Auxetic metamaterials have evolved growing beyond the cellular material structures and offering unique combinations of both auxetic and mechanical properties. However, the understanding and ...development of these promising material structures have been predominantly based on analytical evaluation methods, as practical implementation was restricted by manufacturing limitations. The manufacturing limitation has changed quite significantly with the advent of additive manufacturing methods, particularly with direct metal printing, such as selective laser melting. Ever since, the auxetic meta material structures began attracting renewed interests, and several different new structural forms and hybridisation of certain existing models followed, focussing specifically on enhancing auxeticity as well as other mechanical attributes. A recently developed and novel S-shaped unit cell based auxetic structure is hybridised with star re-entrant unit cells and analysed both numerically and experimentally based on physical forms produced by selective laser melting as part of the research reported here. Numerical and experimental results correlated sufficiently, depitcing significant enhanced auxeticity as high as −3, and mechanical property with the hybrid structures.
•Some hybrid auxetic models embedding the star re-entrant units into the matrices of the S-shaped model were designed.•The physical parts were built using the selective laser melting and were analyzed experimentally and numerically.•The H1 model resulted in enhanced compressive strength and Young's modulus values, compared to the S-shaped model.•The H2NC model depicted −3 Poisson's ratio which is double of the S-shaped structure.•Overall hybridization proved to be effective in tailoring the auxetic and mechanical properties of the S-shaped structure.
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•Quasi-static compressions of Ti6Al4V novel 2D auxetic lattices fabricated by PBF-EB.•ATC and TLR structures outperformed double DAH lattice structures in SEA.•Residual stresses are ...in the order of DAH > TLR > ATC, inversely correlating with SEAs.•TLR fabricated at high-speed scan exhibited small fluctuations in SEA.
The metal-based 2D auxetic lattice structures hold the potential for multifunctional tasks in aerospace applications. However, the compression response of those manufactured by powder bed fusion process is underexplored. This study proposes a comprehensive comparison of in-plane quasi-static compression performance of 2D auxetic lattice structures, utilizing three designs (anti-tetrachiral (ATC), double arrow-headed (DAH), and tree-like re-entrant (TLR)), manufactured with stiff Ti6Al4V by the electron beam powder bed fusion process (PBF-EB) with various manufacturing speeds. The results revealed unique failure patterns and superior energy absorptions among 2D lattice structures in the literature. TLR design enhanced energy absorption by overcoming failures between DAH columns and exhibited the lowest standard deviations in specific energy absorption (SEA) values (9.75 %–12.62 %). Besides, Kernel average misorientation (KAM) values followed the order of DAH, TLR, and ATC, and inversely correlated with SEA values. ATC structures with the lowest KAM outperformed DAH and TLR by 47.5 % and 6.44 %, respectively. Scan speed variations affected SEA and porosity values differently for each lattice design while exhibiting similar microstructure characteristics. The findings in this study propose a significant contribution to the development of aerospace sandwich structures where harsh environments exist and employment of 2D topologies are required.
Environmental concerns surrounding petrochemical-based materials are constantly increasing and they are pushing the industry to find renewable sources and sustainable solutions. Recent years have ...also witnessed a focus on the development of multifunctional and metamaterials such as auxetic structures. This work describes the manufacturing of castor-oil-biobased polyurethane foams and their use as platforms to make double-arrowhead auxetic (DAH) metamaterials via laser ablation. A microstructural analysis is conducted on the biobased foam, revealing a micro-cellular structure formed by rounded and closed cells. The proposed DAH structures are innovative because they possess two different micro and macro cellular scales. The effects of the geometric parameters (angles) of the macro-cells of the DAH structures on the equivalent density, elastic modulus, yield stress, ultimate stress, modulus of toughness, and Poisson ratio are investigated. Configurations with a lower unit cell α angle of 45° exhibited improved properties, including the maximum tensile performance (yield stress of 33.2 kPa, ultimate stress of 38.9 kPa, and modulus of toughness of 10.1 μJ/mm3) and the lower negative value for the Poisson ratio (-0.16), although with a higher density (70 kg/m3). In contrast, DAH structures made with a 70° α angle resulted in a reduced equivalent density of 48 kg/m3 and increased specific strength (0.61 kPa.m3/kg). In terms of specific stiffness, the 45° α DAH structures outperform the other configurations, with a specific elastic modulus close to 10 kPa.m3/kg.
