Ultra-lightweight Structures Research Articles

Crashworthiness and stiffness improvement of a variable cross-section hollow BCC lattice reinforced with metal strips

The ultra-lightweight structures with high mechanical properties and energy absorption behaviors are focused on the innovation structural design. The design strategy of implementing novel unit cells within architecture materials such as lattice materials enables the attainment of unparalleled combinations of mechanical properties and functionalities while minimizing weight. The most common method to design light-weight lattice materials is optimizing the geometric configurations of the unit cells. However, changing the geometric boundary of lattice structures can also be a good solution to improve the mechanical characteristics and energy absorption behaviors of the lattice materials. In this work, a novel variable cross-section hollow (VCH) lattice was established to improve the energy absorption capacity. By adding metal strips on the edge of the VCH lattices, a new reinforced variable cross-section hollow (RVCH) lattice with metal strips was developed to further enhance the stiffness and energy absorption capacity. The stainless VCH and RVCH lattices were additively manufactured by Selective Laser Melting (SLM) using an EP-M450H metal 3D printer. The quasi-static compressive characteristics and energy absorption behaviors of RVCH lattices were studied experimentally. Finite element modeling was implemented to study the energy absorption mechanism of RVCH lattices. A parametric analysis based on the finite element models was conducted to study the influence of different metal strips on the energy absorption capacity of RVCH lattices. The results show that the RVCH lattices with metal strips attaching to the vertical edges can signally enhance energy absorption of lattice structures with good load uniformity. Furthermore, the natural frequencies analysis indicate that the higher bending stiffness and the tensile stiffness can be achieved for RVCH lattices. This novel lattice is more stable as a load-bearing structure and more efficient as an energy absorber, which can provide guidance in designing innovative lattice structures with excellent mechanical properties and energy absorption capacity.

Exploring the influence of initial design domain dependencies in concurrent multiscale topology optimization for heat conductivity maximization

Advancements in additive manufacturing have led to the widespread adoption of topology and shape optimization in the design process across various industrial applications. Topology optimization has emerged as a promising approach for creating ultra-lightweight structures with exceptional functionality. By designing porous structures and addressing the layout of pores, concurrent multiscale topology optimization schemes offer a means to achieve such outcomes. This paper focuses on the development of a multiscale topology optimization tool and presents a numerical investigation of the initial design domain's impact on maximizing heat conductivity through concurrent multiscale topology optimization. The study highlights the significant influence of the initial design domain in the microscale on the final microscale design. Moreover, modifying the initial micro design domain has a direct impact on the macro design domain. The findings underscore the importance of considering the initial design domain when aiming to maximize heat conductivity through concurrent multiscale topology optimization. To facilitate the replication of the results presented in this paper, comprehensive details regarding parameter settings and implementation aspects are provided.

Three-Dimensional Printed Highly Porous and Flexible Conductive Polymer Nanocomposites with Dual-Scale Porosity and Piezoresistive Sensing Functions.

Highly flexible, deformable, and ultralightweight structures are required for advanced sensing applications, such as wearable electronics and soft robotics. This study demonstrates the three-dimensional (3D) printing of highly flexible, ultralightweight, and conductive polymer nanocomposites (CPNCs) with dual-scale porosity and piezoresistive sensing functions. Macroscale pores are established by designing structural printing patterns with adjustable infill densities, while the microscale pores are developed by phase separation of the deposited polymer ink solution. A conductive polydimethylsiloxane solution is prepared by mixing polymer/carbon nanotubes with non-solvent and solvent phases. Silica nanoparticles are utilized to modify the rheological properties of the ink, making direct ink writing (DIW) feasible. 3D geometries with various structural infill densities and polymer concentrations are deposited using DIW. The solvent is evaporated during a stepping heat treatment, leading to non-solvent droplet nucleation and growth. The microscale cellular network is developed by removing the droplets and curing the polymer. Up to 83% tunable porosity is achieved by independently controlling the macro- and microscale porosity. The effect of macroscale/microscale porosity and printing nozzle sizes on the mechanical and piezoresistive behavior of the CPNC structures is explored. The electrical and mechanical tests demonstrate a durable, extremely deformable, and sensitive piezoresistive response without sacrificing mechanical performance. The flexibility and sensitivity of the CPNC structure are enhanced up to 900 and 67% with the development of dual-scale porosity. The application of the developed porous CPNCs as piezoresistive sensors for detecting human motion is also evaluated.

