Effect of angle interlocking/plain weave compound fabric system on mechanical properties of aramid epoxy resin composites

Preform is one of the important factors affecting the mechanical properties of textile composites. To further reflect the influence of the preform structure on the mechanical properties of composite materials, the angle interlocking and plain weave structures are configured in a certain proportion to form a composite fabric with both structures. The results show that the compound fabric preform has a positive effect on the improvement of the mechanical properties of AERC. An increase in the proportion of angle interlocking organization configuration in the compound fabric system is beneficial to the increase in tensile strength of AERC. In this experiment, when the ratio of angle interlocking structure and plain weave structure in the compound fabric system is 3:1, the tensile strength of AERC increases significantly, which is 80.98% higher than that of AERC with angle interlocking preformed structure. When the preform is made of compound fabric, the bending properties of AERC are greatly improved, and the bending strength is also increased 1.20 times compared to the AERC with angle interlock structure as the preform. When the ratio of angle interlock weave and plain weave in the compound fabric system is 1:3, the bending strength of AERC increases 1.81 times the maximum. The proportion of two fabric structures in the compound fabric system has a significant impact on the impact properties of AERC. The increase in the proportion of plain weave in the compound fabric system is not conducive to the increase in impact strength of AERC, and the large proportion of corner interlock configuration is conducive to the increase in bending strength of AERC. When the ratio of angle interlocking structure to plain weave structure is 2:1, the AERC has the maximum impact strength of 33.14 KJ/m2, which is increased by 26.28%.

structure reinforced composites by adjusting the weft density, binding yarn buckling structure, material composition, and volume content of the reinforcing fiber.For instance, Liu [19] designed preforms with different angle interlocking structures, analyzed the effects of weft bending degree and reinforcing fiber content on the mechanical properties of glass fiber reinforced composites, and compared them to orthogonal glass fiber reinforced composites.Through three-point bending test, it is found that the angle interlock braided structure has a higher flexural strength (50%), modulus (40%), and failure resistance than the orthogonal woven structures, and the through-thickness angle interlock woven structure has the best flexural failure resistance among all textile structures.S. Abbasi studied the effect of metallic and composite z-filaments on the mechanical properties of composite materials and found that the presence of z-shaped yarns can significantly improve the fatigue resistance of laminated composite materials.In addition, it was found that because the metal z-filaments can sustain higher plastic deformation than their carbon counterpart, they are less effective than carbon z-filaments at increasing fatigue resistance under both mode I and mode II loadings [20].Meanwhile, researchers also simulated the damage form of composites with 3D structures through various analysis models to predict the mechanical properties of composites [21][22][23][24].However, there is currently limited research on the 3D preform structure design.For instance, Liu used the 3D preform structure design to combine the two structures of a shallow intersection direct connection and a shallow intersection bending connection to improve the mechanical properties of angle interlocking composites [25].
The angle-interlocking structure connects the multi-layer warp and weft into a stable overall structure by connecting the warp and weft in the horizontal and thickness directions [26], respectively.Because the angleinterlock fabric can be woven on an ordinary loom or a reformed ordinary loom, it has strong designability and processability.Therefore, the angle-interlocking structure has become the most common structural form in the preparation of 3D woven preforms.Although there are several defects in 2D woven laminate composites compared to 3D woven composites, Reifsnider and Guess established that the tensile elastic modulus of 3D woven composites is lower than that of laminate composites.Therefore, in this study, weaving technology is used to compound 2D plain woven fabric based on an angle interlocking organization.Composite woven fabric organization structure forms with angle interlocking organization and 2D organization structures are prepared.Additionally, the number of in-plane bending yarns in the preform structure are reduced, and the in-plane tensile and compressive properties of 3D angle interlocking composites are improved.Furthermore, a new organizational structure for angle-interlocking woven preforms while ensuring the integrity of the preform structure is presented.

Experimental 1.Materials
The resins used in this study were JL235 epoxy resin and JH242 curing agent provided by China Changshu Jiafa Chemical Co., Ltd.The preform fabric was made of aramid fiber yarn obtained from DuPont.