•A biobased polyurethane foam was successfully synthesized using castor-oil derivatives.•Bulk density, Poisson ratio, tensile modulus and strength are 75 kg/m3, 0.28, 3.72 MPa and 467 kPa, respectively.•Double-arrowhead (DAH) structures are obtained via laser ablation.•DAH structures with a lower α angle of 45° exhibited enhanced mechanical properties.•The latter achieved an equivalent density of 70 kg/m3 and a lower Poisson ratio of −0.16.
Auxetic materials or structures, especially 3D cellular lattice architectures with negative Poisson's ratio (NPR) have attracted great attention due to their unprecedented mechanical behaviors and ...promising applications in recent years. Many 3D auxetic architected cellular materials have been manufactured and characterized with polymers and metals using additive manufacturing technology as well as some fibre reinforced polymers (FRP) composites via assembly method, however metal 3D printed auxetic structures with high-quality are rarely reported and FRP composites using assembly method contain a variety of defects which result in a knock-down effect on mechanical properties. Auxetic structures made from polymers are normally of low stiffness and strength and FRP composites are of low toughness and impact resistance, which causes limitations on their structural and functional applications. If rationally designed structures can be made from high-performance metal materials the specific stiffness and strength of which are much higher than polymers, their specific stiffness and strength can be significantly increased which will make them more suitable for various potential applications. This paper presents novel 3D double-U hierarchical structures (DUHs) based on double-V hierarchical structures (DVHs) with tunable Poisson's ratio and Young's modulus by tailoring the geometry parameters. Experimental, numerical and theoretical results show that DUHs have smooth geometry that can reduce manufacturing damage and stress concentration under elastic loading. Another advantage of DUHs is that the curved configurations have enhanced auxetic behavior and higher static collapse stress than DVHs under crushing. The advanced 3D printing technology and those excellent mechanical properties of 3D DUHs meet the requirements of structural and functional applications.
A novel auxetic structure-based stiffener for stainless steel–carbon reinforced polymer (CFRP) T-joints with outstanding strength and toughness improvement is proposed. The stiffener was composed of ...a bulk web and re-entrant structure flange that was 3D printed using a selective laser melting 3D printer. The stiffener was bonded to a CFRP skin, and a pull-off test was performed to evaluate the pull-off strength, stiffness, and toughness of the joint. The results demonstrated that the proposed structure has specific strength and toughness that were 3.4 and 77 times greater than those of the baseline joint, respectively. The improvement in strength was due to the delayed damage initiation caused by the mismatch between the bending stiffness of the bulk web and the stiffener under the first re-entrant structure. However, the damage initiation in the baseline joint was located at the stiffener edge. Moreover, the negative Poisson's ratio of the re-entrant structure arrested the delamination growth after damage initiation, which enhances the toughness of the joint. By optimizing the structure of the stiffener, a joint with 2.66 and 38.2 times larger specific strengths and toughness compared to the baseline joint with the same specific stiffness was achieved.
•Novel auxetic metal stiffener for metal-composite T-joints is presented.•The proposed auxetic stiffener showed 38.2 higher toughness than baseline joint.•The specific strength of the proposed joint is 266% larger than the baseline joint.•Delamination was arrested at the skin-adhesive interface for the auxetic joint.
•Auxetic cementitious composite (ACC) achieved out-standing compressive strain-hardening behavior.•Extremely high recoverable deformability equipped the ACC with strikingly energy dissipation ...ability.•A tailored composite action of developing damage-resistant composite is found from the novel ACC.
A novel highly compressible auxetic cementitious composite (ACC) is developed in this work. Contrary to conventional cementitious materials, such as plain concrete and fiber reinforced concrete, the ACC shows strain-hardening behavior under uniaxial compression: the stress continuously increases with strain up to approximately 40 % strain. On one hand, in the early compression stage, the ACC exhibit highly recoverable deformability of 10 % strain under cyclic loading (20 times higher than the constituent cementitious material). In addition, the ACC shows fatigue damage until the stiffness/strength and energy dissipation plateau values are reached after 500 cycles. At 2.5 % strain amplitude, the plateau stiffness/strength is approximately 120 MPa/3 MPa, while these values are only 25 MPa/1.2 MPa at 5 % strain amplitude. In contrast, the energy dissipation plateau of the ACC is independent from the amplitude and remains at 0.05 J/cm3. On the other hand, due to the strain-hardening behavior, the ACC exhibits significantly improved energy dissipation capacity compared to both the conventional cementitious materials and the auxetic frame. This behavior is achieved by a tailored composite action: integrating cementitious mortar with 3D printed thermoplastic polyurethane (TPU) auxetic frame. A rotating-square auxetic mechanism was designed for the TPU frame for the ACC to achieve the tailored cracking behavior. The horizontal ACC cells enable large deformability by enlarging the crack width under the confinement of the auxetic frame, while the vertical cells work as stiffening phase to ensure load resistance. Owing to the outstanding mechanical properties, the ACC shows great potential to be applied in engineering practice where high compressive deformability is required, for instance yielding elements for squeezing tunnel linings.
