Abstract
This study reports on the development of a novel form of yarn reinforcement for making super tough, flexible composites. The concept is based on the rupturing of components of the composite yarn many times before totally breaking. The composite yarn structure has a configuration of three main components − helical, straight and sinusoidal. The components are not restricted by the type of raw material, and any type of fibre or blend can be used. Experimentally, the hollow-spindle spinning system was used to make nine variants of this composite yarn. These variants were tested for their tensile behaviour at an extension rate of 250 mm/min. It was found that the core component of two of the nine variants had eight breakages, while the core component of the remaining variants had four to five breakages before the final break of the core component. The main mechanism that allows the core component yarn to break so many times is a locking applied to it by the wrapping yarn during stretching. The gain in strength for these variants was 21–81% in comparison with a typical twistless composite yarn. Flexible composites made of such yarn configurations can find numerous uses in construction, geotextiles, ballistic protective garments, aerospace, automotive and shipping industries.
Introduction
It is known that to make textile composites, many types and forms of fibrous materials are being used to reinforce polymeric matrices. In the composite industry, these fibrous materials are, for simplicity, called fibres or tows. The choice of type and form of fibrous reinforcement in a composite is decided by several criteria. These criteria include the properties of the fibres, the ease of use or handling, safety, price and the end use of the composite. Fibrous reinforcement in fibre-reinforced polymer-matrix (FRPM) composites has an important role to play in composite performance because the fibres carry the load imposed on the whole FRPM composite. They improve the mechanical properties of the polymeric matrix by providing stiffness, strength and dimensional stability. 1 And also, depending on the fibres used, they may improve or change the physical properties of the composite. These properties include thermal conductivity, heat resistance, flame resistance, electrical conductivity and water absorption.
Traditionally, many FRPM composites have structural applications where high levels of strength and stiffness are required. To make these composites, high-performance fibres are being used. These fibres are also known as high-function fibres, super fibres or extreme fibres. Examples of high-performance fibres are carbon, aramid and glass fibres. However, not all composite materials are required to have high stiffness or are made using high-performance fibres. For instance, the use of natural fibres to make FRPM composites is expanding in many industries such as the construction industry. This is because natural fibre RPM composites are a low-carbon-impact alternative to glass fibre composites and other structural materials such as steel and concrete. 2 Consequently, their use is perceived as a potential solution to the problems of sustainable economic growth and energy. 2 There is also a demand for FRPM composites with high flexibility for certain unique applications. This last class of FRPM composites is known as flexible RFPM composites or flexible composites. Flexible composites are those FRPM composites that sustain high levels of usable deformations or strains and still provide high load-carrying capacity and fatigue loadings. 3 The main types of flexible composites are cord/rubber composites, composites containing simply arranged wavy fibres and also some coated fabrics. 3 Flexible composites can be found in a wide range of applications and for specialised uses as in high mechanical actuator systems. 4 In one study, these actuators were made by mixing a flexible matrix with carbon fibres in a composite tube form where both the stiffness and elongation become dependent on the internal pressure applied, the materials used and the orientation of the fibres. 4 Another application was found in pressure-driven nano-filtration membranes where the flexible composite structure itself was made using hybridization of Kevlar® into regenerated silk fibroin. 5 Other applications are found in making hoses, flexible diaphragms, racket strings, surgical replacements, geotextiles, reinforced membranes, pneumatic tyres and conveyor belts. 6 Coated fabrics can be used for air or cable-supported building structures, tents, parachutes and decelerators in high-speed aeroplanes, bullet-proof vests, tarpaulins, inflated structures (e.g. boats and escape slides) and safety nets. 3
Typically, flexible composites are made using elastomeric polymers, which by convention have values of strain at break that exceed 100%. 7 Examples of which are thermoplastic elastomers (TPEs), such as polyurethane thermoplastic elastomers (TPUs), thermoplastic copolyester elastomers (COPE), melt-processable rubber (MPRs), thermoplastic vulcanizates (TPVs) and synthetic and natural rubber. Besides the flexible matrix, the fibres must be able to deform by (a) using short or discontinuous fibres; (b) using continuous fibres that are arranged in suitable orientation that allows them to rotate as the load increasesand (c) using continuous fibre architectures that are wavy or have crimp, for example, woven fabrics, knitted fabrics, braided fabrics or other wavy forms. 3 It is essential for flexible FRPM composites to be made using polymeric matrices with a high level of strain. However, the majority of researchers focus on ordinary FRPM composites that have some flexibility, and they call them flexible FRPM composites without their matrix being from a true flexible polymeric material.6,8
