Enhancement of mechanical and electromagnetic absorbing properties of carbon/glass hybrid composites with a three-dimensional quasi-isotropic braided structure

Abstract

Two carbon/glass hybrid composites with different reinforced structures were designed and their mechanical and electromagnetic absorbing properties (EMAPs) were investigated in this paper. It was found that the tensile, bending, and double-notch shear strength of the three-dimensional (3D) quasi-isotropic (QI)-braided composite were 4.50%, 9.64%, and 14.29% higher than those of the QI-laminated composite, respectively. This was because Z-binder yarns in the 3D QI-braided composite can lock all yarn sets together to bear external stress and inhibit crack propagation in interlamination. The EMAPs of the 3D QI-braided composites were larger than that of the QI-laminated composite in the entire Ku band. This was because the Z-directional glass fibers in the 3D QI-braided composite were beneficial for electromagnetic transmission. The uniform arrangement of five sets of yarns (+45°, –45°, 90°, 0°, and Z-yarns) resulted in the 3D QI-braided composites having better QI-EMAPs and QI mechanical properties in plane and outstanding interlayer performance than the traditional carbon fiber laminated composite.

Keywords

hybrid composite three-dimensional braided composite quasi-isotropic structure mechanical strengths microwave absorption

With the development of advanced radar detection technology, having such advantages as high resolution and high reliability, and the emergence of precision guided weapons, various weapon systems on the battlefield are faced with severe threats.^1–3 As an effective means to improve the survival and penetration capability for weapons, such as warplanes and missile, and enhance the overall operational efficiency, stealth technology has been highly valued by the world's military powers.^4,5 Radar absorbing structures (RASs) are a new type of structure-functional composite material developed on the basis of advanced composites.⁶ RASs can meet the needs of modern radar absorbing materials (RAMs), such as being “wide, thin, light and strong,”^7–9 and also can be tailored to various complex shapes, such as wings, tails, and air inlets.⁶ Therefore, RASs are a main development direction of contemporary stealthy materials.^10,11

Carbon fiber polymer composites (CF-PMCs) with high specific stiffness and high specific strength are typical structural materials used widely in aerospace industries.^12–14 However, electromagnetic (EM) waves are reflected by the CF-PMCs due to the impedance difference between the free spaces.^15,16 Accordingly, E-glass fibers (E-GFs) have high resistivity and low permittivity and they do not interfere with EM wave transmission characteristics, hence E-GF-PMCs are often be used as low-observable radomes.¹⁷ Also, GF has higher elongation at break than that of CF, and CF/GF hybrid composites can avoid the brittle fracture and improve ultimate tensile strain of CF-PMCs.^18,19 In addition, CFs as electric loss RAMs can absorb some EM waves. In fact, continuous CF is a strong reflector of EM waves due to its high degree of graphitization.⁷ Therefore, hybrid composites with GF and CF were designed to obtain appropriate electrical resistivity and to decrease the EM reflection.

The mechanical and electromagnetic absorbing properties (EMAPs) of CF/GF-PMCs vary with the change of structural parameters,^16,20,21 and this feature allows designers to tailor the properties through the mixed arrangement and weaving of CF and GF to meet the exact needs of RASs. Liu et al.¹⁶ investigated the electrical resistivity of CF/GF-PMCs with different structural parameters, and found that the EMAPs of interlaminar hybrid composites were larger than those of intraply hybrid composites. Hunjra et al.²² made a sandwich construction of RASs comprising GF-PMCs containing carbon black as the front face skin and CF-PMCs as the back face skin, and achieved a broad range attenuation of –10 dB. Although the hybrid composite laminates satisfy the EMAPs, they are susceptible to delamination failure due to their poor interlaminar strength.^23,24 Fan et al.⁷ designed a three-dimensional (3D) orthogonal CF/GF woven composite and investigated its mechanical properties and EMAPs. The results showed that the 3D orthogonal woven composites enhanced the mechanical and absorbing properties due to the binding effect and EM transmission effect of E-GF in the thickness direction. However, they found that the 3D orthogonal composite had different EMAPs when the incident electric field was perpendicular to the weft yarn direction and the warp yarn direction due to the different CF numbers in these directions. However, the deeper answer is that the magnetic susceptibility (μ) of CF-PMCs is controlled by the polarization angle (θ), which is between the incident electric field and the CF axis. The magnetic susceptibility ranged from non-magnetic at θ = 90°, μ = 0, up to strongly diamagnetic at θ = 30°, μ = –0.75, over the 8–18 GHz bandwidth.²⁵ This means the inhomogeneous distribution of CFs in the composites, such as unidirectional and orthogonal composites, will lead to different EMAPs when the incident electric field comes from different directions. For example, in some directions, the composites have excellent EMAPs, while in some directions they do not. This is unacceptable for stealth weapon systems, because the radar waves may come from any direction in war. In addition, the composites are often in complex stress states in some practical engineering applications; for example, an aircraft wing is under ever-changing alternating loads when flying. Therefore, quasi-isotropic (QI) laminates were developed to improve the in-plane properties by orienting the fiber in the preform.^26–28 Moreover, multiaxis 3D woven preform was fabricated with five sets of yarns, +bias, –bias, warp, filling, and Z-yarns, to enhance both the in-plane properties and interlaminar performance. Its in-plane properties are improved by orienting the fiber in the preform and the Z-yarns locked all yarn sets to provide structural integrity.²⁹