Fabric architecture
Figures 1(a) and (b) depict the structures corresponding to the angle-interlocking (5 warp 4 weft) and plainweave organizations, respectively.Figure 1(c) displays the organizational structure of A + B composite angle interlock system composed of an angle interlock cycle organization and a double-layer plain weave cycle organization, whereas figure 1(d) illustrates the organizational structure of the simple A + B angle interlocking system.Figure 2 shows the three-dimensional structure of angle interlock and plain preform fabric.To analyze the influence of the compound fabric system on the mechanical properties of the composites, the proportions of the two types of structure configurations in the composite structure system were adjusted.The configuration mode of angle interlocking and plain weave in the composite tissue is recorded as NaA + NbB, wherein A and B indicate the angle interlocking organization and the plain weave, respectively; N represents the configuration proportion in the composite structure, i.e., the number of complete tissue cycles of each organization in the composite structure; and Na and Nb represent the configuration ratio of the angle interlocking and plain weave structures, respectively.The preform composite system of configuration is shown in table 1.
The preformed fabric with an angle-interlocking structure was prepared using a loom from Shanghai Shuangjiu Industrial Co., Ltd.[figure 3(a)].All small samples adopted the same reed number, and the warp density of all the preform fabrics was 120 pieces cm −1 .The preform is shown in figure 3(b).
The structural parameters of the preform fabric prepared in this study are listed in table 2. The composite angle interlock consists of two organizational structures, and the warp and weft yarns of the two organizational structures are interwoven and bent differently.The warp yarns are indicated as Warp1 and Warp2, where they represent the warps in the plain weave and corner interlock structures in the composite weave, respectively.The binding yarn is that in the angle-interlocking organization.Table 1.Preform compound fabric system configuration.

Sample number N a N b
Preform structure

Preparation of composite materials
Aramid epoxy resin composites were prepared through resin transfer molding (RTM) using the prepared fabric as the preform and epoxy resin as the matrix.To ensure consistency of the mass percentage of the reinforcing fiber in the composites, the same mass hammer was used to cover and press the composites during curing.Table 3 lists the thickness values of the preform fabric with different organizational structures and the mass percentage of the reinforcing fiber in its composites.The test results indicate that the mass percentage of the reinforcing fiber in the prepared aramid epoxy resin composites was approximately 40%, but the mass content of the reinforcing fiber in the single-angle interlocking structure composites was high.

Mechanical testing
Tensile test was performed according to the ASTMD3039 standard [27], and the sample was cut along the direction of the preform; the standard size of the sample was 225 mm × 25 mm.A tensile test was conducted using a CSS-88100 electronic universal testing machine at a tensile speed of 2 mm min −1 .Five samples were tested for each sample and the average value was obtained.The bending strength test was performed according to the three-point mode in ASTM d7264 (790), and the sample was cut along the meridional direction of the preform [28].The standard size of the sample was 160 mm × 20 mm.A CSS-88100 universal testing machine was used for the bending test.The span-to-thickness ratio was maintained at 16:1 and the crosshead speed was  set to 2 mm min −1 .Impact performance test was performed according to the ASTMD 6110 standard [29].The sample was cut along the direction of the preform, and the standard size of the sample was 75 mm × 10 mm, impact speed was 3.8 m s −1 , pendulum energy was 7.5 J, and elevation was 160°.