A survey of the literature indicates that only a small number of researchers have conducted studies on true flexible FRPM composites in comparison with other types of FRPM composites. In one study, finite element analysis (FEA) was adopted to understand the influence of 2D and 3D reinforcement architectures of ultrahigh molecular weight polyethylene (UHMWPE) fibres on the overall ballistic performance of FRPM composite armour. 9 The results show that besides material type, the reinforcement architecture of UHMWPE fibres plays a critical role in the overall ballistic performance of these FRPM composite panels because it affects their structural and damage/failure response. 9 In another study, a mechanically bonded nonwoven preform was used to reinforce a thermoplastic polyurethane (TPU) matrix with Lyocell®, polyethylene terephthalate (PET), polyamide (PA) or para-aramid fibres. 10 In that study, the researchers focused on the FRPM composite tensile strength and E-modulus rather than the strain at break, and they investigated the relationships of these two properties with the properties of the nonwoven itself, that is, punch density, fibre type and direction of the nonwoven. The effect of fibre waviness on the elastic moduli of flexible FRPM composites was investigated in another study with the flexible composites composed of SYLGARD 184® silicone elastomer as the matrix material with the reinforcement being from Thornel-300® carbon fibres. 11 That investigation considered the fibres arranged in sine-wave-like sheets and the waves in either the iso-phase or random-phase. The fibres were put into a wavy, sinusoidal configuration using a mould. The study 11 showed a good agreement between the theoretical and experimental data for both the longitudinal and transverse tension in the iso-phase model. Further, a comparison between model and experimental data of the longitudinal tension for both the iso-phase and random-phase models indicated that the iso-phase model gave the best results. 11
Other researchers have reported new methods for making flexible composites including the use of a non-orthogonal warp-weft orientation in the woven ply within the composite. The use of these specially made woven fabrics requires a material to stabilise the non-orthogonal orientation of the warp and weft yarns by a coating or by the use of a suitable resin matrix. Exemplary candidates of these stabilising materials include silicone rubber, urethane rubber, urethane, polyurethane and combinations. 6 Although the use of non-orthogonal weft-warp woven yarns may impart some flexibility to the final composite materials, the magnitude of flexibility may be relatively small in comparison with the common types of flexible FRPM composites that are made using intrinsically flexible resin matrices and reinforcements (i.e. with intrinsically high extension levels). This is particularly true for a few materials suggested in a US patent concerning the polymeric matrices 6 that include polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol and fluoropolymers; these all are, inherently, not highly flexible matrices, though. Since the number of methods of making super flexible textile composites are limited, this study reports on a novel form of structure of reinforcement suitable for making super flexible, super tough composite materials. Potentially, the fibres can be of any material type and any yarn form: twistless, twisted, textured, single, plied or cords.
The concept of the novel continuous composite yarn structure
The textile structure proposed in this study as a continuous reinforcement for making flexible FRPM composites takes the form of a composite yarn that is made of several components. The components are yarns in their own right, as shown in Figure 1. The first component is the core component (or foundation component), and this component can have one or more yarns. The second component is called the undulating component, and it can also be one or two yarns. Both the core component and the undulating component are fastened together using a third wrapping component, also known as the binder. The wrapping component is shown as the red helix in Figure 1. The basic building unit of the novel multiple-thread composite yarn structure.
The manufacturing method for making the final composite yarn can be either by twistless, twisted, textured, single, plied or cords. The formation of this composite yarn structure can be accomplished using either wrapping or twisting. Using wrapping, this structure is made in a one-stage manufacturing process using the hollow-spindle spinning system or the combined ring-spinning and hollow-spindle process. The twisting route requires more than one manufacturing stage: traditional doubling and twisting processes. This multiple-yarn structure composite yarn can be made of several material types; thus, it is a hybrid yarn. Traditionally, this yarn structure is used for making garments, fashionable clothes and furniture due to its aesthetic properties and as such it is known as ‘novel yarn’ or ‘fancy yarn’. Its aesthetic features depend on the undulating pattern of the yarn structure, the combination of individual component materials and colours, the number of wraps of the wrapping component and the thickness of the whole yarn.
