An investigation of the fatigue behaviour of the re-entrant auxetic structures made of the aluminium alloy AA 7075-T651 is presented in this study. The analysed auxetic structures represent a new ...class of cellular structures that show anomalous deformation responses such as negative Poisson’s ratio. In the proposed work, the influence of the unit cell orientation on the crack path and fatigue life was studied using experimental and computational approaches. The Low Cycle Fatigue (LCF) tests were performed at load control with the load ratio 0.1 in tension. In the LCF tests five loading levels were selected, and at least two tests were performed at each loading level. The same loading conditions were then applied in the computational model in the framework of the ANSYS software package, where a nonlinear kinematic material model was applied to obtain the stress–strain relationship. The strain-life approach, with consideration of the Morrow mean stress correction, was then used to obtain the fatigue life of the analysed auxetic structures. The experimental and computational results showed that the unit cell’s orientation has a minor influence on the fatigue life of both analysed auxetic structures, but impact on the direction of the fatigue failure path significantly.
•The LCF-behaviour of aluminium auxetic structures was investigated.•The different loading levels were considered.•An elastic–plastic kinematic material model was applied in the computational model.•The strain-life approach with a Morrow correction was used to obtain the fatigue life.•Comparison between the computational and experimental results showed a reasonable agreement.
In this work, polyurethane aerogels are fabricated in re-entrant honeycomb forms to achieve auxeticity and much higher flexibility than corresponding aerogel monoliths. For this purpose, a set of ...re-entrant honeycomb-shaped hollow molds is first printed from high-impact polystyrene (HIPS) using a fused filament fabrication technique and subsequently filled with polyurethane sol synthesized from an aliphatic triisocyanate and a diol selected from among butanediol, pentanediol, hexanediol, or octanediol. A sol–gel transition process yields the gel in a re-entrant honeycomb shape, which is isolated by dissolving the HIPS mold and supercritically dried using carbon dioxide. The effects of diol chain length on the properties of aerogels, such as bulk density, pore size, and tensile properties are investigated. The auxetic aerogels show different deformation behavior and much higher elongation at break than dogbone-shaped aerogel monoliths. Such unique properties of auxetic structures can extend applications of aerogels as acoustic absorbers and smart bandages that require high porosity, tunable pore structures, and high flexibility.
The present work reports the first attempt of developing auxetic structures using high performance fibres through knitting technology. Polyamide (PA) and para-aramid (p-AR) fibres and their ...combination were knitted in to purl structures using flatbed knitting machine, varying different structural (such as loop length, cover factor and yarn density) and machine parameters (such as take-down load). The influence of different parameters on negative Poisson's ratio (NPR) was thoroughly investigated. It was observed that NPR improved strongly with the increase in loop length of knitted structures. NPR also increased with the decrease in cover factor and increase in course density of knitted fabrics. An increase in take-down load also improved NPR for tightly knitted samples, but led to initial decrease and subsequent increase in NPR for medium and higher loop lengths; except for p-AR fabrics, which showed a decrease in NPR with take-down load for higher loop lengths. Tensile properties of the developed auxetic structures were also found to depend strongly on fibre type and loop length, and the highest tensile performance was achieved with lower loop lengths and p-AR yarns. The p-AR fabrics produced using lower loop length and lower take-down load resulted in the highest NPR of −0.713. Therefore, the developed knitted structures produced using high performance yarns and showing strong auxetic effects can have huge potential for industrial applications, especially in personal protection materials, such as cut resistance fabrics, bullet proof vest, helmets, and so on.
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•Weft knitting technology using high performance fibres can be used to develop auxetic structures.•The negative Poisson's ratio are influenced by fibre type, loop length, cover factor, yarn density and machine parameters.•These parameters should be taken into account in the analysis and optimization of the proposed auxetic structures.•The present study can open a new way for producing auxetic fabrics using high performance yarns for various applications.
The main objective of this research work is focused on the vibration analysis of auxetic sandwich cylindrical shell structures resting on an elastic foundation. In the analysis, the sandwich shell ...structure is composed of three layers in which the middle layer consists of auxetic material with a negative Poisson’s ratio, and the two skin layers are isotropic homogeneous materials. The motion equation is extracted according to the first-order shear deformation theory (FSDT) and the Hamilton principle. The governing equations of coupled partial differential equations are solved by the generalized differential quadrature (GDQ) method, and the natural frequencies are determined. By comparing the experimental results with the numerical results calculated by commercial finite element software, the validity of the proposed theoretical model is verified. Finally, the influences of geometrical parameters and elastic foundation on the vibration behavior of sandwich shell structures have been investigated.
Auxetic structures possess high energy absorption, especially under impact loads that are influenced by their negative Poisson's ratio (NPR). This negative ratio is due to their specific ...microstructural designs. In this article, newly modified designs for an existing chiral auxetic structure have been proposed and their mechanical and energy absorption performances have been investigated. In the proposed designs, the geometry of slots in an available peanut-shaped auxetic structure has been modified and some extra masses have been removed to introduce innovative auxetic structures with higher NPRs, higher energy absorption, and lower structural weight. By performing finite element simulations in ABAQUS software, it has been found that the proposed modifications in the design of the available auxetic structure result in up to 7.3% and 76.6% increases in Poisson's ratio and energy absorption respectively. The accuracy of simulations has also been confirmed by performing a comparison study with some available results of experimental tests.
