Materials with auxetic properties are currently being researched in the textile industry due to their physical properties that make them useful for a variety of purposes. An article published in the journal Polymers explored the creation of new multifunctional materials from conventional composite fabrics using traditional textile technologies and laser cutting.
To study: In-Plane Strain Behavior and Open Area of Rotating Squares in Auxetic Compound Tissue. Image Credit: dkHDvideo/Shutterstock.com
When stretched, auxetic materials tend to expand rather than contract, giving them a multitude of functional uses. Auxetic materials possess a negative Poisson’s ratio and their auxetic properties are a result of their unique structure which, when stretched, resembles a honeycomb shape. One of the areas of research in the textile industry is the creation of materials with auxetic properties at the level of fibres, threads and fabrics.
At the yarn and fabric level, auxetic materials can be produced in two ways. Either the yarn/fabric can be produced with induced auxetic geometry using standard fibers and yarns during the production process (e.g. spinning, weaving or knitting) or using fibers which themselves possess auxetic properties.
The first attempt in the laboratory to create materials with auxetic properties was carried out in the early 1990s by Alderson & Evans. In this attempt, an ultra high molecular weight polyethylene was produced which had a Poisson’s ratio of -1.2. This auxetic polymer was composed of a network of rectangular nodes connected by freely articulated inextensible fibrils/rods in a microporous node-fibril structure. The material was produced using a three-step thermal process.
Deformation of the fabric with a geometry of rotating square unit cells under loading (dashed lines – the initial size of the sample) with particles of different sizes passing through the open area of the fabric depending on the applied tension force (a and b are rhombus diagonals). Image Credit: Dubrovski, PD et al., Polymers
Since the first experiments, there has been an increasing number of studies on the fabrication of various materials with auxetic properties. Anderson et al. used a modified conventional industrial scale melt spinning technique to produce auxetic polypropylene and polyethylene. In 2002, Ravirala et al. produced auxetic polyamide and polyester fibers by modifying an earlier method of making auxetic polypropylene fibers. He et al. in 2005 produces auxetic liquid crystalline polymers.
Work over the past decade by Hook, Sloan, Ge, and Ng, among others, has developed and improved helical auxetic wires (HAYs). These auxetic materials consist of a thick wire core and a thinner, stiffer helically wound wire which, under tension, begins to unwind, with the core wire moving laterally. This increases the width of the wire and achieves the negative Poisson ratio characteristic of auxetic materials. The auxetic wire undergoes a resultant lateral extension.
Production of low-cost auxetic fibers using conventional fibers and yarns
Another way to produce auxetic fibers at low cost is to make auxetic fabrics using conventional fibers and yarns. By inducing a unique tissue structure, a multifunctional auxetic material can be made. Many studies have been published that have developed these low cost auxetic fabrics.
Poisson’s ratio of sample AF 12.5 with longitudinal deformation (a) and the corresponding deformation of the fabric in the machine direction (b) (LS–longitudinal strain (%), LD–longitudinal displacement (mm)). Image Credit: Dubrovski, PD et al., Polymers
Foldable geometry and re-entrant geometry have been used in studies to develop new knitted and woven fabrics that display auxetic behavior in the plane. In 2011 Hu et al. created auxetic weft knit fabrics that had rotating rectangles using rotating unit geometry. A partial interlock knit was used to form the rectangular units which were bound in the direction of the column using an elastic thread binding technique.
The rotating rigid unit cell geometry was first proposed by Grima et al. in the early 2000s. This geometry connects the units (squares, rectangles and triangles) with corner hinges. Other rotating rigid units (parallelograms and rhombuses) have been studied by Grima et al. to create auxetic tissues.
The study published in Polymers expanded on the author’s earlier work on conventional nonwoven fabrics modified with rotatable rigid cell geometry and auxetic behavior. The authors found that nonwoven samples that had smaller cell sizes had the highest negative Poisson’s ratio, making them highly auxetic.
The research presented deals with the auxetic behavior of tissues but focuses on compound tissues and the open area under tensile loading as a consequence of cell rotation. To the knowledge of the authors, the development of auxetic compound tissues is absent from the current literature. Additionally, no current study has attempted to transform conventional compound tissue into auxetic tissue. Compound fabrics behave differently under tensile forces than non-compound fabrics; moreover, they have a more complex structure.
Additionally, while the geometry of rotating cells has been widely studied, it has not been so widely studied on textile substrates, especially composite fabrics. The goal of the study was to create a new auxetic material that could be used as a new filtration material that has an adjustable open surface that can be used to filter multiple particles.
In the research, a conventional compound fabric, for example a woven fabric reinforced needlepunched nonwoven, was cut with a laser to form rotating square structures and induce auxetic behavior in the compound material. The authors analyzed and discussed properties such as the Poisson’s ratio of the material, the in-plane deformation behavior under tensile load, and the relationship between the applied force and the open area of the fabric.
The relationship between particle size and longitudinal displacement. Image Credit: Dubrovski, PD et al., Polymers
The authors concluded that the larger unit cells in the tissue exhibited a higher mean negative Poisson ratio, a major reduction in tensile strength due to the introduction of an auxetic geometry which limits their application to low loads. tensile strength, and a higher particle size range for strain load dependent filtering. linked to larger unit cell size in compound auxetic tissues.
Finally, the authors stated that future research will focus on the parametric study of geometric parameters and their influence on the deformation and mechanical behavior of the material, as well as the consideration of different structural models of rotating units.
Dubrovski, PD et al. (2022) In-Plane Strain Behavior and Open Area of Rotating Squares in Auxetic Compound Tissue [online] Polymers 14(3) 571 | mdpi.com. Available at: https://www.mdpi.com/2073-4360/14/3/571