Results and discussion
2.1.Tensile properties Figure 4(a) shows the tensile strengths of the composites with different preformed structures.It was established that the tensile strength of aramid composites with a composite structure was greater than that of composites with a single-angle interlocking structure, with a maximum and minimum increase of 80.98 and 40.35%, respectively.Although the composite structure improved the tensile strength of the composite, the tensile strength did not increase with increase in the proportion of plain weave in the preformed fabric composite system.In contrast, the tensile strength of the aramid-reinforced composites increased with increase in the configuration ratio N a .The single-angle interlocking preform fabric contained a large number of binding yarns.
Owing to the buckling of the binding yarns, the load bearing in the warp direction of the preform fabric was affected.The shrinkage of the warp yarn in the angle-interlock structure in the fabric was 0%, which is approximately in the straight-line state.When bearing a tensile load, the angle inter-locking structure reflected the tensile properties of the yarn to the greatest extent, which increased the tensile strength of the composite.It can be observed from table 2 that the weft content in the preform composite system decreases after the preform fabric was structurally compounded.This indicates that more warp yarns bear the load in the warp direction of the composite system, which improves the tensile strength in the warp direction of the composite preform.In the composite structure system, the mass content of the binding yarn in the system gradually decreased with increase in the N b configuration proportion, which provides favorable conditions for improving the warp tensile strength of the composites.The mass content of Warp2 decreased gradually with increase in the proportion of the plain weave system, whereas the mass content of Warp1 increased.Because the buckling of yarn in the plain weave structure in Warp1 was significantly greater than that in Warp 2, the tensile strength of the compound fabric system preform structure decreased when the configuration proportion reached a certain degree.When the configuration ratio N a was increased, although the mass content of the binding yarn in the composite system increased, the content of Warp2 in the composite system increased and the tensile strength of the composite increased.Figure 4(b) shows the tensile stress-strain curve of the composite.When the preformed structure adopted the compound fabric system, the tensile strain and tensile fracture work of the composite increased significantly.The tensile fracture specific work of angle interlock composites without microstructure composite was 475.94 KJ m −2 .The maximum and minimum tensile fracture specific works of angle interlock composites after composite increased by 1.35 times and 27.5%, respectively.The increase in configuration ratio N a increased the tensile fracture work of composites.Additionally, the tensile modulus of composites with different preformed microstructures was different.In the composite structure system, the tensile modulus of the composite angular interlocking structure composite was less than that of the single angular interlocking preform structure composite when the configuration proportion N b was large.In contrast, the tensile modulus of the composite angular interlocking structure composite was greater than that of the single angular interlocking composite when the configuration proportion N a was large.This could be attributed to two reasons.First, owing to the existence of binding yarn, the double-layer plain weave in the composite angular interlock does not offer the reinforcement of traditional laminated structures.However, a tubular double-layer plain-weave structure is formed through the binding yarn in the angular interlock; therefore, its integrity and stability are better from those of traditionally laminated composites.Furthermore, in the composite preform system, the stability of angle-interlock decreased with an increase in the configurational proportions of the multi-layer plain-weave structure.Second, the warp shrinkage in the angle interlock was less than that in the plain weave.When subjected to external force load, the tensile deformation reflected in the multi-layer plain weave structure was greater than that in the angle interlock structure.Additionally, there was a difference in the number of fully recycled yarns between the two organizations.In this test, the angle-interlock and multi-layer plain structures had five and four complete warp cycles, respectively.Therefore, when different composite configuration ratios were adopted, a change in the number of complete cycle yarns of the two structures changed the tensile properties.

Bending performance
Figure 5(a) shows the bending strength of the aramid epoxy resin composite (AERC).When the preform adopted the compound fabric, the bending strength of the AERC was significantly improved, with maximum and minimum increase of 1.81 times and approximately 1.20 times, respectively.Through comparison, it was established that in the preform fabric structure composite system of N a A + N b B, the bending strength of the AERC increased with an increase in N b in the structure system and decreased with an increase in N a .This is because after the preform structure is compounded, the content of the binding yarn in the compound fabric system decreases with an increase in the configuration ratio of the multi-layer plain weave structure, which improves the compression resistance of the composite.
By comparing the bending strength curves of the composites, it was established that the bending modulus of the AERC with a compound fabric structure was greater than that of the composites with a single angular interlocking structure [figure 5(b)]; the bending modulus of AERC with 2A+B configuration increased by 2.67 times.The bending moduli of the composites were different when the microstructure configuration ratio of the preform was different.In the preform fabric composite structure system, the plain-weave configuration was relatively large, i.e., when N b was large in the composite structure system, the composite exhibited a large bending modulus.Moreover, it was established that the bending strain of the composites with multi-layer plain weave structure was significantly less than that of the composites with angular interlocking structure.Although the combination of the plain texture structure in the preform improved the bending strength of the composite, the stability of the plain texture structure was not as good as that of the angular interlocking structure.Therefore, the overall performance of the preform decreased when the plain texture configuration ratio increased.When subjected to a bending load, the plain texture structure was prone to delamination, which affected the strain performance of the composite.