Conventional elastomeric polymers used as substrates for wearable platforms have large positive Poisson's ratios (≈0.5) that cause a deformation mismatch with human skin that is multidirectionally ...elongated under bending of joints. This causes practical problems in elastomer‐based wearable devices, such as delamination and detachment, leading to poorly reliable functionality. To overcome this issue, auxetic‐structured mechanical reinforcement with glass fibers is applied to the elastomeric film, resulting in a negative Poisson's ratio (NPR), which is a skin‐like stretchable substrate (SLSS). Several parameters for determining the materials and geometrical dimensions of the auxetic‐structured reinforcing fillers are considered to maximize the NPR. Based on numerical simulation and digital image correlation analysis, the deformation tendencies and strain distribution of the SLSS are investigated and compared with those of the pristine elastomeric substrate. Owing to the strain‐localization characteristics, an independent strain‐pressure sensing system is fabricated using SLSS with a Ag‐based elastomeric ink and a carbon nanotube‐based force‐sensitive resistor. Finally, it is demonstrated that the SLSS‐based sensor platform can be applied as a wearable device to monitor the physical burden on the wrist in real time.
In this study, a skin‐like stretchable substrate (SLSS) based on an auxetic structure and a glass‐fiber‐reinforced polymer with a negative Poisson's ratio is developed. Requirements for the mechanical properties of materials are proposed through numerical simulation and verify the deformation mechanism using digital image correlation analysis. Finally, the SLSS is used as a wearable device by integrating multiple sensors.
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Additive manufacturing has revolutionized the entire design cycle across a wide range of industries. The majority of materials utilized in 3D printing result in rather rigid ...structures with a fixed set of properties. Four-dimensional printing (4D printing) addresses this limitation by relaxing the geometric/properties rigidity over time. The careful selection of structural designs and the use of active and smart materials enables a unique set of shapes and properties to be programmed and achieved. This deviation from rigid structures with a fixed set of properties enables novel and unique applications in robotics, deployable structures, biomedical, and aerospace industries. The current work is focused on an interesting class of structures, 2D auxetic cellular solids, which exhibit a negative Poisson’s ratio. 4D printing of such structures allows for tunable structural stiffness, Poisson’s ratio, and rigidity. This work aims to experimentally investigate the tunability of stiffness and Poisson’s ratio in 4D printed auxetic structures fabricated using a thermoplastic shape-memory polymer. The attained structural stiffness was tuned in the range of 0.179−0.242 kN/mm while the in-plane Poisson’s ratio was controlled from -0.33 up to even positive magnitudes of +0.69. This wide range of elastic properties was obtained from a single structure programmed to different deformation levels. Experimental evaluation of 4D printed SMP structures under constant cyclic programming/recovery conditions induced residual strains which consequently affected the mechanical properties by degrading the Poisson’s ratio and structural stiffness. The magnitude of induced residual strains depends on the level of applied deformation during programing and the number of applied programing/recovery cycles. At the local level, full-field measurements revealed the localization of strains within the complex heterogeneous structure of the axially loaded auxetic honeycomb samples.
The behaviour of the auxetic structure under external load was regarded as the behaviour of the compliant (flexible) mechanism. The multi-objective topological optimization, based on genetic ...algorithms and the finite element method, was used to find the optimal shape of such two-dimensional compliant mechanism in this study. The optimization was performed on a quarter of a double-symmetric representative unit cell, which is a building block of the symmetrical auxetic structure. Static linear computational simulations were performed to determine the mechanical response of cell topologies. The proposed method leads to a set of best solutions positioned on the Pareto front with different topologies and gives a broad overview of possible designs of new auxetic structures. The method is highly effective and can be easily extended to large deformation formulations, nonlinear elasticity or elasto-plasticity.
Inducing auxetic nature is one of the emerging techniques to enhance impact tolerance in 3D woven composites due to their unique bidimensional-energy dissipation capability. The conventional 3D ...orthogonal structure has inherent auxetic nature; however, such auxeticity is completely restricted due to brittle thermoset resins-based composites. To overcome this problem, three types of 3D woven auxetic structures were developed and used as reinforcement with thermoplastic resins, that is, polycarbonate (PC) and polyvinyl butyral (PVB). The results revealed that the warp interlock structure showed the highest auxeticity, while the bidirectional interlock structure showed the least auxeticity. Charpy and low-velocity impact tests were performed to evaluate the effect of auxeticity on the impact properties of corresponding composites. Warp interlock with PC showed 49% and 47% higher Charpy impact strength and force, respectively, than bidirectional interlock with PC resin. Similarly, the low-velocity impact test results of warp interlock showed 32% and 32.5% higher impact force and absorbed energy, respectively, in warp direction than bidirectional interlock with PC resin.
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