Abstract:

Objective Angle interlock composites have been widely used in defense engineering fields because of their excellent interlayer delamination performance, strong designability, near-clean forming and other characteristics, and can still maintain good structural integrity after post-processing. In recent years, major equipment components in the air, sky and sea have increasingly high requirements for lightweight, and, therefore, it is urgent to carry out innovative structural design and application of angle interlock composites to effectively reduce structural parameters and improve load-bearing efficiency. Among them, angle interlock composite with variable densities provides an effective means to realize the "light-strength coordination" of components.

Method In order to explore the influence of warp density or weft density on compression mechanical properties of angle interlock carbon/epoxy composites, the T700-12K carbon fiber was selected. Then three-dimensional (3-D) woven angle interlock composites with variable warp density or weft density and constant warp and weft density were prepared by resin transfer molding (RTM). On this basis, combined with digital image correlation technology (DIC), the out-of-plane compression test was carried out, and the damage morphology and damage distribution inside the samples were analyzed by scanning electron microscopy and computed tomography (Micro-CT). Finally, the progressive damage evolution process and failure mechanism of out-of-plane compression were sorted out.

Results All samples showed yield failure characteristics. In addition, by comparing with the compression specific strength and compression specific modulus of 5 types of composites, it was found that the compression specific modulus of samples with variable densities was significantly improved. Among them, the compressive specific strength of the warp yarn is dense at the top and sparse at the bottom composites (BJ-MS) sample was 4.36% higher than that of the warp yarn is sparse on the top and dense on the BJ-SM sample, and the compressive specific strength of the weft yarn is sparse on the top and dense on the BW-SM sample was 15.72% higher than that of he weft yarn is dense at the top and sparse at the BW-MS sample. In the warp and weft density remain unchanged composites (JZ) sample under compression load, the maximum strain first appeared at both ends of the sample. When the strain is 4.06%, a large range of stress concentration occurred in the middle region. Thus the continuously deformed resin matrix was first destroyed along the thickness direction, and then the load was transferred to the fiber bundle. Finally, there is an obvious kink phenomenon on the curved warp yarn. In the BJ sample under compression load, a large curved strain band appeared in the region with large warp density (strain 4.06%). With increase of the displacement, BJ sample finally developed a large range of delamination in the region with large warp density. For the BW sample under compression load, there is not only a large curved strain band, but also a kinking phenomenon in the area of small weft density (strain 4.06%). In addition, the damage of BW samples is slight in the area of larger weft density. According to the CT results, the interfacial cracks of the angle interlock composites were curved warp and straight weft. The damage volume and proportion of JZ samples were the largest, which were 26.90 mm³ and 3.09%, respectively. Moreover, as seen from the CT injury image the damage modes of the angle interlocking composites included matrix cracking, fiber debonding, fiber fracture and delamination.

Conclusion It can be concluded from the research that the variable density structural design is beneficial to improve the out-of-plane bearing capacity, and decrease the damage degree of the angle interlock composites. Because the bending degree of yarn varies with yarn density, the stress distribution along the thickness direction is affected. In addition, a variety of nondestructive testing methods can effectively reveal the damage process and mechanism of angle interlock composites. In the further study of three-dimensional woven angle interlock carbon/epoxy composites, it is necessary to develop a high-fidelity numerical simulation method. By establishing an accurate meso-structural model, researchers can effectively realize the prediction of progressive damage and failure mechanism.

Key words: angle interlock fabric, carbon/epoxy composite, variable density structural design, out-of-plane compression, computed tomography technology, damage mechanisms