Energy Absorption Applications Research Articles

Tuneable mechanical performance and reusability of 4D-printed heterogeneous metamaterials using shape memory biomass-derived polymer

ABSTRACT Herein, hybrid triply periodic minimal surfaces (TPMS) metamaterials were structurally designed and additively manufactured via 4D printing using a biomass-derived shape memory polymer. Two TPMS topologies including gyroid and diamond with homogeneous and graded structures were separately generated by adjusting the C parameter. The mechanical performance and deformation behaviour between the uniform and graded structures were compared, and the compressive shaping-recovery cycles were conducted. The results showed that the gyroid (C:0.7→0.4) and diamond (C:0.7→0.4) samples exhibited superior reusability to the uniform counterparts. Subsequently, heterogeneous metamaterials assembled by gyroid and diamond subsections were established and 4D-printed. It was observed that both the yielding and shape recovery were preferred occurring in the gyroid part, while the diamond subsection played a stronger supporting effect. Therefore, the compressive performance and deformation mode could be feasibly tuned by the hybridisation of TPMS, which broadened the functional and sustainable applications in energy absorption, intelligent protection and medical implants.

Investigating mechanical properties and energy absorption of elastomeric polymer nanocomposites containing cellulose nanofibers in tensile, quasi-static compression, and high strain rate tests

Abstract In this study, mechanical properties and energy absorption of elastomeric nanocomposites reinforced with cellulose nanofibers are investigated from tensile, Quasi-static Compression (QSC), and Split-Hopkinson Pressure Bar (SHPB) tests. For this purpose, the design and preparation of rubber nanocomposites with different loadings of cellulose nanofibers (CNFs) were carried out, and the optimal cure temperature (T90) of the rubber compound containing cellulose nanofibers was determined from the rheometer test. In the continuation of this study, the effects of adding cellulose nanofibers on the tensile strength, elongation to break, and energy absorption of the proposed nano-composites were investigated. The results showed that the nanocomposite containing 6 phr increases the ultimate strength and elastic modulus of 300% by 33.5% and 22.7%, respectively, compared to the control rubber (0 phr). Similarly, these numbers are about 10 and 65% for loading 12 phr cellulose nanofibers. From the results of the quasi-static compression test for different amounts of cellulose nanofibers at a strain rate of 50%, it was found that the lowest and highest compressive stress due to the resistance of elastomeric nanocomposites is related to the control sample (0 phr) and the 12 phr sample, respectively. Also, from high strain rate tests of Split Hopkinson Pressure Bar, it was found that the fracture mechanism of flexible composites containing cellulose nanofibers changes in response to a high-speed impact, and the samples respond to high-pressure impact with brittle fractures. It was also found that rubber nanocomposites reinforced with cellulose nanofibers are very sensitive to strain rates. As the strain rate increases, the energy absorption of rubber nanocomposites increases. The optimal loading (6 phr) of cellulose nanofibers in rubber compounds makes them suitable for energy absorption applications. Cellulosic nanofibers provide acceptable dispersion of nanomaterials through good interaction with natural rubber and lignin-carbon fillers. Therefore, through the physical interweaving of fillers with polymer chains, CNF provide better binding of polymer chains to improve properties.

DFT calculations of new Li-based halides Li2AgAlY6 (Y = Cl, Br, I) for energy conversion applications

Recently, lead-free halide double perovskite (DP) materials have got scholarly attention owing to their environmentally sustainable characteristic, excellent stability and potential applications in solar cells and renewable energy. In this present article, we computationally examined the mechanical, optoelectronic and thermoelectric characteristics of a discernible compound Li2AgAlY6 (Y = Cl, Br, I) using the density functional theory (DFT). The evaluation of tolerance factor (tf), lattice constant (a0), and formation enthalpy (Hf) confirms the structural and thermodynamic stability of the investigated double DPs, also elastic constants further elaborate the mechanically stable nature. The studied compounds have direct band gap of 3.6 eV, 2.6 eV and 1.2 eV for Li2AgAlCl6, Li2AgAlBr6 and Li2AgAlI6 respectively, indicating the diverse energy absorption applications from ultraviolet to visible region. We used semi-classical Boltzman theory to determine the figure of merit (ZT) and corresponding Seebeck Coefficient (S), which validates the electrical and thermoelectric conductivity for the compounds under investigation. The current investigation establishes a theoretical foundation for the examined DPs, essential for comprehending and comparing forthcoming experimental inquiries aimed at exploring diverse optoelectronic and thermoelectric applications.

