3D Natural Fiber Reinforced Composites

Abstract

Three-dimensional (3D) natural fiber reinforced polymer composite materials are gaining popularity in the modern world due to their environment-friendly nature, lightweight, low cost, life-cycle superiority, and biodegradability. Natural fiber reinforced composites are widely used in various engineering applications, and this research area is constantly evolving. However, due to the inherent properties of natural fibers, researchers face numerous challenges in the development and application of natural fiber reinforced composites. These difficulties include fiber quality, thermal stability, water absorption capacity, and compatibility issues with polymer matrices. Ecological and financial concerns are stimulating new research areas in the field of natural fiber reinforced composites. Also, significant research has been conducted recently to improve the performance of natural fiber reinforced composites. This chapter explains the types of reinforcement and composite fabrication techniques used for 3D natural fiber reinforced composites. Also, discussed the literature survey of 3D natural fiber reinforced composites to examine the effect of binder yarn, stuffer yarn, and weave design on the mechanical performance of composites. Furthermore, 3D woven-shaped preforms and their associated composites are also discussed.

1 Introduction

Composite can be defined as the combination of two or more distinct materials in terms of both physical and chemical nature, these constituents cannot lose their identity, but enhance the overall performance of the composite material. Normally composite is designed for specific jobs such as high strength, lightweight, and resistance to corrosion or electricity. Composite materials showed better properties than their constituents in all perspectives, therefore, their applications are increased day by day with respect to conventional materials [1,2,3]. The main constituents of the composites are matrix and reinforcement. The functionality of the matrix is to protect the composite material from the external environment, dissipate the externally applied load throughout the structure, and keep the reinforcement together. While the contribution of reinforcement in the composite is to provide mechanical strength to the composite. Composite and its constituents are shown in Fig. 1.

Fig. 1

Composite and its constituents, reinforcement, and matrix [4]

The history of composite materials is very ancient, thousands of years ago people use composite materials, the first man-made composite was developed in Iraq by Mesopotamians in 3400 B.C. Ancient societies developed plywood from the combination of glue and wood strips, place the wood strips at one another and joined each layer with another through glue. The same concept was followed by Egyptians to develop death masks in 2181 B.C. from the combination of fabric (papyrus or linen) and plaster. With the time, to increase the strength of boats, mud pottery and bricks they use straw as reinforcement with mud [5]. Around 1200 A.D., the Mongols started to develop composite bows, which were extremely efficient during that period. Pine resin-bonded cattle tendons, bamboo, horn, wood, and silk were used to make these composites. Polymerization enabled synthetic resins to solidify after the industrial revolution because of the industrial revolution, synthetic resin was converted into solid form with the help of polymerization, this knowledge leads to the development of many plastics such as vinyl, polyester and phenolic. Leo Baekeland was the chemist who invented Bakelite [6], it is thermally and electrically insulator, in many industries it is used in the form of composite with other materials where thermal or electrical insulation is required. In 1930, with the invention of glass fiber, the application of composites is rapidly increased, first fiber reinforced industry was also developed in this era. Unsaturated polyester was patent in 1936, but still it is used in the composite industry. In 1960, the composite industry further boosts with the invention of high-performance fibers such as para-aramids. Successful application of composites in the Boeing 787 opened the doors for composites to use in high tech applications [7,8,9].

2 Reinforcement

Natural fibers, used as reinforcement in composites, are either in the form of 2D or 3D woven structures. Woven structures are dimensionally stable and having better mechanical properties as compared to knitted and non-woven structures, therefore, it is preferred to be used as reinforcement in the composites. The difference between 2D woven structures and 3D woven structures is the presence of yarns in the 3rd axis. 3D woven structures have a significant thickness due to the presence of yarns in the Z-direction. These yarns strongly joined the multilayers together and give more dimensional stability to the structure. To achieve a certain level of thickness in 2D woven structures, multiple layers are joined together, but these layers are independent of each other, therefore delamination can easily occur in 2D woven reinforced composites. 2D woven structures have poor through thickness properties due to absence of yarns in the Z-directions, as the layers are independent from each other so cracks can easily propagate in 2D woven reinforced composites. To address these issues researchers, prefer to use 3D woven structures as reinforcement in composites. 3D woven structures have better through thickness properties due to the presence of binding yarns [10].

Woven structures consist of two sets of yarns which interlace with each other at right angle. Properties of woven structures depend on the fiber’s properties, yarn spinning, and weave design. Woven structures can be classified on the basis of weave design, fabric dimensions, weaving technique and end product. A weave design explains the interlacement of warp and weft yarn. Woven structures are broadly divided into two types i.e., 2D woven structures and 3D woven structures.

2.1 Two Dimensional (2D) Woven Structures

2D woven structures or reinforcements have only two dimensions, length, and width while negligible thickness. In 2D woven structures only two sets of yarns interlace with each other to develop a structure as shown in Fig. 2. The warp yarns are normally denoted by X-axis while weft yarns are denoted by Y-axis. Interlocking of yarns give dimensional stability to the structure and improve the mechanical performance of the structure. 2D laminates showed lower resistance to delamination, cracking under low and high-speed impact loading due to their inferior interlaminar fracture toughness.

