Effect of Braiding Architectures on the Mechanical and Failure Behavior of 3D Braided Composites: Experimental Investigation.

Benefiting from the multi-directional load-bearing capability, the three-dimensional braided composites (3DBC) have found a wide application in primary structures. It is therefore of great importance to fully understand their mechanical behavior and failure modes. In the present paper, the tensile and compressive tests were carried out, according to standardized testing methods, for eight types of 3DBC, which were manufactured by resin transfer molding (RTM). It was found that the mechanical properties of the 3DBCs decreased with an increasing braiding angle. When the braiding angle was 20°, 3D 5-directional braided composite (3D5dBC) exhibited the best mechanical properties, while for the braiding angle of 40°, the mechanical properties of 3D6dBC were the most prominent. Moreover, the tensile strength of the 3DBCs is approximately two times as much as the compressive strength; however, the compressive modulus is always 10% higher than the tensile modulus. The failure modes of the 3DBCs with a braiding angle of 20°greatly depended on the braiding structures. However, they tend to be consistent when the braiding angle increases to 40°.

Effect of Absorbent Foam Filling on Mechanical Behaviors of 3D-Printed Honeycombs.

Polylactic acid (PLA) hexagonal honeycomb structures were fabricated by using 3D-printing technology. By filling with absorbent polymethacrylimide (PMI) foam, a novel absorbent-foam-filled 3D-printed honeycomb was obtained. The in-plane (L- and W-direction) and out-of-plane (T-direction) compressive performances were studied experimentally and numerically. Due to absorbent PMI foam filling, the elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass of absorbent-foam-filled honeycomb under L-direction were increased by 296.34%, 168.75%, 505.57%, and 244.22%, respectively. Moreover, the elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass, under W-direction, also have increments of 211.65%, 179.85, 799.45%, and 413.02%, respectively. However, for out-of-plane compression, the compressive strength and energy absorption per unit volume were enhanced, but the density has also been increased; thus, it is not competitive in energy absorption per unit mass. Failure mechanism and dimension effects of absorbent-foam-filled honeycomb were also considered. The approach of absorbent foam filling made the 3D-printed honeycomb structure more competitive in electromagnetic wave stealth applications, while acting simultaneously as load-carrying structures.

Enhancement by Metallic Tube Filling of the Mechanical Properties of Electromagnetic Wave Absorbent Polymethacrylimide Foam.

By the addition of a carbon-based electromagnetic absorbing agent during the foaming process, a novel electromagnetic absorbent polymethacrylimide (PMI) foam was obtained. The proposed foam exhibits excellent electromagnetic wave-absorbing properties, with absorptivity exceeding 85% at a large frequency range of 4.9⁻18 GHz. However, its poor mechanical properties would limit its application in load-carrying structures. In the present study, a novel enhancement approach is proposed by inserting metallic tubes into pre-perforated holes of PMI foam blocks. The mechanical properties of the tube-enhanced PMI foams were studied experimentally under compressive loading conditions. The elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass were increased by 127.9%, 133.8%, 54.2%, and 46.4%, respectively, by the metallic tube filling, and the density increased only by 5.3%. The failure mechanism of the foams was also explored. We found that the weaker interfaces between the foam and the electromagnetic absorbing agent induced crack initiation and subsequent collapses, which destroyed the structural integrity. The excellent mechanical and electromagnetic absorbing properties make the novel structure much more competitive in electromagnetic wave stealth applications, while acting simultaneously as load-carrying structures.

Paraxial propagation dynamics of the radially polarized Airy beams in uniaxial crystals orthogonal to the optical axis.

We study the paraxial propagation of the radially polarized Airy beams (RPAiBs) in uniaxial crystals orthogonal to the optical axis analytically and numerically. The propagation trajectory, the intensity and the radiation forces of the RPAiBs are investigated and the properties are elucidated by numerical examples in this paper. Results show that the RPAiBs evolve into the beams produced by the x-direction electric field (RPAiXBs) and the y-direction electric field (PRAiYBs) which are totally different in uniaxial crystals. During the propagation, the intensity of the RPAiXBs transfers from the side lobe in the x-direction to the main lobe and finally returns to the side lobe in the x-direction again, but that of the RPAiYBs transfers from the side lobe in the y-direction to the main lobe and flows to the side lobe in the x-direction at last. The effect of the intensity focusing for the RPAiXBs can be modulated by the ratio of the extraordinary index (ne) to the ordinary index (no) in anisotropic medium, which contributes to the intensity focusing of the RPAiBs in a short distance a lot. We can adjust the intensity distribution especially the focusing position, the propagation trajectory and the radiation forces distributions of the RPAiXBs through choosing an appropriate value of the ratio of ne to no to meet the actual usage accordingly.