Weave technologies and impact performance of varied shaped cellular textile composites

Abstract

Cellular structural composites have been widely used in aerospace, radar satellite, rocket missile, automobile, ship, rail transit, construction, and packaging engineering etc. Textile reinforced composite is a niche market of the composites family with its advantages of light weight and good mechanical properties. In this article, textile reinforced cellular composites are fabricated and tested under low velocity impact to investigate their energy absorption performances by experimental method. Cellular structure fabric with internal tunnels of triangle, hexagon, trapezoid, and mixed shapes are weaved and consolidated by epoxy resin. The cellular structure composites are impacted under low velocity impact and it is verified that the four types of composites are show different impact performances. The triangle cell composite is sensitive to the out-coming penetration, while the trapezoid cell material is flexible to deform to absorb the energy. The other two structure cell composites are easily damaged by the output impact. The outcomes of this research study provide experimental results for the future investigation of similar composite materials.

Keywords

Textile reinforced composite cellular structure fabric internal structure low velocity impact

In nature honeycombs are stacked by hexagonal cells with consistent simple structure and stable performance.¹ Research studies on honeycombs have verified that, compared with other materials, honeycomb structural materials have greater structure integrity, good impact resistance, high specific strength, heat insulation, and heat dissipation performance.^2–5 It is reported that fiber reinforced honeycomb composite impregnated with resin, ceramics, or other polymers as matrix, grants further physical performance such as being lightweight and corrosion-resistant.^6–8 Cellular material is easy to design in its cross-section direction and, by adjustment, the cross section can be in the hexagon, square, triangle, and other shapes. Furthermore, if the cellular composite is produced by textile technology, the problems of delamination layer by layer inside the cellular structure can be resolved.⁹ From the point of view of mechanics, cellular material can offer a larger bearing capacity with less material consumption compared to other materials.^10–12 When a cellular structure is subjected to a load perpendicular to the plate surface, its bending stiffness can be comparable to that of a solid plate with the same material, and even has higher superior performance.^13–17 Related to its application, cellular materials are often subjected to low velocity impact^18,19 and their energy absorption and fracture performances have become one of the most important indexes in the field of composite materials research.^20–22

This article is aimed to design, fabricate, and test the textile reinforced cellular structure composites with different cross sections under low velocity impact and the results will be analyzed for future applications. Four three-dimensional cellular woven fabrics with triangle, trapezoid, hexagon, and rectangle shapes were designed and weaved according to the weave plan. These fabrics were used as preforms and resins were impregnated as matrix. The samples were placed under low velocity impact test under 25 J energy level. The results indicate that the sample with a trapezoidal shape can absorb more energy with less structure damage than other composites.

Design of cellular structure fabric

Generally, cellular structure fabrics are created based on the multi-layer weaving principle.²² Multi-layer woven structures consist of two or more layers of independent structures, which can be stitched together by the weave design wherever needed. In conventional multi-layer structures, stitches are created uniformly in order to form a structure that is solid and strong. In the case of integrated textile cellular structures, the adjacent fabric layers are weaved and separated according to the specified cross-sectional shape. It can be opened up to a hollow structure with tunnels formed between the adjacent layers. By changing the tunnel's cross-sectional shape and direction, a variety of cellular structures can be constructed.

Cross-sectional shapes

The cross section of a cellular structure can be designed to have some simple geometrical shapes. Based on the consideration of structural integrity, fabric continuity, and the topological structure, the practical cross-sectional shapes of tunnels are generally selected to be triangle, rectangular, trapezoid, or hexagon.

Triangle

Zhu and Han²³ developed a multi-layer structure with a triangular cross section by stitching the intermediate layer to the top and bottom layers alternately as shown in Figure 1. Each solid line of Figure 1 represents a section of fabric. The advantage of such cellular structures is that they have flat top and bottom layers, and different sections may have different lengths.

Figure 1.

Triangular cross section.

Hexagon

Gong et al.²⁴ patented a honeycomb woven structure of which the possible cross section is shown in Figure 2. The cross-sectional shape is hexagonal, and each adjacent fabric layer can be woven together to form one or two layers according to the designed cross-sectional shape. It is clear that this type of hollow fabric will not give even top and bottom surfaces, unless some special measures are taken.

Figure 2.

Hexagonal cross section.