In order to meet the high and complex requirements of RASs in modern war, we designed a “sandwich-type” composite reinforced by 3D QI-CF/GF braided fabric that has five fiber sets. The + 45°, –45°, 90°, and 0° fibers enhanced in-plane properties and Z-binder yarns locked all yarn sets to improve the interlaminar performance. To determine the role of the reinforced structure on the mechanical properties and EMAPs of hybrid fiber, a QI-laminated CF/GF hybrid composite (QI-laminated composite) was designed to compare with the 3D QI braided CF/GF hybrid composite (3D QI-braided composite). The mechanical properties and EMAPs of the composites were investigated by a universal testing machine and a waveguide measurement system, respectively.

Experimental details

Materials

The fibers for the intermediate four layers of the two types of CF/GF hybrid fabric were T700-12K CF (Toray, Japan). The fibers for the top two layers, the bottom two layers, and the binder yarns were 400 tex E-GF, produced by Shaanxi Huate New Materials Co., Ltd, China. The BH301 bismaleimide resin used in the experiment was supplied by Jiangsu Hengshen Co., Ltd, China. Table 1 gives the properties of CF, GF, and bismaleimide resin.

Table 1.

The properties of constituent materials

Materials	Type	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation at break (%)	Density (g/cm³)
Carbon fiber	T700	4900	240	2.1	1.80
Glass fiber	E	3430	73	4.8	2.54
Bismaleimide resin	BH301	92	4.5	2.4	1.25

Preparation of composites

The 3D QI-CF/GF braided fabric was newly made by the substitution method.³⁰ There were five sets of yarns: + 45°, –45°, 90°, 0°, and Z-yarns. These yarns were interlaced to form the structure where 90° (warp) yarns were laid longitudinally and 0° (filling) yarns were placed adjacent to warp yarns. The + 45° yarns were placed adjacent to warp yarns, and –45° yarns were placed adjacent to +45° yarns. The cycle continued to form the thickness of 4 mm. Then Z-yarns were inserted into the fabric in the thickness direction, locking all yarn sets to provide structural integrity. The architecture of the 3D QI-CF/GF braided fabric is illustrated in Figure 1(a). For the QI-CF/GF laminated fabric, the CF or GF was woven into unidirectional fiber cloth using a weaving loom, and then the unidirectional clothes were laid in the sequence of 0°, 90°, + 45°, –45° until reaching the 4 mm thickness. The QI-CF/GF laminated fabric is illustrated in Figure 1(b). The technological parameters of the 3D QI-CF/GF braided fabric and the QI-CF/GF laminated fabric are shown in Table 2. The fiber volume fraction was obtained by the weighing method.³¹ The void contents of the 3D QI-braided composite and the QI-laminated composite are 1.17% and 0.83%, respectively.

Figure 1.

Schematics of the three-dimensional quasi-isotropic carbon fiber/glass fiber (QI-CF/GF) braided fabric (a) and the QI-CF/GF laminated fabric (b).

Table 2.

