, it was found that the introduction of a foaming agent sheet and foaming at the joining interface can significantly reduce the maximum disassembly load with fracturing at the joining interface. The above results indicate that the use of a foaming agent sheet can easily add an easy-disassembly characteristic to the area near the joining interface in the FSW process. [ABSTRACT FROM AUTHOR]

Aluminum foam is a multifunctional material with excellent shock absorption and heat-insulating properties and is expected to be used in many industrial fields. Since aluminum foam is fabricated by foaming aluminum, variations in pore structures can be observed even when foaming is conducted under the same conditions. Therefore, the establishment of a quality assurance method for the properties of aluminum foam as a product is a major issue. In this study, we attempted to estimate the plateau stress of aluminum foam by machine learning using only X-ray computed tomography (CT) images. A supervised learning neural network model was created using the obtained X-ray CT images of the entire three-dimensional structure and a set of plateau stresses obtained from actual compression tests of the corresponding aluminum foam. Using the created model, we showed that the plateau stress of a new aluminum foam compression specimen, which was different from the aluminum foam compression specimens used for training, can be estimated from X-ray CT images only. That is, it was shown that the mechanical properties of materials with complex geometries, such as foam, can be predicted nondestructively. In the future, it is expected to predict the mechanical properties of the foam by X-ray transmission and visual photographs, which are easier to obtain. We also showed that as the resolution of X-ray CT images improved, the mean error became smaller, namely, the estimation accuracy improved. [ABSTRACT FROM AUTHOR]

This study investigates how varying cell size affects the mechanical behaviour of photopolymer Triply Periodic Minimal Surfaces (TPMS) under different deformation rates. Diamond, Gyroid, and Primitive TPMS structures with spatially graded cell sizes were tested. Quasi-static experiments measured boundary forces, representing material behaviour, inertia, and deformation mechanisms. Separate studies explored the base material's behaviour and its response to strain rate, revealing a strength increase with rising strain rate. Ten compression tests identified a critical strain rate of 0.7 s−1 for "Grey Pro" material, indicating a shift in failure susceptibility. X-ray tomography, camera recording, and image correlation techniques observed cell connectivity and non-uniform deformation in TPMS structures. Regions exceeding the critical rate fractured earlier. In Primitive structures, stiffness differences caused collapse after densification of smaller cells at lower rates. The study found increasing collapse initiation stress, plateau stress, densification strain, and specific energy absorption with higher deformation rates below the critical rate for all TPMS structures. However, cell-size graded Primitive structures showed a significant reduction in plateau and specific energy absorption at a 500 mm/min rate. [ABSTRACT FROM AUTHOR]

Cellular materials offer industries the ability to close gaps in the material selection design space with properties not otherwise achievable by bulk, monolithic counterparts. Their superior specific strength, stiffness, and energy absorption, as well as their multi-functionality, makes them desirable for a wide range of applications. The objective of this paper is to compile and present a review of the open literature focusing on the energy absorption of periodic three-dimensional cellular materials. The review begins with the methodical cataloging of qualitative and quantitative elements from 100 papers in the available literature and then provides readers with a thorough overview of the state of this research field, discussing areas such as parent material(s), manufacturing methods, cell topologies, cross-section shapes for truss topologies, analysis methods, loading types, and test strain rates. Based on these collected data, areas of great and limited research are identified and future avenues of interest are suggested for the continued maturation and growth of this field, such as the development of a consistent naming and classification system for topologies; the creation of test standards considering additive manufacturing processes; further investigation of non-uniform and non-cylindrical struts on the performance of truss lattices; and further investigation into the performance of lattice materials under the impact of non-flat surfaces and projectiles. Finally, the numerical energy absorption (by mass and by volume) data of 76 papers are presented across multiple property selection charts, highlighting various materials, manufacturing methods, and topology groups. While there are noticeable differences at certain densities, the graphs show that the categorical differences within those groups have large overlap in terms of energy absorption performance and can be referenced to identify areas for further investigation and to help in the preliminary design process by researchers and industry professionals alike. [ABSTRACT FROM AUTHOR]

Triply periodic minimal surfaces (TPMSs) are found in many natural objects including butterfly wings, sea urchins, and biological membranes. They simultaneously have zero mean curvature at every point and a crystallographic group symmetry. A metamaterial can be created from such periodic surfaces or used as a reinforcement of a composite material. While a TPMS as a mathematical object has been known since 1865, only novel additive manufacturing (AM) technology made it possible to fabricate cellular materials with complex TPMS shapes. Cellular TPMS-based metamaterials have remarkable properties related to wetting/liquid penetration, shock absorption, and the absence of stress concentrators. Recent studies showed that TPMSs are also found in natural crystals when electron surfaces are considered. Artificial crystal-inspired metamaterials mimic such crystals including zeolites and schwarzites. These metamaterials are used for shock, acoustic waves, and vibration absorption, and as structural materials, heat exchangers, and for other applications. The choice of the crystalline cell of a material, as well as its microstructure, plays a decisive role in its properties. The new area of crystal-inspired materials has many common features with traditional biomimetics with models being borrowed from nature and adjusted for engineering applications. [ABSTRACT FROM AUTHOR]