Trapezoid

Cellular structures can be made to have trapezoidal cross sections that are shown in Figure 3. This kind of structure, similar to the one with triangular cross-sectional shapes, has flat top and bottom surfaces.²⁵ The trapezoidal shape is very flexible and it can be changed into a triangle or rectangle by adjusting the angle and length of the sides, as shown in Figure 4. It allows many levels of cells and different sizes and shapes of cells to provide the required structural and mechanical properties, limited only by the loom capacity.

Figure 3.

Trapezoid cross section.

Figure 4.

Rectangle cross section.

Mixed shape

The above-mentioned cellular woven structures consist of only one type of cell shape. A more complex cellular structure can be constructed by combining several cross-sectional shapes as shown in Figure 5.

Figure 5.

Hexagonal and triangular or rectangular combined cross section.

Creation of weaves

Tunnel direction

According to the weave principle, by changing the stitching pattern between the two adjacent layers, tunnels can be formed in any direction in the fabric plane.¹³ If the adjacent layers are stitched continuously along the weft/warp direction, tunnels will be created in the weft/warp direction. Using more complex stitching patterns, two or more interesting tunnels can be formed simultaneously. Although all cellular structures are achievable based on the weaving principle, the present discussion will be confined to those structures whose tunnels have uniform cross-sectional shapes and those where the tunnels run only in the weft direction. Such structures can be fully defined by their cross-sectional shapes.

Triangle

The multi-layer technique is used in the creation of the cellular structure fabric. Connecting the adjacent fabric layer at specific places with the given length forms a network of fabric layers in the cross section. When the fabric is opened up in the thickness direction, a triangle structure is created. This is illustrated in Figure 6(a), where the cellular structure with two triangle cells in its horizontal column is made from five layers of fabrics. In this research study, the plain weave is chosen for the single fabric layers because of its advantages such as simplicity, structural integrity, and good acceptance by the technical end users. Figure 6(b) illustrates the weaves used for manufacturing a complete unit cycle of fabric. In a complete cycle, the number of warp yarns is 10, and the number of weft yarns is 83. Cotton yarn is used to weave the fabric with its linear density of 102 tex, the warp density for the fabric is 7/cm. By this design, the top length of the triangle is 1.7 cm, if opened up with a 60° opening angle, the height of the single triangle is 1.47 cm.

Figure 6.

Design of triangle cellular structure fabric: (a) cross-section view along warp direction and (b) weave of the fabric design.

Rectangular

In the rectangular-shaped cellular structure fabric, adjacent layers are connected at intervals in order to achieve the hollow effect. There are four layers of warp yarns to form the fabric, three layers of weft yarns are arranged from top to bottom, interlaced with warp yarns to form an integrated cellular structure. The basic weave for each fabric layer is also plain weave. In a complete cycle, the number of warp yarns is 10, and the number of weft yarns is 60. The yarn is cotton with its linear density of 102 tex, and the warp density is set up as 7/cm. If open up as shown in Figure 7, the top line of the rectangle is 1.5cm and the height is 1.5cm too.

Figure 7.

Design of cellular structure fabric: (a) cross-section view along warp direction and (b) weave of the fabric design.

Trapezoid

Five layers of warp yarns are designed to fabricate the geometric structure as trapezoid inside the fabric (see Figure 8). The length of the trapezoid top is 3.5 cm and the bottom length is 1.75 cm, the height of the trapezoid is designed as 1.5 cm. To manufacture this kind of cellular structure fabric, the number of warp yarns in a weave cycle is 10, and of the weft yarn is 149.

Figure 8.

Cross-section view along the warp direction with trapezoid cross section.

In a complete cycle, the number of warp yarns in the weave cycle R_j = 10, and the number of weft yarns in the weave cycle R_w = 149. The fabric warp weave cycle diagram is shown in Figure 8. It passes through 7 roots/cm. The hole is trapezoidal: the upper side is 1.7 cm long, the lower is 3.5 cm, and the height is 1.5 cm. (see Figure 3).

Figure 9.

Cross-section view of triangle-hexagon mixed shape cellular fabric.

Hexagon

One repeat of the hexagon structure can be divided into four regions, regions I, II, III, and IV, as shown in Figure 10. Region I corresponds to the section of the hexagon structure where the adjacent layers join together at an alternate interval; Region II where the fabric layers are all separated from each other; region III is again the joining section but the joining layers are different from those in region I; region IV is the same as region II. Because of the nature of weaving, the cellular fabric is woven with all cells flattened as indicated in Figure 11, and the cellular structure is achieved when the fabric is opened and consolidated as shown in Figure 12.