Foreword to the special issue on new developments in structural stability.

Foreword to the special issue on new developments in structural stability.

Modeling dynamic behavior of dielectric elastomer muscle for robotic applications.

Recently, as a strong candidate for artificial muscle, dielectric elastomer actuators (DEAs) have been given the spotlight due to their attractive benefits from fast, large, and reversible electrically-controllable actuation in ultra-lightweight structures. Meanwhile, for practical use in mechanical systems such as robotic manipulators, the DEAs are facing challenges in their non-linear response, time-varying strain, and low load-bearing capability due to their soft viscoelastic nature. Moreover, the presence of an interrelation among the time-varying viscoelasticity, dielectric, and conductive relaxations causes difficulty in the estimation of their actuation performance. Although a rolled configuration of a multilayer stack DEA opens up a promising route to enhance mechanical properties, the use of multiple electromechanical elements inevitably causes the estimation of the actuation response to be more complex. In this paper, together with widely used strategies to construct DE muscles, we introduce adoptable models that have been developed to estimate their electro-mechanical response. Moreover, we propose a new model that combines both non-linear and time-dependent energy-based modeling theories for predicting the long-term electro-mechanical dynamic response of the DE muscle. We verified that the model could accurately estimate the long-term dynamic response for as long as 20min only with small errors as compared with experimental results. Finally, we present future perspectives and challenges with respect to the performance and modeling of the DE muscles for their practical use in various applications including robotics, haptics, and collaborative devices.

Innovative Method of Embedding Optical Fiber inside Titanium Alloy Utilizing Friction Stir Forming

The development of FBG (Fiber Bragg Grating) sensors is essential for intelligent parts of airplanes to achieve ultra-lightweight structures and intelligent aviation control. Awaiting solution is embedding technology of optical fiber sensors into the base material of the parts; however, it has been challenging to embed fibers into high melting temperature point alloys like titanium-based materials without having any defects. Present research fulfills the mentioned demands effectively by utilizing Friction Stir Forming (FSF). Precisely, optical fiber has been placed into a guide slit inside the titanium sheet. Then, FSF was applied to the surface of the titanium. As a result, titanium plasticizes and flows into the guide slit. This mechanically interlocks the optical fiber inside the titanium alloy. This solid-state stirring process can bury sensors inside the titanium to protect the sensor from harsh environments. Embedded sensors will detect the strain and temperature of the host titanium in real-time. Moreover, this research can have further considerable effects on the areas like aerospace and artificial intelligence systems that demand real-time condition monitoring systems.

Topology optimization and 3D printing of micro-drone: Numerical design with experimental testing

The expanding capabilities and decreasing costs of additive manufacturing have resulted in the increased adoption of micro-unmanned aerial vehicles (micro-UAVs) among professionals and hobbyists. Due to safety regulatory requirements of UAV operations, weight is generally the overriding design features of interest for micro-drones, but it often comes as a trade-off against the durability, loading constraints, and other subsystem equipment. Nevertheless, ultra-lightweight structures can be realized through the adoption of both 3D-printing and topology optimization without compromising the structural integrity and overall strength and this article explores the use of these two technologies for designing and manufacturing optimized ultralight micro-UAVs. First, material properties of Nylon 12 (PA12) manufactured using selective laser sintering (SLS) were accurately characterized via mechanical testing and ultrasonic means. These properties were verified by comparing the mechanical response of 3-point and 4-point bending tests with corresponding finite element (FE) simulation. Next, topology optimization was performed to produce an optimized structure of a Z-split configured lightweight micro-quadcopter. The optimized design is then 3D-printed and subsequently validated through a load test for verification against the optimized FE simulation-based design. A close correlation was obtained between the numerical and experimental data, suggesting that topology optimization with 3D printing can be safely and reliably adopted for the design and rapid prototyping of micro-UAVs, whilst catering to different specifications and requirements.

Determining parameters of sandwich composite panels for earth remote sensing satellite bus

The paper discusses the feasibility of replacing conventional sandwich panel with carbon fiber skins and aluminum honeycomb core in the structure of an Earth remote sensing satellite bus. Thermal loading conditions for sun-synchronous orbit were considered and math simulations were run for stress-strain behavior of a satellite bus element built of the proposed versions of sandwich composite panels. Simulations results might be of interest for design of ultra-lightweight structures of the satellite bus, solar arrays and antenna reflector. Key words: Earth remote sensing spacecraft, Sun-synchronous orbit, math simulations, polymer composite materials.

