Structural design and characterization of highly elastic woven fabric containing helical auxetic yarns

Abstract

Auxetic textiles have been the focus of much attention due to their great promise for advanced protective clothing, flexible energy harvest devices, and functional textiles. Herein, plain fabric, basket fabric, and a derivative weave with the warp and weft yarns arrangement in a series of zigzags were prepared by incorporating different initial wrap density helical auxetic yarns in the weft direction using a commercial semi-automatic loom. The derivative weave using HAYs with a 150 m⁻¹ initial wrap density as the weft yarn not only possesses superior auxetic behavior but also has good performance in strength and elasticity—essential properties useful for textile daily application. This fabric exhibits a high auxetic effect (ν = −0.585), low elastic deformation (total deformation of 8.4% at 20% strain), excellent flexibility, and high break load. Moreover, by taking account of the key geometric parameters, a systematic discussion of the fabrics has been completed to evaluate the effect on the auxetic behavior; this clarified that changing the fabric structure and initial wrap density of a HAY is an effective strategy to tailor auxetic behavior without compromising the intrinsic properties of components. On the basis of our research, auxetic textiles can be considered a promising candidate for next-generation smart textiles and advanced functional textiles.

Keywords

helical auxetic yarn negative Poisson's ratio auxetic fabric tensile performance elastic deformation

Auxetic textiles have advanced rapidly in the past few decades due to their intuitive behavior of expanding laterally when subjected to a tensile stress. This contradictory deformation mode provides many benefits over conventional positive Poisson's ratio behavior; for example, materials which have a negative Poisson's ratio value always have increased shear stiffness, synclastic curvature (dome shaped), fracture toughness, and indentation resistance, especially for excellent energy absorption and damping properties.^1–7 All these performances enhance their potential applications in the manufacture of advanced protective clothing (e.g. body armour, helmet, etc.), shape adaptive materials, and smart textiles.^1–4

As the Poisson's ratio is a physical parameter that is independent of the material scales, auxetic behavior can be achieved from the molecular to macroscopic scale. A great amount of effort has been devoted to the preparation of auxetic fibers (yarn, fabric, composite) by creating auxetic geometries including helical,^5–7 rope-like,^8,9 honeycomb,^10,11 and porous polymer-like structures^12–14 or by combining with 3D printing,^15,16 thermomechanical treatment,^17,18 and chemical modification technologies.¹⁹ After the structure of helical auxetic yarn (HAY) was pioneered by Hook in 2003,²⁰ both auxetic structures^21–23 and auxetic yarn^24–26 have been greatly developed. Attracted by the contradictory mechanical deformation behavior of HAYs, researchers began designing all kinds of textiles with a negative Poisson's ratio (NPR) including knitted fabrics,^27–29 woven fabrics,^30–32 and composites.^33–35 Meanwhile, the key structure parameters of these auxetic textiles were investigated too. Furthermore, focusing on the innovative ideas, Hu's group developed another type of bi-component auxetic yarn with a rope-like multi-plied structure.⁹ Subsequently, these multi-plied yarns were incorporated within the woven fabric to obtain a series of fabrics with auxetic behavior.³⁰ By measuring and characterizing the auxetic behavior of the yarns and fabrics, they found that the in-plane NPR behavior of a woven fabric can be inherited from its constituent auxetic yarns, but with a significant reduction due to a combination of different factors. These factors include the embedding of auxetic plied yarns during fabric fabrication, the constraint of weft yarns, and the overlapping effect of auxetic yarns upon extension. Some research results also show that the weave structure of the woven fabric has a great impact on the NPR of the auxetic fabric, and a fabric with a long floating length of HAYs is beneficial to a significant auxetic behavior.^30,31

However, current technologies for auxetic textiles are associated with challenges that other tradition textiles do not face, such as the labor-consuming and time-consuming manufacturing process of the fabric as well as how to insert HAYs in every kind of textile effectively. Therefore, it is both urgent and challenging to construct fabrics with NPR behavior through continuously improving their geometric structure, controllability, and stability to push forward their promising application in flexible wearable energy harvest and conversion devices, such as triboelectric generators, and functional textiles, such as “intelligent particle filters,” in terms of their outstanding superiority in energy harvest, synclastic curvature, variable permeability, elasticity, and durability.^2,3