Figure 10.

Cross-section view along the warp direction with hexagon cross section.

Figure 11.

Cross-section view of rectangle-hexagon, trapezoid-hexagon mixed shape cellular fabric.

Figure 12.

Cellular structure textile composites with different internal structure.

In the complete weave recycle as shown in Figure 10, where the number of warp yarn is 10 and the number of weft yarn is 160 leads to the length of the hexagon wall is 1.15 cm, if opened up by 120° opening angle, the height of the single hexagon cell is 2 cm.

Mixed shape

The principle for weaving cellular structure fabric with mixed geometric shape is the same as the one to weave a single shaped fabric as mentioned above. For example, to design a fabric with triangle and hexagon cross sections mixed as shown in Figure 9, the cross-section view of warp yarn is illustrated in Figure 9. Ten layers of warp yarns are involved in fabricating this structure. The top and the bottom part of the structure is triangle shapes which are constructed by warp yarns interchanged with single weft yarns. The middle part of the structure if opened is a hexagon shape, where the warp yarns are interlaced with weft yarns at designed intervals. Similar to the triangle-hexagon mixed shape, the cellular fabric with rectangle-hexagon or trapezoid-hexagon mixed shapes can also be successfully designed and manufactured (see Figure 11)

Fabric consolidation

The yarn to weave the multi-layer fabric is cotton (90 tex) with the yarn twist 23.8 turns/10 cm. To open up the fabric as the structures have been designed, vacuum assisted resin transfer molding (VARTM) technology is adopted to consolidate the cellular structure textile composite. The matrix is epoxy resin mixed with hardener, and the mix to mass ratio is 2:1. Liquid paraffin wax is filled into cardboard, which is folded in the same shape as the triangle, rectangle, trapezoid, and hexagon. After the wax is cooled down, this is used as support to insert into the cellular fabric for opening the structure. The samples are produced by the VARTM process (see Figure 13) and final products are shown in Figure 12.

Figure 13.

Vacuum assisted resin transfer molding (VARTM) process to consolidate the samples.

Low velocity impact test

Test method

The drop weight impact test was carried out by impact tester according to the Chinese test standard GB11548. The total weight of the impact object is 10 kg. The impact energy of 25 J was used to investigate the specimen structure deformation. During the impact, force sensors and data acquisition equipment were used to record the velocity and impact force-time curve at the moment of impact. According to the curve, the absorbed energy vs time curve and load vs displacement curve were derived. The derivation method was as follows:

When the data collection interval is very small, it can be assumed that the impact force changes linearly during the collection interval. Assuming that the impact force v F_i at time is t_i then according to the impulse theory, the impactor velocity v_i at time t_i is as follows.

v_{i} = v_{i - 1} - \frac{F_{i} + F_{i - 1}}{2 m} \cdot (t_{i} - t_{i - 1})

(1)

at time t_i, the impactor displacement d_i is as follows:

d_{i} = d_{i - 1} + \frac{v_{i} + v_{i - 1}}{2} \cdot (t_{i} - t_{i - 1})

(2)

at time t_i, the specimen energy absorption E_i is as follows:

E_{i} = E_{0} - \frac{1}{2} m \cdot v_{i}^{2}

(3)

inside the above equations (1)–( 3), where m is the total mass of the impactor include the impactor head and main body; E₀ is the initial impact energy; the initial impact velocity is calculated by

v_{0} = \sqrt{\frac{2 E_{0}}{m}}

; the impact force F_i and timing t_i are recorded by the data acquisition equipment.

A few of the cellular structure textile composites with different cross sections were prepared for the low velocity impact test. The test equipment was a low velocity drop weight hammer (DIT302E) produced by Shenzheng Wance Ltd. The specimen was clamped by two fixtures, and the hole in the middle of the fixture was 12.7 mm in diameter. The scheme of the specimens is listed in Table 1.

Table 1.

