Su Ju

The use of cutting carbon fiber fabric/epoxy composites as band-pass frequency selective surfaces

Sandwich structures with frequency selective surfaces have been widely used to fabricate low-observable radomes. Traditional frequency selective surfaces were made of metals, such as copper and aluminum. There were problems when used the frequency selective surfaces with the sandwich structures, such as a bonding layer and thermal mismatch. Because of the thermal mismatch between the sandwich structures and the metals, mechanical properties of the sandwich structures usually decreased. To avoid these disadvantages, a new type of composite frequency selective surface was put forward in this study. A four-legged slot-array frequency selective surface and a square-aperture array frequency selective surface were fabricated by cutting carbon fiber/epoxy composites. Free-space method and finite-element method were carried out to evaluate the electromagnetic transmission characteristics of the frequency selective surface specimens, respectively. Results show that the composite frequency selective surface with four-legged slot array can realize the function of frequency selection, and its minimum transmission loss can be decreased by increasing the electrical conductivities of the composite material, adjusting the thickness, and increasing the aperture-to-cell ratio of it. For the composite frequency selective surface with square-aperture array, grating lobes come closely after its resonant frequency, although its minimum transmission loss is very close to zero.

Flexural properties of lightweight FRP composite truss structures

This article deals with the flexural behavior of a lightweight, long-span, and triangular cross-section fiber-reinforced polymer (FRP) composite truss. Three-point bending tests are conducted for a 6 m long, triangular cross-section, glass fiber/epoxy composite truss and characteristic structural responses identified. Numerical analysis has been performed, validated by the experimental results. Stiffness to weight ratio of this truss structure made by different materials has been compared by numerical analysis under the same geometry dimension constraints. A parametric study has been employed to determine the effects of different geometric parameters on the flexural performance of this lightweight FRP composite truss. The composite truss is of bilinear behavior under three-point bending. The linear load capacity and flexural stiffness of this composite truss structures are identified. A relationship between the individual geometric parameters and the flexural properties has been obtained.

Exploration relation between interlaminar shear properties of thin-ply laminates under short-beam bending and meso-structures

Carbon fiber reinforced polymer matrix composite laminates with standard thickness plies (0.125 mm) usually have weak interlaminar shear strength, meanwhile, for thin-thickness laminate structures such as aircraft wing skin, it is difficult to design a balanced laminate with the standard plies. It is a possible way to improve the interlaminar shear performance of carbon fiber reinforced polymer composite laminates and enlarge the design space of the thin-thickness structures by using thin-plies technology. In this paper, the interlaminar shear strength of carbon fiber/epoxy laminates with thin prepreg thickness subjected to short-beam bending is investigated. Unidirectional, cross-ply and quasi-isotropic laminate specimens were prepared by using prepregs with different ply thicknesses. Results show that, with decreasing of the ply thickness, higher interlaminar shear strength and smaller coefficient of variation of the data are obtained. Compared to laminates made by standard thickness prepreg, the laminates with thin-thickness prepreg exhibit more homogeneous microstructures and more regularly interlaminar shear stress distribution. This indicates that inherent anisotropy of the laminate composites is weakened in the thin-ply laminates and show pseudo-isotropic behavior. Especially in the case of ply thickness less than 0.020 mm, the interlaminar shear stress distributions of the cross-ply and quasi-isotropic laminate are almost the same with that of isotropic materials according to the classic laminate theory. On the other hand, as expected, the design space of the thin-thickness laminate structures will be increased since more ply number are allowed and superior interlaminar properties can be obtained due to the pseudo-isotropic behavior of the thin plies.

Investigation of Tensile Property of Thin Ply Composite Laminate with Open-Hole Assisted by Acoustic Emission Technology

As the application of carbon fiber reinforced polymer composite (CFRP) laminates in aerospace and aircraft structures, opening hole is inevitable in design, fabricate and assemblage. However, the open-hole procedure has a bad effect on mechanical properties of the composite laminate. In this paper, characters of acoustic emission (AE) and curve of stress-strain of composite laminate with open-hole under tensile test were obtained. It was found that onset damage stress of a thin ply laminate detected using acoustic emission was higher than that of the standard ply laminate with the decreasing of ply thickness, while ultimate open-hole tensile strength of the thin ply laminate was lower than that of the standard ply laminate. More homogeneous microstructure and balanced stacking sequence make the thin ply laminate have higher initial damage stress. AE signals of matrix damage and delamination are presumably classified by the amplitude, which validate superior damage suppression and delamination resistance in the thin ply laminates. Stress concentration around the open-hole cannot be relaxed under tensile test as higher damage suppression and delamination resistance in the thin ply laminate. That can lead the thin ply laminate has lower open-hole tensile strength.