3D-Printed Micro/Nano-Scaled Mechanical Metamaterials: Fundamentals,Technologies, Progress,Applications, and Challenges

Micro/nano-scaled mechanical metamaterials have attracted extensive attention in various fields attributed to their superior properties benefiting from their rationally designed micro/nano-structures. As one of the most advanced technologies in the 21st century, additive manufacturing (3D printing) opens an easier and faster path for fabricating micro/nano-scaled mechanical metamaterials with complex structures. Here, the size effect of metamaterials at micro/nano scales is introduced first. Then, the additive manufacturing technologies to fabricate mechanical metamaterials at micro/nano scales are introduced. The latest research progress on micro/nano-scaled mechanical metamaterials is also reviewed according to the type of materials. In addition, the structural and functional applications of micro/nano-scaled mechanical metamaterials are further summarized. Finally, the challenges, including advanced 3D printing technologies, novel material development, and innovative structural design, for micro/nano-scaled mechanical metamaterials are discussed, and future perspectives are provided. The review aims to provide insight into the research and development of 3D-printed micro/nano-scaled mechanical metamaterials.     

3D optomechanical metamaterials

Ideally, many materials should have a “knob” that allows for changing its properties at will, including the possibility to flip the sign of its behavior. This “knob” could be used to continuously tune the properties or in the sense of a digital switch. Such extreme level of stimulus–responsiveness has come into reach with recently increased possibilities of manufacturing complex rationally designed artificial materials called metamaterials on the micrometer scale. Here, we present mechanical metamaterials composed of liquid–crystal elastomers, whose director field is arranged into a designed complex three-dimensional (3D) pattern during the 3D laser printing process. External light from a blue LED, with intensities in the range of 10–30 W/cm², serves as the stimulus. In the first example, we repeatedly flip the sign of the Poisson’s ratio of an achiral architecture within classical elasticity. In the second example, we flip the sign of the twist per strain in a chiral metamaterial beyond classical elasticity. The presented examples overcome major limitations in responsive mechanical metamaterials and we foresee many possible three-dimensional responsive micro-architectures manufactured along these lines.     

Microarchitected Stretching‐Dominated Mechanical Metamaterials with Minimal Surface Topologies

Historically, the creation of lightweight, yet mechanically robust, materials have been the most sought‐after engineering pursuit. For that purpose, research efforts are dedicated to finding pathways to emulate and mimic the resilience offered by natural biological systems (i.e., bone and wood). These natural systems evolved over time to provide the most attainable structural efficiency through their architectural characteristics that can span over multiple length scales. Nature‐inspired man‐made cellular metamaterials have effective properties that depend largely on their topology rather than composition and are hence remarkable candidates for a wide range of application. Despite their geometrical complexity, the fabrication of such metamaterials is made possible by the emergence of advanced fabrication techniques that permit the fabrication of complex architectures down to the nanometer scale. In this work, we report the fabrication and mechanical testing of nature‐inspired, mathematically created, micro‐architected, cellular metamaterials with topologies based on triply periodic minimal surfaces (TPMS) with cubic symmetries fabricated through direct laser writing two‐photon lithography. These TPMS‐based microlattices are sheet/shell‐ and strut‐based metamaterials with high geometrical complexity. Interestingly, results show that TPMS sheet‐based microlattices follow a stretching‐dominated mode of deformation, and further illustrate their mechanical superiority over the traditional and well‐known strut‐based microlattices and microlattice composites. The TPMS sheet‐based polymeric microlattices exhibited mechanical properties superior to other micrloattices comprising metal‐ and ceramic‐coated polymeric substrates and, interestingly, are less affected by the change in density, which opens the door for fabricating ultralightweight materials without much sacrificing mechanical properties.