Scheme of the test specimens

Cross-section	L (cm) × W (cm)	Quantity
Triangle	10 × 10	5
Trapezoid	10 × 10	5
Hexagon	10 × 10	5
Rectangle	10 × 10	5

Discussion of the test results

Impact force performance

Figure 14 shows the impact force-time history curve of hollow structure textile composites with different structures at 25 J energy. It can be seen that the impact force-time curves produced by materials with different unit structures are completely different. Figure 14(c) of triangle shaped cellular composite is a typical impact force-time history curve. In the initial stage of impact, the impact force increases linearly with time, and the specimen stiffness changes significantly when the characteristic impact force (F₁) is reached. As the impact force continues to increase, the structure is constantly damaged, and the stiffness of the specimen decreases dramatically after reaching the maximum impact force (F_m), at which time fiber fracture occurs inside the specimen. Therefore, the time of large deformation and fiber fracture of the specimen can be determined by the two characteristic impact forces (F₁ and F_m) of the impact curve. In addition, due to the elasticity of the hollow structure material, the impact head will rebound during the descent, so the negative value in the curve read means the change of the direction of the force, which is reflected in the hexagonal and trapezoidal structure.

Figure 14.

Impact force curve of different shaped cellular composites. (a) Triangle shaped cellular composite; (b) rectangle shaped cellular composite and (c) trapzoid shaped cellular composite and (d) hexagon shaped cellular composite.

It can be seen from Table 2 and Figure 14(a)–(d) that the impact force (F₁) required by the initial large deformation of the hollow structure textile composites of the four structures is significantly different under the impact energy of 25 J. The maximum impact force (F_m) produced by the punch also varies. Under the impact of the weak impact force of 0.04 kN, the triangular structure material immediately appears in response, the structure changes, and an inflection point appears on the curve of the force value. When the impact force reaches about 0.34 kN there is a sharp decline, at this time the material internal fiber fracture occurred. It can be seen that in the case of high energy, the triangle structure is more sensitive in the reaction to the punch, and it is very easy to produce initial structural mutation and fiber fracture.

Table 2.

Two characteristic impact forces (F₁ and F_m) of the impact curve

	F₁ (KN)	F_m (KN)
Triangle	0.04	0.34
Rectangle	0.18	0.66
Trapezoid	0.41	1.21
Hexagon	0.84	1.46

Compared with the triangular structure material, the impact force required by the initial structural mutation of the rectangular structure material (F₁) is 0.18 kN, indicating that the rectangular structure composed of four nodes needs greater impact force to produce initial large deformation itself. However, the impact force (F_m) required for the fiber fracture caused by the punch to the rectangular structure material is about 0.66 kN, which is twice that of the triangular structure. In the triangular and rectangular structures, the impact curve does not appear to show negative value, indicating that the punch is not bounced, which indicates that under the energy impact of 25 J, once the punch causes fiber fracture, the penetration phenomenon of the material will occur quickly.

As can be seen from Table 2, the trapezoidal and hexagon structures of the composites which are under impact energy showed similar behavior. Their punches are rebounding and the material caused by the impact of the initial structure mutation (F₁) is less than 1 (KN). The impact force (F_m) required by the fracture damage of these two kinds of composites are relatively close, both are within 1.5 (kN), far higher than the other two kinds of structures. It can be seen that the trapezoid and hexagon shapes have stronger resistance to the punch. Compared with the other two structures, the material is less prone to initial mutation and fiber fracture, and also has certain elasticity.

Energy absorption performance

Figure 15 shows the absorbed energy-time history curves of samples with four structures at 25 J impact energy. It can be seen that the energy absorption curves experienced by cellular textile composites with different structures are greatly different. The energy curve of the trapezoidal specimen is a typical curve. At the initial stage of impact, part of the kinetic energy of the punch is converted into the elastic potential energy of the specimen with the increase of time, and the other part is used to produce damage such as fiber fracture. When the maximum converted energy E_max is reached, if the sample is not broken, the kinetic energy of the punch becomes 0 J, and the elastic potential energy stored in the specimen begins to be transformed into the kinetic energy of the punch again, and is separated from the punch at a certain energy level, resulting in the rebound of the punch. In this case, the difference between the maximum converted energy E_max and the kinetic energy of the punch is the final absorbed impact energy E_ab of the specimen. This energy is mainly used to cause damage to the specimen, including layer to layer penetration, indentation and fiber fracture. Therefore, the severity of specimen damage can be determined by observing the impact energy (E_ab) absorbed by the specimen. However, if the punch penetrates the sample directly from the surface to the bottom, the maximum converted energy E_max of the punch is all used to cause damage to the sample. At this time, the energy absorption ratio (E_ab/E_max) of the specimen reaches 100%, and it can also be judged that the sample has been penetrated.