Fig. 2

Two dimensional (2D) woven structure

2.2 Three Dimensional (3D) Woven Structures

3D woven structures or reinforcements can be defined as the interlacement of three set of yarns i.e., warp, weft, and binding yarns. 3D woven structures have a significant thickness due to the presence of yarns in third dimension. 3D woven structures are also called multilayered structures, because it contains more than 2 layers which are joined together through binding yarn, the distance between two binding points is larger than the ground weave of multilayered structure. Figure 3 showed the 3D woven structure, the orange color yarn is the binding yarn while black and blue yarn is the warp and weft yarn in the structure respectively.

Fig. 3

Three dimensional (3D) or multilayer woven structure [11]

A 3D woven structure is formed of warp (0° direction) and weft (90° direction) stuffers that are bound together by a series of warp binders. By varying the binding pattern, different 3D woven structures are produced. The performance of woven preforms depends on the orientation of the binding patterns.

Multilayer 3D woven structures

 Multilayer 3D woven structures are classified as:

• Orthogonal interlock structures

• Angle interlock structures

• Hybrid (warp-weft) interlock structures

• Bidirectional interlock structures.

Orthogonal interlock structures

The orthogonal interlock structure is produced from the two sets of yarns, no special binding yarns were used in the orthogonal interlock structure. The warp yarns are used as binding yarn, to join multi layers together. In orthogonal interlock, z-direction yarns are drawn through the warp and weft yarns, intersecting the layers at a 90° angle. The yarns are interlaced homogeneously in each of the three planes to provide quasi-isotropic properties or an unbalanced amount in each direction when anisotropic properties are required [12]. Orthogonal 3D woven structures are further divided into two types, i.e., orthogonal through the thickness and orthogonal layer to layer. In orthogonal through the thickness structure, some of the warp yarns from the top layer move down towards bottom layer and joined all the layers, similarly some of the warp yarns from bottom layer move up towards top layer and joined all the layers that come in the middle. In this way only top and bottom layer is responsible for joining all the layers in through thickness structure. In layer-to-layer structure some of the warp yarns from the first layer interlock with 2nd layer, similarly some of the warp yarns from 2nd layer interlock with 3rd layer and so on. Figure 4 showed the cross-sectional view of through thickness structure while Fig. 5 showed the cross-sectional view of layer-to-layer structure.

Fig. 4

Orthogonal through the thickness multilayer interlock structure

Fig. 5

Orthogonal layer to layer multilayer interlock structure

Angle interlock structures

Angle interlock structure can be defined as, when the binding yarn move through layers in certain angle then the multilayered structure is called angle interlock multilayer structure. It is further divided into two types based on the depth of binding yarn. When the binding yarn moves from top layer and pass-through bottom layer then it is called angle interlock through thickness structure as shown in Fig. 6. On the other hand, if the binding yarns move within layers, then it is called an angle interlock layer to a layer structure as shown in Fig. 7.

Fig. 6

Multilayer angle interlock through thickness structure

Fig. 7

Multilayer angle interlock layer to the layer structure

An angle interlock structure usually has larger geometrical repeat unit as compared to the orthogonal interlock structure. The mechanical performance of a 3D woven structure is majorly depends on the pattern of binding yarn. Angle interlock structures have higher pliability and forming capability as compared to the orthogonal interlock structure, while in case of fiber volume friction in composite orthogonal interlock structure showed better results as compared to the angle interlock structure. In multilayered structure, the thickness, strength, and stiffness of 3D woven structures significantly depend on the properties of binding yarn. 3D woven structures showed better through thickness, mechanical properties as compared to 2D woven structures.

Hybrid interlock structures

In general, 3D woven structures (reinforcements) are made up of only one type of interlocking pattern. But in hybrid interlock structures, a combination of two, three or four basic types of 3D multilayer interlocking patterns i.e., orthogonal layer to layer, orthogonal through the thickness, layer to layer angle interlock and through thickness angle interlock, can be used. Similarly, hybridization can also be done by combining warp and weft interlocks in one structure, keeping in view the required properties. The specific sequence of yarns placement can also be achieved in both directions. 3D multilayer hybrid structure can be produced by combining both; different types of interlocking patterns and yarn materials keeping in view the cost and target properties. Since interlocking and straight warp (stuffer) yarns help to improve the in plane as well as out of plane mechanical properties of the 3D woven structures. Therefore, these (interlocking and stuffer) yarns could be added in different places of the woven structures to improve their properties. These 3D hybrid woven structures with different types of interlocking patterns can also be produced on conventional dobby/jacquard looms.

Bidirectional interlock structures

In warp interlock structure, as the name suggested that warp yarn is responsible for binding the layers together, while weft yarn remains straight. Therefore, in warp interlock structure, warp yarn has higher crimps %, while weft yarn has negligible crimps %. In case of weft interlock structure, weft yarn is responsible for interlocking of layers. Therefore, in weft interlock structure the crimp % in warp yarn is reduced as compared to the warp interlock structure, while weft yarn has higher undulation or crimp % due to interlocking of layers. Bidirectional structure is the combination of warp and weft interlock structure; therefore, it has maximum crimps % because both warp and weft yarn interlock the structure at alternative sequence. Figure 8 showed the cross-sectional view of bidirectional interlock structure. In bidirectional structure the crimp % in both warp and weft wise is almost same, therefore the mechanical performance of the structure in both direction is almost same. While in case of warp or weft interlock, the structure behaves entirely different in warp and in weft direction due to differences in the undulation of the yarn in both directions. Bidirectional interlock structure-based composites are used in high performance applications due to balance performance of composite in warp and in weft direction.