Cellular approaches facilitate the flexible design of materials with desirable properties that may be manufactured from single polymers, metals, and, recently, fiber-reinforced composites. This paper describes cellular materials with metal and carbon fiber-reinforced plastic (CFRP) struts. Composite lattice tubular structures were automatically braided, and the effects of axial CFRP yarns on the compressive behaviors of braided lattices were studied. Metal wire-reinforced plastic and metal rods served as axial yarns when preparing hybrid triaxially braided metal-CFRP lattices. The effects of the axial yarns in polyurethane (PU) foam-filled samples were also assessed. Ten braided structures were manufactured, and their compressive behavior and energy absorption were evaluated by axial compression tests. Finite element (FE) simulations were used to investigate deformation, buckling, and damage. Addition of axial CFRP yarns improved the compressive properties; the specific absorbed energy (SAE) increased by 16%. Hybridization further enhanced the SAE by 62%. [ABSTRACT FROM AUTHOR]

The interest in exploring new microarchitected materials that take advantage of precise geometrical construction to achieve unique properties has recently arisen due to the accelerated development of additive manufacturing. Exploring and understanding the mechanical behavior of additively manufactured structures remains crucial to expand the capabilities of this technology. Herein, a methodology to design, fabricate, and characterize the flexural behavior of additively manufactured structures reinforced with cellular and fractal patterns is presented. A total of 16 different arrangements are designed, tested, and numerically simulated, including 4 cellular arrangements, 10 fractal, and 2 nonreinforced structures. Samples are fabricated out of polylactic acid via material extrusion, subjected to three‐point bending loading and simulated via finite element models. Further, the fracture modes and deformation mechanisms from the experimental tests to obtain a broad overview of their flexural capabilities are examined. An increase in stiffness of up to 36% is observed for the cellular‐reinforced structures, while the fractal‐reinforced structures present great recovery, flexibility, and unconventional bending behaviors. [ABSTRACT FROM AUTHOR]

Aluminum foam has relatively low tensile and flexural strengths because it is composed of many pores with thin cell walls. One method of strengthening aluminum foam is to fabricate a composite material with a dense lightweight resin. In this study, the fabrication of composite materials by directly printing resin on an aluminum foam surface using a 3D printer was attempted. The resin was directly printed on both heated and unheated aluminum foam. It was shown that composite materials consisting of aluminum foam and resin can be fabricated by directly printing resin with a 3D printer on both heated and unheated aluminum foam. The resin was softened during the printing process in the case of directly printed resin on heated aluminum foam, allowing more resin to penetrate into the pores than in the case of directly printed resin on unheated aluminum foam. In addition, it was shown that resin can be directly printed on the aluminum foam with a high bonding strength, as a large amount of resin penetrated into the pores, resulting in an anchor effect. That is, composite materials consisting of aluminum foam and arbitrary-shaped resin with relatively high bonding strength can be fabricated when a large amount of resin is allowed to penetrate into the pore. [ABSTRACT FROM AUTHOR]

A finite element (FE) modelling approach is developed to reproduce the cyclic elastoplastic response and to assess the low cycle fatigue (LCF) life of two cellular materials (strut-based, gyroid) investigated in a previous experimental campaign. The cyclic response of different FE models (unit cell, one layer structure) is compared in terms of computational cost and modelling accuracy. The most satisfactory model is further updated based on the actual relative density of fabricated cellular materials. The LCF assessment exploits a volume-based strain energy density (SED) criterion, calibrated after comparing static properties of strut-based and bulk materials. The cyclic elastoplastic response is well reproduced for both cellular materials, whereas the estimated fatigue lives are in closer agreement for the gyroid structure than the strut-based one.

In comparison to monolithic materials, cellular solids have superior energy absorption capabilities. Of particular interest within this category are the periodic lattice materials, which offer repeatable and highly customizable behavior, particularly in combination with advances in additive manufacturing technologies. In this paper, the crashworthiness of engineering multi-layer, multi-topology (MLMT) resin lattices is experimentally examined. First, the response of a single- and three-layer single topology cubic and octet lattices, at a relative density of 30%, is investigated. Then, the response of MLMT lattices is characterized and compared to those single-topology lattices. Crashworthiness data were collected for all topology arrangements, finding that while the three-layer cubic and octet lattices were capable of absorbing 9.8 J and 7.8 J, respectively, up to their respective densification points, the unique MLMT lattices were capable of absorbing more: 19.0 J (octet-cube-octet) and 22.4 J (cube-octet-cube). These values are between 94% and 187% greater than the single-topology clusters of the same mass.