In this work, a series of highly elastic, multi-scale auxetic fabrics, which had different structure designs, were fabricated by incorporating HAYs in the weft direction. Of particular note, a derivative weave with the warp and weft yarns arranged in a series of zigzag was prepared. The HAYs were used in the preparation of fabrics with negative Poisson's ratio (NPR), which had different initial wrap densities. As proof of auxetic behavior, a real-time recording of the woven fabrics structured at different elongation levels was characterized using a high-resolution camera. Among all the as-prepared woven fabrics, fabrics showed a maximum NPR value of about −0.585 and total elastic deformation of 8.4% at a strain of 20% when the maximum NPR value of HAYs in the weft direction was −2.5. Meanwhile, the key geometric parameters used to tailor auxetic behavior have been identified and systematically discussed, and it is proved that the fabric structure can make an obvious difference to the auxetic behavior. In addition, the HAY initial wrap density is another parameter that can be utilized to optimize the NPR performance. Our work provides a reform of mainstream convention textiles and creates considerable potential for more practical applications of auxetic textiles.

Formation and characterization of HAYs

Geometric definitions and fabrication of HAYs

Firstly, to fabricate auxetic yarns with a helical configuration as proposed by Hook,²⁰ a HAY geometric structure is constructed by incorporating 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). The initial diameters of the core and wrap yarns are defined as D and d, respectively. The outer diameter of the HAY is characterized by D_y, and the cyclic frequency of the wrap yarn can be described by using the wrap density ρ_w, cyclic pitch L, or by considering the helical angle θ.

At zero strain, the wrap yarn and the core yarn are contacted in a uniform configuration, with the core yarn along the axial direction and the wrap yarn helically wound around the core yarn, as shown in Figure 1(b). Upon suffering an axial tensile load, the wrap yarn straightens and thus displaces the core yarn with the core helically wound around the wrap. So, an increase in the outer diameter of the HAY can clearly be seen in Figure 1(c).

In our previous study, it was found that the NPR behavior is more significant with a lower diameter and a higher tensile modulus of wrap yarn.^6,7,24 Thus, HAYs in which a low modulus, initially straight core yarn was uniformly wrapped with a lower diameter and a higher modulus wrap yarn were manufactured. To quantitatively investigate the auxetic behavior of the as-prepared HAYs, three types of HAYs with different initial wrap densities were manufactured, and the specifications of these HAYs are listed in Table 1, where the diameter of the components was measured via microscope. Furthermore, the wrap angle and the cyclic pitch are displayed in Table 1, and considering the intuition of the wrap angle and the practicality of the cyclic pitch, real photographs of the as-prepared HAYs in their initial state are shown in Figure 2(a) to (c). In addition, we take exactly one cycle of a HAY to correlate the wrap angle θ to the cyclic pitch L to improve the effectiveness and controllability of the manufacturing process. The cyclic pitch and component diameters of a HAY can be described by using a simple right-angled triangle after unfolding through imagining the wrap yarn sticking to a flat surface as it unravels from the core.^6,25 As shown in the inset of Figure 2(d), the predicted values of the wrap angle (tan θ)_pre can be calculated by

(tan θ) pre = \frac{π (D + d)}{L}

(1)

Table 1.

The specifications of HAYs

Yarn code	Core parameter	Wrap parameter	Wrap density ρ_w (m⁻¹)	Linear density (tex)	Cyclic pitch L (mm)	Wrap angle		Relative error E (%)
Yarn code	Core parameter	Wrap parameter	Wrap density ρ_w (m⁻¹)	Linear density (tex)	Cyclic pitch L (mm)	(tanθ)_pre	(tanθ)_exp	Relative error E (%)
Y1	spandex 1680D D = 0.58 mm	polyester (150D/36f) D = 0.25 mm	150	161	8.587	0.304	0.285	6.25
Y2			300	164	4.546	0.574	0.529	7.84
Y3			450	166	2.434	1.071	1.000	6.63

Then, the relative error (E) is given by

E (%) = \frac{| {(tan θ)}_{exp} - {(tan θ)}_{pre} |}{{(tan θ)}_{pre}} \times 100 %

(2)

Using equations (1) and (2), we obtained a histogram of the experimental and predicted values of initial tan θ for all of the fabrics. All results are shown in Figure 2(d) and Table 1; by comparing (tan θ)_pre and (tan θ)_exp in Table 1, the smaller difference Δ and the low relative error confirmed the validity of the geometric calculations. It also contributed to the improvement of the weaving efficiency.