Design and Development of Metasurface Materials for Enhancing Photodetector Properties.

Recently, metasurface-based photodetectors (metaphotodetectors) have been developed and applied in various fields. Metasurfaces are artificial materials with unique properties that have emerged over the past decade, and photodetectors are powerful tools used to quantify incident electromagnetic wave information by measuring changes in the conductivity of irradiated materials. Through an efficient microstructural design, metasurfaces can effectively regulate numerous characteristics of electromagnetic waves and have demonstrated unique advantages in various fields, including holographic projection, stealth, biological image enhancement, biological sensing, and energy absorption applications. Photodetectors play a crucial role in military and civilian applications; therefore, efficient photodetectors are essential for optical communications, imaging technology, and spectral analysis. Metaphotodetectors have considerably improved sensitivity and noise-equivalent power and miniaturization over conventional photodetectors. This review summarizes the advantages of metaphotodetectors based on five aspects. Furthermore, the applications of metaphotodetectors in various fields including military and civil applications, are systematically discussed. It highlights the potential future applications and developmental trends of metasurfaces in metaphotodetectors, provides systematic guidance for their development, and establishes metasurfaces as a promising technology.

Mechanical properties and energy absorption of medium-entropy alloy triply periodic minimal surface cellular structures fabricated via selective laser melting

Triply periodic minimal surfaces (TPMSs) are extensively used as effective tools for the microarchitectural designing of cellular structures. CoCrNi medium-entropy alloy, known for its high strength and high ductility, is an ideal candidate for fabricating TPMS-based structures. However, few studies have focused on this topic. This study investigated the compressive mechanical and energy absorption properties of CoCrNi D-sheet TPMS cellular structures fabricated via selective laser melting (SLM). The surface morphology of the fabricated D-sheet TPMS structure was observed through scanning electron microscopy, and the deformation and failure mechanisms of the structure were analyzed by the quasi-static compression test. The results indicated that the SLM-ed CoCrNi D-sheet TPMS structure exhibited stable collapse mechanisms compared to other metal-based TPMS structures. Meanwhile, the energy absorption characteristics of the modified finite element (FE) model agreed well with the experimental results. Furthermore, the impact of the level constant on the energy absorption performance of the D-sheet TPMS structure was investigated using the FE model. Thus, an optimal D-sheet (OD-sheet) TPMS structure with lower density and higher energy absorption capacity was obtained. Additionally, the Gibson–Ashby prediction model was established to aid in selecting CoCrNi OD-sheet TPMS structures with controlled relative densities for energy absorption applications.

Electroplating Additively Manufactured Honeycomb Structures to Increase Energy Absorption Under Quasi-Static Crush

Honeycomb (HC) has been used in energy absorption applications due to its high stiffness and low density. Metallic HC are used for energy absorption applications, however, these metallic structures can be challenging to manufacture if complex geometric features designed to improve energy absorption are used, which motivates the use of additive manufacturing (AM). Metal AM methods include powder bed fusion (PBF) and direct energy deposition (DED). In addition to capital equipment cost, these processes possess challenges that include a required inert environment, powder handling, final part porosity, residual stresses, and nonuniform surface finish. These concerns can be alleviated through the use of polymer AM, however, polymeric parts exhibit brittle failure and have a lower stiffness than metallic HC structures. In this study, a low-cost 3D polymer printing method, stereolithography (SLA), is combined with a conventional electroplating process to fabricate a metal-plastic composite HC structure with energy absorption capability much greater than of a plastic HC structures of the same nominal volume. SLA parts have a smooth surface, so that the surface finish is at least as uniform after electroplating as the SLA part. The energy absorption characteristics of the electroplated HC is studied to determine how these energy absorbing materials can be manufactured at reduced cost. Our study confirms that the metal-plastic composite HC increases both the crush strain range and the mean crush stress of these samples, resulting in metal-plastic composite HC structures with substantially increased energy absorption. This study also examines how buckling initiators (BIs), or diamond shaped holes located at 50, 75, and 100% of the height of the hexagonal cell vertices, can influence energy absorption performance. This study shows that it is feasible to fabricate electroplated HCs, using an SLA preform, to achieve a substantial increase in energy absorption over using SLA alone.