Aluminum foam is a lightweight material and has excellent shock-absorbing properties. Various properties of aluminum foam can be obtained by changing the base aluminum alloy. Multi-layer aluminum foam can be fabricated by varying the alloy type of the base aluminum alloy, but with different foaming temperatures, within a single aluminum foam to achieve multiple properties. In this study, we attempted to fabricate a two-layer aluminum foam with the upper layer of a commercially pure aluminum A1050 foam and the lower layer of an Al-Si-Cu aluminum alloy ADC12 foam by using an optical heating device that can heat from both the upper and lower sides. Two types of heating methods were investigated. One is to directly stack the A1050 precursor coated with black toner on top of the ADC12 precursor and to foam it from the top and bottom by optical heating. The other is to place a wire mesh between the ADC12 precursor and the A1050 precursor and place the A1050 precursor on the wire mesh, thereby creating a space between the precursors, which is then foamed by optical heating from the top and bottom. It was shown that both precursors can be foamed and joined, and a two-layer A1050/ADC12 foam can be fabricated for both types of heating methods. In the method in which two precursors were stacked and foamed, even if the light intensity of the halogen lamps on the top and bottom were adjusted, heat conduction occurred between the stacked precursors, and the foaming of each precursor could not be controlled, resulting in tilting of the joining interface. In the method of foaming using a wire mesh with a gap between two precursors, it was found that by adjusting the light intensity, the two precursors can be foamed almost simultaneously and achieve similar pore structures. The joining interface can also be maintained horizontally. [ABSTRACT FROM AUTHOR]

One of the main types of uses of cellular materials is for energy absorption and dissipation in applications, such as safety and packaging, to protect people and goods during impact situations. In such cases, the use of cellular materials is justified by their capacity to largely deform under limited loads. This is often achieved, alone or within energy absorbing structures, with the additional advantage of cheap components that are relatively simple to manufacture and assemble. As in most engineering applications, weight reduction is sought after and, as in the case of other materials, this objective can be attained by optimizing the use of the material. Optimization of a cellular material for energy absorption means obtaining an optimal mechanical characteristic that can be obtained by properly designing it in terms of the type of base material and cell properties. Cell properties are mainly related to density and their optimal selection can be made by means of energy criteria. The aim of the present paper is to discuss such optimality criteria based on what are termed efficiency diagrams to produce an effective design tool. Additionally, based on empiric observations on the behavior of several classes of polymeric foams, a simplified selection method is proposed to hasten the selection criteria. [ABSTRACT FROM AUTHOR]

Cellular solids' acoustic properties, such as dispersion, direction dependence, and phase velocities, are inherently coupled to the dominant deformation modes in their ligaments. Understanding and exploiting this coupling can improve the design process of applications utilizing cellular solids' acoustic properties. This work aims to provide insights into this coupling using finite element simulations. In particular, this work investigates the effect of the two main deformation mechanisms observed in cellular materials, i.e. bending and stretching, on the dispersive and direction dependent characteristics of 2D elastic waves traversing them. Two related honeycomb-based lattices are used in this work, namely a bending dominated lattice and a stretching dominated lattice. Results show that asymmetric waves in the bending dominated lattice are more direction dependent and less dispersive than in the stretching dominated lattice, whereas symmetric waves are generally independent of direction and dispersive in both bending and stretching dominated lattices. In addition, results show that phase velocities of symmetric and asymmetric waves scale linearly with relative density in the bending dominated lattice and nonlinearly with relative density in the stretching dominated lattice. Moreover, maximum phase velocities in the stretching dominated lattice are observed to be higher than in the bending dominated lattice. [ABSTRACT FROM AUTHOR]

Metamaterials open up a spectrum of artificially engineered properties otherwise unreachable in conventional bulk materials. For electromechanical energy conversion systems, lightweight materials with high hydrostatic piezoelectric coupling coefficients and negative Poisson’s ratio can be obtained. Thus, in this contribution, we explore the possibilities of piezoelectric metamaterials design by employing structural optimization. More specifically, we apply a sequential framework of topology and shape optimization to design piezoelectric metamaterials with negative Poisson’s ratio for electromechanical energy conversion under uniform pressure. Topology optimization is employed to generate the initial layout, whereas shape optimization fine tunes the design and improves durability and manufacturability of the structures with the help of a curvature constraint. An embedding domain discretization (EDD) method with adaptive domain and shape refinement is utilized for an efficient and accurate computation of the state problem in the shape optimization stage. Multiple case studies are conducted to determine the importance of desired stiffness characteristics, symmetry conditions and objective formulations on the design of piezoelectric metamaterials. Results show that the obtained designs are highly dependent on the desired stiffness characteristics. Moreover, the addition of the EDD-based shape optimization step introduces significant changes to the designs, confirming the usability of the sequential framework. [ABSTRACT FROM AUTHOR]

Abstract Architected materials design across orders of magnitude length scale intrigues exceptional mechanical responses nonexistent in their natural bulk state. However, the so‐termed mechanical metamaterials, when scaling bottom down to the atomistic or microparticle level, remain largely unexplored and conventionally fall out of their coarse‐resolution, ordered‐pattern design space. Here, combining high‐throughput molecular dynamics (MD) simulations and machine learning (ML) strategies, some intriguing atomistic families of disordered mechanical metamaterials are discovered, as fabricated by melt quenching and exemplified herein by lightweight‐yet‐stiff cellular materials featuring a theoretical limit of linear stiffness–density scaling, whose structural disorder—rather than order—is key to reduce the scaling exponent and is simply controlled by the bonding interactions and their directionality that enable flexible tunability experimentally. Importantly, a systematic navigation in the forcefield landscape reveals that, in‐between directional and non‐directional bonding such as covalent and ionic bonds, modest bond directionality is most likely to promotes disordered packing of polyhedral, stretching‐dominated structures responsible for the formation of metamaterials. This work pioneers a bottom‐down atomistic scheme to design mechanical metamaterials formatted disorderly, unlocking a largely untapped field in leveraging structural disorder in devising metamaterials atomistically and, potentially, generic to conventional upscaled designs.

