Fabrication and Testing of 2-Dimensional Z-Expandable Auxetic Textile. Structures for Impact Protective Clothing Applications

Transcription

1 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 1 Fabrication and Testing of 2-Dimensional Z-Expandable Auxetic Textile Structures for Impact Protective Clothing Applications ASTM Student Project Grant Harini Ramaswamy University of Minnesota-Twin Cities Advisor: Lucy Dunne 12/31/2014

2 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 2 ABSTRACT Auxetic textiles become thicker when subjected to a stretch and are incorporated in functional clothing design (Alderson, 2005). These counter-intuitive smart materials grow in dimensions, in a direction that is perpendicular to the applied force. Among several applications in technical textiles, auxetic materials are incorporated in elbow pads, knee pads and body armor for impact protection and shock absorption. Unlike conventional substances that compress at the point of impact and become vulnerable to breakage when subjected to a tensile stress, auxetic planar materials push towards the point of impact, thereby making the material more resistant to breakage. Since the ratio of the transverse to longitudinal strains (Poisson s ratio) for these materials are negative, these are also referred to as Negative Poisson s Ratio (NPR) materials. Most existing auxetic structures are two-dimensional and grow in the Y axis when stretched along the X axis or vice-versa. Limited research has been directed towards the creation and testing of three-dimensional auxetics that grow in the Z-direction, when subjected to stresses along the X or Y axis. Further, the manufacture of 2D and 3D auxetics is generally complex (Mslija, A., &. Lantada, D. A.). The purpose of this study was to engineer Z-expandable auxetic structures that can be manufactured easily from a sheet-like textile material, for incorporation in an application such as a kneepad. An adaptation of the ASTM D Standard Test Method for Breaking Strength and Elongation of Textile Fabrics and ASTM D Standard Test Method for Thickness of Textile Material was used to determine the engineering Poisson s ratio-- a negative value of which confirms auxeticity. The stresses that come into play during growth and recovery were identified.

3 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 3 INTRODUCTION Classification The word Auxetics has been derived from the Greek word Auxetikos meaning that which tends to increase. These materials exist in various forms ranging from the microscopic to macroscopic as shown in Figure 1. Figure 1: Classification of Auxetics (Scott et al, 2000) Poisson s Ratio The Poisson s ratio is defined as the negative ratio of the transverse strain to the axial strain in the direction of loading (Wan et al. 2004). Conventional materials have positive Poisson s ratio, whereas auxetic materials have negative Poisson s ratio (Goud, 2010). Production of Auxetics Auxetic textiles may be produced in several ways. Inherently auxetic textile fibers could be used to make a fabric that exhibits auxetic behavior. Alternatively, conventional fibers could be made into an auxetic structure as well (Alderson et. al., 2012). A recent study employed various warp knitted 3D spacer auxetic fabrics, which were constructed using a novel geometrical structure with parallelograms. The highest auxetic behavior was examined when stretched in the weft direction and the lowest was observed in warp direction.

4 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 4 The auxetic behavior also reduced with increase in tensile strain. It was also revealed that the auxetic fabric will retain 65% of its effect after 10 cycles of extension. These novel auxetic geometric configurations make it attractive for potential applications like sports and protection (Wang et al., 2013). In another recent research study, rotating square auxetic structures (Figure 2) with enhanced mechanical properties were designed and manufactured for the application of stents for palliative treatment of esophageal cancer. Polyurethane foam sheets were laser cut to the desired structures using the CNC guided laser cutter and the compressive stress-strain behavior was tested on the Instron. Employing the auxetic cell geometry improved the stenting outcomes. The material could get wider when stretched, offer stiffness without being brittle and minimize stresses (Bhullar S.K., et al., 2013). Figure 2: Rotating Square Auxetics with holes (left) and without holes (right) (Bhullar S.K., et al., 2013, p 44) Properties Auxetic materials are suitable for fitting the human body. The ability of the structure to open out when stretched leads to enhanced air permeability under tension. (Wang, et. al., 2013). Auxetic materials exhibit synclastic behavior in that they curve in the same direction of the bending force (Lakes, 1987; Evans, 1990; Cherfas, 1990). In the case of wearable auxetics, this means that the structure would offer more conformability while offering impact protection.