The mechanical properties of this kind of structure depend on several factors including the undulating pattern of the yarn structure, the combination of individual component materials, the number of wraps of the wrapping component, the thickness of the input yarn and the false twist of the structure. Extensive work was conducted to understand these properties12-14 as detailed in Part II of this paper. Assuming that this structure has three input yarns (one each for the core component, the undulating component and the wrapping component), one may think mistakenly that such a structure would break three times if it was tested for its tensile strength. And given the relative lengths of the components, one may think the components would break one by one, starting with the core component first, followed by the wrapping component and finally by the undulating component. However, the real mechanism is more complicated than such a simple assumption. It is evident from a previous study 13 on the variants of this composite yarn structure that these variants break several times and that they keep carrying the imposed tensile load until the whole structure fails; the variants were wavy yarns, gimp yarns and generic-overfed yarns, but the study was limited to the effect of false twist on yarn features rather than their mechanical properties. 13 Therefore, this study is dedicated to reveal the multiple-break mechanism of the composite yarn and to provide a roadmap for taking advantage of this effect for making truly flexible FRPM composites. The proposed composite yarn structure can potentially be made of any combination of fibres for the three components. In particular, fibres and individual components must allow for a strong interaction between the components and specific end uses which can help in deciding the type of material for each component. Typical uses for flexible FRPM composites made of such composite yarn may be in energy absorption, vibration dampening, protection, impact and ballistics.
Methodology
Individual component properties and specification of the composite yarn
For the purpose of showing the tensile load-elongation behaviour of the composite yarn structure, an initial composite yarn (called yarn 0) was made of hybrid materials, and it has a simple wavy structure. The technology used to make it was a hollow-spindle system Gemmill and Dunsmore MK3, and this composite yarn was made in one-stage process as explained in a previous study.
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The overfeed ratio of the undulating component (relative to the core component) was 110%, and the number of wraps of the wrapping component was 225 wrap per meter (wpm). The core component of the structure is made of two single, rotor-spun, cotton yarns, each of which having a linear density of Ttex= 29.5 tex. The undulating component was a two-ply cotton yarn with a resultant linear density value Ttex= R36.9/2 tex. The wrapping component was a 16.7 tex textured polyester multi-filament of 34 filaments. This composite yarn, shown in Figure 2, was not false twisted on the hollow-spindle machine while making it. To show the impact of changing the individual components, the overfeed ratio, the number of wraps and false-twist, a set of eight different composite yarns were made using the design of experiment method, the details of which are given in Table 1. Images of these eight composite yarns are shown in a previous study,
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while Figure 3 shows other potential variants of the same structure. The initial composite yarn made using an overfeed ratio of 110% and number of wraps of 225 wrap per meter (i.e. composite yarn 0). Parameters of the set of eight composite yarns. Several variants of the composite yarn made using various types of individual components, overfeed ratio and number of wraps.
Testing of the composite yarn and its individual components
Tensile properties of the individual components.
Results and discussion
Defining the number of complete breaks of the core component of the composite yarn
The composite yarn can break several times until the whole structure fails. This is because each component breaks completely more than once but without a full failure of the whole composite yarn structure. This is confirmed in the tensile load-elongation graphs of a previous study on variants. 13 To distinguish what is a complete component break from partial breaks happening to the constituent fibres/filaments of the components, the peaks corresponding to complete breaks in the tensile load-elongation graphs were considered as the important peaks. These important peaks mark the point at which the rupture of all constituent filaments (or fibres) occurs suddenly, sharply and completely with no partial breaks. Much smaller peaks are due to individual fibres or small bundles breaking and they are negligible. 16
Typically, the composite yarn has a general break pattern as shown in Figure 4. Such a break pattern is characterised by three zones: the first of which is where the core component breaks many times. The second zone is where, in most cases, the wrapping component breaks usually more than once but less than the core component. The third zone is where the undulating component breaks. The high number of breaks is usually of the core component followed by the wrapping component and finally the undulating component. The most important breaks are those related to the core component which allows the composite yarn structure to maintain its capability to carry the imposed tensile load even if the core component breaks more than once. In fact, in many occasions, the core component breaks eight times before any of the other components start breaking. The last break of the core component that should be considered in this respect is the complete break that marks a sudden drop in the total tensile load to approximately marginally higher than zero. Such a final complete break of the core component marks the point at which the whole yarn structure loses its loading bearing mechanics while the elongation of the whole structure increases sharply until the wrapping component starts carrying the tensile load acting on the structure, indicated by a sharp load peak. When the wrapping component breaks and fails, the undulating component starts carrying the load before failing and finally producing a complete rupture to the whole composite yarn. The synergy of the components is of particular importance because it can lead to higher breaking tensile forces of the structure than that of any of its individual components. General break pattern of the composite yarn suggested in this study.