     

3D Pixel Mechanical Metamaterials

Metamaterials have unprecedented properties that facilitate the development of advanced devices and machines. However, their interconnected building structures limit their applications, especially in the fields that require large deformation, rich programmability and efficient shape‐reconfigurability. To break this limit and exploit more potentialities of metamaterials, an innovative material design strategy is proposed, named mechanical pixel (MP) array design. Similar to a screen that displays images by adjusting the colors of pixels, the metamaterials can form and reconfigure 3D morphologies by tuning the heights (lengths) of the MPs in the array. The strategy is demonstrated in a multistable metamaterial by experimental tests, theoretical analysis, and numerical simulations. Using this strategy, a large macroscopic shear deformation is obtained, and remarkable enhancements in the mechanical programmability, shape‐reconfigurability and adaptability, and reusable shock‐resistance are exhibited. Moreover, mechanical design and property prediction for the metamaterials are both greatly simplified due to the pixelated design. For a piece of the 3D pixel metamaterial with m n‐unit MPs, the number of programmable displacement–force curves increases from n+1 to 2^m?n+1, and the number of stable morphologies grows from n+1 to at least (n+1)^m. This strategy can be used to enhance the merits and further excavate the potential of versatile metamaterials.     

Multiscale Geometric Design Principles Applied to 3D Printed Schwarzites

Schwartzites are 3D porous solids with periodic minimal surfaces having negative Gaussian curvatures and can possess unusual mechanical and electronic properties. The mechanical behavior of primitive and gyroid schwartzite structures across different length scales is investigated after these geometries are 3D printed at centimeter length scales based on molecular models. Molecular dynamics and finite elements simulations are used to gain further understanding on responses of these complex solids under compressive loads and kinetic impact experiments. The results show that these structures hold great promise as high load bearing and impact‐resistant materials due to a unique layered deformation mechanism that emerges in these architectures during loading. Easily scalable techniques such as 3D printing can be used for exploring mechanical behavior of various predicted complex geometrical shapes to build innovative engineered materials with tunable properties.     

Inconel 718镍基高温合金热加工动态再结晶建模与计算

目的 建立Inconel 718镍基合金动态再结晶组织演变三维介观尺度计算模型.方法 采用Gleeble-1500型热力模拟试验机进行恒温恒应变速率压缩实验,获得Inconel 718高温合金热变形的真应力-应变曲线,并结合光学显微镜分析热变形后的材料的塑性流动规律与组织演化特征,获得材料参数.结果 基于流变和组织行为...     

Strain Localisation in iPP/MWCNT Nanocomposites Using Digital Image Correlation

Polypropylene/multiwall carbon nanotube nanocomposites with different rates of weight incorporation (0–1%) were prepared by melt compounding and cast extrusion. The effect of maleic anhydride (5 wt%) grafted on polypropylene is studied through mechanical tests at different scales and morphological observations. In particular, the micromechanism of deformation was investigated through instrumented tensile experiments (at a macro and micro scales) using of a non‐contact method known as digital image correlation. The objective of this paper is first to characterise global behaviour (Young modulus, tensile strength, and ultimate properties) and second to go further in local analysis. In particular, optical instrumentation enables estimation of strain profile distribution onto the sample in a constricted area. Statistical parameters extracted from these local profiles are promising tools to enhance mechanical properties in link with microstructural composition. Tensile tests confirm composite reinforcement at a low level of nanocomposite incorporation, and local analysis enables quantitative measurements of adding maleic anhydride in formulations. The results reveal that addition of maleic anhydride delays strain localisation in the necked area.     

Folding at the Microscale: Enabling Multifunctional 3D Origami‐Architected Metamaterials

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura‐Ori tubes. The origami‐architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson’s ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson’s ratio sign during deformation. The findings here reported underscore the scalable and multifunctional nature of origami designs, and pave the way toward harnessing the power of origami engineering at small scales.     

Illuminating Origins of Impact Energy Dissipation in Mechanical Metamaterials

Elastomeric mechanical metamaterials have revealed striking ability to attenuate shock loads at the macroscopic level. Reports suggest that this capability is associated with the reversible elastic buckling of internal beam constituents observed in quasistatic characterizations. Yet, the presence of buckling members induces non‐affine response at the microscale, so that clear understanding of the exact energy dissipation mechanisms remains clouded. In this report, the authors examine a mechanical metamaterial that exhibits both micro‐ and macroscopic deformations under impact loads and devise an experimental method to visualize the resulting energy dissipation mechanisms. By illuminating the dynamic distribution of strain in the metamaterial, the authors uncover a rational way to program the macroscopic deformation and enhance impact mitigation properties. The results emphasize that mechanical metamaterials clearly integrate materials science and structural engineering, encouraging future interdisciplinary studies to capitalize on the opportunities.