The NPR behavior characterization of HAYs

Herein, we conducted auxetic behavior measurements via a self-assembly stretching system, as shown in Figure 3(a) and (b). Firstly, the sample was held by a fixed clamp and a movable clamp with a gauge length of 100 mm and stretching speed of 1 mm s⁻¹. Simultaneously, a microscope was mounted in front of the HAYs to capture images at a rate of 1 s, which corresponded to a 1% interval of the tensile strain ε_x of the tested sample. Three experiments were conducted for each sample. Then, all photos captured during the experiments were imported into the ImageJ software. Five tests have been done in different sections of the HAYs to reduce the measurement error. Finally, the average value of the measurement results was taken as the effective diameter of the HAYs under tension.

Figure 3(c) shows real photos of the HAYs with different initial wrap densities at different strain levels. It can be seen that the strain of the components exchanging their position is delayed with the increase of wrap density from 150 m⁻¹ to 450 m⁻¹; this is mainly due to the unwrapping time showing an upward trend with the increase of initial wrap density.

Based on the test results above, the diameter of the HAY in its original state D₀ without strain and stretched state D were obtained. Thus, the lateral strain ε_y was further expressed as

ɛ_{y} = \frac{D - D_{0}}{D_{0}}

(3)

Similarly, the axial strain ε_x is expressed further as

ɛ_{x} = \frac{L - L_{0}}{L_{0}}

(4)

Then, the Poisson's ratio (ν) of the HAYs could be calculated by

ν = - \frac{ɛ_{y}}{ɛ_{x}}

(5)

Using equations (3) to (5), we determined the lateral strain ε_y and Poisson's ratio ν of each HAY under different strain levels. Figure 3(d) shows the lateral strain of the as-prepared HAYs versus changing axial strain for different initial wrap densities. It was found that the lateral strain of all HAYs was negative at the beginning of stretching, which was derived from the elongation of the core yarn. Afterward, the lateral strain of samples Y1 and Y2 became positive, corresponding to the NPR. However, the lateral strain of sample Y3 remained negative throughout the whole stretching process in spite of some fluctuation. This may be explained by the fact that the diameter of the core yarn has been greatly reduced due to the large elongation before the wrap yarn migrates to the yarn core, and then, the diameter increase of the HAY caused by the positional exchange of the component yarns can no longer make the diameter of Y3 increase, which is consistent with the images in Figure 3(c).

Figure 3(e) describes the Poisson's ratio–axial strain curves of HAYs with an initial wrap density ρ_w equal to 150, 300, and 450 m⁻¹, respectively. All three HAYs show similar Poisson's ratio–axial strain curves with different maximum NPR values, and the HAYs with 150 m⁻¹ initial wrap density (Y1) show superior auxetic behavior with a maximum NPR value of about −2.5, in contrast with sample Y3 (450 m⁻¹; almost no auxetic behavior) and sample Y2 (300 m⁻¹; about −2.1). This is because the components of the HAYs exchange their relative position at a small strain level arising from the lower initial wrap density, which also consistent with that observed in Figure 3(c). Furthermore, we found that changing the initial wrap density of the wrap yarn is an effective strategy for boosting the auxetic behavior and maximum NPR value without replacing the raw material and damaging the intrinsic properties of the yarn. Consequently, HAYs with a different initial wrap density but a similar helical structure are an ideal raw material for fabricating auxetic fabric by controlling their interwoven structure.

Fabrication of elastic woven fabrics based on HAYs

To date, weaving or knitting conventional yarns in an auxetic geometrical architecture as well as fabricating woven fabric through auxetic yarn directly are two main methods to create fabrics with auxetic behavior. The first method is limited by the complexity of the fabric structure and the difficulty of fabrication. Therefore, the auxetic fabrics in our work were prepared by incorporation of HAYs in the weft direction.

