Jiayu Xiao

Effects of seawater immersion on water absorption and mechanical properties of GFRP composites

Water absorption behavior and mechanical properties of specimens of an epoxy resin casting and the glass fiber reinforced epoxy matrix composites (GFRP) with two fiber volume fractions (Vf) immersed into artificial seawater were investigated. The composite specimens absorbed less water than the resin casting because water absorption by the matrices was the main way in the composites. The composites with a Vf of 44% absorbed more water than the composites with a Vf of 34% because there were more micropores inside the fiber bundles and capillary paths in the higher fiber volume fractions. Due to the reversible and irreversible changes in the resin casting and the failure of the fiber/matrix interface, the tensile strength, the flexural strength, and the ILSS of the composite specimens after 42 days’ immersion decreased 13%, 43%, 50%, respectively. And the tensile strength, the flexural strength and the ILSS of the specimens after desorption were 97%, 38%, and 43% of the original state, respectively.

Matrix modification with silane coupling agent for carbon fiber reinforced epoxy composites

To improve interfacial adhesion between carbon fiber and epoxy resin, the epoxy matrix is modified with N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (YDH602) and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (YDH792), respectively. And the effect of matrix modification on the mechanical performance of carbon/epoxy composites is investigated in terms of tensile, flexural and interlaminar properties. The flexural properties indicate that the optimum concentration of silane coupling agents YDH602 and YDH792 for the matrix modification is approximately 0.5 wt% of the epoxy resin system, and the mechanical properties of the YDH792-modified epoxy composites is better than that of the YDH602-modified epoxy composites at the same concentration. Compared to unmodified epoxy composite, the incorporation of 0.5 wt% YDH792 results in an increase of 4, 44 and 42 % in tensile, flexural and interlaminar shear strength (ILSS) values of the carbon/epoxy composite, respectively, while the corresponding enhancement of tensile and flexural modulus is 3 and 15 %. These improvements in mechanical properties can be considered to be an indication of better fiber/matrix interfacial adhesion as confirmed by SEM micrographs of the fracture surface after interlaminar shear testing. The viscosity of the modified epoxy resin system can be reduced by incorporation of silane coupling agent YDH792, which is beneficial for fiber impregnation or wetting during liquid composite molding process.

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.

Compression performance of integrated 3D composite sandwich structures

A new type of 3D composite sandwich structure with foam core reinforced by composite columns, which can be called integrated 3D composite sandwich structure, is designed, fabricated, and studied. The integrated 3D composite sandwich structure inherits the advantages and improves the shortcomings of the traditional foam core composite sandwich structures. All types of the samples with different consisting materials and structural parameters are tested and analyzed under compressive load. The results show that the compressive properties of the integrated 3D composite sandwich structures are obviously better than those of the traditional foam core composite sandwich structures and due to the fact that the composite columns and the foam support each other. Moreover, the support effect is more obvious for the better toughness foam. And the structural parameters have significant influence on the out-of-plane compressive properties, especially the diameter of composite columns and the distance between composite columns. However, the variety of these parameters results in a corresponding change of the mass and then effect the specific compressive properties. Consequently, it is necessary to find a balance among these parameters in order to get the optimal properties.

A new resin with improved processability and thermal stability

To maintain outstanding thermal stability, amino- and hydroxyl-containing phthalonitrile monomers, 4-(4-aminophenoxy)-phthalonitrile (APN) and 4-(4-hydroxyphenoxy)-phthalonitrile (HPN) were selected and synthesized. Their structures were confirmed by proton nuclear magnetic resonance spectroscopy. Their curing polymers were characterized by Fourier transform infrared spectroscopy. The self-catalytic curing behaviors of the monomers were investigated by differential scanning calorimetry (DSC) at different heating rates. From the results, APN exhibits a higher curing temperature, while HPN exhibits a longer curing time. Then, mixtures of these monomers were investigated by DSC. The result shows that the 50/50 mixture exhibits different autocatalytic behaviors: the curing temperature is lower than that of APN and the curing time of the mixture is shorter than that of HPN. Furthermore, thermogravimetric analysis shows that the polymer from the mixture exhibits higher temperature of 5% weight loss (T5%) and char yield value at 800°C than those of the polymers from each monomer. All these results indicate that the new mixture resin exhibits improved processability with excellent thermal stability, attributed to the synergistic effect between similar monomers; the synergistic effect optimizes the cure reaction kinetics and promotes cross-linking reactions, thereby producing an excellent resin; this approach is a new method for improving the processability without sacrificing thermal stability.