Technological parameters of the three-dimensional (3D) quasi-isotropic (QI)-carbon fiber (CF)/glass fiber (GF) braided fabric and the QI-CF/GF laminated fabric

Reinforced structure	Yarns liner density/tex			Preformed unit density/(yarn/cm)			Layers	Fiber volume fraction/%
Reinforced structure	0 °/90 °	±45 °	Z-binder yarns	0 °/90 °	±45 °	Z-binder yarns	Layers	Fiber volume fraction/%
3D QI-CF/GF braided fabric	800	400 × 2	400	5	6	5	8	47.74 ± 1.16
QI-CF/GF laminated fabric	800	400 × 2	—	6	6	—	8	46.23 ± 1.42

Vacuum assisted resin transfer molding (VARTM) was used to produce the high-performance composite in this study. The process involved placing the 3D QI-CF/GF braided fabric (250 mm × 180 mm × 4 mm) and the QI-CF/GF laminated fabric (250 mm × 180 mm × 4 mm) into their own molds, closing the molds, checking for leaks, and heating up to 120℃. Once the molds and the pipes connected to the molds were sufficiently heated, a vacuum of approximately 0.6 MPa was applied to the molds and the resin trap was allowed to stabilize for 5 min, and then the resin was injected into the molds. The injection process was continued until a sufficient volume of resin was seen in the resin trap, to indicate that the molds had been completely filled. The molds were isolated from the resin pot and the resin trap and then put into an air-circulating oven. The manufacturer recommended cure cycle was employed: the first step of the cure cycle was 3 h at 180℃, followed by 3 h at 230℃. The fiber volume fractions of the 3D QI-braided composite and the QI-laminated composite were 47.74% and 46.23%, respectively.

The large pieces of 3D QI-braided composite and QI-laminated composite were cut into EM absorption samples (30 mm × 25 mm × 4 mm) in four different directions (0°, 90°, + 45°, –45°) and the tensile samples (250 mm × 25 mm × 4 mm) and three-point bending samples (80 mm × 15 mm × 4 mm) in the warp direction with a computer numerical control (CNC) water jet cutting machine (Shenyang All-Powerful Science and Technology Joint Stock Co., Ltd, China). The double-notch shear samples, as shown in Figure 1, were machined by wire-electrode cutting. The cross-section microscopic pictures of the 3D QI-braided composite and the QI-laminated composite are shown in Figure 2. The typical profile of Z-binder yarn within the 3D QI-braided composite can be seen clearly in Figure 2(a).

Figure 2.

Cross-section microscopic pictures of the three-dimensional quasi-isotropic (QI)-braided composite (a) and the QI-laminated composite (b).

Characterization

Tensile, three-point bending, and double-notch shear properties were tested at room temperature with a universal testing machine (Shenzhen Suns Technology Co., Ltd, China) following ISO 527-5, ISO 14125-1998 and ASTM D3846, respectively. The loading rates were 2 mm/min for the tensile and bending tests, and 1 mm/min for the shear test. For each property, three samples were tested, and the average value was taken. The fracture morphologies of samples were observed by a VHX-5000 Ultra-field Microscopy System (KEYENCE International Trading (Shanghai) Co., Ltd).

EM properties were tested with a waveguide measurement system following ASTM D4935−18. The waveguide measurement system, which was composed of two coaxial cables, a waveguide tube for a broadband frequency range of 11.9–18 GHz (Ku-band),³² a data acquisition system, and a vector network analyzer (Agilent Technologies, N5232A), was used to evaluate the EM characteristics of the composites. The network analyzer consisted of a synthesized sweeper and a scattering parameter (S-parameter) test set. The S-parameter test set in the network analyzer was linked to the waveguide tube through precision coaxial cables. A schematic configuration of the waveguide measurement system of the experimental setup is shown in Figure 3. The test sample was illuminated with a polarized EM wave at normal incidence, and the reflection and transmission were investigated by measuring reflection scattering parameter (S₁₁) and transmission scattering parameter (S₂₁). The principle of EM wave interaction with radar absorption material was similar to our precious work,³³ but the EM wave struck the composite surface in the perpendicular direction. In this study, four directions (0°, 90°, + 45°, –45°) of the samples were tested and, for each direction, three samples were tested.

Figure 3.

Schematic configuration of the waveguide measurement system. EM: electromagnetic.

Results and discussion

Tensile properties

Figure 4 shows typical tensile load–displacement curves obtained from the 3D QI-braided composite and the QI-laminated composite samples. The curve of the 3D QI-braided composite approximately follows a linear relationship up to ultimate failure. The curve of the QI-laminated composite showed a similar trend and followed a linear relationship in the initial phase, but climbed in the form of a saw-tooth before reaching the maximum load. The saw-tooth pattern indicated that typical delamination damage happened in the QI-laminated composite. Also, the break loads and elongations of the 3D QI-braided composite were greater than those of QI-laminated composite. The test results of tensile strength and tensile elastic modulus of the 3D QI-braided composite and the QI-laminated composite are shown in Table 3. The average tensile strength and modulus of the 3D QI-braided composite were 615.25 MPa and 34.36 GPa, which were 4.50% and 2.72% higher than those of the QI-laminated composite, respectively.

