PSI - Issue 17

Michał Kwietniewski / Structural Integrity Procedia 00 (2019) 000 – 000

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Michał Kwietniewski et al. / Procedia Structural Integrity 17 (2019) 58–63

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Peer-review under responsibility of the ICSI 2019 organizers.

Keywords: auxetic; composite; elastomer, FEM analysis

Nomenclature Poisson ’s ratio model diameter change original diameter original model length, before stretch change of length 1 Introduction.

Auxetics have more complex behaviour from conventional materials, their Poisson ’s ratio is negative. During unidirectional stretching, this material increases value of cross-section area in the perpendicular plane. The phenomenon of auxetic strengthening of material is often used to build blast wave protective panels, because it apparently increases the "compaction" at the point of load application, which positively translates into local increase of strength. In addition, auxetics material found application in filtration technology, protective helmets, sports clothing, ropes, and other shock absorbing materials. Research works on these materials are constantly progressing, which translates into a great potential for new applications. From the point of view of human protection, the most popular application is the use of auxetic fabrics, which, in the normal mode of use, behaves like a conventional woven material. However, when subjected to load, it increases its strength. There are two basic ways of auxetic fabrics production. The first one is based on the use of auxetic threads (auxetic polypropylene thread) for the construction of fabric; the second one is the use of conventional materials, which is formed to auxetic material as proposed by Alderson et al (2002). Hook (2011) presented a helical wrapping of many high stiffness warp threads around a thicker and less rigid weft, assuming that both materials separately do not exhibit auxetic characteristics. During longitudinal stretching, more stiffer warp threads straighten and cause less stiffness weft to be bent. The stretching diagram is shown in Fig. 1. This thread construction exhibits auxetic characteristics and can be produced on conventional weaving machines. The aim of the work is the observation of the auxetic phenomenon, while stretching the weave of the Helical Auxetic Yarn cooperating with the surrounding elastic matrix. A very important relationship that characterizes the auxetic materials is the negative Poisson ’s value. This material feature makes quite difficult to create the appropriate numerical model. It generates many problems with obtaining convergence. However, the HAY includes non-auxetic materials that do not require a special constitutive model. Only the results of deformations obtained during the analysis determines the naming of the entire model with the auxetic term.

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Fig. 1 Helical Auxetic Yarn a) unloaded, b) loaded; Sloan (2011).