Load-elongation graphs of the composite yarns made for this study
Examples of the load-elongation graphs are shown for each of the composite yarns, which highlight their unique breaking pattern behaviour. Figure 5 shows the load-elongation graphs for three specimens, sampled from the initial composite yarn, that is, yarn 0. This figure shows that each specimen had seven complete breaks before the final failure occurred in the core component. Beyond this failure, the load-bearing capacity of the whole composite yarn dropped suddenly while the elongation increased sharply until the yarn started carrying the load again by the wrapping component in synergy with the undulating component and the remaining segments of the broken core component. Figure 6 shows the tensile load-elongation graphs for the set of the eight composite yarns (from 1 to 8). This figure shows that the core component of each of the composite yarns breaks completely more than once. For instance, in the case of yarn 1, up to eight complete breaks occur. The number of complete breaks of the core component for the rest of the samples differs for two reasons. The first is the difference in properties between the input fibres as shown in Table 1, in particular the materials used and their form. The second is related to the composite yarn itself and its manufacturing conditions, in particular the overfeed ratio of the undulating component, the number of wraps of the wrapping component and the number of components. The numerical analysis of these differences is fully detailed in Part II of this study. Load-elongation graph for the initial composite yarn (specimens 1, 2 and 3 were sampled from the same initial composite yarn). Load-elongation graphs for the set of eight composite yarns; all these composite yarns are made using different materials, number of wraps, overfeed ratios and technological parameters of the hollow-spindle machine.
Gain in strength of the composite yarn at its first break in comparison with the break strength of its core component.
Where P1 is the theoretical tensile strength at break of the threads used for the core component but taken as individual components, P2 is the resulting average value of tensile strength of the core component within whole composite yarns due to the synergy of components and factors (these load values are obtained from Ref. 12 ).
In some cases, as it is shown in yarns 0, 1, 4 and 6, for every successive break of the core component, the breaking tensile force or load is higher than that of the previous break. Close observations during testing indicate that the composite yarns are being locked in succession by the wrapping yarn during loading, and it is this self-locking mechanism structure that makes it unique. This desirable behaviour does not happen with yarns, but only when the overfeed ratio used for the undulating component is low (<150%) and also the combined effect of the overfeed ratio and the number of wraps should make the undulations as small as possible. This case of increasing the carrying of the tensile breaking forces is superior to that of more complex textile structures which are proven to have fewer number of breaks. For example, in one study, a 3D warp interlock fabrics made with flax rovings had only two distinct breaks. 17
In the case of composite yarns,2–5,7,8 the wrapping component has broken before the undulating component. The broken/ruptured parts of the wrapping component were locked into the structure by the intact undulating component and the remaining parts of the ruptured core component. In the case of the composite yarns 1 and 6, the undulating component has broken once or twice, and this was concurrent with the breakage of the wrapping component. This suggests that the broken segments of these components were caught or locked in together by the remaining segments of the ruptured core component. Although important in their own right as yarns remain capable of carrying load, these last two situations are of less importance to the stress-strain properties of the final composite. As already discussed, the number of breaks of the composite yarn may change by altering the engineering characteristics of the structure beside the individual components properties. The impact of the overfeed ratio can be seen from the graphs. The highest number of eight complete breaks of the core component is observed at the lowest overfeed ratio of 110%, while low breaks of two to three at an overfeed ratios of 150%. Other factors that affect the number of complete breaks of the core component are presented in Part II of this study.