Figure 4.

Tensile load–displacement curves of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite.

Table 3.

Tensile strength and modulus of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite

Samples	Tensile strength/MPa				Tensile modulus/GPa
Samples	1	2	3	Average mean	1	2	3	Average mean
Braided^a	602.77	632.23	610.75	615.25 ± 15.24	32.56	34.49	36.02	34.36 ± 1.42
Laminated^b	586.44	603.83	575.96	588.74 ± 14.08	33.33	34.85	32.17	33.45 ± 1.10

Representing the 3D QI-braided composite.

Representing the QI-laminated composite.

During tension, cracks appeared in the resin firstly accompanied by audible sound. As is known to all, the tensile strength of bismaleimide resin is much less than that of CF or GF, so the bismaleimide resin began to break and more cracks appeared in the composites as the load increased. For the QI-laminated composite, the cracks in interlamination easily led to delamination. As shown in Figures 5(d)–(f), it is clear to see that the QI-laminated composite sample after tensile failure showed severe delamination and large damage areas. For the 3D QI-braided composite, however, the resin cracks initiated at the outside of the binding points and propagated along the binder tow in the thickness direction. The thin layer of resin between the surface +45° yarns and –45° yarns started to fail and formed a debonding feature. As the load increased, debonding of the ±45° yarns, 0°/90° yarns, and the Z-binder yarns occurred and the tensile fracture of the ±45° yarns, 0°/90° yarns, and the Z-binder yarns resulted in the final failure. The damage areas of the 3D QI-braided composite (Figure 5(a)) were significantly smaller than those of the QI-laminated composite (Figure 5(d)). Figures 5(c) and 5(f) show the real-time depth composite images, which were used to obtain the depth information of the rugged plat by changing the focal length. It can be seen that the depths of cracks for the 3D QI-braided composite (Figure 5(c)) were smaller than for QI-laminated composite (Figure 5(f)). This was because the Z-binder yarns in the 3D QI-braided composite can block crack growth in the interlaminar region and bound the ±45° yarns and the 0°/90° yarns together to resist the external stress. That is why the tensile strength and tensile modulus at break of the 3D QI-braided composite were higher than those of the QI-laminated composite.

Figure 5.

Photomicrographs on cross-sections of the three-dimensional quasi-isotropic (QI)-braided composite (a) and the QI-laminated composite (d) after tensile failure; (b) and (e) show the zoom on cracks corresponding to (a) and (d), respectively; (c) and (f) show the real-time depth composite images corresponding to (b) and (e), respectively.

Bending properties

The three-point bending load–displacement curves of the 3D QI-braided composite and the QI-laminated composite are shown in Figure 6. The QI-laminated composite exhibited a linear elastic behavior at the initial stage, and the load linearly increases with the displacement until the maximum load point, and then there was a sharp decline after the peak value. From the fracture morphologies analysis of the QI-laminated composite in Figures 7(b) and (d), we knew that the sharp decline of the load was caused by the delamination damage. For the 3D QI-braided composite, the bending load increased linearly with the displacement at the initial stage, but climbed in the form of a gentle slope before reaching the maximum load, and then the curve had a short, sharp decline and then decreased slowly in a slope. This was because the Z-binder yarns locked all yarn sets in the thickness to resist crack propagation between layers, and there was no delamination in the bending failure (Figures 7(a) and (c)). Because of this, the average bending strength and modulus of the 3D QI-braided composite were 9.64% and 13.33% higher than those of the QI-laminated composite (Table 4), respectively. The average bending toughness of the 3D QI-braided composite were 45.58% higher than that of the QI-laminated composite, as shown in Table 5.

Figure 6.

Bending load–displacement curves of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite.

Figure 7.

Photomicrographs on cross-sections of the three-dimensional quasi-isotropic (QI)-braided composite (a) and the QI-laminated composite (b) after bending failure; (c) and (d) show the zoom on cracks corresponding to (a) and (b), respectively.

Table 4.