Flexural performance of integrated 3D composite sandwich structures

A new type of 3D composite sandwich structure with foam core reinforced by composite columns, which can be called an integrated 3D composite sandwich structure, was designed, fabricated, and studied. The integrated 3D composite sandwich structure inherited the advantages and improved the shortcomings of the traditional foam core composite sandwich structures. All types of the samples with different consisting materials and structural parameters were tested and analyzed under bending load. The results show that the flexural properties of the integrated 3D composite sandwich structures are obviously better than those of the traditional foam core composite sandwich structures and due to the fact that the composite columns strengthen and stiffen the foam core. Moreover, this effect is more obvious for the better mechanical properties foam. And the structural parameters have significant influence on the flexural properties, especially the thicknesses of face sheet and foam core. However, the variety of these parameters results in a corresponding change of the mass and then effect the specific flexural properties. Consequently, it is necessary to find a balance among these parameters in order to get the optimal properties.

Construction of super-hydrophobic iron with a hierarchical surface structure

Wettability of an iron surface is crucial for the wide applications of iron in practice. In this work, a hierarchical structure highly similar to that of the underside of a bamboo leaf was constructed on an iron surface via the template method and controllable etching. After modification by stearic acid, the iron surface with hierarchical structure showed excellent water repellency, with an average contact angle of 156° and a sliding angle of 3°. X-ray diffraction (XRD) techniques and Fourier transform infrared spectroscopy (FTIR) are applied to examine the chemical components of an iron surface.

Experiments on shear performance of integrated 3D composite sandwich structures

A new type of 3D composite sandwich structure with foam core reinforced by composite columns, which can be called integrated 3D composite sandwich structure, was designed, fabricated and studied. The integrated 3D composite sandwich structure inherited the advantages and improved the shortcomings of the traditional foam core composite sandwich structures. All types of the samples with different consisting materials and structural parameters were tested and analyzed under tension shear load. The results show that the shear properties of the integrated 3D composite sandwich structures are obviously better than those of the traditional foam core composite sandwich structures and due to the fact that the composite columns bond the two face sheets and the core together. Moreover, the stitch-bonding effect is more obvious for the better shear properties foam. The structural parameters have significant influence on the shear properties, especially the diameter of composite columns and the distance between composite columns. However, the variety of these parameters result in a corresponding change of the mass and then effect the specific shear properties. Consequently, it is necessary to find a balance among these parameters in order to get the optimal properties.

Thermal Residual Stresses in Single-sided Bonded Composite Patching

Thermal residual strains/stresses in the single-sided bonded specimens, resulting from the mismatch in coefficients of thermal expansion between the carbon/epoxy composite patch and the aluminum alloy substrate, were experimentally and numerically investigated. A two-dimensional plane-stress finite element model was developed to predict the thermal residual stresses. The finite element analysis (FEA) results agree well with the experimental results, which indicates the FEA is an efficient method to estimate the residual stresses in the single-sided patched structures. The FEA results show that the residual stress in the substrate near the adhesive interface and the peak shear stress in the adhesive layer can be efficiently reduced by increasing the adhesive layer thickness, and as the patch to substrate stiffness ratio increases, the residual stress decreases and the peak shear stress increases.Thermal residual strains/stresses in the single-sided bonded specimens, resulting from the mismatch in coefficients of thermal expansion between the carbon/epoxy composite patch and the aluminum alloy substrate, were experimentally and numerically investigated. A two-dimensional plane-stress finite element model was developed to predict the thermal residual stresses. The finite element analysis (FEA) results agree well with the experimental results, which indicates the FEA is an efficient method to estimate the residual stresses in the single-sided patched structures. The FEA results show that the residual stress in the substrate near the adhesive interface and the peak shear stress in the adhesive layer can be efficiently reduced by increasing the adhesive layer thickness, and as the patch to substrate stiffness ratio increases, the residual stress decreases and the peak shear stress increases.

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.