     

Multi‐Stable Mechanical Structural Materials

Energy absorbing is an important and desirable property in mechanical and civil engineering. Here, a proof‐of‐concept method is presented as a new approach to achieve artificial mechanical materials with tunable compressive behavior for energy absorbing constructed from unit cells with a snap fit structure. The artificial structure undergoes a series of stable configurations derived from the sequential insertion of the plug into the groove of the snap fit. Both, experimental and simulation results manifest the multi‐stable and tunable mechanical properties of the structure. The mechanical energy dissipated by the proposed structure is demonstrated to be dependent on the lead‐in angle of the snap fit and the deflection ratio of the groove, as well as on the coefficient of friction between the plug and the groove of the snap fit. The system designed, herein, exhibits mechanical properties that can be tuned not only by adjusting the geometric parameters, but also by tuning the coefficient of friction between the plug and the groove, allowing the mechanical properties to be tailored post‐fabrication. Furthermore, the proposed model can be extended to the macro‐, micro‐, or nanoscales. These findings provide a simple method to obtain artificial materials with tunable energy absorbing properties, which can be applied in areas such as the design of automobile bumpers and foldable devices that facilitate their transportation.

     

Programmable Self‐Locking Origami Mechanical Metamaterials

Developing mechanical metamaterials with programmable properties is an emerging topic receiving wide attention. While the programmability mainly originates from structural multistability in previously designed metamaterials, here it is shown that nonflat‐foldable origami provides a new platform to achieve programmability via its intrinsic self‐locking and reconfiguration capabilities. Working with the single‐collinear degree‐4 vertex origami tessellation, it is found that each unit cell can self‐lock at a nonflat configuration and, therefore, possesses wide design space to program its foldability and relative density. Experiments and numerical analyses are combined to demonstrate that by switching the deformation modes of the constituent cell from prelocking folding to postlocking pressing, its stiffness experiences a sudden jump, implying a limiting‐stopper effect. Such a stiffness jump is generalized to a multisegment piecewise stiffness profile in a multilayer model. Furthermore, it is revealed that via strategically switching the constituent cells' deformation modes through passive or active means, the n‐layer metamaterial's stiffness is controllable among 2ⁿ target stiffness values. Additionally, the piecewise stiffness can also trigger bistable responses dynamically under harmonic excitations, highlighting the metamaterial's rich dynamic performance. These unique characteristics of self‐locking origami present new paths for creating programmable mechanical metamaterials with in situ controllable mechanical properties.     

Behavior of 3D orthogonal woven CFRP composites. Part II. FEA and analytical modeling approaches

In this paper, a 3D macro/micro finite element analysis (FEA) modeling approach and a 3D macro/micro analytical modeling approach are proposed for predicting the failure strengths of 3D orthogonal woven CFRP composites. These approaches include two different scale levels, macro- and micro-level. At the macro-level, a relatively coarse structural model is used to study the overall response of the structure. At the micro-level, the laminate block microstructure is modeled in detail for investigating the failure mechanisms of 3D orthogonal woven CFRP composites. The FEA and analytical models developed previously [Tan P, Tong L, Steven GP. Modeling approaches for 3D orthogonal woven composites, Journal of Reinforced Plastics and Composites, 1998:17;545–577] are employed to predict the mechanical properties of 3D orthogonal woven CFRP composites. All models presented in this paper are validated by comparing the relevant predictions with the experimental results, which were reported earlier in Part I of the paper [Tan P, Tong L, Steven GP. Behavior of 3D orthogonal woven CFRP composites. Part I. Experimental investigation, Composites, Part A: Applied Science and Manufacturing, 2000:31;259–71]. The comparison shows that there is a good agreement for the mechanical properties. An acceptable agreement exists for the failure strength in the x or stuffer yarn direction even though the FEA model gives a lower bound and the analytical model gives an upper bound. However, for the failure strength in the y or filler yarn direction, the difference between the predicted and experimental results is significant due to primarily ignoring of the waviness of filler yarn in the models. A curved beam model, which considers the waviness of the filler yarn, will be presented in Part III of the paper.     