5 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 5 Applications Auxetic polymeric materials are often used in combination with other materials for personal protective sportswear such as crash helmets, knee pads, shin pads, ballistic protection and gloves due to their ability to absorb energy (Liu, et al, 2010). The Defence Clothing and Textile Agency (DCTA) in Colchester has been looking at applications of auxetic textiles for military purposes as shown in Figure 4 (Liu, 2006). Figure 4: Auxetic Materials for Ballistic Protection (Alderson, 1999) It is significant to note that compared to auxetic fibers and auxetic yarns, studies of auxetic fabrics and sheet structures are limited (Wang et.al, 2013). Existing 2D auxetics are generally X-Y expandable (i.e., when stretched along the X-axis, these structures grow in dimensions along the Y axis and vice-versa). There is limited research directed towards developing auxetics that transform from 2D planar structures to 3D, and the growth of auxetics in the Z-direction, when stretched along the X or Y axis that can be incorporated in impact protective clothing. Figure 5: x, y and z axes

6 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 6 METHODOLOGY Figure 6 highlights the scope for innovation in auxetics related to this project. Figure 6: Mind map showing scope for innovation in Auxetics The objectives of this project were to create auxetic textile structures that: (i) Demonstrate growth in the Z direction (normal to the plane), when stretched along the X or Y axis, thereby transforming from 2D to 3D in the process. (ii) Are elastic in nature (i.e., auxetics return to their original configuration once the stresses that they are subjected to are removed). In other words, the deformation that these structures undergo would not be permanent and the growth along Z- direction is retained only as long as the structure is subjected to a stress along the X or Y direction. (iii) Can be easily fabricated from a sheet-type textile material such foam and (or) fabric.

7 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 7 Ideation Various material and design directions were explored as part of open and structured ideation. Explanation for the creation of these structures is beyond the scope of this report. Herringbone Structures Wave and Arc Structures Swastika ( 卐 ) inspired structures (i)paper prototypes with slits (ii)open-celled foam prototypes (iii)closed-celled foam prototypes (iv) Origami prototypes (iv)fabric and industrial felt/foam integrated prototypes Figure 7: Thumbnails of some materials and methods used

8 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 8 Manufacturing Rotating Triangles The following schematic diagrams shows the simplest repeating unit of the rotating triangles structure, which combines four triangular sub-units of polyurethane foam that were sewn together in areas indicated by the red dots. Following this, holes were punched in these triangles and an embroidery floss was threaded through for tensioning purposes, as indicated by the yellow arrows. In order to prevent the floss from cutting into the polyurethane foam, the punched holes were secured with metallic eyelets. Figure 8: Simplest Unit of Rotating Triangles Slot Pop-up The slot pop-up structure was made from two sheets of foam with slots cut out as shown in the diagram. Corresponding flaps from the two structures were spot welded with glue or sewn together with a single stitch. Figure 9: Slot pop-up

9 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 9 Testing Rotating triangles and slot pop-up that were refined and considered for final testing are presented in this study. Images and videos were captured. The images below show front and side views of the prototypes, when subjected to a stretch test on the Instron. Figures 8 (i)-(vi)rotating Triangles (labelled from left to right) (i)(ii)relaxed Views (iii)(iv)intermediate Stretch Positions (v)(vi)completely stretched Figures 9 (i)-(iv) Slot Pop-up (labelled from left to right) (i)(ii) Relaxed View (iii)(iv) Completely stretched The ASTM D Standard Test Method for Breaking Strength and Elongation of Textile Fabrics and ASTM D Standard Test Method for Thickness of Textile Material were adapted for the purpose of this study. The structures considered were made to 9.25 inches (length) x 5 inches (width) and inches (thickness), from closed-cell polyurethane sheets.