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•Novel layered aux-hex honeycomb (LAHH) filled tube (LAHHFT) structures are designed and investigated.•Aux-hex interaction and LAHH-tube interaction lead to superior energy absorption ...performance.•Theoretical models are carried out to describe the double interaction effect.•Deformation modes of the LAHHFT structures are recognized and theoretically interpreted.
In this work, a novel layered aux-hex honeycomb (LAHH) filled tube (LAHHFT) structure is proposed and investigated. A series of LAHH and LAHHFT structures with different number of aux-hex interfaces is designed and analyzed via extensive finite element analyses (FEAs). The results indicate that the designed LAHH and LAHHFT structures possess significantly higher effective Young's modulus, mean crushing stress (MCS) and specific energy absorption (SEA) than the single honeycombs and single honeycomb filled tubes under the same relative density. Double interaction effect, i.e. aux-hex interaction and LAHH-tube interaction, contributes to the superior improvement. Quantification of the interaction effect shows that the relative improvement of the MCS and SEA due to the double interaction effect could reach up to 617% and 557% for the LAHHFT structures. Theoretical models are carried out to predict the mechanical and crushing stress properties of the proposed hybrid structures considering interaction effect, which agree well with the FEA results. Besides, the interaction also affects the deformation modes of the LAHH and LAHHFT structures, which in turn influences the energy absorption performance. Three different crushing deformation modes including global bending mode, straight folding mode and mixed deformation mode occur to the LAHHFT structures. A theoretical model is also established and provides a good explanation to the change of deformation modes. This study offers a new route and a useful guideline for the design of future high-performance light-weight materials and structures.
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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.
Honeycombs are engineered cellular materials that often show superior specific strength, stiffness and energy absorption compared to solid materials. As a consequence they have found numerous ...applications across engineering fields. The development of additive manufacturing (AM) technologies has initiated an abundance of studies into novel honeycombs as historic manufacturing constraints are lifted. Investigations have been focused on improving or tailoring a given property but very few have focused on isotropy, and little has been done to bring together different patterns under the same manufacturing and experimental conditions. In this study, AM has been used to manufacture nominally identical honeycombs based on differing unit cells, in a range of orientations and densities. Elastic and plastic properties for the hexagon, triangle, square, re-entrant and double-V honeycombs have been obtained through mechanical testing. The elastic properties of these honeycombs have been modelled for all possible in-plane loading directions using minimal computational resources. The effect of orientation and density has been presented, confirming the level of in-plane isotropy for dense honeycombs with regards to Young’s modulus, Poisson’s ratio, yield strength and compressive strength. Insights have also been gained into how these properties vary with relative density. These results provide a basis for comparison with future work on honeycombs.
•Two novel auxetic structures with different arrangements of a new enhanced trigram frame unit-cell were proposed.•The mechanical performance of the designed structures was evaluated through ...theoretical analysis, finite element simulation, and experimental tests.•Single and multi-objective optimization methods were implemented to optimize the geometry parameters of the selected auxetic structure.•The exceptional mechanical characteristics of the present proposed structures, surpassing previous designs in terms of stiffness and specific stiffness, highlight their tremendous potential for load-bearing applications.
Superior stiffness is deemed essential for structures intended for several applications. The stiffness enhancement can be achieved by adopting structures that deform based on the stretching-dominated mechanism instead of the bending-dominated mechanism. This paper proposes a novel auxetic unit cell design and two distinct lattice structures dominated by stretching when subjected to quasi-static compressive and tensile loadings. The structures are manufactured via the fused deposition modeling (FDM) 3D printing method with the use of polylactic acid (PLA) filament as the patent material. The investigation also includes theoretical analyses and finite element (FE) simulations, all yielding consistent results. Response Surface Methodology (RSM) is also employed to understand the influence of three geometric parameters. Besides, single-objective optimization is conducted to maximize the stiffness through geometry design, and multi-objective approaches are implemented to improve the stiffness while simultaneously reducing relative density. Finally, different unit cells, which deform based on the stretching-dominated mechanism, are selected to represent their respective classifications, enabling a comparison of their potential stiffness capacity with the designed structures in this study. Remarkably, it is highlighted that the proposed unit cells and structures outperform others in terms of specific stiffness, making them a promising choice for use in structural engineering.
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