A Methodology for Accurately Measuring Mechanical Properties on the Micro‐Scale

Abstract: A methodology has been developed for accurately measuring the mechanical properties of materials used on the micro‐scale. The direct tension test method using a dog bone‐type specimen has been employed, as it is the most effective and straightforward method to obtain results including a full stress–strain curve. The goal of this investigation was to develop a universal, yet simple and reliable, methodology to be used for accurate characterisation of mechanical properties for a wide variety of materials. Specimens from single crystal silicon were fabricated using photolithography by means of deep reactive ion etching. This material was chosen as it is expected that on both the micro‐ and macro‐scales, Young's modulus will have the same value. Hence, the accuracy of the methodology may be unambiguously examined. The test set‐up includes a small test machine containing a load cell whose maximum capacity is 5 N and is capable of direct gripping and displacement control. The specimens were found to have a trapezoidal cross‐section that was accurately measured using a scanning electron microscope. The strains were obtained by means of digital image correlation using images obtained via optical microscopy. The quantities measured include Young's modulus E, the fracture strength σ_f and the fracture strain ε_f. The average value of E obtained in the micro‐tests agrees well with the reference value obtained on the macro‐scale.     

Manufacture and Mechanical Properties of Batwing-Type Al Cellular Metamaterials by Selective Laser Melting

Designing metallic cellular structures with triply periodic minimal surfaces (TPMS) is a novel approach for preparing multifunctional and lightweight metamaterials. TPMS-structured Batwing Al cellular metamaterials are fabricated using selective laser melting. The mechanical performance, failure/deformation modes, and energy absorption capacity of the metamaterials are investigated. It is demonstrated in experimental results that the unit cell structure has a significant influence on the mechanical properties of the metamaterials, and that the sample with large wall thickness exhibits excellent mechanical properties and energy absorption capacity. Furthermore, the Gibson–Ashby equation is developed to estimate the mechanical properties of the Batwing-type Al cellular metamaterials. Herein, a theoretical foundation is provided in these findings for the engineering application of phase-pure Al, which is typically unsuitable as a structural material due to its low yield strength.     

Impact resistance and energy absorption mechanisms in hybrid composites

The response of hybrid composites to low-velocity impact loading has been investigated. The energy absorbing mechanisms of laminates containing various fibers were studied primarily by means of the instrumented falling dart impact testing technique. Static indentation tests and scanning electron microscopy (SEM) were also employed to assist in the identification of failure mechanisms. The composites containing polyethylene (PE) fibers, which were of high strength and high ductility, were found to be effective in both dissipating impact energy and resisting through penetration. Polyester (PET) fiber reinforced epoxy also exhibited superior impact characteristics even though the PET fabric layers without epoxy did not have good modulus or ductility. Good energy absorbing capability was also observed in epoxy reinforced with woven fabrics made of high-performance Nylon fibers. Nylon, PE and PET fibers were found to enhance the impact resistance of graphite fiber composites. Upon impact loading, the composites containing either PE or PET fibers in general exhibited a great degree of flexural plastic deformation and some level of delamination, thereby dissipating a significant amount of strain energy. Hybrids containing Nylon fabric showed analogous behavior, but to a lesser degree. The stacking sequence in hybrid laminates was found to play a critical role in controlling plastic deformation and delamination. This implies that the stacking sequence is a major factor governing the overall energy sorbing capability of the hybrid structure. The penetration resistance of hybrid composites appeared to be dictated by the toughness (strength plus ductility) of their constituent fibers. The fiber toughness must be measured under high strain rate conditions.     

3D Printing of Ultralight Biomimetic Hierarchical Graphene Materials with Exceptional Stiffness and Resilience

Biological materials with hierarchical architectures (e.g., a macroscopic hollow structure and a microscopic cellular structure) offer unique inspiration for designing and manufacturing advanced biomimetic materials with outstanding mechanical performance and low density. Most conventional biomimetic materials only benefit from bioinspired architecture at a single length scale (e.g., microscopic material structure), which largely limits the mechanical performance of the resulting materials. There exists great potential to maxime the mechanical performance of biomimetic materials by leveraging a bioinspired hierarchical structure. An ink‐based three‐dimensional (3D) printing strategy to manufacture an ultralight biomimetic hierarchical graphene material (BHGMs) with exceptionally high stiffness and resilience is demonstrated. By simultaneously engineering 3D‐printed macroscopic hollow structures and constructing an ice‐crystal‐induced cellular microstructure, BHGMs can achieve ultrahigh elasticity and stability at compressive strains up to 95%. Multiscale finite element analyses indicate that the hierarchical structures of BHGMs effectively reduce the macroscopic strain and transform the microscopic compressive deformation into the rotation and bending of the interconnected graphene flakes. This 3D printing strategy demonstrates the great potential that exists for the assembly of other functional materials into hierarchical cellular structures for various applications where high stiffness and resilience at low density are simultaneously required.     