Fig. 8

Bidirectional interlock structure of a warp and b weft interlock structure [13]

3D shaped woven structures

Shaped weaving is a special type of weaving, in which certain shapes can be achieved as a final product such as V shape, T shape and H shape. In shape weaving warp and weft yarns were used just like in conventional weaving to develop shaped fabric. Shaped fabric can be single or multilayered depend on the required shape, V shape can be achieved through single layer, but T and H shape are only achieved through multilayered structures. 3D weaving loom can be used for shaped weaving, but due to some constraints such as cost, availability, productivity, and lack of training, industry and researchers prefer to use conventional jacquard or Dobby loom for the development of shaped fabrics. Figure 9 showed the cross-sectional view of T and H shaped structures.

Fig. 9

Shaped woven structures (H shape and T-shape) [14]

3 Composite Fabrication Techniques

Different types of fabrication techniques are used for the manufacturing of composites. Each fabrication technique has its own set of advantages. Some allow for enormous volumes of production in a short amount of time, while others have low capital cost and materials costs. When deciding which composite manufacturing technique would produce the greatest outcomes, an engineer must make an informed decision. Molding is used in most composite manufacturing procedures to form the resin and reinforcement into the desired finished shape. When selecting a composite fabrication process, keep in mind that not all methods are equally suited for all purposes [15]. 3D woven natural fiber reinforced composite fabrication techniques are discussed below.

3.1 Hand Layup

Hand layup technique is the most basic and convenient composite fabrication technique. It is mostly used for thermoset resin because it has lower viscosity so can easily penetrate through reinforcement without need of extra pressure or temperature. In this process reinforcement layers are placed in laminate stack, then thermoset resin is poured on it and distribute throughout reinforcement so that no end remains dry. Wet layup is a variation of this procedure in which the plies are covered with resin before being laid, and the stack is subsequently “debulked” or compressed. The hand lay-up technique was used in 3D woven natural fiber reinforced composites, because in this process the applied pressure is low so the reinforcement can maintain its 3D structure. Disadvantages of hand layup technique are, difficult to achieve finished surface, higher V_f cannot be achieved in hand layup technique [16]. Yasir et al. [17] developed green composites from four layered orthogonal layered to layered reinforcement with green epoxy as a matrix. The hand lay-up technique was used for the manufacturing of composite sample. 20 bar pressure was applied to the sample to remove the air gaps and achieve the constant fiber volume friction.

Kashif et al. [18] used the same hand layup technique to develop composites from jute-based 3D woven reinforcements. Layer to layer structure, through the thickness structure and combination of both structures (Hybrid) were used as 3D reinforcement, to study the effect of reinforcement on the mechanical performance of composite. Umair et al. [19] developed T and H shaped composites from jute reinforcement by using the Hand layup technique. Eight types of 3D structures were developed in the study, orthogonal layer to layer, orthogonal through thickness, angle interlock layer to layer and angle interlock through thickness. From the above literature it can be concluded that hand layup technique is very versatile technique, different types of 3D reinforcement can be used with thermoset resins to develop composites. Figure 10 showed the hand layup process of composite fabrication.

Fig. 10

Hand layup process of composite fabrication [20]

3.2 Compression Molding

Compression molding is used for both thermoset and thermoplastic polymers for composite fabrication. It is used for high scale production and the initial cost of stencil (die) is high but it can be used for long term so it can compensate the capital cost. In compression molding technique the reinforcement is sandwiched between resin and placed in the compression molding plates. Pressure and temperature are increased, resulting melting of thermoplastic resin and evenly distribution of the resin throughout the reinforcement. In case of thermoset resin, the compression molding decreases the curing time with the temperature and pressure, so the overall time of sample development is reduced. In case of thermoplastic resin, the main issue with thermoplastic polymer is high viscosity. In compression molding the applied temperature and pressure reduced the viscosity of the polymer and the pressure pushes the resin to penetrate throughout the reinforcement [21]. For 3D reinforcement the pressure is applied step wise to avoid the deformation of Z-direction yarns in the reinforcement. Imran et al. [22] developed composite sample for impact applications using para-aramid as reinforcement and PVB as matrix, while glass microspheres and silica micro particles were used as micro filler to further enhance the mechanical performance of composites. Composite fabrication technique used in this study was hand layup and then for curing and even distribution compression molding was used. In compression molding the temperature was set at 180 °C and the pressure of 2 tons was applied for 25 min. Umair et al. [23] developed green composite from green epoxy and 3D woven jute reinforcement with novel weaving designs. Novel weave design consists of orthogonal interlock, angle interlock, and hybrid structures. Green epoxy was applied through hand layup technique, and then for further curing and even distribution plates were placed in compression molding machine. The pressure in compression molding was 15 bar for 3 hours. Figure 11 showed the compression molding machine.