Evaluation of quasi-static and dynamic compressive properties of Ti-6Al-4 V functionally graded shell lattices with minimal surfaces

Functionally graded shell (FGS) lattices have recently captivated enormous research interest in engineering applications for their outstanding mechanical and energy-absorbing performances. This paper designs FGS lattices featuring I-WP triply periodic minimal surfaces using implicit functions. FGS and uniform shell (US) samples made of Ti-6Al-4 V powder are manufactured utilizing laser powder bed fusion technology. Quasi-static compression and drop-weight impact experiments are conducted to evaluate the mechanical and energy-absorbing performances as well as deformation mechanism of FGS and US samples. The results demonstrate that the deformation modes of the samples under both loading conditions are similar, which is basically attributed to the drop-weight impact velocities being lower than the critical speeds calculated by the rigid-power-law hardening (RPLH) model. Additionally, the effects of loading velocity on the deformation mechanisms of US and FGS lattices are simulated, and homogenization, transition, and shock deformation models are predicted by the RPLH model. The FGS sample exhibits distinct layer-by-layer failures and eliminates the abrupt shear band under dynamic and quasi-static loadings. Compared to the quasi-static results, a notable strain rate hardening effect is observed under the dynamic condition, mainly due to the hardening of the row material. Consequently, the elastic modulus, yield strength, compression strength, and cumulative energy absorption of the US sample under drop-weight impact loading improved by 18.74 %, 56.52 %, 45.31 %, and 6.62 %, respectively. However, the FGS sample shows less sensitivity to strain rate owing to the layer-by-layer deformation mechanism, and their elastic modulus, yield strength, compression strength, and cumulative energy absorption only increased by 11.84 %, 34.72 %, 36.85 %, and 3.86 %, respectively. The FGS lattices exhibit a low peak stress during the compression process, which is favorable for industrial applications in crashworthiness and energy absorption.

Laser powder bed fusion of bio-inspired metamaterials for energy absorption applications: A review

A significant amount of research has been done in the last few decades to reduce the risk of injury for occupants and the structures that are subjected to impact loading. Metamaterials have been proven to be useful in energy-absorbing structures to improve a structure's crashworthiness performance by reducing the negative impacts during collision. The metal additive manufacturing industry, especially Laser Powder Bed Fusion (LPBF), has made it easier to produce complex metamaterials with remarkable mechanical characteristics like lightweight, high specific strength, and effective energy absorption. This review paper investigates the transformative potential of bio-inspired metamaterial designs, which are additively manufactured using LPBF machines, for use in protective energy-absorbing structures. First, biomimicry in engineering is briefly discussed. The review focuses on the energy absorption performance of different designs, like thin-walled structures and different bio-inspired metamaterials. It discusses the effects of base metal, process conditions, and manufacturing defects. Optimization methods to enhance the design and crashworthiness of these bio-inspired energy absorbers are investigated. Various characterization methodologies including experimental techniques and numerical simulations, are highlighted, with a particular emphasis on integrating manufacturing defects into simulations. Finally, possible applications and future trends in aerospace, automotive, construction, and medical applications are reviewed. Despite decades of research into energy absorbers, there remains a lack of a comprehensive review on the use of additively manufactured metamaterials as energy absorbers, particularly those inspired by nature. This review paper addresses this gap by examining recent studies in the field, assessing the effectiveness of various bio-inspired metamaterial designs, their crashworthiness, and the associated characterization methods under different loading scenarios.