10 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 10 Tensile Testing Machine The testing was performed on the Instron 5544 Constant Rate of Elongation (CRE) type machine in which the specimen is subjected to elongation of 0.50 inches at a uniform rate. Measurement The amount of stretch or elongation that the specimen undergoes during tensile testing is expressed in the form of strain. For the purposes of this study, engineering strain (the ratio of the change in length to the original length) of the specimen was determined for the Y and Z axes. Using these values, the engineering Poisson s ratio (ratio of the transverse strain to the longitudinal strain) was determined. Clamping or Holding Devices Specimens were mounted on the clamps manually. Clamp liners were used in order to preclude slippage and minimize specimen failure in the clamped areas. Calibrating Devices The machine had a steel rule running along the longitudinal direction to measure length. A ruler was also attached along the transverse direction in order to measure the change in thickness. In the case of the rotating triangles structure, thickness was determined by measuring the distance between a pair of cardboard sheets without parallax error. Length and thickness measurements were recorded. Images and videos of the mounted structures in various positions were also captured. Figure 10: Measuring Thickness of Samples

11 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 11 Mechanics during Growth and Recovery The stresses that come into play along the longitudinal and transverse directions were identified as shown in Table 1. Type of stress during growth and recovery along longitudinal direction (Y-Axis) Type of stress during growth along transverse direction (Z-Axis) Type of stress during recovery along transverse direction (Z-Axis) Rotating Triangles Tension Buckling/bending (combination of tension and compression) Buckling/bending (combination of tension and compression) Slot Pop-up Shear Radial Radial Table 1: Mechanics of Structures during Growth and Recovery

12 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 12 RESULTS AND DISCUSSION For various lengths of stretch, corresponding thicknesses were recorded, until two consecutive thickness values were obtained, as shown in Table 2 indicating saturation. At 12.5 inches, in the case of the rotating triangles structure, the structure did not expand beyond 2 inches and in the case of the slot pop-up, the slots started reversing beyond an extension of 10.5 inches and reached 0.25 inches. No. Length (Inches) Thickness (Inches) Rotating Triangles Slot Pop-up (when mounted in relaxed position) and (actual) 0.25 (when mounted in relaxed position) and (actual) Table 2: Thickness obtained for Various Lengths Rotating Triangles: Length vs. Thickness Slot Pop-up: Length vs. Thickness Graphs for Rotating Triangles and Slot Pop-up

13 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 13 Auxeticity of both the structures were determined from the Engineering Poisson s ratio values, based on the formula provided below. Strain Transverse T2-T1 Engineering Poisson s Ratio = - = - T1 Strain Longitudinal L2-L1 L1 It is to be noted that in the case of a conventional non-auxetic material, when the value of Strain Longitudinal is positive, Strain Transverse is negative because a stretch along the longitudinal direction will result in a compression along the transverse direction. The formula is usually assigned a negative sign, so that the resultant Poisson s ratio is positive. However, in the case of auxetic materials, since a stretch along the longitudinal direction would result in expansion along the transverse direction, the Strain Longitudinal and Strain Transverse will carry positive signs, and therefore, the resultant Poisson s ratio will be negative. The samples were subjected to ten cycles of minimum and maximum (Table 3) stretch and the average values of these were used to calculate the Poisson s ratio. Rotating Triangles Slot Pop-up Length Thickness Length Thickness Average Minimum Stretch(inches) 9.25 (L1) (T1) 9.25 (L1) (T1) Average Maximum Stretch (inches) 11 (L2) 2 (T2) 10.5 (L2) 1.25 (T2) Engineering Poisson s Ratio Table 3: Minimum and Maximum Stretch Values The structures considered for this study are anisotropic because they do not have identical properties along every direction. Poisson s ratio for isotropic materials that stretch uniformly in all directions ranges between -1 to 0.5. Anisotropic materials can an arbitrary value of any magnitude, as is the case for these two structures (Ting, T. C. T., & Chen, T., 2005).

14 2D Z-EXPANDABLE AUXETIC TEXTILES FOR IMPACT PROTECTION 14 CONCLUSIONS Conventional auxetics grow along the Y axis when stretched along X axis and vice-versa. The manufacture of auxetics is generally complex. This endeavor demonstrates successful creation of easily manufacturable 2D Z-expandable auxetic structures that can be incorporated in an impact protective application such as kneepad. These materials would grow thicker and retain their expanded configuration along the Z axis (transverse direction) for as long as there is a stretch along the Y axis (longitudinal direction). Various material and design directions were explored during the course of open and structured idea generation phases. Rotating Triangles and Slot Pop-up structures were fabricated from Polyurethane foam. The ASTM D for elongation and ASTM D method for thickness were adapted and applied. The mechanics during growth and recovery were also identified. The results reveal that the structures are anisotropic (do not have identical properties along every direction) and are therefore characterized by unusual Engineering Poisson s ratio values.

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