It is interesting to note that the multi-breaking pattern of the composite yarn is different from that of a typical ply yarn or spiral yarn (i.e. core component and wrapping component), and this is mainly due to having the third undulating component. In a typical ply yarn, such as a sewing thread, the number of breaks would be only two or three combined with isolated fibre breakages. 16 It is shown in one study that the tensile properties of similar three-component products made on the ring-spinning system indicated only two distinct breaks for the three-component wrap yarn and three-component bouclé yarn, while a three-component spiral yarn had only one complete break. 18 Since these three products were made by using only two components, only one complete break of the whole structure is achieved. This makes these types of product of less value to the composite industry.
Mechanical analysis of the progressive failure of the composite yarns and flexible matrix
In an ordinary FRPM composite made of unidirectional straight fibres, the stress-strain behaviour can be estimated using the stress-strain graphs for the constituent yarns and matrix as given in Figure 7.
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Generally speaking, and using classic mechanics, the properties of the final composite structure can be decided using the rule of mixture (ROM) as follows
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in equations (1) and (2). These two equations assume perfect contact between the fibrous reinforcement and the polymeric matrix. If the matrix is added to the reinforcement in perfect conditions, this assumption will stand valid. Schematic stress-strain curves for classical composite materials made using straight, brittle fibres and ductile matrix materials.
In the direction of fibres
While in the direction transverse to the fibre direction
The rule of mixture can be used to estimate the tensile properties of a flexible composite made using the composite yarn, assuming perfect contact between the matrix and the yarn. Typically, the first few of these breaks pertain to the core component, followed usually by one rupture or more of the wrapping component, and the last few breaks pertain to the undulating component, as shown in Figure 4. When the composite yarn is used to reinforce flexible plastics so as to make flexible FRPM composites, the situation is expected to be more complicated than above. This is because the multiple breaks may be translated into more than one break of the final FRPM composite. Two possible scenarios are expected. In the first scenario, which is the rarest, all the composite yarns making the flexible composite structure break approximately in the same manner and in harmony, that is, iso-phase or in-phase, as shown in Figure 8. In the second scenario, the breaks of the composite yarn are out-of-phase, that is, randomly phased, as in Figure 9. In both scenarios, the tensile properties of the final textile composite made using our composite yarn and a flexible thermoplastic matrix may be decided using equation (1). The use of the flexible thermoplastic matrix under perfect processing conditions would ensure a complete bonding between the composite yarn and the matrix, even under stretching. However, the use of an ordinary thermoplastic or thermoset matrix may not be as useful in the case of a flexible matrix. The first scenario of predicted tensile properties of flexible FRPM composite made using the composite yarn. The second scenario of predicted tensile properties of flexible FRPM composite made using the composite yarn.
In the first scenario, momently, as the core yarn breaks for the first time, there will be a quick, but not immediate, reduction in the load-carrying capacity of the whole composite yarn by more than 10%. The flexible matrix will then by exposed to a higher load than before by a break of the core component; thus, it will stretch rather than carry the load. Therefore, a reduction in the stress-carrying capacity of the whole composite structure is expected, as shown in Figure 8. This reduction in the tensile behaviour of the whole composite will continue for only a short period of time until the flexible matrix stretches to a level suitable for the composite yarn structure to self-lock and regain its capability to carry the load. This point is shown in Figure 8 as the lowest point of the first trough. This pattern of breaks of the whole composite structure may repeat several times following the first break of the core component. Additionally, the whole composite yarn structure will still be able to carry tensile loading until the final failure of its core component. Beyond this failure, the tensile properties of the whole composite structure would fall to a level not suitable for the intended end-use performance of the flexible FRPM composite. Though due to the breaking performance of the wrapping component and undulating component, as we have already explained, the whole composite structure would still be able to carry tensile loads. The benefit of this is increasing the engineering performance and safety factor of this flexible composite.
In the second scenario, the breaks of the composite yarn structure are out-of-phase, that is, randomly phased, as in Figure 9. This scenario is the most common situation and the expected tensile properties of an ultimate flexible FRPM composite would be marked as shown in Figure 9. These two possible scenarios are analogous to the scenarios shown in Figure 1(a) of a previous carbon/glass hybrid composites study.