In recent years, there has been considerable focus on ensuring that energy is used in the most efficient manner possible. This is due to the fact that globally, over 70% of energy is generated from fossil fuels. Consequently, the matter of designing and utilizing materials that will negate energy losses within the construction industry is of paramount importance. Simultaneously, the necessity for a sustainable approach to the design and production of materials is strongly emphasized. This paper presents an innovative approach to the use of a combination of mineral and plant-based fillers in polyurethane foam technology as a thermal insulation material with the potential to be used in construction to reduce energy consumption. Polyurethane composites containing fly ash from biomass combustion and the addition of rice, sunflower, and buckwheat husks as plant fillers were proposed. The structure of the obtained materials was studied, and the most important physical properties were analyzed. These included apparent density, dimensional stability, water absorption, and the effects of UV radiation and water influence on the carbon, hydrogen, nitrogen, and oxygen content. Moreover, the mechanical properties of the materials were investigated, including compressive strength and brittleness. Additionally, the foams were subjected to flammability tests using a cone calorimeter. Furthermore, additional parameters were determined, including the limiting oxygen index and the vertical and horizontal flammability tests. The results demonstrate the beneficial effects of combining mineral and vegetable fillers in polyurethane foam.

Cellular soundproofing materials have been critical for lowering incidence pressure waves, which have led to their wide application in studios, auditoria, automotive and aerospace applications, among others. This work, therefore, premiers the utilisation of machine learning backpropagation algorithm of artificial neural networks (ANN) to develop and train a series of datasets of sound absorption properties (noise reduction coefficient, sound absorption average, and area under the sound absorption spectrum) obtained via numerical modelling and simulations. ANN models were established to analyze these sound absorption properties in relation to several non–acoustical parameters associated with cellular materials, including their thickness. Modelling and simulation revealed the relative importance of non–acoustic parameters on sound absorption spectra for these structures. ANN models predicted output signals that almost overlapped with their respective “real” signals with correlation coefficients over 95 %. Model confidence was proposed to analytically predict sound absorption phenomena accurately for cellular materials characterized by permeability ranging between 0.5–3.0 × 10−09 m2 or viscous characteristic lengths ranging between 80 and 300 μm. Analytical models based on optimised synaptic weights, biases, non–acoustic parameters, and material thickness could assist manufacturers design soundproofing materials for noise reduction and reverberation control.

One of the areas that have benefited the most from the advent of additive manufacturing is the development of customized cellular materials, scaffolds and lattices. Although these different groups of materials are typically considered separately, they can be categorized as mechanical metamaterials. Among the different additive manufacturing techniques, perhaps the most popular is that of Fused Filament Fabrication. Numerous works have been reported in the literature in which this fabrication technique has been used to produce such materials. Inspired by the increasing volume of work dealing with the subject, we present a review of the manufacturing and characterization of cellular and lattice-based mechanical metamaterials using Fused Filament Fabrication. An overview of the topologies, their effective mechanical properties and intrinsic manufacturing aspects are presented. The methods for failure analysis at different scales are also discussed. Finally, studies comparing the production of mechanical metamaterials using Fused Filament Fabrication and other additive manufacturing techniques are presented, in addition to recommendations and current trends in the production of these structures by Fused Filament Fabrication.

Simulating the mechanical behavior of cellular materials stands as a pivotal step in their practical application. Nonetheless, the substantial multitude of unit cells within these materials necessitates a considerable finite element mesh, thereby leading to elevated computational expenses and requisites for formidable computer configurations. In order to surmount this predicament, a novel and straightforward equivalent calculation method is proposed for the computation of mechanical properties concerning both random and ordered hyper-elastic cellular materials. By amalgamating the classical finite element approach with the distribution attributes of cells, the proposed equivalent calculation method adeptly captures the deformation modes and force-displacement responses exhibited by cell materials under tensile and shear loads, as predicted through direct numerical simulation. This approach reflects the deformation characteristics induced by micro-unit cells, elucidates an equivalent principle bridging cellular materials and equivalent materials, and substantially curtails exhaustive computational burdens. Ultimately, this method furnishes an equivalent computational strategy tailored for the engineering applications of cellular materials. [ABSTRACT FROM AUTHOR]

Many biological materials may be considered as cellular materials, for example, soft tissues, but also bones, wood, or foam‐like medulla in a bird feather. The cells may be filled with a fluid or just with air, have different size, shape, and wall thickness. Less is known about how the mechanical and morphological parameters of the cellular materials influence their biologically relevant functions. A simple descriptive 2D numerical model of a cellular material based on Voronoi diagrams is presented here which aims prompt investigation how structural parameters influence its physical properties. The preliminary numerical simulations demonstrate that with an increasing wall thickness the energy dissipation increases and with an increasing cell density also an average stress increases. Other interesting results are that an increased dissipation constant of cellular material reduces the stress, and that in hierarchical foams the dissipation takes place mostly in the thick walls. The presented approach allows screening the influence of further material features and structural parameters, such as for example, different kinds of gradients or different properties of biological or biologically inspired cellular materials at different loading conditions. Direct comparison of different real biological structures could be also performed, especially when real experimental measurements are impossible. [ABSTRACT FROM AUTHOR]