Build and raster orientation effects on CFRP onyx/aramid impact absorption

Composite materials fabricated via additive manufacturing are becoming more relevant in ready-for use products used in engineering applications like aerospace structures, propellers, electric vehicles, or sandwich cores. Continuous Fiber Reinforced Polymeric (CFRP) composites are 3D-printed composites with tailor-made properties due to the capability of the printing process to deposit matrix and fiber whenever required. However, CFRP products behave differently and have lower mechanical properties than traditional composites. This study aimed to analyze the impact strength of CFRP specimens with gyroid infill and the effects of two build (flat and on-edge) and two raster (0° and 45°) orientations on impact behavior. The gyroid infill helps to obtain a light-weight structure and it has been used in energy absorption applications showing excellent mechanical behavior in thermoplastic 3D-products. The specimens were manufactured using Onyx as the matrix and aramid fiber as the reinforcement materials. The impact energy absorption of CFRP composites was measured using unnotched Izod impact specimens. The impact tests results were statistically analyzed, revealing that the build orientation directly and significantly affects the impact behavior, resulting in higher impact absorption when flat orientation is used to produce CFRP composites. The impact strength of CFRP composites increased 8 times, and 2 times for flat and on-edge oriented specimens, respectively compared to pure Onyx specimens. The variation in impact energy absorption between raster orientations in both build orientations was not significant, the difference in flat-oriented specimens at 0° and 45° was only 0.2 J, and between on-edge-oriented specimens at 0° and 45° was only 0.089 J. Also, the after impact specimens were analyzed to categorize the different failure modes observed. The after-impact analysis showed poor impregnation between aramid and onyx layers, causing delamination, fiber bridging, and fiber exposure failures. The combination of Aramid, Onyx, and a non-solid infill (gyroid) demonstrated positive results in impact behavior, obtaining high-impact absorption (165 kJ/m2) with no more than 56 % of fiver volume. The impact properties information of CFRP composites made with aramid fibers is still very scarce, joined to the lack of information on the impact properties of CFRP composites with non-solid infill, like gyroids or sinusoidal path infills. The results of this research open the possibility of using non-solid infill in CFR process that can be used to manufacture and test ready-for-use CFRP products in tasks that require high-strength, low-weight structures and impact energy absorption.

Non local analytical and numerical modelling of re-entrant auxetic honeycomb

Auxetic materials, characterized by their negative Poisson’s ratio, have been extensively studied for applications in energy absorption and mechanical reinforcement. Re-entrant honeycomb structures, a subtype of auxetic materials, have demonstrated superior mechanical characteristics. However, understanding the mechanical behavior of these structures at the nanoscale remained a significant challenge. To address this gap, the authors explored the influence of size on the deflection behavior of re-entrant auxetic honeycomb structures through non-local continuum mechanics. Their analytical model, incorporating the Euler-Bernoulli beam model and considering four non-dimensional geometrical parameters, was validated through numerical simulations and a comprehensive review of existing literature. The study aimed to provide valuable insights into the design and engineering of re-entrant auxetic honeycomb structures across diverse applications, contributing to the advancement of non-local elasticity theory and deepening the understanding of the mechanical behavior of auxetic structures at the nanoscale. The research laid a foundation for further exploration and optimization of re-entrant honeycomb structures, facilitating their effective utilization in fields such as MEMS/NEMS by leveraging the dimensionless parameters identified in the study.

Design, Manufacturing, and Analysis of Periodic Three-Dimensional Cellular Materials for Energy Absorption Applications: A Critical Review.

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.

Quasi-static and dynamic behavior analysis of 3D CFRP woven laminated composite auxetic structures for load-bearing and energy absorption applications

This paper investigated the quasi-static and dynamic behavior of 3D auxetic metamaterial structures made from carbon fiber reinforced polymer (CFRP) laminated composite. The aim of the study was to enhance design methodologies for load-bearing and energy absorption applications of these 3D novel structures, filling the research gap in understanding their response to quasi-static and especially dynamic loadings. The two novel 3D structures were designed and fabricated by using an interlocking assembly method based on the 2D auxetic CFRP sheets, which were formed with hybrid double-arrow-head with re-entrant and star unit-cells and made with plain weave carbon epoxy prepregs. The finite element (FE) method was adopted to analyze the mechanical characteristics of the structures under the quasi-static and dynamic loading, and Hashin failure criteria were used to define damage in the structures. The study showed that the designed 3D auxetic CFRP structures simultaneously exhibit superior auxeticity, load-bearing, and energy absorption capacity.

Tailorable Energy Absorbing Cellular Materials via Sintering of Dry Powder Printed Hollow Glass Microspheres