Fig. 11

Compression molding machine

3.3 Commingling Technique

Commingling technique is generally used for thermoplastic composites. Since, thermoplastic matrix based composites have higher impact strength, unlimited shelf life, can easily be reshaped, no need of extra time for curing process. But the main issue with thermoplastic resins is their higher viscosity and which hinders its penetration throughout reinforcement. To resolve this issue commingling technique is used. In this process both matrix and reinforcement are in the form of filament and weaving of these reinforcement and matrix filaments is done simultaneously. After applying temperature, the matrix yarn will melt and easily penetrate in the whole structure at low pressure [24]. Habib et al. [25] studied the effect of fabric structural design on impact and short beam shear (SBS) properties of Jute, hemp and flax fibers based thermoplastic composites. Three types of structures i.e., woven, woven commingled and knitted commingled were studied. SBS and impact tests were performed to evaluate the performance of developed samples. Polypropylene was used as matrix, the density of polypropylene is high so it is difficult to flow through structure, therefore commingled technique was used to resolve this issue and the results showed that commingled based sample showed improved SBS and impact strengths.

Feng et al. [26] used the same concept of commingling technique to form composite samples from pineapple leaf fibers (PALF) and kenaf fibers with polypropylene matrix. PALF and Kenaf both are cellulosic fibers so initially moisture was removed from the fibers to avoids the air gaps or voids issue in the composite. In the next step fibers are mixed with polypropylene fibers at different ratio ranging from 10 to 50%. After mixing the fibers pallets are placed in compression molding machine under 5 MPa pressure and temperature was set at175 °C for 8 min. High temperature melt the polypropylene fibers which joined the Kenaf and PALF fibers in the sheet. Commingling can be done at fibers, yarn and at fabric levels by mixing the reinforcement with thermoplastic matrix. Figure 12 showed the commingling technique for composite fabrication.

Fig. 12

Commingling process of composite fabrication at fiber stage in self reinforced composite [26]

4 Effect of 3D Woven Fabric Parameters on the Performance of Their Corresponding Composites

4.1 Effect of Z Yarn Stitching Density on 3D Woven Composites

Flax yarn of 38.5 tex was used to develop 3D woven reinforcement and EnviPOXY^® 530 (epoxy) was used as matrix. In this work four types of 3D woven structures were developed to investigate the effect of stitching point density on the mechanical performance of the composites. Each structure is divided into three sections, top section (TS), core section (CS) and bottom section (BS). Top and bottom section is orthogonal through thickness while core section is layer to layer as shown in Fig. 13. Repeat cross section of design consists of 12 warp and 60 weft yarns. Each section has two layers, while the core section or middle layer is responsible for joining top middle and bottom layers together. The binding warp yarn (binding point 1) which is present in 3rd layer (L3) of the repeat makes an interlocking point with 2nd layer (L2). This interlocking is referred as stitching or binding points.

Fig. 13

Schematic of weft wise cross section of 3D woven interlock sample (SP1) [27]

Some of the warp yarns in core section were used as binding yarn to stitch the CS layer with TS and BS. These binding points improve the delamination resistance in the structure. Sample SP1 has 3 stitching point in repeat cross section, while SP2 has 5 points, SP3 has 7 points, and SP4 has 9 stitching points in repeat cross section as shown in Fig. 14 [27].

Fig. 14

Schematic of cross sections SP2, SP3, SP4 and P (1/1 Plain) [27]

There was 36 ± 1 and 131 ± 2 threads per centimeter warp and weft yarns, respectively. The produced fabrics had an areal density of 611 ± 5 g/m² for all woven reinforcement samples i.e., SP1, SP2, SP3, SP4. While plain-woven (P) reinforcement had 12 ends/cm and 42 picks/cm giving an areal density of 200 ± 2 g/m². The warp and weft densities were selected with respect to limit of the loom and ease of manufacturing. It is important to note that in one cm² of the SP1, SP2, SP3 and SP4, total stitching points were 20, 33, 46 and 59, respectively. Four composites were produced from 3D interlock woven fabric. Three layers of plain fabric were used to fabricate composite sample for comparison purpose. Yarn architecture of 3D woven samples modelled on TexGen software are shown in Fig. 15.

Fig. 15

Yarn architecture of 3D woven samples modelled on TexGen [28]

Tensile strength results showed that the structure having minimum number of interlocking points, or a smaller number of crimps in the yarn has higher tensile strength, because the force dissipation capability of straight yarn is much higher as compared to crimped yarn. Crimp or interlocking points act as weak point because the force accumulate at that point so there is chance that the structure may collapse at that point. Out of four structure SP1 has minimum number of cross-linking points therefore it showed the highest tensile strength results as shown in Fig. 16. From SP2 to SP4 the cross-linking points gradually were increased. Therefore, the tensile strength is decreased from SP1 to SP4 samples. Sample P has the lowest tensile strength results which showed that 2D laminates has poor tensile strength as compared to 3D structures. In weft direction all the samples has higher tensile strength as compared to warp direction due to higher thread density in the weft direction [27].