A review on developments of deployable membrane-based reflector antennas

The gossamer space structures are very large and ultra-lightweight structures. A large aperture-based space structure is highly efficient in capturing signals from a wide coverage area. However, their deployment in space has been a critical challenge to date. Extensive research has been carried out on various types of gossamer structures used for earth exploration and deep space applications. Space antenna, in general, should have a large electrical aperture, be light in weight with small stowage volume, be easily deployable in space, and thermally stable. The antenna consists mainly of a reflector as a major part and a torus for supporting the reflector, electronic control unit, and struts. This paper covers an extensive review of gossamer space structure considering different categories of large deployable antennas. More attention has been given to a large membrane-based antenna having a parabolic reflector. This work includes the study of designs, analyses, and development of prototypes and successful deployment of space structures in orbit. We emphasize understanding the behaviour of thin membrane, their static and dynamic characteristics, wrinkling control, shape control of thin membrane reflector, and recent advances in deployment techniques of large deployable antenna structures.

Metrology of additively manufactured lattice structures by X-ray tomography

Additive manufacturing allows complexity of manufactured metal structures. Lattice structures hold the most promise for high complexity, tailorable and ultra-lightweight structures. In medical applications, these structures find application especially in bone implants – allowing matching of local elastic modulus of implant to that of bone while also allowing osseointegration. With this new complexity comes new manufacturing quality control and metrology challenges. Traditional metrology tools cannot access the entire structure and the only reliable method to inspect the inner details of these structures non-destructively is by X-ray tomography.

Progressive damage and failure modeling in composite cylindrical pyramidal lattice structure

In this paper, the mechanical behavior of a carbon fiber composite cylindrical pyramidal lattice structure has been predicted based on initial failure criteria and progressive damage under compressive loading. For this purpose, the three-dimensional Hashin failure criteria with instantaneous and gradual unloading models have been employed by the user subroutine UMAT in ABAQUS/Standard. In the instantaneous and gradual unloading models, progressive damage has been controlled by binary and exponential damage functions, respectively. Besides, to validate finite element models, the results have been compared with the experimental tests. Therefore, a new mold has been designed and manufactured for the experimental test of composite cylindrical pyramidal lattice structures. The new mold allows the use of the maximum inherent strength of the fiber-reinforced composite. The predicted force–displacement curves are in good agreement with the experimental test results. Hence, it is possible to use these FE models in the design of ultra-lightweight structures.

Using Influence Matrices as a Design and Analysis Tool for Adaptive Truss and Beam Structures

Due to the already high and still increasing resource consumption of the building industry, the imminent scarcity of certain building materials and the occurring climate change, new resource- and emission-efficient building technologies need to be developed. Especially since these effects are further amplified by the growth of population. One proposed technology is that of adaptive load bearing structures. Integrating sensors, actuators and a control unit into the structure enables the structure to react to external loads, when needed. For example, when high but rare loads occur. The goal of the adaptation could be the reduction of deflections or the homogenization of stresses. This allows for ultra-lightweight structures with reduced material consumption and emissions. The two main questions of adaptive truss and beam structures are, first, the question of optimal actuator placement and second, which type of typology (truss, frame, etc.) is most effective for adaptive load bearing structures. One approach for finding the optimal configuration is that of so called influence matrices. Influence matrices, as introduced in this paper, are a type of sensitivity matrix, which describe how and to which extend various properties of a given load bearing structure can be influenced by different types of actuation principles. The method of influence matrices is exemplified by a series of studies on different configurations of a truss structure.

Solution‐Processable 2D Materials Applied in Light‐Emitting Diodes and Solar Cells

AbstractThe last decades have witnessed a remarkable scientific progress in the field of organic and perovskite optoelectronics. Two‐dimensional (2D) materials are an attractive building block for next‐generation devices, thanks to their unique physical, optical, and electric characteristics including atomically thin bodies, high transmittance, ultralight weight, and tunable band structures. The state‐of‐the‐art optoelectronic devices utilizing 2D materials mainly rely on 2D thin films grown by chemical vapor deposition. Although good device performances have been demonstrated, a huge gap between fundamental studies and practical applications remains, because of the high cost and troublesome transfer/restacking processes. Therefore, flexible and transparent light‐emitting diodes (LEDs) and solar cells (SCs) containing solution‐processed 2D materials from top‐down exfoliation methods have recently emerged as promising candidates for future light conversion and emission devices. They combine ease of processing, tailorable optoelectronic features, facile integration with complementary layers, compatibility with arbitrary substrates, and enhanced performances. In addition, the latest processing techniques (such as ink‐jet printing, spray coating) also offer the opportunity for the scaled‐up fabrication of square‐meter‐scale low‐cost device systems. Recent advances, challenges, and future perspectives of solution‐processed 2D materials for usage in emerging LEDs and SCs applications are discussed here.