In order to investigate the auxetic behavior of fabrics, woven fabrics with different patterns were fabricated. Firstly, plain weave and basket weave (2/2) were chosen to evaluate the effect of the structure on the auxetic properties of the fabric. In addition, a derivative weave with the warp and weft yarns arranged in a series of zigzag was designed in our work; the interlacing rules of the warp and weft yarns are shown in Figure 4(c), and Figure 4(a) and (b) illustrate the 3D and 2D planar structure of other fabrics.

All woven fabrics used in our work were fabricated using a semi-automatic loom (SGA598). The real photographs and enlarged view of the as-prepared elastic woven fabrics are shown in Figure 4(d). Meanwhile, the fabric structure and the auxetic properties of the HAYs used in the weft can make a major difference to the NPR of the auxetic fabrics. So, HAYs with an initial wrap density ρ_w of 150 m⁻¹ and 300 m⁻¹ were chosen as weft yarn based on the results in Figure 3(e), and commercial aramid was used as the warp yarn. Then the prepared fabric samples were designated into six types for convenience of subsequent discussion and analysis; the specifications corresponding to these fabrics are listed in Table 2.

Table 2.

The design specifications of woven fabrics

Fabric code	Warp yarn		Weft yarn		Fabric density		Fabric structure
Fabric code	Material	Linear density	Material	Wrap density ρ_w (m⁻¹)	Ends (cm⁻¹)	Picks (cm⁻¹)	Fabric structure
F1	Aramid F-12	23tex	Y1	150	20	10	plain
F2			Y1	150			basket
F3			Y1	150			derivative
F4			Y2	300			plain
F5			Y2	300			basket
F6			Y2	300			derivative

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 an INSTRON 5967 tensile tester. Taking into account the high elongation performance of the weft yarn, here all measurements were carried out at a gauge length of 50 mm and stretching speed of 50 mm min⁻¹; for each sample, three experiments were conducted, and the schematic and real photograph of the fabric sample tensile system are shown in Figure 5(a) and (b), respectively.

During the tensile test, we captured images through a microscope before and after the components of the HAYs exchanged their position to better understand the deformation mechanism of HAYs in the weft direction of the fabric. Figure 6(a) to (f) show the micrographs of the as-prepared fabrics with different pattern structures during the stretching process. It can be seen that the component yarns of the HAYs in these fabrics do not exchange their position synchronously. This was due to different restrictions on the HAYs resulting from fabrics of different constructions.

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 (Figure 5) to accurately record longitudinal and lateral fabric strain, and images of the tested sample were captured at one second intervals during the experiment.

In addition, a square with a side length of 10 mm was drawn in the middle of each sample to measure the dimension change of the square, as shown in Figure 7(a) and (b). Finally, the collected images were imported into the ImageJ software. 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 (5).

Elastic deformation properties test

Elasticity is definitely required for textiles in their long-term usage. The elastic deformation is utilized here to characterize the structural stability of the fabrics. To better understand the elastic deformation of fabric samples, two characteristic parameters, i.e. instantaneous elastic deformation and slow elastic deformation, were used here to evaluate the elastic property of the fabrics. Firstly, fabric samples with a testing length of 50 mm were held in the tensile system and stretched to a certain elongation at a speed of 500 mm min⁻¹; then, this elongation was kept for 3 min, after which the tension was unloaded at the same speed to its initial position for 30 s relaxation and 3 min relaxation; finally, we measured the length of the fabrics: L₁ and L₂. Thus, the instantaneous elastic deformation and slow elastic deformation of fabrics can be obtained by

Instantaneouselasticdeformation (%) = \frac{L_{1} - L_{0}}{L_{0}} \times 100

(6)

Slowelasticdeformation (%) = \frac{L_{2} - L_{0}}{L_{0}} \times 100

(7)

where

L_{0}

is the original fabric length before stretching (mm);

L_{1}

is the fabric length after 30 s relaxation (mm); and

L_{2}

is the fabric length after 3 min relaxation (mm).

Figure 8(a) to (d) illustrates the test process diagram for instantaneous elastic deformation and slow elastic deformation for different elongations. Three tests were conducted for each sample.

Results and discussion

Representative tensile behavior of woven fabrics made of HAYs

As for textiles, tensile performance is an essential factor for the application of auxetic fabrics. Figure 9 shows the load-displacement curves of all samples and their corresponding HAYs in the weft direction. It is evident that the load of these fabrics follows the same principle as a function of the displacement, and three main regions can be observed in a typical load-displacement curve, including a linear region at the beginning of stretching, followed by a nonlinear region, and another linear region.