Fig. 16

Tensile strength of 3D woven and laminated composite samples in warp and weft directions [28]

Similarly, tensile modulus of composites with more stitching points decreases as compared to the sample with low stitching point. SP1 has higher tensile testing results than plain woven sample (P), which showed that 3D woven reinforced composites has better tensile properties as compared to the 2D laminated composites. Furthermore, flexural strength was higher in weft direction due to higher thread density of weft yarns while increased from SP1 to SP4 composite samples as shown in Fig. 17 because SP1 has least number of stitches per cm². By increasing the number of joining points per unit area, the stitching yarns tightly hold the weft yarns together and make it more compact. Structure with higher compactness required higher force to bend it, therefore SP4 showed higher value of bending stiffness as compared to the structure that has lower stitching points per unit area. For example, when a structure with 4 stitching points is subjected to bending load, the yarns tend to open at 4 different points. While the structure having one stitching point resists at a single location only. Therefore, higher number of stitching points allow the structure to withstand higher out of plane bending load.

Fig. 17

Flexural strength of 3D woven and laminated composite samples in warp and weft directions [28]

Figure 18 showed the force versus time curves of 3D woven and laminated composite samples during drop weight impact test. The initial region of the peak shows the elastic behavior, i.e., force increasing with time. In this region, maximum force was showed by SP4 and minimum by laminated composite sample. The subsequent region in the curve represents plastic behavior where sample has undergone permanent deformation. SP4 showed the highest peak force while sample P showed the lowest peak force. From the samples it was observed that initially breakage was started in matrix followed by the yarn failure. The SP1 and SP2 structure failed soon after the first peak. Due to the absence of any stitching yarns in the laminated composite P, the curve decreased gradually in the plastic region showing the sample delamination. In case of composite SP4, maximum number of stitching points offered more resistance to delamination and ultimately failure [27].

Fig. 18

Force versus time curves of 3D woven and laminated composite samples [28]

4.2 Effect of Binder and Stuffer Yarns on 3D Woven Composites

Jute yarn having linear density of 278 tex was used to developed seven types of different 3D woven structures on dobby loom. Specification and cross section of each structure is shown in Table 1 and Fig. 19, respectively.

Table 1 Specifications of 3D woven reinforcement

Fig. 19

Cross sectional views of F1 to F6 woven reinforcement and schematic view of F7 woven reinforcement [23]

These seven types of 3D woven structures namely F1 to F7 were used as reinforcement and bio-degradable epoxy was used as matrix to fabricate composites. For composite fabrication hand layup technique was used and for further curing and consolidation the composite plates were placed in compression molding machine under 15 bar pressure for 3 h. Fiber volume fraction (V_f) was maintained at 32% ± 0.5. Figure 20a and b showed the curves of tensile stress against extension % of developed 3D woven reinforced composites in warp and weft directions, respectively. Weft wise results of all sample were higher as compared to warp wise due to higher number of yarns per unit length in weft direction as compared to warp direction. Overall, a quasi-linear fracture behavior was observed in both directions warp and weft directions in all composite samples. The only difference is the highest value of tensile strength and elongation % because each reinforcement structure has different interlocking pattern and weave design. The curves showed that in warp direction the samples reached to fracture point after the extension of 1.2–2%, similarly in weft direction the fracture point starts after 2–3% extension. As the force applied on the composite sample, initially the sample resists against it with slightly elongation then reached to brittle fracture as shown in Fig. 20.

Fig. 20

Tensile stress versus extension (%) curves of 3D woven composites a warp wise b weft wise [23]

In 3D natural fiber reinforced composites, the failure mechanism initiates with matrix cracks followed by start of fiber failure. Delamination cannot be observed due to interlocking in the 3D structure. The breaking point of composite is sharp like brittle material. Expect H3 sample, in all other structures the stitching yarns hold the structure by moving above or below the weft yarns. The undulation in the stitching yarn produced some crimps in the structure. While in case of H3 sample, the binding yarns are present in both warp and weft directions, which generates nearly equal crimps in both directions i.e., warp and weft. The results concluded that out of seven structures, OTT structures has the lowest crimps in the yarn therefore it has the highest value of tensile strength in both directions i.e., warp and weft. Figure 21 showed the fracture behavior of OTT and H3 samples.

Fig. 21

Fracture analysis of composite samples [23]

Figure 22a and b showed the flexural stress versus deformation % curves of composite samples in warp and weft directions, respectively. The results revealed that within elastic region, increase in stress is faster while deflection in the samples is lower. After attaining the peak value of flexural stress, sudden drop in flexural stress was observed in all samples in both directions. Except OLL, all other samples showed brittle behavior at the point of fracture in warp and weft directions. In case of OLL sample, strain hardening region was observed. Weft-wise flexural stress is higher as compared to warp-wise due to the higher number of yarns per unit length in the weft direction. ATT sample showed the highest flexural strength in weft direction while in warp direction ATT and ALL showed the highest and comparable flexural stress. Within hybrid structures, H1 showed the highest value of flexural stress and H3 sample showed the intermediate value while H2 sample showed the lowest value of flexural stress [29].