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To put it in context, its deviation from the rule of mixture is presented in Figure 1(b). The addition of the undulating yarn as a third component is the reason for triggering multi-breaks of the core component in the case of the composite yarn presented in this study. This performance is not seen in any ordinary spiral yarns made of a straight component/yarn and a helically arranged component/yarn. The addition of a third wavy component in our case creates the right conditions for triggering a self-locking mechanism of the composite yarn structure and also for maximising the performance of this mechanism. The continuous interaction of the composite yarn with flexible polymeric matrices ensures that the flexible polymeric matrix keeps transferring the load to the yarn as it meant to do before any break. This means that the final composites will still be able to carry the load, and at the same time, it will keep extending without affecting the load-bearing capacity of the flexible composite. Further, when the combination of the composite yarn and matrix is appropriate, the load carried by the undulating component may increase after each break (as shown for yarn 0 in Figure 5 and also for composite yarns 1, 3, 6 and 8 in Figure 6). Therefore, the capacity of the final flexible composites to carry the load may also further increase.
Reinforcement architecture of FRPM flexible composites
The full reinforcement architecture for these composites can be made as a preform by assembling the composite yarns into the intended textile architecture using a suitable process; a 3D nonwoven manufacturing process using mechanical bonding with a thread is suggested for this purpose. Other relevant processes are those that use biaxial yarn non-interlacing or multiaxial stitch bonding systems, for example, a Malimo multiaxial stitch bonding machine which is also known as needling or assembling by sewing. 22 In such a nonwoven stitching route, the composite yarns are held in place, in layers and according to preselected orientations, by stitching with fine threads. Making a reinforcement architecture from the composite yarns this way is cheaper than the expensive braiding, weaving or knitting processes. When making the final composite architecture, the required processing temperature must not be higher than the melting or combustion temperature of the fibres in order to prevent the melt or burning of the fibres that make the composite yarn and the stitching thread. The resulting FRPM composites may be classified as either continuous intermingled hybrid composites 23 or even intrayarn hybrid composites. These technologies can be scaled-up easily to make large commercial parts.
Conclusions
This study introduces a new form of flexible textile reinforcement for the composite industry. Typically, this structure is made of three components. One component has an approximately straight configuration, called the core component; the second component has an undulating configuration like a sinusoidal wave, called the undulating component; and the third component wraps around the previous two components to bind them together, and, thus, it has a helical configuration, called the wrapping component. Typically, the materials used for the three components are different from each other in order to maximise the performance of the structure by means of the number of breaks. Nine variants of this composite yarn were made using the hollow-spindle system and using synthetic and natural yarns as components at various machine settings. The number of wraps of the wrapping component was in the range of 225 –350 wpm and the overfeed ratio of the undulating component was in the range 110–158%. Upon testing of nine composite yarn variants, using a tensile-testing machine, several breakages of the structure occurred prior to complete failure of the composite. The majority of these breaks are related to the core component, while a smaller number of breaks relate to the wrapping and the undulating components. The number of breaks of the core component was between 2 and 8, while both the wrapping and the undulating components broke between 1 and 3 times each. Consequently, the failing mechanism of this structure consists of three zones which promote the carrying of the load before failure longer than other composite structures. The useful breaks for carrying tensile loadings are those related to the core component in zone 1, while the other breaks are useful to delay the final flexible FRPM composite from failing, in zones 2 and 3. This will maximise safety by delaying failure and allowing parts to be replaced.
Interestingly, composite yarns 0, 1 and 6 experience a pattern of progressive breaks in which the breaking force increases following the first break of the core component, and keeps increasing until the final complete break of the core component after which a severe drop in the tensile load is experienced. It has been observed that the composite yarns are being locked in succession by the wrapping yarn during loading, and it is this self-locking mechanism architecture that makes this composite yarn structure unique. This desirable behaviour occurs when the overfeed ratio used for the undulating component is low (<150%). In this study, eight breaks was the highest number of breaks of the core component achieved, whilst two and three breaks occurred in the rest of the samples. Part II of this study is dedicated to maximise the number of complete breaks of the core component and to understand the effect of the factors influencing it.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