Bending strength and modulus of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite

Samples	Bending strength/MPa				Bending modulus/GPa
Samples	1	2	3	Average mean	1	2	3	Average mean
Braided^a	366.30	349.34	370.24	361.96 ± 11.11	14.75	14.14	15.48	14.79 ± 0.55
Laminated^b	332.93	339.34	318.08	330.12 ± 10.91	12.99	13.42	12.75	13.05 ± 0.28

Representing the 3D QI-braided composite.

Representing the QI-laminated composite.

Table 5.

Bending toughness of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite

samples	Bending toughness/(KJ/m³)
samples	1	2	3	Average mean
Braided^a	17.70	17.01	18.27	17.66 ± 0.52
Laminated^b	9.33	10.19	9.32	9.61 ± 0.41

Representing the 3D QI-braided composite.

Representing the QI-laminated composite.

Shear properties

The short beam shear test is often used to reveal the interlaminar shear property of laminated composites; however, the 3D QI-braided composite is an integral structure, so the short beam shear test may not reflect its real interlaminar shear strength. The major advantage of the double-notch shear technique is that the interlaminar shear failure occurs consistently. Also, the specimen configuration is simple and specimen preparation is relatively easy.³⁴ Therefore, the double-notch shear test was adopted in this study. Figure 8(a) shows the double-notch shear load–displacement curves of the 3D QI-braided composite and the QI-laminated composite. The shear load–displacement curves of the both composites show approximate linear elastic behavior at the initial stage. After the peak value, the shear load decreased sharply for the QI-laminated composite. However, the 3D QI-braided composite displayed clear platforms after the maximum load. This was because shear failure had occurred in most of the stress regions after the maximum failure load in the 3D QI-braided composite; however, some Z-binder yarns did not fully break and continued to bear the subsequent shear stress. The above deduction can be proved by the fracture morphologies of the 3D QI-braided composite and the QI-laminated composite in Figure 9. It can be seen that there were only telescoped 0° and 90° yarns in the QI-laminated composite (Figures 9(c) and (d)), while there were some fractured Z-binder yarns in addition to telescoped 0° and 90° yarns in the fracture morphology of the 3D QI-braided composite (Figures 9(a) and (b)). As the Z-binder yarns can bear more stress than the resin, the interlaminar shear strength of the 3D QI-braided composite was 14.29% higher than that of the QI-laminated composite (Figure 8(b)).

Figure 8.

Shear load–displacement curves (a) and interlaminar shear strength bar graph of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite (b).

Figure 9.

Fracture morphologies of the three-dimensional quasi-isotropic (QI)-braided composite (a) and the QI-laminated composite (c) after double-notch shear failure; (b) and (d) show the zooms on cracks corresponding to (a) and (c), respectively.

EM properties

The EMAPs of the material can be evaluated in terms of the refection loss (RL), which is expressed by the following equation^15,35

RL = 20 log | S_{11} | (dB)

The EM wave RL curves of the 3D QI-braided composite in four directions are shown in Figure 10. The –45°, 0, + 45°, and 90° representing the electric field direction were perpendicular to the –45°, 0°, + 45°, and 90° yarn directions of the 3D QI-braided composite, respectively. In the entire Ku waveband, the RL of the 3D QI-braided composite was below –2 dB, and the minimum RL for –45°, 0, + 45°, and 90° were –7.04, –6.82, –7.02, and –6.84 dB at the frequency of 18 GHz, respectively. Furthermore, the 3D QI-braided composite had similar RL curves in four different directions, which indicated that the 3D QI-braided composite was a QI material for EM waves. This was because of the uniform yarn arrangement in the +45°, –45°, 90°, 0° directions. The uniform yarns in the 3D QI-braided composite will lead to in-plane QI mechanical properties of 3D QI-braided composite. The above results also indicated that the waveguide measurement system can be used as a nondestructive testing method to detect the in-plane distribution of CFs for CF-PMCs.

Figure 10.

Electromagnetic wave reflection loss curves of the three-dimensional quasi-isotropic-braided composite in four directions.