This article examines amorphous glass-based foams as lightweight core materials for crash-resistant structures that offer tailorable energy absorption capabilities. Hollow glass microspheres (HGMs) of different densities are layered using dry powder print- ing (DPP), an additive manufacturing process, and subsequently sintered to consolidate these microspheres into a cellular foam structure. The tuning of energy absorption is achieved in these foams by layering hollow microspheres with different densities and different thickness ratios of the layers. The mechanical response to quasi-static uniax- ial compression of the bilayer foams is also investigated. Bilayer samples a distinctive two-step stress-strain profile that includes first and second plateau stress, as opposed to a single constant density which does not. The strain at which the second plateau occurs can be tuned by adjusting the thickness ratio of the two layers. The resulting tailorable stress-strain profile demonstrates tailorable energy absorption. Tailorability is found to be more significant if the density values of each layer differ greatly. For comparison, bilayer samples are fabricated using epoxy at the interface instead of the co-sintering process. Epoxy-bonded samples show a different mechanical response from the co-sintered sample with a different stress-strain profile. Designing the bilayer foams enables tailoring of the stress-strain profile, so that energy-absorption requirements can be met for a specific impact condition. The implementation of these materials for energy absorption, crashworthiness, and buoyancy applications will be discussed.

Twist design of lattice structure fabricated by powder bed fusion to adjust the energy absorption behavior

Additive manufactured lattice structures offer great potential for impact resistance applications. They exhibit excellent energy absorption characteristics during the elasto-plastic deformation process, providing protection to internal devices. However, the mechanical response of lattice structures undergoes substantial variations during the deformation process, thereby imposing limitations on their energy absorption behavior. This study introduces a novel twist design to modify the energy absorption behavior of rectangular and hollow cylindrical lattice structures. Powder bed fusion was used to fabricate the twisted lattice structures. The study employed a combination of compressive simulations and experimental investigations to systematically explore the impact of the twist angle on the peak crushing force, energy absorption, and crash load efficiency of the structures. Adjusting the twist angle results in a reduction of the peak crushing force and a transition towards a stable loading profile during the deformation process. Moreover, the crash load efficiency is also reduced. The twist design concept presented in this paper provides insight into designing and optimizing lattice structures for energy absorption applications.

Experimental Investigation on Fabrication of Cermet Hollow Spheres for Use as Reinforcement in the Production of Lightweight and Strong Metal Foams for High Energy Absorption Applications

Experimental Investigation on Fabrication of Cermet Hollow Spheres for Use as Reinforcement in the Production of Lightweight and Strong Metal Foams for High Energy Absorption Applications

Experimental and numerical investigation on novel three-dimensional printed bio-inspired hexagonal lattices for energy absorption and stiffness behavior

In the current study, a new type of novel lattice structures with the concept of bio-mimicking three different nature-inspired designs from the honeycomb patterns, the overlapping phenomenon of the scales present on fish dermal layers, and scutes pattern observed from the top skin layers present on crocodile species are developed. These lattice structures are designed within a cubic volume of 30 mm. The design of lattice unit cells is made with two different geometrical sizes of 4 and 6 mm with different overlapping areas of 20, 30, 40, and 50% calculated from each cell area. All the unit cells’ walls are maintained at 0.4 and 0.6 mm only. The specimens are modeled and manufactured through the VAT photopolymerization process, one of the significant additive manufacturing principles for great dimensional accuracy with negligible defects in 3D printing (3DP). The research approach used to develop lightweight bio-inspired structures has been laid out, starting with observing design that served as inspiration. The cured 3D-printed specimens are examined under quasi-static compressive loading carried out on all specimens as per ASTM 1621 standards to measure the crashworthiness response of designed specimens. The energy ingestion capacity of all the specimens is assessed. A good correlation is observed between the experimental and numerical results for better validation. The best design parameters among all 16 specimens are identified for applying energy absorption applications in the automobile and aviation fields in real-time.

3D-printed thermoplastic polyurethane/polyvinylidene fluoride gradient stiffness and hierarchical cellular structures with tailored energy absorption behavior

Currently, designing materials and structures is the main means of improving their energy-absorption properties; however, optimization methods based on structural design can only be applied to specific structures. In this study, two optimization methods of energy-absorbing structures (EASs) are proposed from the material system and manufacturing method for fabricating gradient stiffness and hierarchical cellular structures, respectively. The gradient stiffness structure was prepared using different ratios of thermoplastic polyurethane (TPU)/polyvinylidene fluoride (PVDF) filaments in different locations in the EAS. The gradient stiffness structure exhibited energy-absorption behavior corresponding to the strength of the printed material; this behavior decreased in the order of material strength from low to high, thus achieving controlled yield deformation behavior. Hierarchical honeycomb structures were prepared by combining three-dimensional (3D) printing with supercritical CO2 foaming technology, which facilitated the simultaneous formation of both centimeter and micron-scale cellular structures. When the PVDF content was 30%, the average cellular size of the microcellular structure was only 3 μm, and the prepared hierarchical honeycomb structures exhibited higher energy-absorption efficiencies compared with the general honeycomb structure. The energy absorption per unit mass values in three different compression directions of the hierarchical honeycomb structures were 152.75%, 67.33%, and 56.91% higher, respectively, than that of the TPU honeycomb without foaming. Therefore, the gradient stiffness and hierarchical honeycomb structures prepared from TPU/PVDF blends have various applications in energy absorption, personal protection, and transportation.

