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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 2013-2014

Harini Ramaswamy

University of Minnesota-Twin Cities

Advisor: Lucy Dunne

12/31/2014


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 D5034-09 ‘Standard Test Method for Breaking

Strength and Elongation of Textile Fabrics’ and ASTM D1777-64 ‘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.


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.


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.


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


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.


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


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


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 D5034-09 ‘Standard Test Method for Breaking Strength and Elongation of

Textile Fabrics and ASTM D1777-64 ‘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 0.125 inches (thickness), from closed-cell polyurethane sheets.


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


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


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

1 9.25 0.25 (when mounted in relaxed

position) and 0.125 (actual)

0.25 (when mounted in relaxed

position) and 0.125 (actual)

2 9.5 0.75 0.5

3 9.75 1 0.75

4 10 1.50 1

5 10.5 1.75 1.25

6 11 2 0.75

7 11.5 2 0.5

8 12 2 0.25

9 12.5 2 0.25

Table 2: Thickness obtained for Various Lengths

Graphs for Rotating Triangles and Slot Pop-up

0

0.5

1

1.5

2

2.5

9.25 10.25 11.25 12.25 13.25

Rotating Triangles: Length vs. Thickness

0

0.2

0.4

0.6

0.8

1

1.2

1.4

9.25 10.25 11.25 12.25 13.25

Slot Pop-up: Length vs. Thickness


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) 0.125 (T1) 9.25 (L1) 0.125 (T1)

Average Maximum Stretch (inches) 11 (L2) 2 (T2) 10.5 (L2) 1.25 (T2)

Engineering Poisson’s Ratio -79 -67

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).


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 D5034-09 for

elongation and ASTM D1777-64 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.


REFERENCES

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