Abstract Porous structures are useful elements in several applications, such as in tissue engineering and in the refinement of optimized elements, with the benefit of offering minimum weight, diminishing costs, and reducing material use. This paper presents a method to design porous structures in two dimensions based on grayscale images, in which the porosity and pore size can be changed as a function of a distribution of densities (gray intensity). Generative design tools, Voronoi diagrams and Delaunay triangulations, are developed in which the size of the cells is related to the gray intensity distribution within the image. The method was tested on grayscale images from X-ray and topological optimization algorithms to obtain CAD models suitable for additive manufacturing. These models were subsequently manufactured through an additive manufacturing process. The results show that the developed tool is able to successfully obtain cellular structures; moreover, the 3D-printed specimens were evaluated through both FEM and physical tests to compare which of the trabeculae shapes was better for loading transmission. [ABSTRACT FROM AUTHOR]

Cellular materials offer industries the ability to close gaps in the material selection design space with properties not otherwise achievable by bulk, monolithic counterparts. Their superior specific strength, stiffness, and energy absorption, as well as their multi-functionality, makes them desirable for a wide range of applications. The objective of this paper is to compile and present a review of the open literature focusing on the energy absorption of periodic three-dimensional cellular materials. The review begins with the methodical cataloging of qualitative and quantitative elements from 100 papers in the available literature and then provides readers with a thorough overview of the state of this research field, discussing areas such as parent material(s), manufacturing methods, cell topologies, cross-section shapes for truss topologies, analysis methods, loading types, and test strain rates. Based on these collected data, areas of great and limited research are identified and future avenues of interest are suggested for the continued maturation and growth of this field, such as the development of a consistent naming and classification system for topologies; the creation of test standards considering additive manufacturing processes; further investigation of non-uniform and non-cylindrical struts on the performance of truss lattices; and further investigation into the performance of lattice materials under the impact of non-flat surfaces and projectiles. Finally, the numerical energy absorption (by mass and by volume) data of 76 papers are presented across multiple property selection charts, highlighting various materials, manufacturing methods, and topology groups. While there are noticeable differences at certain densities, the graphs show that the categorical differences within those groups have large overlap in terms of energy absorption performance and can be referenced to identify areas for further investigation and to help in the preliminary design process by researchers and industry professionals alike.

This study investigates how varying cell size affects the mechanical behaviour of photopolymer Triply Periodic Minimal Surfaces (TPMS) under different deformation rates. Diamond, Gyroid, and Primitive TPMS structures with spatially graded cell sizes were tested. Quasi-static experiments measured boundary forces, representing material behaviour, inertia, and deformation mechanisms. Separate studies explored the base material’s behaviour and its response to strain rate, revealing a strength increase with rising strain rate. Ten compression tests identified a critical strain rate of 0.7 s−1 for “Grey Pro” material, indicating a shift in failure susceptibility. X-ray tomography, camera recording, and image correlation techniques observed cell connectivity and non-uniform deformation in TPMS structures. Regions exceeding the critical rate fractured earlier. In Primitive structures, stiffness differences caused collapse after densification of smaller cells at lower rates. The study found increasing collapse initiation stress, plateau stress, densification strain, and specific energy absorption with higher deformation rates below the critical rate for all TPMS structures. However, cell-size graded Primitive structures showed a significant reduction in plateau and specific energy absorption at a 500 mm/min rate.

Triply periodic minimal surfaces (TPMSs) are found in many natural objects including butterfly wings, sea urchins, and biological membranes. They simultaneously have zero mean curvature at every point and a crystallographic group symmetry. A metamaterial can be created from such periodic surfaces or used as a reinforcement of a composite material. While a TPMS as a mathematical object has been known since 1865, only novel additive manufacturing (AM) technology made it possible to fabricate cellular materials with complex TPMS shapes. Cellular TPMS-based metamaterials have remarkable properties related to wetting/liquid penetration, shock absorption, and the absence of stress concentrators. Recent studies showed that TPMSs are also found in natural crystals when electron surfaces are considered. Artificial crystal-inspired metamaterials mimic such crystals including zeolites and schwarzites. These metamaterials are used for shock, acoustic waves, and vibration absorption, and as structural materials, heat exchangers, and for other applications. The choice of the crystalline cell of a material, as well as its microstructure, plays a decisive role in its properties. The new area of crystal-inspired materials has many common features with traditional biomimetics with models being borrowed from nature and adjusted for engineering applications.