Fig. 22

Flexural stress versus deformation (%) curves of 3D woven composites a warp wise b weft wise [23]

Similar increasing trend of flexural modulus was observed in all composite samples. Weft wise flexural modulus was higher as compared to warp wise of all samples, it is due to a smaller number of undulations in the yarn in weft direction. Within the warp direction results it could be observed that ATT sample showed the highest value of flexural modulus. While in hybrid structures H1 showed the highest value of flexural modulus comparable to ATT sample. In weft direction the results revealed that ATT reinforced composite sample showed the highest value of flexural modulus. Within hybrid structures, H3 showed the highest value of flexural modulus in weft direction.

The impact performance of all seven developed composites were tested against 3 and 6 J drop weight impact energy, Fig. 23a showed the impact results of composites sample under 3 J, while Fig. 23b showed the impact performance of composite samples under 6 J impact. Figure 23a depicts that within basic four 3D woven structures, ALL sample showed the highest value of force against displacement as compared to ATT, OLL and OTT. It is due to the angular movement of binder yarns in the structure, which gave more resistance to the structure against the free fall of impact mass. Within angle interlock the float length of binder yarn also significantly affect the impact performance of the structure, binder yarn with shorter float length in angle interlock resist more to the impact force as compared to longer float binder yarn in angle interlock or truly orthogonal structure. Therefore, ALL composite sample showed higher impact force as compared to ATT based composite at similar displacement. Whereas in case of orthogonal structures, OTT sample showed better results as compared to OLL. Minor cracks were observed in all composite samples [29].

Fig. 23

Force versus displacement curves of 3D woven composites a at 3 J b at 6 J [23]

In comparison of hybrid structures-based composites, H3 showed the highest value of force at maximum displacement, while H2 showed the lowest force at maximum displacement. The float length of binder yarn in H3 is minimum, therefore it has higher impact force as compared to H1 and H2 samples. Impact force against maximum displacement of H3 is comparable to ALL sample.

Similar behavior to bear impact force was observed in case of 6 J impact force test as in case of 3 J impact test. But overall maximum force value against displacement is higher in case of 6 J as compared to 3 J impact test. Within four basic structures, ALL showed the highest value of impact force against displacement as compared to rest of three basic structures. Within hybrid structures-based composites samples, H3 showed highest value of impact force against displacement.

Optimization of multilayered woven structures for enhanced mechanical performance

Nine (09) different types of orthogonal interlock 3D woven structures were developed on dobby loom. Jute yarn having linear density of 8 lb/spindle was used to develop all four layered orthogonal interlock structures. 3D woven structures with sample notation and description are mentioned in Table 2 and cross-sectional view of each design is shown in Fig. 24. Nine samples are divided into three groups A, B, and C. group A contains the sample of orthogonal layer to layer with warp, weft, and hybrid interlocks, similarly group B consists of orthogonal through thickness samples with warp, weft, and hybrid interlocks, and group C consists of Hybrid weave design with hybrid interlocking patterns.

Table 2 3D woven samples description and notation

Fig. 24

Cross-sections of 3D woven preforms (i) S1 (ii) S2 (iii) S3 (iv) S4 (v) S5 (vi) S6 (vii) S7 (viii) S8 (ix) S9 [18]

Tensile strength results of all three groups of 3D orthogonal interlock structures are highlighted in Fig. 25. From the results it was concluded that by increasing the crimp % in the structure, the tensile strength decreases. There is inverse relation between crimp % and tensile strength. It is due to poor force dissipation throughout the length in crimp or undulated yarn as compared to straight yarn. From group A results, it was observed that warp interlock structure showed lower tensile strength in warp direction as compared to weft interlock layer to layer structure. It is due to a smaller number of undulations in weft yarn as compared to warp yarn in the structure. Hybrid structure of group A showed the average value of tensile strength as compared to warp and weft interlock structure. Similarly in group B, same trend of tensile strength was observed with through the thickness interlock samples. The structures that have higher undulations showed lower tensile strength. Warp interlock structure showed lower tensile strength as compared to weft interlock structure in warp direction. In case of weft interlock structure, the tensile strength is lower in warp direction as compared to weft direction, while undulation is higher in weft direction, it is due to higher tension on warp yarn during weaving process, which effect its mechanical properties therefore it has lower tensile strength. In group C, the lowest tensile strength was shown by LL/TT (wf/wf), it is due to highest number of undulations in the structure [18].

Fig. 25

Tensile strength of 3D woven structures in warp and weft directions [18]

Figure 26 showed the elongation % of developed 3D orthogonal interlock woven structures. Elongation in the structure depends on the undulation of the yarns in the structure, higher the undulation in the structure higher will the elongation in the structure. Because after applying force on it, the crimp part of yarn tries to become straight hence the resultant length of the yarn increases, ultimately the elongation % also increases. In group A the highest value of elongation was shown by LL-hybrid structure, because in this structure both warp and weft yarns interlock the structure, resulting higher crimp % in the structure. Group B consists of the structures that have through the thickness interlocking, as the binding yarn moves from top layer to bottom layer of the structure and the binding yarn faces higher tension as compared to ground weave yarns. After releasing the fabric from loom, the interlocking yarns become more relax than the ground weave yarns, due to the release of tension applied by the loom parts during weaving process. In group B TT-wf structure showed higher elongation % as compared to other structures due to lobular effect in the structure. In group C, hybrid structure of LL/TT-wp/wf showed the highest elongation due to higher number of crimps in the structure, because both warp and weft yarns interlock the structure, and secondly the changing of weave from LL to TT creates more float, therefore it has higher elongation % as compared to rest of the structures [18].