As shown in Figure 9(a) to (c), the load is clearly going up to the maximum load, with an initial concavity and a consequent convexity characteristic. Firstly, an initial short linear region is mainly caused by resistance to friction and bending of yarns. The deformation mechanism of a fabric made of HAYs is different from a traditional one. Its deformation behavior is mainly dependent on the load-displacement properties of HAYs when subjected to a load in the HAYs direction. Thus, a relatively lower slope in the non-linear region represents the stretching of soft yarn and unwrapping of the helical, that is to say, the spandex in the core of the HAY bears the load and exhibits a relatively small increase in force and high elongation; this tensile behavior is similar to that of a single HAY. And while the stiff yarns in the HAYs transfer to the yarn core, they begin to withstand a great load and a rising slope appears in both the curves of the HAY and fabric, but the load of the fabric increases in a higher magnitude compared to a single HAY of fabric. The reason for this phenomenon arises from the deformation behavior of HAYs in the fabric not being completely synchronized. Finally, further elongation takes place as the yarn is stretched until it fails completely. Moreover, the ultimate displacement as well as the load of the derivative weave with the warp and weft yarns arranged in a zigzag were preferred over those of plain weave and basket weave, which can be explained by the long floats and few intersection points of the fabric, endowing the HAYs with better freedom. This more fully utilizes the tensile behavior of the HAY and increases the synchronism of the deformation of the HAYs during the whole tensile process. Also, it demonstrates the superiority of the as-designed derivative fabric in the field of protection.

On comparison of the tensile behavior of fabrics made from HAYs under different initial densities, major differences can be observed, as shown in Figure 9(a) to (c) and Figure 9(d) to (f) . It can be seen from Figure 9(a) and (d) that when the initial wrap density of the HAYs increased from 150 to 300 m⁻¹, the ultimate displacement of the HAY increased by about 50% with the breaking load remaining unchanged; this is mainly due to the fact that the soft yarn in a HAY with a higher initial wrap density undergoes a longer stretch region before the wrap yarn exchanges position with the core yarn, i.e. the wrap yarn migrating to the position of core yarn and the core yarn migrating to the position of wrap yarn. Similarly, the same rules can be seen in the ultimate displacement of the auxetic fabric with a ∼10% increase which results from the constrained elongation of the HAYs in the fabric. Additionally, there are also some variations in the load of the fabric, which are due to the fact that the constraints imposed by fabric with different structures are not the same and an asynchrony of HAYs deformation in fabrics exists.

Representative auxetic behavior of woven fabrics made of HAYs

Auxetic fabric is a candidate protective material for wearable devices. Before investigating the effect of their design parameters on the auxetic behavior, samples F3 and F6 were first used to discuss the auxetic behavior of the fabric. Figure 10 shows the lateral strain and Poisson's ratio as a function of the applied strain for both fabrics and their respective HAYs.

As shown in Figure 10(a) and (b), the HAY Y1 exhibits a negative lateral strain at a strain below 0.02, which corresponds to a positive Poisson's ratio value, whereas the fabric F3 demonstrate a negative value and positive lateral strain in the same strain range; the reason for this mainly comes from the tension applied to the warp and weft yarns during the weaving process. The auxetic effect of fabric F3 continues with the increase of the strain and then decreases to zero in magnitude until the strain reaches 0.44 over a strain approximately two times that of the HAY. This phenomenon can be attributed to the fact that the auxetic effect of the HAYs in the fabric occurs asynchronously when the same strain is applied, which can be observed in Figure 6.

Interestingly, different from samples F3 and Y1, the fabric F6 and yarn Y2 all exhibit a negative lateral strain and a positive Poisson's ratio value at a strain below 0.02. In addition, we can observe a similarly obvious increase in Poisson's ratio ν for sample Y2 and F6, after which a symmetrically sharp decrease in ν is shown. This is the onset of auxetic behavior, as shown in Figure 10(c) and (d). Then the yarn and fabric indicate an auxetic behavior at 0.04 and 0.05 strain when an NPR value is observed, respectively. This difference is mainly because of the fact that a larger strain is required to exhibit the auxetic effect for yarn Y2 which has a large initial wrap density compared to Y1, which also can be clarified in Figure 3. Additionally, resulting from the constraining of the warp yarn and overlapping effect, the auxetic behavior of the fabrics are greatly reduced in contrast to their respective HAYs, which have a maximum NPR value of −2.5 and −2.1, respectively.