Impact performance
Figure 6 shows the impact strength of the angle-interlock composite.Not all composites with the preform structure of the compound fabric exhibited higher impact strength than ordinary angle interlocking composites.The impact strength of the aramid composites with the preform structure of the A + 3B configuration was lower than that of the ordinary angle-interlocking composites.The impact strength of the aramid composites with composite structures dominated by angular interlocking configurations was significantly improved.However, the impact strength of composite angle interlock composites with a plain weave configuration was improved slightly, with a maximum increase of only 6.86% and a minimum decrease of 10.63%.In the two configurations, the impact strength of the aramid-reinforced composites decreased with an increase in the configuration ratio N a of the composite system.This is because in the common-angle interlocking preform structure, there are many straight yarns that are not easy to deform and absorb less energy.Owing to the composite structure of the preform fabric, the content of non-deformable yarn in the fabric was reduced and the energy absorption performance was enhanced.The increase in the plain texture material cycle, i.e., the increase in the plain texture of the composite system and the decrease in the integrity of the angle interlocking composites affected the increase in the impact strength of the composites.

Conclusion
The preform of angle interlocking/plain weave composite fabric is beneficial to the improvement of the mechanical properties of AERC, but the proportion of angle interlock and plain weave in the compound fabric system has a significant difference in the influence of mechanical properties.The use of compound fabric systems can greatly improve the tensile properties of AERC.It not only improves the tensile strength, but also greatly improves the tensile fracture work of AERC.Compared to the AERC with angle interlocking structure as the preform, the tensile strength of AERC increased by 40.35% at the minimum and 80.98% at the maximum.After using the compound fabric system as the preform, the tensile fracture work of the AERC increased by a maximum of 1.35 times.In the compound fabric system, the increase in the proportion of angle interlocking weave configuration is beneficial to the improvement in the tensile strength of AERC, whereas the increase in the proportion of plain weave configuration has a negative effect on the tensile fracture modulus of AERC.The compound fabric preform can greatly improve the bending properties of AERC, as well as the bending strength.The increase in the proportion of plain weave in the compound fabric system is conducive to the improvement in the bending strength of AERC.When the proportion of corner interlock and plain weave is 1:3, the bending strength of AERC increases 1.81 times.The bending modulus of AERC is also greatly improved by using the compound fabric system, but it is greatly affected by the proportion of two types of weaves in the AERC.When the proportion of plain weave in the compound fabric system is large, the bending strain of AERC is smaller.The proportion of two types of weave configuration in the compound fabric system has a great influence on the impact performance of AERC.When the proportion of angle interlocking weave in the compound fabric system is large, the impact strength of AERC is improved, and the maximum increase is 26.40%.The increase in the proportion of plain weave in the compound fabric system is not conducive to the improvement in the impact strength of AERC.The increase in plain weave will affect the integrity of the compound fabric system, which will reduce the impact strength of AERC.
In the past, the research on the preformed focused on the influence of the preformed organizational structure and changes in structure parameters on the mechanical properties of composite materials.In addition, the preformed organizational is the same structure or a homologous organizational.In this study, the characteristics of preforms were combined with different organizations, 2.5D fabric and flat fabric were configured to design a compound fabric preform with both angle interlocking and plain weave characteristics, and the influence of the parameters of the compound fabric system on the mechanical properties of AERC was discussed, which provided a theoretical support for the structural design of composite preforms with different mechanical performance requirements.In this study, only the influence of the proportion ratio of two types of organization structures in the compound fabric system on the mechanical properties of the composite material was discussed.It was found that the bending of the bundled yarn in the compound fabric system had a great influence on the mechanical properties of the composite material.Therefore, in the later stage, the structural parameters of the compound fabric system such as the buckling depth of the yarn in the compound fabric system can be further discussed, and the influence of the compound fabric system on the mechanical properties of the composite material can be analyzed to realize a design of the composite preform, which can meet the requirements of different mechanical properties.

Figure 1 .
Figure 1.Structural diagram of fabric structure.(a) Angle interlock organization (b) Plain weave organization (c) A+B composite structure organization (d) Illustration of A+B composite structure organization.

Figure 2 .
Figure 2. Illustration of fabric structure.Angle interlock organization (b) Plain weave organization.

Table 2 .
Combination parameters of preform.

Table 3 .
Preform fabric thickness and fiber content in composites.