Aluminum foam is expected to be one of the candidates for lightweight materials for structural components as it is lightweight and has excellent shock absorption and sound absorption properties. However, aluminum foam has low tensile and flexural strength due to its thin cell walls. Therefore, aluminum foam is used by combining with dense materials. In particular, with the recent trend toward multi-materials, research on the combination with lightweight resins is expected. In this study, we attempted to join aluminum foam fabricated by the precursor method to a thermoplastic resin polycarbonate (PCTA) plate by friction welding. It was found that the aluminum foam and PCTA plate can be joined in about 1 min by friction welding, by rotating the aluminum foam at 2000 rpm and pressing 1 mm into the PCTA plate. In addition, in the friction welding of aluminum foam and PCTA plate, it was found that the pores of the aluminum foam were maintained without being collapsed. The anchoring effect is presumably caused by the penetration of PCTA softened by the frictional heat generated by the friction welding into the pores. Furthermore, tensile tests of the joined samples showed that fracture occurred either at the joining interface or at the base material of the aluminum foam, and that the joining strength was equivalent to the tensile strength of the aluminum foam itself. [ABSTRACT FROM AUTHOR]

Interest in components with detailed structures increased with the progress in advanced manufacturing techniques in recent years. Parts with graded lattice elements can provide interesting mechanical, thermal, and acoustic properties compared to parts where only coarse features are included. One of these improvements is better global buckling resistance of the component. However, thin features are prone to local buckling. Normally, analyses with high-computational effort are conducted on high-resolution finite element meshes to optimize parts with good global and local stability. Until recently, works focused only on either global or local buckling behavior. We use two-scale optimization based on asymptotic homogenization of elastic properties and local buckling behavior to reduce the effort of full-scale analyses. For this, we present an approach for concurrent local and global buckling optimization of parameterized graded lattice structures. It is based on a worst-case model for the homogenized buckling load factor, which acts as a safeguard against pure local buckling. Cross-modes residing on both scales are not detected. We support our theory with numerical examples and validations on dehomogenized designs, which show the capabilities of our method, and discuss the advantages and limitations of the worst-case model. [ABSTRACT FROM AUTHOR]

Crumpled metallic thin foils are very promising as weight‐saving and energy‐absorption materials that can easily be fabricated. To achieve practical industrial applications, it is necessary to improve the understanding of the process of crumpling and the mechanical behavior of crumpled materials considering their complex internal structures. Herein, two possible strategies for computational simulations of crumpled material under closed‐die compression are presented for the first time. The first one entails computations performed at the scale of the foil (direct method), while the second one considers the structure as a continuum with porosity. The analysis performed shows that the continuum‐based approach is more suitable for representing the macroscopic mechanical behavior of crumpled materials with the relative densities ranging from 2% to 40%. An additional benefit is the low computational cost and high efficacy of the porous continuum approach. However, the direct method is shown to be the preferable computational tool when the internal structural patterns changes need to be adequately reproduced, e.g., for a better prediction of the mechanical response under complex loading conditions. [ABSTRACT FROM AUTHOR]

Cellular materials have a wide range of applications, including structural optimization and biomedical applications. Due to their porous topology, which promotes cell adhesion and proliferation, cellular materials are particularly suited for tissue engineering and the development of new structural solutions for biomechanical applications. Furthermore, cellular materials can be effective in adjusting mechanical properties, which is especially important in the design of implants where low stiffness and high strength are required to avoid stress shielding and promote bone growth. The mechanical response of such scaffolds can be improved further by employing functional gradients of the scaffold's porosity and other approaches, including traditional structural optimization frameworks; modified algorithms; bio-inspired phenomena; and artificial intelligence via machine learning (or deep learning). Multiscale tools are also useful in the topological design of said materials. This paper provides a state-of-the-art review of the aforementioned techniques, aiming to identify current and future trends in orthopedic biomechanics research, specifically implant and scaffold design. [ABSTRACT FROM AUTHOR]

Background: Digital Volume correlation (DVC) consists in identifying the displacement fields that allow for the best possible registration of volume images of a sample captured at various loading stages. With cellular materials, the use of DVC faces an intrinsic limit: in the absence of an exploitable texture on (or in) the struts or cell walls, the available speckle pattern will unavoidably be formed by the material architecture itself. This leads to the inability of classical DVC techniques to measure kinematics below the cellular scale, i.e. at the sub-cellular or micro scales. Objectives: Here, we extend a newly developed architecture-driven DIC technique [1] for the measurement of 3D displacement fields in real cellular materials at the scale of the architecture. Methods: The proposed solution consists in assisting DVC by a weak elastic regularization using, as support, an automatic finite-element image-based mechanical model. Results: Complex (locally buckling) kinematics of a polyurethane foam under compression are accurately measured during an in-situ test. The method is essential to evidence the class of dominance (stretching versus bending) of the foam. Conclusion: The proposed method allows to confirm that the foam used is bending-dominated, which is not possible with a classical mesoscopic DVC approach. This method is a good candidate for the analysis of complex local deformation mechanisms at the architecture scale. [ABSTRACT FROM AUTHOR]

