Papers by Keyword: Auxetic

Abstract: This research improves the mechanical properties of laminates in ship hulls made of glass fiber reinforced plastics (GFRP) with the design of auxetic sheets, to take advantage of the property in their geometry to reduce the damage energy due to surface impacts absorbed by the laminate. 3D printing of second generation auxetic components to produce modified specimens. Laboratory reproductions of mechanical damage were compared with those of specimens extracted from a ship under construction. The mechanical properties of the bending and tensile tests demonstrated that the insertion of the core in the laminate protected the matrix from damaged energy, prolonging its useful life. Comparative results are presented, which will allow GFRP hull designers to insert auxetic sheet cores into their design. Mechanical tests allowed us to compare the progress of delamination.

Abstract: A new viscoelastic design known as auxetic sheet is proposed for the protection of GFRP planing hull vessels. The study of specimens made with auxetic sheets with different configurations in their geometry is presented to determine which one will be capable of absorbing the greatest amount of energy. For this, uniaxial compression tests were carried out using Hooke's 3D law on the designed specimens. The execution showed that the auxetic specimen in the "M" shape was the one capable of absorbing the greatest amount of energy from the applied force, obtaining the greatest capacity to absorb energy. The compression was considered quasistatic with the purpose of simulating an instantaneous slamming impact and being able to process the uniaxial test curve for the calculation of the elastic energy according to the level of damage in the specimen.

Abstract: This paper introduces the braided helically wrapped yarns with negative Poisson’s ratio (NPR) exhibiting a stable structure and good practicability to avoid slippage of wrapped component from the core. The geometry and auxetic behavior of the braided helical structure for two different combinations of core materials with similar combination of wrap materials and different braiding angles have been analyzed. Two elastomeric materials i.e. Polyurethane (PU) and Polyester, were used as monofilament cores whereas two stiffer multifilament wrap yarns employed were, ultra-high molecular weight polyethylene (UHMWPE) and polyethylene terephthalate (PET). Multiple braided yarns were investigated to analyze materials’ behavior towards NPR. It was observed that the NPR strongly depended on the inherent properties of the core and wrap materials in addition to the braiding angle. The NPR value was higher for a core with relatively higher elasticity (e.g., PU elastomer compared with polyester). It was also noticed that a lower wrap angle and lower braiding speed exhibited higher NPR. The maximum NPR value using PU core wrapped at a low angle of 9° was-1.70, when tested at a low strain rate of 0.5%.

Abstract: In this paper, vibration and wave propagation characteristics of auxetic metamaterial of the star-shaped structure system are studied. The plane wave expansion method is adopted to investigate and calculate the band structures of the auxetic star-shaped structure system. Besides, the frequency ranges of the complete bandgaps are also obtained and discussed. Then, the COMSOL® finite element software is utilized to solve wave propagation characteristics within the periodic auxetic star-shaped honeycomb structures. After that, the auxetic star-shaped structure devices made by 3D printed machine are used to measure the experimental results and also compared with numerical results. It can be seen that good agreements can be found in the discussions.

Abstract: The performance of a metamaterial is dominated by the geometric features and deformation mechanisms of its microstructure. For a certain mechanism, the geometric features have bounds in which the performance of a metamaterial such as negative Poisson’s ratio (NPR) can be designed. Previous investigation on buckling-induced auxetic metamaterial revealed that there is a geometric limit for its microstructure to exhibit auxetic behaviour in infinitesimal deformation. However, the limit for auxetic metamaterials undergoing large deformation is different from that under small deformation and has not been reported yet. In this paper, the geometric limit was investigated in an elastic and infinitesimal deformation range using linear buckling analysis. Furthermore, experimentally validated finite element models were used to identify the geometric limits for auxetic metamaterials undergoing large deformation. Depending on the control parameters of the topology, the bounds were represented by a line strip for one control parameter, an area for two control parameters and spatial domain surrounded by a 3D surface for three parameters. The limit was determined by the shape and size of the void of the metamaterials and it was identified through the large deformation analysis as well as the linear buckling analysis. We found that there was a significant difference in the geometric bounds obtained through those two methods. The results from this study can be used to design an auxetic metamaterial for different applications and to control the auxetic performance.

Abstract: The auxetic lattices are structures, which have the negative Poisson’s ratio. When material has negative Poisson’s ratio, has also auxetic properties - during process of stretching, are made wider and during compressing are made narrower. This structures are cellular and negative Poisson’s ratio is depending on the geometry of single auxetic cell. When geometry of the cell is slightly changed also Poisson’s ratio is different. Auxetics have attracted attention of researchers because of their superior dynamic properties. The lattice auxetic structures at one of their natural frequencies exhibit the deformed geometry. It’s can be exploit as resonance to optimization of the power required for the occurrence localized deformations. The dynamic behavior of auxetic and their transmission of the vibration, which is circumscribed by the parameter VTL (Vibration Transmission Loss) will be analyzed in this article.

Abstract: Conventional foam could be converted to auxetic foam under auxeticity process. A new and simple technique to fabricate auxetic foam and to further determine its Poisson’s ratio is described in this paper. It is evident that the present modified technique in fabricating auxetic foam could be adopted to produce desirable auxeticity characteristics. Moreover, the approach used for determination of Poisson’s ratio has considerably merit with great cost effectiveness. This method is, however, specific to auxetic foam sample under compression loading.

Abstract: An innovational structure with auxetic effect was designed from inspiration of a re-entrant honeycomb structure firstly proposed by Gibson et.al. In the newly designed structure, yarns were used to replace the straight and tilted ribs of re-entrant honeycomb structure in order to achieve negative Poissons ratio. The compression testing was carried out in the vertical direction of the structure using an INSTRON 5566 tester to understand its deformation behavior. The results show that the difference in bending stiffness between the warp and weft yarns is the main reason for getting negative Poissons ratio of the structure.

Abstract: Analytical and Molecular Mechanics methods have been used to study the deformation mechanisms acting at the molecular level in the auxetic polymorph of crystalline silica (a-cristobalite). The analytical models indicate that a-cristobalite deforms by concurrent tetrahedral dilation and cooperative rotation when stretched along the x3 axis, and that a second phase is predicted to exist for this loading scenario, having a geometry similar to that of ‘idealised’ b-cristobalite. This is supported by preliminary Molecular Mechanics simulations, which also indicate that the cooperative rotation predicted for loading along x3 is not sufficient to describe the deformation mechanism for loading along x1. A negative hydrostatic pressure offset is observed to lead to a change in the sign of the predicted Poisson’s ratio from positive to negative, leading to improved agreement of the Molecular Mechanics model with experiment.