Elastic recovery of woven fabrics made of HAYs

Elasticity is a characteristic for evaluating the ability of a textile to stretch easily and then to return easily to its original shape quickly; it is essential for the durability and dimensional stability of the fabric. Figure 11(a) and (b) illustrate the elastic deformation of all fabric samples at a strain of 20 and 40%, respectively. It can be seen that all of the fabrics in our paper showed a small total elastic deformation with an increase of strain from 20 to 40%; this can be explained by a higher elasticity of the HAY with the spandex in the yarn core. At the same time, clear differences can be observed in the fabrics with different weave structures. This phenomenon can be explained as follows: fabrics with long floats and few intersection points allow the HAYs to deform more easily than those of fabrics with a tight structure upon stretching; yarn friction is susceptible to take place too, which results in a large elastic deformation under different strain loads. Furthermore, fabrics consisting of HAYs with a 300 m⁻¹ initial wrap density in the weft direction possess superior elastic recovery with a smaller total elastic deformation, which can be considered as the time taken for the spandex to migrate to the surface of HAYs being delayed with an increase of wrap density. As a result, the spandex in a yarn with a higher wrap density plays a dominant role as the fabric is stretched.

Plotting the instantaneous elastic deformation and slow elastic deformation alone in Figure 11(c) and (d) clearly shows the instantaneous elastic deformation and slow elastic deformation of hybrid fabrics at 20% strain are less than those at 40% strain; sample F3 showed a maximum instantaneous elastic deformation of about 7.6% at 20% strain among all samples, whereas this value increased to 10.5% as the applied strain was doubled. Similarly, the maximum slow elastic deformation can be found in the same fabric, with values of about 0.8% (20% strain) and 1.6% (40% strain), which also indicates the ideal elasticity of the as-prepared fabric. The fabrics suffer from a loss of their partial elasticity and accumulation of deformation following tests. The elastic deformation decreased when the fabric experienced a long recovery time. Thus, the slow elastic deformation is significantly lower than the instantaneous elastic deformation at the different strain level.

All the results mentioned above indicate desirable elastic performance and dimensional retention of our auxetic fabric; that is, the fabric can return to its original shape quickly to meet the durability of long-term use in small deformations, which is beneficial in expanding the practical application of auxetic fabric in the protective field. Meanwhile, it can also be deduced that the NPR behavior of the fabric is also stable resulting from the superior dimensional retention of the fabric.

Effect of initial wrap density of the HAYs on auxetic behavior of fabrics

Figure 12(a) to (c) shows the effect of varying the initial wrap density of the HAYs on the auxetic behavior of the fabric samples. For comparison, the maximum NPR values of fabrics with HAYs in the weft are also shown in the radar chart in 12(d). The initial wrap density used was nominally 150, 300 m⁻¹ with a ratio of 1:2. While increasing the initial wrap density of HAYs from 150 to 300 m⁻¹, all fabrics display a limited auxetic behavior as well as a lower maximum NPR value (Figure 12(d)) during the whole strain range.

In our previous investigation, HAYs with a 150 m⁻¹ initial wrap density (Y1) possessed a higher auxetic effect (Figure 3), and it is evident from the results that fabrics F1 and F3 produced with it also have superior auxetic behavior, given that the NPR of fabric F1, F3 is more obvious than that of F4, F6 at all strains. However, there is a different tendency taking place in fabric F2, F5 with an intersect phenomenon on the curve of Figure 12(b). The reason for this may be on account of the overlap effect and uneven strain distribution during yarn deformation, which leads to an expansion of the fabric in the thickness direction.

All the results mentioned above imply that the initial wrap density of HAYs, that is, the migration intensity of the components in HAYs, is a parameter that can use to optimize the auxetic behavior of the fabric made therefrom.