Figure 11 shows the RL curves of the 3D QI-braided composite and the QI-laminated composite at + 45° yarn direction. It can be seen that the RL of the 3D QI-braided composite was lower than that of the QI-laminated composite over the entire testing range, and this phenomenon was more obvious at high frequency. The maximum RL for the 3D QI-braided composite and the QI-laminated composite were –7.02 and −5.48 dB at 18 GHz, respectively. Xin et al.³⁶ reported that the RL of T300 carbon fiber/epoxy unidirectional composites with 50% fiber volume fraction was from –1 to –2 dB in the frequency range of 8–18 GHz. Hence, it is easy to draw the conclusion that the 3D QI-braided composite and the QI-laminated composite exhibited better EM absorption properties than the carbon fiber/epoxy unidirectional composites over a broadband frequency range of 11.9–18 GHz.

Figure 11.

Electromagnetic wave reflection loss curves of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite.

The logarithmic scale of the ratio E_t (electric field intensity of the transmitted EM wave) to E_i (electric field intensity of incident EM wave) was called the transmission loss (TL) and expressed by the following equation^17,33

TL = 20 log | S_{12} | (dB)

Figure 12 shows the TL curves of the 3D QI-braided composite and the QI-laminated composite at + 45° yarn direction. It can be seen that the TL of the 3D QI-braided composite and the QI-laminated composite were both lower than –46 dB, which means 99.99% of incident radiation was blocked.³³ The TL of the 3D QI-braided composite was higher than that of the QI-laminated composite in the Ku band, and the minimum TL for the 3D QI-braided composite and the QI-laminated composite were –48.00 and –52.29 dB at the frequency of 18 GHz, respectively. This could be because the arrays of Z-directional E-GF yarns (Figure 13) with high resistivity and low permittivity can function as a transmission channel to allow the directional propagation of EM waves. When the EM wave entered the composite, the in-plane CFs captured the specific frequency region of the EM wave and converted it to high-frequency currents.

Figure 12.

Electromagnetic wave transmission loss curves of the three-dimensional (3D) quasi-isotropic (QI)-braided composite and the QI-laminated composite.

Figure 13.

Construction of Z-binder E-glass fiber yarns in the three-dimensional quasi-isotropic-braided composite.

In addition, the 3D interlaced fabric structure is like that of a microwave anechoic chamber,^33,37 where there were many small pyramids between the intersections of the Z-yarns and in-plane yarns. When the incident EM wave arrives at the CFs, it was multi-reflected and gradually attenuated.³⁷ Benefiting from the combination of EM transmission effects of Z-directional E-GF yarns and the microwave anechoic chamber effect of the 3D interlaced fabric structure, the 3D QI-braided composite had higher radar absorption than that of the QI-laminated composite.

Conclusions

A “sandwich-type” composite reinforced by 3D QI CF/GF braided fabric was designed to meet the needs of modern absorbing materials, such as being “wide, thin, light and strong.” In order to determine the reinforced structures effect on the mechanical and EMAPs of hybrid composite, a traditional QI-laminated CF/GF reinforced composite was designed corresponding to the 3D QI-braided composite.

The tensile, bending, and shear properties of the 3D QI-braided composite were superior to those of the QI-laminated composite. This was because the cracks that formed under the stress between layers easily led to delamination and ultimate failure for the QI-laminated composite, while the Z-binder yarns in the 3D QI-braided composite can effectively prevent crack propagation in interlamination and avoid delamination damage.

The 3D QI-braided composite had the QI EM properties due to the uniform arrangement of CF yarns in four directions (0°, 90°, + 45°, –45°). As the Z-binder GF yarns have a good EM wave transmission property in the thickness direction, the EMAPs of the 3D QI-braided composite were higher than that of the QI-laminated composite in the Ku band.

This work provides a simple way to improve the mechanical properties and radar absorption capacity of structural stealth material with the 3D QI-braided structure. However, we only investigated the EM property of the composites in the Ku band, and we need to further design a 3D QI-braided composite to achieve stealth in the entire radar band (X band and Ku band) so that it can be used in weapon systems, such as fighters, warships, and missiles.

Footnotes

Authors' Note

Wei Fan and Lili Xue contributed to this work equally.

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation, China (Grant No: 51603163), the Science and Technology Project of Shaanxi, China (Grant No: 2017JQ5056), the Science and Technology Project of the Textile Industry Association, China (Grant No: 2017041), the Scientific Research Program Funded by the Shaanxi Provincial Education Department, China (Grant No: 18JS041), the Young Talent fund of the University Association for Science and Technology in Shaanxi Province, China (Grant No: 20160123), the Thousand Talents Program of Shaanxi Province, and the Sanqin Scholar Foundation of Shaanxi Province, China.

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