Fig. 26

Elongation % of 3D woven structures in warp and weft directions [18]

Figure 27 showed the tensile modulus of developed samples in warp and in weft directions. Tensile modulus has inverse relation with the elongation % in the structure. It showed that the structure having higher crimp % will have lower modulus, because after applying force on yarn, then crimp in the yarn tends to become straight, as a result the length of the yarn increases. Therefore, it will show higher strain % and lower modulus.

Fig. 27

Tensile modulus of 3D woven structures in warp and weft directions [18]

Figure 28 showed the stiffness results of developed orthogonal interlock structures. Stiffness in the structure indicates that the structure cannot easily be molded or shaped. In group A, the results showed that warp interlock layer to layer structure showed the highest stiffness value while weft interlock layer to layer structure showed the lowest stiffness. In group B, warp interlock through the thickness structure showed the highest stiffness value not only within group but it also showed the highest stiffness value among all nine samples. In group C both hybrid structures i.e., LL-TT-wp and LL-TT-wp-wf/wp-wf have comparable values of stiffness [18].

Fig. 28

Stiffness of 3D woven structures [18]

4.3 Effect of Weaving Patterns on Damage Resistance of 3D Woven T and H Shaped Reinforcements

Multi-layered T and H shaped structures were developed on conventional dobby loom, using Jute yarn in both warp and weft directions. Linear density and tenacity of jute yarn was 490 tex and 5550 cN/tex, respectively. Each shaped fabric consists of four-layer structure. Both T and H shapes were developed from four basic multilayer weave designs i.e., orthogonal layer to layer interlock (OLL), angle interlock layer to layer (ALL), orthogonal through the thickness interlock (OTT) and angle interlock through the thickness (ATT). Total 8 samples were developed, all the samples have same warp and weft density i.e., 10 yarns/cm in both directions. Figure 29 showed the cross-sectional views of shaped fabrics while Fig. 30 showed the top view of developed shaped fabrics [19].

Fig. 29

Cross-sectional view of T and H shaped fabrics [19]

Fig. 30

Top view of developed T and H shaped fabric [19]

Figure 31 showed the sample dimensions for peel test, schematic presentation of test and the real time testing instrument used for peel strength test of developed shaped fabrics [19].

Fig. 31

T-shape sample dimensions for peel test, schematic presentation of test and real-time testing instrument for peel test [19]

Figure 32 showed the force versus deformation curves of OLL shaped structures during peel test. This curve is divided into three zones: A zone denotes elastic zone, B zone denotes plastic zone and C zone denotes fracture zone. Real time images of sample during testing are also represent the respective zones as shown in Fig. 32. In the elastic region of the curve, it was observed that the interlocking yarn resist to the applied force, because with the increase of force the deformation is minimum in this region. At initial stage, the applied force is utilized to straighten the interlocking yarn in T-shape at the point of joint instead of breakage. Due to straightening of yarn the deformation in the structure is increased. Within elastic region, some peaks were observed it may be due to fiber failure at T-joints in the structure [19]. In zone B of the curve, plastic deformation starts. Further increase in the force initiates the failure in the interlocking yarns of the structure. In zone B of the curve maximum resistance of interlocking yarn was observed against applied force then a cluster of yarns is formed in the middle of the T-shape. In the OLL structure, interlocking yarns stitched the consecutive layers at different positions. The repeat size of interlocking yarn is different from the ground weave of the structure. During peel test only interlocking yarns start to break while the warp yarns of ground weave start to form a cluster in the middle and offer further resistance to the applied force. In the necking region of the structure, only weft yarns remain unbroken while warp yarns accumulate in the form of the cluster in the middle. While in the fracture zone complete failure of the structure is observed. In this zone the applied force was rapidly reduce because the structure shows no significant resistance to the applied force.

Fig. 32

Force versus deformation curve of OLL shapes [19]

Figure 33 showed the force versus deformation curve of orthogonal through the thickness structures of T and H shapes. OTT curve also showed three zones like OLL shapes. The elastic region of OTT structure is steeper than OLL structure, it showed that the interlocking yarns in OTT resist more to deformation as compared to the interlocking yarns in OLL structure. Small deformation at high force is observed in elastic region in case of OTT structure, due to the interlocking pattern of stitching yarns. In OTT structure the stitching yarns bind the complete structure in the thickness direction, so there is limited space for the structure to deform under applied force. In elastic region a small peak was noticed, which indicates the initiation of breakage in the interlocking yarns. In plastic region the maximum resistance to applied force was showed by the interlocking yarns after the breakage of stitching yarns. Almost all interlocking yarns are broken in the plastic region and cluster is formed in the middle of the joint T as shown in the Fig. 33. In the last zone of the curve, complete fracture of the structure is observed [19].

