Numerical Analysis of Binding Yarn Float Length for 3D Auxetic Structures

Investigating the Thermo-Physiological Comfort Properties of Weft-Knitted Smart Structures Having a Negative Poisson’s Ratio

Smart auxetic structures are gaining attention in various areas such as architecture, clothing (sports and protective), civil, and medical applications owing to their negative Poisson’s ratio. Compared to ordinary structures, these structures have better properties (shear resistance, formability, energy absorbance, and robust fracture strength). Auxetic structures show the exceptional property of becoming wider in one direction when stretched from another direction. In this research, three different auxetic weft-knitted structures were fabricated using nylon, polyester, acrylic, and cotton yarns on a Shima Seiki flat-knitting machine. The physical properties, negative Poisson’s ratio, and thermo-physiological comfort properties of these fabrics were checked. Negative Poisson’s ratio strain curves of the developed fabrics were plotted; all fabrics, except for nylon, show the negative Poisson’s ratio (NPR). The NPR decreases with increased strain in the longitudinal direction, and polyester exhibits a maximum value of NPR −0.4 in line structure at 30 mm extension. Results also revealed that structures made with nylon and polyester yarns exhibit a better value of air permeability than acrylic and cotton, while acrylic provides the best thermal resistance values than other materials in line structure and polyester yarn shows better overall moisture management capacity (OMMC) performance in zigzag structures.

Effects of Braid Angle and Material Modulus on the Negative Poisson’s Ratio of Braided Auxetic Yarns

Fibers and textiles are ubiquitous in our daily lives, with mechanical properties that match the design specifications for the task for which they are intended; the development of yarns with a negative Poisson’s ratio (NPR) is a hot topic of current research, owing to their potential for use in high-performance textiles (e.g., military, sports, etc.). This study described a simple approach to constructing braided, helically interlaced yarns. When a torque is applied, the yarns prevent the wrapped component from dislodging from the core. The geometry and auxetic behavior of the braided helical structure was analyzed for two different combinations of core materials with similar wrap materials and different braiding angles. Two elastomeric materials (polyurethane (PU) and polyester) served as monofilament cores, while two stiffer multifilament wrap yarns (ultrahigh molecular weight polyethylene (UHMWPE) and polyethylene terephthalate (PET)) served as wrap yarns. In addition, the behavior of yarns braided at seven different angles was investigated to determine the materials’ response to the applied braided configuration’s NPR. The NPR was influenced by the core and wrap materials used and the braiding angle. The NPR value was greater for a core comprising more excellent elasticity (e.g., PU versus polyester); a smaller wrap angle and a slower braiding speed also led to a higher NPR value. The maximum NPR value of −1.70 was obtained using a PU core wrapped at a 9° angle and a strain value of 0.5.

Removing Auxetic Properties in f.c.c. Hard Sphere Crystals by Orthogonal Nanochannels with Hard Spheres of Another Diameter

Negative Poisson’s ratio materials (called auxetics) reshape our centuries-long understanding of the elastic properties of materials. Their vast set of potential applications drives us to search for auxetic properties in real systems and to create new materials with those properties. One of the ways to achieve the latter is to modify the elastic properties of existing materials. Studying the impact of inclusions in a crystalline lattice on macroscopic elastic properties is one of such possibilities. This article presents computer studies of elastic properties of f.c.c. hard sphere crystals with structural modifications. The studies were performed with numerical methods, using Monte Carlo simulations. Inclusions take the form of periodic arrays of nanochannels filled by hard spheres of another diameter. The resulting system is made up of two types of particles that differ in size. Two different layouts of mutually orthogonal nanochannels are considered. It is shown that with careful choice of inclusions, not only can one impact elastic properties by eliminating auxetic properties while maintaining the effective cubic symmetry, but also one can control the anisotropy of the cubic system.

Cancellation of Auxetic Properties in F.C.C. Hard Sphere Crystals by Hybrid Layer-Channel Nanoinclusions Filled by Hard Spheres of Another Diameter

The elastic properties of f.c.c. hard sphere crystals with periodic arrays of nanoinclusions filled by hard spheres of another diameter are the subject of this paper. It has been shown that a simple modification of the model structure is sufficient to cause very significant changes in its elastic properties. The use of inclusions in the form of joined (mutually orthogonal) layers and channels showed that the resulting tetragonal system exhibited a complete lack of auxetic properties when the inclusion spheres reached sufficiently large diameter. Moreover, it was very surprising that this hybrid inclusion, which can completely eliminate auxeticity, was composed of components that, alone, in these conditions, enhanced the auxeticity either slightly (layer) or strongly (channel). The study was performed with computer simulations using the Monte Carlo method in the isothermal-isobaric (NpT) ensemble with a variable box shape.