TEACHING MATERIALS for staff trainess

2.9. Mechanical treatments

Textile materials undergo mechanical processing by calendering under controlled conditions of heat and pressure, with the purpose of achieving different textures or effects in the fabric - the result: a more compact and flat fabric with a smoothing effect (smoothness).

2.9.2. Auxetic materials - helical auetic yarn (HAY)

Auxetic textiles have advanced rapidly in the past few decades due to their intuitive behavior of expanding laterally when subjected to a tensile stress. Performances of these materials such as a negative Poisson’s ratio always have increased shear stiffness, synclastic curvature (dome shaped), fracture toughness, and indentation resistance, especially for excellent energy absorption and damping properties what is suitable for their potential applications in the manufacture of advanced protective clothing (e.g. body armour, helmet, etc.), shape adaptive materials, and smart textiles.

Poisson’s ratio is a physical parameter, it is independent of the material scales, and auxetic behavior can be achieved from the molecular to macroscopic scale.

Preparation of auxetic fibers (yarn, fabric, composite) by creating auxetic geometries including helical, rope-like, honeycomb, and porous polymer-like structures or by combining with 3D printing, thermomechanical treatment, and chemical modification technologies are of great interest.

1. step:

HAY geometric structure is constructed by incorporating of two different components on a hollow-spindle covering system, as shown in Figure 1(a), and the geometric parameters of the HAY under different strain levels are defined in Figure 1(b) and (c).  D and d defined the initial diameters of the core and wrap yarns.

2 step: 

Fabrication of elastic woven fabrics based on HAYs

Weaving or knitting conventional yarns in an auxetic geometrical architecture and fabricating woven fabric through auxetic yarn directly are two main methods to create fabrics with auxetic behavior. Investigation of the auxetic behavior of fabrics, woven fabrics with different patterns is fabricated: plain weave and basket weave (2/2) were chosen to evaluate the effect of the structure on the auxetic properties of the fabric, a derivative weave with the warp and weft yarns arranged in a series of zigzag was designed also and showed at figures a, b, c) illustrated the 3D structure. 

Figure: 3D structure design schematic: (a) plain weave; (b) basket weave; and (c) derivative weave.

3. step:

Measurement methods of properties of elastic woven fabrics 

- Tensile test 

The elastic woven fabrics were mechanically characterized for tension with reference to standard ASTM D3107-1975 and BS 4592-1992 on Tensile tester. 

-  Auxetic behavior test 

To evaluate the NPR behavior of the as-fabricated fabric samples, firstly, a high-resolution camera (SONY A7R3) was placed on a tripod in front of the fabric tensile system. Five tests were carried out for each image in order to ensure higher accuracy of the NPR. The results were summarized and converted to NPR values based on equation (1). A square with a side length of 10mm was drawn in themiddle of each sample tomeasure the dimension change of the square, as shown in Figure (right)