三维机织复合材料的一种梁单元细观力学模型

根据三维机织复合材料中纤维束排列和变形的周期性特点,推导了一种细观梁单元模型。该模型考虑了纤维束的拉 (压) 弯耦合效应和纤维束之间的相互作用,可以描述纤维束和基体中的细观应力分布,并得到宏观的力学性能。针对一种典型的三维机织复合材料,首先根据编织参数,确定其细观几何结构,取最小周期的一段纤维束作为分析胞元,用上述细观梁单元分析了该段纤维束在面内拉伸荷载下的细观应力分布,计算出平均模量, 并用材料试验和细观实体有限元对本模型进行了检验,结果与本文的预测吻合良好。研究表明,拉、弯耦合效应引起的纤维束中的细观弯曲应力同平均轴向应力相比,不可忽略。      

Study on Fracture Behaviour of a Polymer‐Bonded Explosive Simulant Subjected to Uniaxial Compression Using Digital Image Correlation Method

Abstract: Quasi‐static uniaxial compression experiments were conducted on a polymer‐bonded explosive (PBX) simulant. At macro‐scale, the deformation and fracture process of samples were recorded using a charge‐coupled‐device camera. Microscopic examination was conducted to in situ observe the deformation and fracture processes of samples using SEM equipped with a loading stage. Microscopic damage modes, including interfacial debonding and particle fracture, were observed. The digital image correlation (DIC) technique was used to calculate the recorded images, and the macro‐ and micro‐scale displacement and strain fields were determined. Crack initiation, crack propagation, fracture behaviour and failure mechanism of samples were studied. The effects of aspect ratios on fracture behaviour and failure mechanism of PBX simulant were analysed.     

Crumb rubber aggregate coatings/pre-treatments and their effects on interfacial bonding, air entrapment and fracture toughness in self-compacting rubberised concrete (SCRC)

The interfacial-bonding, interfacial transition zone (ITZ), and porosity are regarded as the key factors affecting hardened concrete properties. The aim of this study was to experimentally improve the bonding between the rubber aggregate and cement paste by different methodologies including water washing, Na(OH) pre-treatment, and both cement paste and mortar pre-coating. All methods were assessed by determining mechanical and dynamic properties, then correlating this with ITZ porosity and interfacial gap void geometry, along with quantification of the fracture energy during micro crack propagation using fractal analysis. The results indicated that pre-coating the rubber by mortar gave the best results in terms of fracture toughness and energy absorption showing good agreement between observations made at both micro and macro scales.     

The effect of different forms of strain energy functions in hyperelasticity‐based crystal plasticity models on texture evolution and mechanical response of face‐centered cubic crystals

Micro‐mechanical and macro‐mechanical behavior of face‐centered cubic (FCC) crystals is investigated by using different forms of strain energy functions in hyperelastic material models in crystal plasticity finite element framework. A quadratic strain energy function with anisotropic elastic constants, a polyconvex strain energy function with invariants associated with the cubic symmetry, and a strain energy function from an inter‐atomic potential are considered in hyperelastic material models to describe the elastic deformation of FCC crystals. In our numerical experiments, the trajectories of {111} poles in the pole figure and the accumulated plastic slips of FCC coppers under uniaxial tension and simple shear depend on the choice of strain energy functions when the slip resistance of the slip systems is high. The ability of strain energy functions in this study to represent elastic lattice distortions in crystals varies with the amount of elastic deformation and the shape of deformed lattice. However, numerical results show that the change of macroscopic mechanical behavior of FCC coppers is not significant for the choice of strain energy functions, compared with the change of crystallographic texture evolution. Copyright © 2014 John Wiley & Sons, Ltd.