**1. Introduction**

The problem of electromagnetic (EM) interference and radiation has stimulated the research interest in EM absorbing composites with thin thickness, low density, broadband and strong absorption [1]. The research of EM absorbing composites has gradually turned to structural EM absorbing composites, which have the functions of load-bearing and a broadband EM energy absorbing capability. At present, the research of structural absorbing composites, such as foam EM absorbing composites and honeycomb EM absorbing composites, is being studied to overcome the disadvantages of traditional EM absorbing composites, such as high density, weak absorption and narrow bandwidth [2]. Compared with other structures, the honeycomb structure can not only be used as a part of the main load-bearing structure, but also as a carrier of EM absorbing composites and absorbing medium [3,4]. Li et al. [5] prepared carbon-coated honeycomb absorbing composites and analyzed in detail the effects of honeycomb aperture, honeycomb height, coating thickness, dielectric constant, and dielectric loss factor on EM absorption. Wang et al. [6] integrated shorted carbon fibers into honeycomb frames as a dual-functional material for radar absorption and structural reinforcement, which showed excellent EM absorption at frequencies ranging from 2–18 GHz. Sun et al. [7] used a honeycomb structure of aramid paper impregnated with EM absorbent as the core layer and quartz fiberboard as the skin to form a composite sandwich structure with EM absorption ability bonding. The experimental

**Citation:** Lyu, L.-H.; Liu, W.-D.; Sun, B.-Z. Electromagnetic Wave-Absorbing and Bending Properties of Three-Dimensional Honeycomb Woven Composites. *Polymers* **2021**, *13*, 1485. https:// doi.org/10.3390/polym13091485

Academic Editor: Tarek M. Abou Elmaaty

Received: 5 April 2021 Accepted: 30 April 2021 Published: 5 May 2021

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results showed that the minimum reflection loss (RLmin) was −29.5 dB, and the EAB was 13.1 GHz.

At present, honeycomb structure EM absorbing composites are mostly prepared by the lamination method, which has some disadvantages which were under high temperature and high humidity environments or alternating external forces, the honeycomb structure EM absorbing composites are easy to break [8–10]. Therefore, the traditional honeycomb structure EM absorbing composites have difficulty achieving load-bearing and a broadband EM energy absorbing capability.

However, three-dimensional (3-D) honeycomb woven composites are composites which are made of 3-D honeycomb woven fabric as reinforcement and resin or other polymers as the matrix. Three-dimensional honeycomb woven fabric is a kind of 3-D structure textile material. In the fabric structure of the textile material, the concept of vertical yarn was introduced along the thickness direction of the material, and the vertical yarn was used to connect the warp yarn and the weft yarn. There were fiber bundles and reinforcement structures through the thickness direction to enhance the integrity of the textile materials. Therefore, 3-D honeycomb woven composites have good integrity and structural stability and can solve the interlayer problems of traditional honeycomb structure EM absorbing composites [11]. Three-dimensional honeycomb woven fabrics have been applied in a number of practical applications, for example fibrous porous media [12]. Bayraktar et al. [13] prepared a 3-D honeycomb woven composite and carried out low-speed impact tests on fabric samples using a drop-weight impact tester. The experimental results showed that the energy absorbed by the honeycomb structure was more significant than that absorbed by the plate sample. The honeycomb woven fabric showed good mechanical properties. Lv et al. [14] prepared 3-D honeycomb structure composites using glass fiber filament yarn/BF filament yarn on the ordinary loom and tested their bending properties, providing a reference structural optimization design and performance analysis of 3-D honeycomb woven composites. Zahid et al. [15] wove three kinds of 3-D honeycomb woven fabrics with different honeycomb sizes, and the breaking strength and elongation at break of the 3-D honeycomb woven fabrics were measured. The effect of honeycomb size on the mechanical properties should be considered in the design and weaving of 3-D honeycomb woven fabrics.

Current studies have shown that 3-D honeycomb woven composites have good antidelamination abilities. However, there was no research on the EM absorbing property of 3-D honeycomb woven composites. Therefore, the low-cost design and preparation of 3-D honeycomb woven EM absorbing composites with the integration of load-bearing and absorbing structures and functions can realize the collaborative design of mechanical properties and EM absorbing properties the composite.

Therefore, in this study, three different thicknesses (7.5 mm, 15 mm, 22.5 mm) of 3-D honeycomb fabrics with ordinary looms were woven through practical design. The CB/CIP/EP as the matrix and the 3-D honeycomb woven EM absorbing composites were prepared by the VARTM process. Then, the coaxial method was used to study the EM properties of CB/CIP. The 3-D honeycomb woven EM absorbing composite used the United States Naval Research Laboratory (NRL) arch method to test the RL on 2–18 GHz. Next, the bending properties were tested by a microcomputer-controlled electronic universal testing machine. Finally, it discussed particles and absorbent material thickness absorbing the composites' performance and the influence of mechanical properties, and experimental results obtained from the failure modes. An effective method to coordinate the design of absorbing and mechanical properties was proposed.

#### **2. Materials and Methods**

#### *2.1. Materials and Equipment*

EP JC-02A and the solidification reagent JC-02B (Changshu Jiafa Chemical Co., Ltd., Changshu City, China). 800 tex CF filament SYT49 (Zhongfu Shenying Carbon Fiber Co., Ltd., Lianyungang City, China). 800 tex BF filament (Zhejiang Shijin Basalt Fiber Co., Ltd., Hangzhou City, China). Ordinary loom SGA 598 (Jiangyin Tong Yuan spinning machine Co., Ltd., Wuxi City, China). The universal testing machine TH-8102S (Suzhou Tuobo Machinery Equipment Co., Ltd., Suzhou City, China).

#### *2.2. Preform Design and Weaving*

The warp structural drawings of 3-D honeycomb woven fabrics with three different layers are shown in Figure 1. As shown in Figure 1, the bottom warp and weft yarns were made entirely of 800 tex CF filament, and the rest of the warp and weft yarns were wholly made of 800 tex BF filament.

**Figure 1.** (**a**) Warp structural drawings of 7.5 mm 3D honeycomb woven fabric; (**b**) warp structural drawings of 15 mm 3D honeycomb woven fabric; (**c**) warp structural drawings of 22.5 mm 3D honeycomb woven fabric.

The chain drafts of 3-D honeycomb woven fabrics with three different thicknesses were drawn up according to the warp structural drawings, and they are shown in Figure 2. The weaving parameters of 3-D honeycomb woven fabrics are shown in Table 1.

**Figure 2.** (**a**) Chain draft of 7.5 mm 3-D honeycomb woven fabric; (**b**) chain draft of 15 mm 3-D honeycomb woven fabric; (**c**) chain draft of 22.5 mm 3-D honeycomb woven fabric.


**Table 1.** Weaving parameters of 3-D honeycomb woven fabrics.