Effect of weave structures on auxetic behavior of fabrics

Apart from the initial wrap density, the weave structure also plays a significant role in the determination of the NPR behavior of auxetic fabric. Figure 13 depicts the Poisson's ratio–axial strain curves and maximum NPR values of fabrics with changing weave structure. The prominent differences can be seen in Figure 13(a) and (c) although the same HAYs (initial wrap density of about 150 m⁻¹) were used in the weft direction of the fabric; the plain weave fabric (F1) and derivative weave fabric (F3) exhibit NPR once the strain is applied, and their maximum NPR values, about −0.348 and −0.585, respectively, are also illustrated in Figure 13(c), while the basket weave (2/2) fabric (F2) shows an initial increase in ν versus strain, after which an identical decrease appears, yet the auxetic behavior takes place with a maximum NPR value of −0.280 in a small strain range.

Fabrics F4, F5 and F6 were manufactured with an intended initial wrap density of the HAYs of about 300 m⁻¹ in Figure 13(b). All three variations in weave structure show similar low strain behavior in the sharp increase and subsequent decrease in ν to a minimum value of −0.250, −0.153, and −0.426, respectively. As shown in Figure 13(d), a change in weave structure produces a nearly 2.7 times change in the maximum NPR value. The onset of auxetic behavior of sample F6 is observed at a comparably low strain with the highest auxetic behavior in comparison with that of sample F4, F5 which exhibit small auxetic behavior at a marginally high strain.

The results reveal that the weave structure is an important parameter we can use to tailor the NPR of the auxetic fabric. Furthermore, it can be concluded that fabrics with long floats and few intersection points are desirable for generating excellent NPR effects and a higher NPR value.

Conclusion

In summary, we have achieved the fabrication of HAYs into woven fabric by selecting different HAYs and designing different structures which may make a difference to the NPR effect. Plain fabric, basket fabric, and derivative fabric with HAYs used as the weft yarn were fabricated to demonstrate the auxetic behavior. After comparison of various fabrics, the maximum NPR value of the derivative fabric with the warp and weft yarns arranged in a series of zigzags reached −0.585, which is much higher than the value of plain fabric and a basket fabric limited by intersections. In addition, the tensile behavior and elasticity of the textiles are extremely important for daily use; thus, the complete load-displacement curve and elastic deformation at a strain of 20 and 40% of hybrid fabrics were systematically investigated. We found that the deformation mechanism of all fabric follows the same rules and can be discussed in three regions. Importantly, the ultimate displacement as well as the breaking load of the derivative fabric mentioned above were superior when contrasted with those of plain and basket fabrics. The ultimate displacement of fabric increased by about 10% with the initial wrap density of the HAYs increasing from 150 to 300 m⁻¹. At the same time, a maximum total elastic deformation of 8.4%, which is composed of instantaneous elastic deformation (7.6%) and slow elastic deformation (0.8%), at a strain of 20% could be observed in the same fabric, which demonstrates the practicability and superiority of the as-designed derivative fabric for a wide range of applications.

Furthermore, the weave structure and initial wrap density of the HAYs are proved to be an effective strategy in optimizing the auxetic behavior and improving the maximum NPR value without replacing the raw material and compromising the intrinsic properties of the components. More specifically, a small initial wrap density of the HAYs (large migration intensity of the components in HAYs), long floats, and few intersection points are desirable for generating excellent NPR effects and a higher NPR value. At the same time, the architecture design of the auxetic textiles can provide inspiration for their development in sensors or smart wearable devices. Our research not only promotes the diversification of auxetic textiles used in flexible wearable devices and functional textiles but also provides considerable guidance to continuously improve their structure, controllability, and stability to push forward their application in more fields.

Supplemental Material

TRJ881814 Supplemental Material - Supplemental material for Structural design and characterization of highly elastic woven fabric containing helical auxetic yarns

Supplemental material, TRJ881814 Supplemental Material for Structural design and characterization of highly elastic woven fabric containing helical auxetic yarns by Junli Chen, Zhaoqun Du and Tianyuan Li in Textile Research Journal

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is jointly supported by “the Fundamental Research Funds for the Central Universities (2232018G-01),” by the National Key Research and Development Program of China (Grant No.2016YFC0802802), by the Fundamental Research Funds for the Central Universities and Graduate Student Innovation Fund of Donghua University (CUSF-DH-D-2019056) and by the Open Project Program of Key Lab for Sport Shoes Upper Materials of Fujian Province (Fujian Huafeng New Material Co., Ltd.) of China (No. KLSSUM1902).

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