Fig. 33

Force versus deformation curve of OTT shapes [19]

Figure 34 showed the force versus deformation curve of shaped structures developed from angle interlock layer to layer structure. In the elastic region the curve is sharp due to the high resistance of interlocking yarns, while deformation is due to straightening of interlocking yarns. In this region the interlocking yarns become straight in the response to applied force, while breakage of the yarn starts in plastic region. In plastic region the maximum resistance against the applied force is observed which is called peak force, after that the force reduces due to breakage or pull out of interlocking yarns from the structure. In ALL structure, the interlocking yarns stitched the consecutive layers at certain angle to achieve this angle some weft yarns were skipped, therefore during applied force the stitching yarn can easily be pulled out instead of breakage. Consequently, it showed high deformation up to 300 mm. The fracture zone showed the complete failure of the structure and the force was rapidly reduced [19].

Fig. 34

Force versus deformation curve of ALL shapes [19]

Figure 35 showed the force versus deformation curve of angle interlock through the thickness structure during peel test. ATT curve indicates that in elastic region, sharp curve behaviour was observed in the elastic region between force and deformation. It is due to the high resistance offered by the interlocking yarns in the structure. Further increase in the force starts deformation in the structure which is due to the straightening of stitching yarns. In the elastic region some small peaks were observed, which indicates the fiber failure in the interlocking yarns.

Fig. 35

Force versus deformation curve of ATT shapes [19]

Plastic region of the curve indicates that structures start to deform with the increase in the applied force. When the interlocking yarns become straight, peak resistance is observed to the force, after that instead of straightening of yarn failure starts. Since, resistance is only showed by the interlocking yarn, therefore the ground weave warp yarns form a cluster in the middle of the joint T. After peak force, the applied force reduces while deformation in the structure increases which is due to the slippage and breaking of interlocking yarns. In the fracture zone, complete failure of the structure was observed [19]. Figure 36 showed the relation between work done and deformation of different 3D woven shapes. It can be concluded that OLL structure showed the highest work done before failure, while Structure OTT showed the minimum work done before complete failure [19].

Fig. 36

Work done during failure of different 3D multilayer shapes [19]

4.4 3D Woven-Shaped Preforms and Their Associated Composites

Six layered shaped structures were developed from Jute yarn having linear density of 47 tex. The structures were developed on dobby loom with same warp and weft density. The interlocking patterns used to develop shaped structures were through the thickness and layer to layer. Hand layup technique was used to developed shaped composites. Unsaturated polyester resin was used as resin with 0.1% concentration of cobalt as initiator, and 1% concentration of methyl ethyl ketone per oxide as accelerator [14]. For comparison purpose 2D laminated shaped composite was also developed. Figure 37 showed the highlighted portions of the developed samples for testing and compared its results with laminated composite. Table 3 showed the peel off test results of developed samples [14].

Fig. 37

T and H shaped composite samples produced using 3D woven preforms [19]

Table 3 Mechanical properties of T and H shaped composites

Table 3 showed that orthogonal layer to layer interlock T shaped (S1C) and orthogonal layer to layer interlock H shaped (S3C) have the same mechanical performance. Because both samples have same type of 3D reinforcement, fiber volume friction and curing cycle. Same trend was observed in the case of S2C (T) and S4C (H) samples. Because both samples have orthogonal through the thickness reinforcement with T and H shapes, respectively. While sample S5C was developed with 2D laminated reinforcement. At reinforcement level, all structures showed higher elongation, but after conversion into composite the resin restricts the elongation of the samples. In sample S5C the applied force generate cracks in the matrix and delamination occurs in the composite sample, because in reinforcement there is no stitching point in the structure, therefore it showed minimum resistance [14]. Figure 38 showed the elastic modulus and breaking load results of developed shaped composite samples.

Fig. 38

Comparison of young’s moduli and breaking loads of T and H shaped composites [19]

The results showed that 3D shaped composite has a higher value of modulus, maximum load and stress at peak load as compared to laminated composite at equal fiber volume friction. Because the laminated structures have no stitching points within layers, therefore layers can easily separate from each other can causes failure in the whole structure. In 3D shaped structures LL showed better results as compared to TT shaped composites [14].

5 Challenges

Although natural fibers are derived from renewable sources, and polymer composites based on natural fibers are sustainable as compared to synthetic fiber reinforced polymer composites. But there are several drawbacks associated with the use of natural fibers as reinforcement in the composites. These drawbacks include raw fiber quality variation, excessive moisture absorption, and thermal stability [30, 31]. Natural fibers have high moisture absorption rate, which is their main disadvantage. This decreases the interfacial connection between the polymer matrix and the fiber causing deterioration of mechanical properties. The high moisture sensitivity of lingo-cellulosic fiber results in dimensional instability [32, 33] and limiting their usage as reinforcement in composite materials. In many circumstances pre-treatment of the fiber surface or the introduction of a surface modification during composite preparation is necessary to overcome this difficulty and to increase the fiber-matrix adhesion. The impact of various types of chemical treatment on the physical and mechanical properties of natural fiber reinforced composite has been studied extensively in the literature [34,35,36]. Different fiber treatments result in considerable increase in tensile and flexural, strength and modulus. The chemical treatment of fibers can significantly minimize moisture absorption. Chemicals such as alkali (sodium hydroxide), isocyanate, KMnO4 (permanganate), CTDIC (cardanol derivative of toluene diisocyanate), peroxide, enzyme, and others have been used for the treatment showed significant changes in the mechanical and physical properties [12].