Nature-Inspired, Ultra-Lightweight Structures with Gyroid Cores Produced by Additive Manufacturing and Reinforced by Unidirectional Carbon Fiber Ribs.

This article reports on a nature-inspired, ultra-lightweight structure designed to optimize rigidity and density under bending loads. The structure’s main features were conceived by observing the scales of the butterflies’ wings. They are made of a triply periodic minimal surface geometry called gyroid and further reinforced on their outer regions with a series of ribs. In this work, the ribs were substituted with carbon fiber-reinforced bars that were connected to the main structure with an innovative concept. Stereolithography was used to print a plastic component in one piece that comprised the core and the connection system. Bending tests were performed on the structures along with a Finite Element Method optimization campaign to achieve the optimum performance in terms of stiffness and density. Results show that these architectures are among the most effective mechanical solutions in respect to their weight because of their particular arrangement of material in space.

Binder jetting additive manufacturing of copper foam structures

In binder jetting additive manufacturing (BJAM), the part geometry is generated via a binding agent during printing and structural integrity is imparted during sintering at a later stage. This separation between shape generation and thermal processing allows the sintering process to be uniquely controlled and the final microstructural characteristics to be tailored. The separation between the printing and consolidation steps offers a unique opportunity to print responsive materials that are later “activated” by temperature and/or environment. This may allow a new paradigm in multi-scale, multifunctional materials. This concept is preliminarily demonstrated using a foaming copper feedstock, such that the copper is printed, sintered and then foamed via intraparticle expansion in separate steps. The integration of foaming feedstock in BJAM could allow for creation of ultra-lightweight structures that offer hierarchical porosity, graded density, and/or tailored absorption properties. This work investigates processing protocol for copper foam structures to achieve the highest porosity. The copper feedstock was prepared by distributing copper oxides through the copper matrix via mechanical milling, and that powder was then printed into a green geometry through BJAM. The printed green parts were then heat treated using different thermal cycles to investigate the porosity evolution relative to various heating conditions. The heat treated parts were then examined for their resulting properties including porosity, microstructural evolution, and volumetric shrinkage. Parts that were initially sintered in air and then annealed in a hydrogen atmosphere led to higher porosity compared to those sintered in hydrogen alone. It was also found that the annealing of parts at 600 °C for 2 h resulted in the highest final porosity (59%) and the lowest volumetric shrinkage of 5%. Anisotropy in linear shrinkage in X, Y, and Z direction was also observed in the heat treated parts with the largest linear shrinkage occurring in the Z direction.

Influence of augmented tuning of core architecture in 3D woven sandwich structures on flexural and compression properties of their composites

The concept of 3D woven sandwich structures is the most amenable technology that attracts the current design engineers with their advantages over foams and honeycomb core-based composites. A thorough investigation on the mechanical endurance of sandwich structures reinforced with woven spacer fabrics reveals that their compression and shear properties are at stake with the increase in their core heights. In this work, a new concept for the sandwich structure, 3D spacer fabrics with integrally woven connecting walls in three different configurations were designed and fabricated. Further, the woven samples were consolidated into composite structures using epoxy resin and their improved structural effectiveness was studied by loading them under compression and bending loads. From these test results, it was evident that suitable modification of woven connecting wall configuration can successfully produce energy absorbent ultra-lightweight hollow structures. T-peel test carried on a replicate model of the junction formed in the structures revealed that the structures show node lamination prone failures. It is expected that this study will prove to be an experimental database for designing foamless sandwich structural composites with better stability at higher thickness.