We develop a numerical methodology for the calculation of mode-I R-curves of brittle and elastoplastic lattice materials, and unveil the impact of lattice topology, relative density and constituent material behavior on the toughening response of 2D isotropic lattices. The approach is based on finite element calculations of the J-integral on a single-edge-notch-bend (SENB) specimen, with individual bars modeled as beams having a linear elastic or a power-law elasto-plastic constitutive behavior and a maximum strain-based damage model. Results for three 2D isotropic lattice topologies (triangular, hexagonal and kagome) and two constituent materials (representative of a brittle ceramic (silicon carbide) and a strain hardening elasto-plastic metal (titanium alloy)) are presented. We extract initial fracture toughness and R-curves for all lattices and show that (i) elastic brittle triangular lattices exhibit toughening (rising R-curve), and (ii) elasto-plastic triangular lattices display significant toughening, while elasto-plastic hexagonal lattices fail in a brittle manner. We show that the difference in such failure behavior can be explained by the size of the plastic zone that grows upon crack propagation, and conclude that the nature of crack propagation in lattices (brittle vs ductile) depends both on the constituent material and the lattice architecture. While results are presented for 2D truss-lattices, the proposed approach can be easily applied to 3D truss and shell-lattices, as long as the crack tip lies within the empty space of a unit cell.

Cellular materials, in particular metal or polymer foams and beam lattices, are characterized by a complex architecture where the material is concentrated in slender struts. This study is concerned with the construction of numerically efficient sample‐specific digital twin for the analysis and characterization of the mechanical behavior of such meta‐materials. A method to build an explicit, light, and analysis‐suitable isogeometric beam model directly from digital images is proposed. More precisely, as a main contribution, the virtual image correlation method is extended to the case of multiple interconnected beams with varying cross‐sections. Special care is also given to the initialization for better efficiency and robustness. Synthetic and real examples of increasing complexity, that is, curved beams, lattices, and foams, are presented to account for the performance of the developed workflow from digital images to an isogeometric twin for mechanical modeling and simulation. [ABSTRACT FROM AUTHOR]

The growing demand for sustainable and efficient energy harvesting and storage technologies has spurred interest in the integration of triboelectric nanogenerators (TENGs) with supercapacitors (SCs). This combination offers a promising solution for powering Internet of Things (IoT) devices and other low−power applications by utilizing ambient mechanical energy. Cellular materials, featuring unique structural characteristics such as high surface−to−volume ratios, mechanical compliance, and customizable properties, have emerged as essential components in this integration, enabling the improved performance and efficiency of TENG−SC systems. In this paper, we discuss the key role of cellular materials in enhancing TENG−SC systems' performance through their influence on contact area, mechanical compliance, weight, and energy absorption. We highlight the benefits of cellular materials, including increased charge generation, optimized energy conversion efficiency, and adaptability to various mechanical sources. Furthermore, we explore the potential for lightweight, low−cost, and customizable cellular materials to expand the applicability of TENG−SC systems in wearable and portable devices. Finally, we examine the dual effect of cellular materials' damping and energy absorption properties, emphasizing their potential to protect TENGs from damage and increase overall system efficiency. This comprehensive overview of the role of cellular materials in the integration of TENG−SC aims to provide insights into the development of next−generation sustainable energy harvesting and storage solutions for IoT and other low−power applications. [ABSTRACT FROM AUTHOR]

With the growing trend toward using multi-materials to realize lightweight structures, friction stir welding (FSW) has attracted attention as a dissimilar metal joining technology. In dissimilar metal joining, dismantling and separation technologies for joined components at the time of disposal are also required. In this study, we attempted to facilitate the separation of FSW joints between an SS400 plate and an Al-Si-Cu ADC12 die-casting alloy plate by heating and foaming the joints and by reducing the strength of the joints. From the vertical cross sections of the samples in their FSW direction after joining, it was found that the foaming agent powder could be mixed near the joining interface of the ADC12 plates by FSW simultaneously with the joining process. The four-point bending test of the joined samples showed that the joining strength of the as-welded samples with powder added was equivalent to that of the as-welded samples without powder added or that of the ADC12 base material. Heating the ADC12 parts mixed with the foaming agent could foam the area near the joining interface. From the four-point bending test of the obtained samples, it was found that the samples with foamed areas fractured at the foamed area near the joining interface, and SS400 and ADC12 can be easily separated, whereas both as-welded samples fractured at the joining interface or at the ADC12 base material. The samples with foamed areas show markedly reduced maximum load and energy required for separation compared with as-welded samples. Consequently, it was shown that easy separation by foaming the joint can be realized with low load and energy by applying the FSW precursor fabrication method. [ABSTRACT FROM AUTHOR]

The Theory of Critical Distances (TCD) has undoubtedly represented a breakthrough in the brittle failure assessment of engineering materials containing defects, crack, or notches. The basic idea on which the simplest formulation of the TCD is based is to evaluate an effective stress at a characteristic distance from the tip of the defect/crack/notch and compare it with an inherent fracture strength. Is the critical distance related to the material (micro) structure? Whereas a correlation was already proved for homogeneous materials, the current attention to nonhomogeneous ones has brought the question back to the fore. The goal of the present work is therefore twofold: (i) to extend the use of the TCD, through the simple yet effective Point Method (PM), for the static failure assessment of inhomogeneous materials, such as cellular, biological, and additively manufactured (AM) materials; and (ii) to look for a correlation between critical distance and internal (micro) structure. Highlights: The use of TCD is extended to the static assessment of inhomogeneous materials.The failure behavior of natural, biological, and AM materials is investigated.PM failure predictions are generally included in an error interval of ±10%.The critical distance L is related to the material microstructure. [ABSTRACT FROM AUTHOR]

