•A thin walled tube filled with double arrowed gradient auxetic structure is proposed.•Analytical models are established to predict the dynamic characteristics of thin walled tubes filled with ...auxetic structures.•Compared to the numerical simulations, the analytical models are accurate to predict the dynamic responses.
In recent years, auxetic structures have been applied in the energy absorption field due to the unique mechanical properties. Inspired by foam filled structures, this study proposes a thin walled structure filled with double arrowed auxetic structure and investigates the energy absorption characteristics. The gradient configuration is introduced to improve the energy absorption performance. The effects of the gradients on the energy absorption are investigated. A theoretical model is also established to predict the energy absorption to quantify the energy dissipated due to thin walled tubes, gradient auxetic structures and their interactions. The accuracy of the theoretical model to predict the energy absorption characteristics is validated through the numerical simulations.
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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.
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.
AI-assisted study of auxetic structures Grednev, Sergej; Steude, Henrik S.; Bronder, Stefan ...
Acta Polytechnica CTU proceedings,
10/2023, Letnik:
42
Journal Article
Recenzirano
Odprti dostop
In this study, the viability of using machine learning models to predict stress-strain curves of auxetic structures based on geometry-describing parameters is explored. Given the computational cost ...and time associated with generating these curves through numerical simulations, a machine learning-based approach promises a more efficient alternative. A range of machine learning models, including Artificial Neural Networks, k-Nearest Neighbors Regression, Support Vector Regression, and XGBoost, is implemented and compared regarding the aptitude to predict stress-strain curves under quasi-static compressive loading. Training data is generated using validated finite element simulations. The performance of these models is rigorously tested on data not seen during training. The Feed-Forward Artificial Neural Network emerged as the most proficient model, achieving a Mean Absolute Percentage Error of 0.367 ± 0.230.
•An analytical framework is developed to predict the elastic properties of irregular auxetic structures.•A finite element code is developed to validate the analytical predictions.•The effects of the ...degree of irregularity and geometric parameters on the elastic properties are investigated.
This paper presents an analytical framework with a bottom-up multi-step approach to predict the in-plane mechanical properties including the effective Young’s modulus and Poisson’s ratio in two directions. There are deviations for the analytical predictions with respect to the numerical results because of the simplifications of boundary conditions of the representative unit cell. To remedy the deviations, a modification coefficient is embedded to revise the analytical model. A finite element code for obtaining elastic properties of the irregular auxetic structures is developed to validate the revised analytical model. The good agreement between the revised analytical predictions and numerical results affirms the accuracy of the revised analytical model. It is noticeable that the effects of irregularity on the effective Young’s modulus are more prominent than on the Poisson’s ratio.
Auxetic structures exhibit certain abnormal mechanical responses such as negative Poisson's ratios, mainly through the intertwined deformation attributes of inter-connected members of macro ...structural forms. Considering the relative ease of achieving complex forms with the advent of the additive manufacturing technologies, there is a renewed interest in auxetic structures, as the already established as well as new structural shapes are physically produced and tested to ascertain the theoretical predictions. While the focus was mostly around the auxetic responses, the stress distribution patterns and in particular the stress concentration effects were mostly neglected. This paper fills this gap, evaluating the stress concentration aspects of a standard re-entrant auxetic structure. A new structure is designed and proven to be less stressed in comparison with the re-entrant form commonly reported in the literature by means of analytical, experimental, and numerical results. The new structure proposed has proved to be more auxetic up to 15% of the external strain and with Poisson's ratios up to −2.5.
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•The re-entrant cells underwent strut detachment, while the S-structures showed no evidence of the formation of any cracks.•The stress concentration effects more than doubled in the re-entrant unit cells compared to the S- unit cells•The S-structure resulted in a negative Poisson’s ratio as high as 2.5 as against a negative maximum of 1.2 in the case of the re-entrant one.•The re-entrant and the S structures remained auxetic within 5% and 15% of the macroscopic applied strains respectively
Sandwich structures have superior mechanical properties to be applied in civil, marine, mechanical and aerospace engineering fields. Auxetic structures with negative Poisson's ratio (NPR) can ...contract laterally when compressed axially to be excellent fillers for the sandwich structures to improve the energy absorption performance. In this work, a cylindrical sandwich filled with double arrowed auxetic structure is proposed. An analytical model is established to predict the dynamic responses of the sandwich structure under axial impact loading. The theoretical model is composed of three parts: energy dissipated due to the auxetic filler, the thin walled tubes, and their interaction. Numerical simulations are performed to validate the theoretical model and the results indicate that the theoretical model proposed is accurate to predict the crushing platform stress of the cylindrical sandwich under axial impact loading.
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.
As one of the representatives of advanced lightweight cellular materials, the auxetic structure has a great potential for energy absorption due to its unique deformation mechanism, which makes a very ...promising application prospect in the fields of aerospace, vehicle and engineering protection. In order to further increase the energy absorption capacity of auxetic structures, this paper designed and characterized a novel two-dimensional (2D) lightweight rotationally arranged auxetic structure. The elastic properties, plastic collapse stress and specific energy absorption (SEA) of the novel auxetic structure were systematically investigated through theoretical calculations, numerical simulations and experimental method. Our results verify that the proposed theoretical model is accurate and applicable. The rotationally arranged shape and stable triangular design inside unit cells lead to an excellent energy absorption capacity, which is significantly higher than that of conventional auxetic structures (star-shaped honeycombs and re-entrant hexagonal honeycombs). Moreover, the SEA of novel structure has extremum conditions, by which the optimal relative density can be determined. According to different user requirements, the novel structure can be designed as the best energy absorption type or the most concise and lightweight type by adjusting geometrical parameters.
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•The present rotationally arranged auxetic structures show high energy absorption.•Extremum conditions of specific energy absorption are investigated.•Mechanical properties can be designed by adjusting geometric parameters.
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