Figure 15.

Absorbed energy of different structure cellular composite.

Combined with Table 3 and Figure 15, it can be seen that when the impact energy is 25 J, the corresponding energy absorption amounts of triangular sample and rectangular sample are 3.68 J and 5.17 J, and the energy absorption ratios of the two kinds of samples are 100%. This indicates that both samples are broken directly, but the energy absorbed by the rectangular sample is slightly higher than that of the triangle, indicating that greater deformation or fiber fracture occurs inside the rectangular structure. Combined with the impact force analysis of the samples of these two structures, it can also be seen that they did not cause a rebound to the punch, but did not resist the impact under the energy of 25 J, and were directly destroyed.

Table 3.

Three characteristic energy performance index from absorbed energy curve

	E_ab (J)	E_max (J)	$\frac{E_{a b}}{E_{\max}} (%)$
Triangle	3.68	3.68	100
Rectangle	5.17	5.17	100
Trapezoid	10.82	11.45	94.5
Hexagon	9.94	9.94	100

It can be seen from Figure 15 that the energy absorption curve of the hexagonal sample shows two peaks, which indicates that there is a very obvious punch rebound phenomenon in the hexagonal structure sample. This judgment is also verified by combining with the previous impact force curve. Under the impact of 25 J energy, the first wave peak of the impact converted energy of the hexagonal sample is generated at 4.19 J, that is, at this time, the specimen springs back to the punch for the first time. Then, the punch continues to move forward and collides with the sample. At this time, the impact energy absorption is relatively slow and reaches the second peak at about 17 ms. With the release of elastic energy, the final absorbed energy value of hexagonal sample is 9.94 J, which is higher than that of triangle and rectangle sample, indicating that the hexagonal structure has stronger deformation ability and is more likely to generate elastic potential energy.

Compared with the samples of other structures, the trapezoidal sample can absorb the maximum energy(E_ab) of 11.45 J, and the energy absorption rate is 94.5%. According to the impact force curve analysis results, the rebound of the trapezoidal sample on the impulse occurs in the later stage of the impact process, which also explains the curve in Figure 15. After the curve of trapezoidal sample reaches the maximum absorbed energy (E_max = 11.45), it decreases slowly, and the sample releases part of the absorbed elastic potential energy through rebound. From the value of the energy absorption rate as high as 94.5%, the failure of the actual sample structure, trapezoidal sample is still penetrated by the punch, but the damaged area at the bottom is small.

Conclusion

Four types of cellular structure fabrics were weaved in this project, after opening up, the material internal cell cross sections were triangle, rectangle, trapezoid, and hexagon. The fabrics were consolidated by VARTM technology to produce composites.

After the drop hammer impact test with impact energy of 25 J, the triangular structure responded most quickly, and its inflection point started at 0.04 kN, which means that this kind of structure is more sensitive to the out-coming punch. Compared with the triangle-shaped composite, rectangular-structure materials required about four times the impact forces to produce initial large deformation. Composites of trapezoidal and hexagon shape need far higher impact forces to cause their structure deformation, which means these two kinds of structures are strong enough to resist the impact. The rebounding of these two composites also indicated that trapezoidal and hexagon-shaped structures are more elastic.

Regarding the energy absorption behavior, the triangular and rectangular-shaped composites can absorb 100% of impact energies, which means both samples were broken directly. It can also be concluded that the hexagonal structure sample rebounded the punch force more easily.

Compared with the samples of other structures, the trapezoidal sample can absorb the most energies, and its energy absorption rate is as high as 94.5%. Referring to its rebounding behavior in the previous analysis, a conclusion can be drawn that samples with trapezoidal shape not only have better energy absorption capability, but also show better elasticity performance.

Footnotes

Declaration of conflicting interests

The author(s) declared that they have no conflicts of interest in relation to this work.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is sponsored by (1) the Open Project Program Ministry of Education Key Laboratory for Advanced Textile Composite Materials (Tiangong University), No. MATC-2021-007; (2) 3-D Textile Engineering Hubei Province's Research Center, Wuhan, Hubei Province, 430200, China.

ORCID iD

Xiaozhou

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