Owing to its lightweight and excellent shock-absorbing properties, aluminum foam is used in automotive parts and construction materials. If a nondestructive quality assurance method can be established, the application of aluminum foam will be further expanded. In this study, we attempted to estimate the plateau stress of aluminum foam via machine learning (deep learning) using X-ray computed tomography (CT) images of aluminum foam. The plateau stresses estimated by machine learning and those actually obtained using the compression test were almost identical. Consequently, it was shown that plateau stress can be estimated by training using the two-dimensional cross-sectional images obtained nondestructively via X-ray CT imaging. [ABSTRACT FROM AUTHOR]

One of the areas that have benefited the most from the advent of additive manufacturing is the development of customized cellular materials, scaffolds and lattices. Although these different groups of materials are typically considered separately, they can be categorized as mechanical metamaterials. Among the different additive manufacturing techniques, perhaps the most popular is that of Fused Filament Fabrication. Numerous works have been reported in the literature in which this fabrication technique has been used to produce such materials. Inspired by the increasing volume of work dealing with the subject, we present a review of the manufacturing and characterization of cellular and lattice-based mechanical metamaterials using Fused Filament Fabrication. An overview of the topologies, their effective mechanical properties and intrinsic manufacturing aspects are presented. The methods for failure analysis at different scales are also discussed. Finally, studies comparing the production of mechanical metamaterials using Fused Filament Fabrication and other additive manufacturing techniques are presented, in addition to recommendations and current trends in the production of these structures by Fused Filament Fabrication. [ABSTRACT FROM AUTHOR]

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Aluminum foam has relatively low tensile and flexural strengths because it is composed of many pores with thin cell walls. One method of strengthening aluminum foam is to fabricate a composite material with a dense lightweight resin. In this study, the fabrication of composite materials by directly printing resin on an aluminum foam surface using a 3D printer was attempted. The resin was directly printed on both heated and unheated aluminum foam. It was shown that composite materials consisting of aluminum foam and resin can be fabricated by directly printing resin with a 3D printer on both heated and unheated aluminum foam. The resin was softened during the printing process in the case of directly printed resin on heated aluminum foam, allowing more resin to penetrate into the pores than in the case of directly printed resin on unheated aluminum foam. In addition, it was shown that resin can be directly printed on the aluminum foam with a high bonding strength, as a large amount of resin penetrated into the pores, resulting in an anchor effect. That is, composite materials consisting of aluminum foam and arbitrary-shaped resin with relatively high bonding strength can be fabricated when a large amount of resin is allowed to penetrate into the pore.

Aluminum foam is a lightweight material and has excellent shock-absorbing properties. Various properties of aluminum foam can be obtained by changing the base aluminum alloy. Multi-layer aluminum foam can be fabricated by varying the alloy type of the base aluminum alloy, but with different foaming temperatures, within a single aluminum foam to achieve multiple properties. In this study, we attempted to fabricate a two-layer aluminum foam with the upper layer of a commercially pure aluminum A1050 foam and the lower layer of an Al-Si-Cu aluminum alloy ADC12 foam by using an optical heating device that can heat from both the upper and lower sides. Two types of heating methods were investigated. One is to directly stack the A1050 precursor coated with black toner on top of the ADC12 precursor and to foam it from the top and bottom by optical heating. The other is to place a wire mesh between the ADC12 precursor and the A1050 precursor and place the A1050 precursor on the wire mesh, thereby creating a space between the precursors, which is then foamed by optical heating from the top and bottom. It was shown that both precursors can be foamed and joined, and a two-layer A1050/ADC12 foam can be fabricated for both types of heating methods. In the method in which two precursors were stacked and foamed, even if the light intensity of the halogen lamps on the top and bottom were adjusted, heat conduction occurred between the stacked precursors, and the foaming of each precursor could not be controlled, resulting in tilting of the joining interface. In the method of foaming using a wire mesh with a gap between two precursors, it was found that by adjusting the light intensity, the two precursors can be foamed almost simultaneously and achieve similar pore structures. The joining interface can also be maintained horizontally.

One of the main types of uses of cellular materials is for energy absorption and dissipation in applications, such as safety and packaging, to protect people and goods during impact situations. In such cases, the use of cellular materials is justified by their capacity to largely deform under limited loads. This is often achieved, alone or within energy absorbing structures, with the additional advantage of cheap components that are relatively simple to manufacture and assemble. As in most engineering applications, weight reduction is sought after and, as in the case of other materials, this objective can be attained by optimizing the use of the material. Optimization of a cellular material for energy absorption means obtaining an optimal mechanical characteristic that can be obtained by properly designing it in terms of the type of base material and cell properties. Cell properties are mainly related to density and their optimal selection can be made by means of energy criteria. The aim of the present paper is to discuss such optimality criteria based on what are termed efficiency diagrams to produce an effective design tool. Additionally, based on empiric observations on the behavior of several classes of polymeric foams, a simplified selection method is proposed to hasten the selection criteria.