Predicting the Mechanical Behavior of Additively Manufactured Mechanical Metamaterials Using Point Cloud Representation Learning

Abstract Additive manufacturing (AM) has been widely adopted to produce mechanical metamaterials for load bearing, energy absorption, and other applications in various industries such as aerospace, automotive, and healthcare. However, geometric imperfections largely exist in AM. Since the mechanical behavior of metamaterials is primarily dependent on their geometries, it is critical to evaluate how process-induced geometric imperfections affect the mechanical behavior of fabricated metamaterials. Most of the existing approaches for AM quality control concentrate on the detection of defects and are limited in their ability to assess defect-altered mechanical behavior of finished builds. Some studies leverage destructive tests or numerical methods for mechanical behavior assessment, which are costly and time-consuming, and impractical for high-throughput routine quality control. In this paper, a new machine learning framework is developed to predict the mechanical behavior of fabricated metamaterials based on their as-built geometries (represented as high-resolution point clouds). Specifically, the point cloud is first converted into an image profile, which preserves detailed geometric patterns. then, a deep neural network is constructed to encode salient features of the image profile and associate them with the load-deflection curve of the fabricated metamaterial. The effectiveness of the developed framework is experimentally validated through a case study with auxetic mechanical metamaterial. This work has great potential to be extended for in-process prediction of AM builds’ mechanical behavior based on layer-wise point cloud scanning.

A hybrid bio-inspired sandwich structures for high strain rate energy absorption applications

Due to its advantages in terms of enhancing the performance of structures in the desired applications, the bio-inspired design approach has recently attracted the interest of researchers in a number of engineering disciplines. A hybrid bio-inspired design is suggested for the sandwich structures to absorb the energy of the blast loads in the current study. The sandwich structure's core, which often has a regular grid pattern resembling a honeycomb structure, is crucial to how well the panel absorbs energy. In order to achieve the best results, we first chose the structure of the core grid by taking into account potential 2D grids (polygons and multi-pointed stars) through Genetic Algorithm optimization. Next, we combined a bio-inspired bi-tubular thin-walled structure with the core grid to take advantage of its high energy absorption capacity. Finally, the performance of the suggested design is compared with four frequently implemented ones. The results show that the hybrid design has better energy absorption characteristics compared with the bionic and conventional designs presented in the literature.

Isotropic cellular structure design strategies based on triply periodic minimal surfaces

Isotropy is a desired characteristic in cellular structures for load bearing and energy absorption applications that must respond uniformly under external loads in all orientations. Triply periodic minimal surface (TPMS) cellular structures are attracting much attention for such applications due to their demonstrated high performance, tailorable properties, and open cell architecture. However, TPMS structures usually display stiffness anisotropy. In this work, new design strategies are presented for isotropic TPMS-based cellular structures, revealing a large available design space in terms of relative density and relative stiffness. The first design strategy arranges TPMS-based cells in a Simple-Cubic/Face-Centred Cubic inspired pattern, resulting in reduced elastic anisotropy. Two parametric optimisation approaches involving level-set mid-surface offsetting and the functional grading of relative density are then applied in a second step to eliminate any residual elastic anisotropy. Anisotropy is characterised through finite element analysis using the Zener ratio. Four families of cells are optimised, each based on a different TPMS unit cell, and then additively manufactured using the material extrusion process with polylactic acid. Finally, experimental quasi-static compressive tests are conducted to characterise stiffness, strength, and energy absorption properties. Optimised designs are tested in three crystal orientations ([001], [101] and [111]) and manufactured in three orthogonal print orientations. The Primitive TPMS-based design is the stiffest of the four designs reaching 64.4% of the Hashin-Shtrikman upper bound of bulk modulus at 20% relative density. Experimental results validate all four optimised cell designs for elastic isotropy and indicate that all the cell designs are also isotropic in terms of crushing strength and energy absorption.