Cellulosic material obtained from Antarctic algae biomass

Algae biomass is a raw material widely used by many industrial sectors, such as food production, pharmaceuticals, cosmetics, fertilizers, biofuels, and many others. Its usage is mainly due to the phycocolloids content, such as alginates, carrageenans, agar, etc. One of the polysaccharides present in this biomass, and still little explored is cellulose, an important resource with several technological applications, for example: production of nanocellulose, ultralightweight structures, drug delivery, tissue engineering, wound dressings, among others. Thus, we used the Antarctic algae Cystosphaera jacquinottii as raw material for the production of a cellulosic material combining alkaline treatment, bleaching, and freeze-drying. Fourier-transform infrared spectroscopy results revealed that the methodology employed was effective to obtain cellulose. X-ray diffraction analysis showed that the material obtained had crystallinity above 70%. Scanning electron microscopy analyses showed a highly porous morphology, consisting of cellulose nanofibers with a diameter about 32 nm.

Small perforations in corrugated sandwich panel significantly enhance low frequency sound absorption and transmission loss

Numerical and experimental investigations are performed to evaluate the low frequency sound absorption coefficient (SAC) and sound transmission loss (STL) of corrugated sandwich panels with different perforation configurations, including perforations in one of the face plates, in the corrugated core, and in both the face plate and the corrugated core. Finite element (FE) models are constructed with considerations of acoustic-structure interactions and viscous and thermal energy dissipations inside the perforations. The validity of FE calculations is checked against experimental measurements with the tested samples provided by additive manufacturing. Compared with the classical corrugated sandwich without perforation, the corrugated sandwich with perforated pores in one of its face plate not only exhibits a higher SAC at low frequencies but also a better STL as a consequence of the enlarged SAC. The influences of perforation diameter and perforation ratio on the vibroacoustic performance of the sandwich are also explored. For a corrugated sandwich with uniform perforations, the acoustical resonance frequencies and bandwidth in its SAC and STL curves decrease with increasing pore diameter and decreasing perforation ratio. Non-uniform perforation diameters and perforation ratios result in larger bandwidth and lower acoustical resonance frequencies relative to the case of uniform perforations. The proposed perforated sandwich panels with corrugated cores are attractive ultralightweight structures for multifunctional applications such as simultaneous load-bearing, energy absorption, sound proofing and sound absorption.

Intrinsic Aluminum CFRP Hybrid Composites Produced in High Pressure Die Casting with Polymer Based Decoupling Layer

Combining aluminum and carbon fiber reinforced plastic (CFRP) has been a key focus in realizing lightweight concepts. Manufacturing technologies for high load-bearing and ultra-lightweight CFRP structures have reached a high level of innovation. The same goes for near-net-shape high pressure die casting (HPDC) aluminum components, which can be mass-produced in a highly efficient manner. Yet for hybrid composites of these materials, the solutions to date have relied on conventionally mechanical or adhesive joining techniques. The direct joining of these two materials is problematic, due to their electrochemical intolerance and the resulting corrosive degradation. The joining technology therefore is at the center of this challenge. The DFG-sponsored joint research project of Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Fiber Institute FIBRE, and Bremen Institute for Mechanical Engineering BIME, all three institutes located at the campus of the university in Bremen, aims at combining aluminum and thermoplastic CFRP into an intrinsic hybrid composite. This is to be achieved in a single-step primary shaping process, avoiding conventional joining techniques like adhesive bonding or riveting. To this end, CFRP structures are to be recast with aluminum, creating an electrochemically decoupling layer between the two materials. This decoupling layer can therefore be considered as a key factor for realizing hybrid composites. It also needs to have a high process reliability and be long-term and mechanically stable. Polyetheretherketone (PEEK) thermoplast was identified as a suitable material for that purpose, given its stability at high temperatures and electrochemical insulation effect. First test results show the possibility of incorporating CFRP accordingly by HPDC, resulting in a continuous intact decoupling layer of PEEK. The trend indicated that different thermal treatments as well as different aluminum thicknesses of the hybrid casted sample influence the joint strength. On average, in tensile shear tests a joint strength approximately in the range of current single lap adhesive bonds could be achieved.

Three-dimensionally printed cellular architecture materials: perspectives on fabrication, material advances, and applications

Three-dimensional (3D) printing generates cellular architected metamaterials with complex geometries by introducing controlled porosity. Their ordered architecture, imitative from the hierarchical high-strength structure in nature, defines the mechanical properties that can be coupled with other properties such as the acoustic, thermal, or biologic response. Recent progress in the field of 3D architecture materials have advanced that enables for design of lightweight materials with high strength and stiffness at low densities. Applications of these materials have been identified in the fields of ultra-lightweight structures, thermal management, electrochemical devices, and high absorption capacity.