Abstracts:Tri-axial electrospun fibers with self-healing capability are fabricated through a direct, one-step tri-axial electrospinning process. They have been designed to have two distinct protective walls to encapsulate epoxy resin and its hardener as healing agents in separate cores. The presence of an extra layer between encapsulated liquid healing agent and outer layer enables the encapsulation of chemically and physically active healing agents, extends the efficiency and life-time of the healing functionality. Tri-axial electrospun healing fibers are incorporated to add self-healing capability into solo epoxy matrix and also utilized as an interlayer between glass fabric mats in glass fiber reinforced composite. Tri-axial electrospun fiber interlayers provide self-healing functionality at the interface of glass fibers with epoxy matrix, which is highly prone to failure. In addition, various structural health monitoring and non-destructive testing techniques coupled with traditional mechanical testing methods are employed to evaluate the self-healing efficiency of composite structures. In this study, successful and recurring self-healing ability of composite structures at the interface of glass fiber with the epoxy matrix are achieved and confirmed using different characterization techniques.

Abstracts:The effective anisotropic stiffness tensor $C_{\mathit{ijkl}}$ of a woven composite lamina can be calculated by performing a finite element analysis (FEA) of the representative volume element (RVE); however, the complex geometry of heterogeneous RVEs makes meshing tedious. This paper develops two unique multiphase voxel elements (MVEs) that account for multiple materials within an elemental domain by utilizing the material properties at each integration point to develop local strain corrections. Studies performed on simple cuboid geometries show exceptional agreement with standard FEA results, and results from the new voxel elements are more accurate than previous MVEs presented in literature. The application for these new MVEs is demonstrated on plain and twill weave composite laminae, and the results generated are in exceptional agreement with experimental results available in the literature and computationally expensive studies using standard finite elements.

Abstracts:Z-pin through-thickness reinforcement is used to improve the impact resistance of composite structures; however, the effect of loading rate on Z-pin behaviour is not well understood. The dynamic response of Z-pins in mode I and II delamination of quasi-isotropic IM7/8552 laminates was characterized experimentally in this work. Z-pinned samples were loaded at both quasi-static and dynamic rates, up to a separation velocity of 12m/s. The efficiency of Z-pins in mode I delamination decreased with loading rate, which was mainly due to the change in the pin misalignment, the failure surface morphology and to inertia. The Z-pins failed at small displacements in the mode II loading experiments, resulting in much lower energy dissipation in comparison with the mode I case. The total energy dissipation decreased with increasing loading rate, while enhanced interfacial friction due to failed pins may be largely responsible for the higher energy dissipation in quasi-static experiments.

Abstracts:The cure history dependent residual deformation in a thermosetting composite laminate was investigated using bi-lamina strip sample with [0/904] stacking-sequence. The samples were subjected to cure cycles with different heating profiles. Significant dependence of the residual strip deflection was found to relate to (i) the interaction of thermal expansion and cure shrinkage of resin and (ii) dependence of resin modulus development on cure strain rate. The lamina constitutive model was proposed to include cure induced shrinkage and composite hardening during cure. The model was calibrated following the single and multiple ramps cure cycles and applied to a number of two hold stage cure cycles with constant maximum temperature. The heating ramp of the cure cycle was varied allowing decreasing the residual deformation at ambient conditions after cure. The presented methodology can be applied for designing the cure cycle to reduce the amount of residual deformation in composite materials from manufacturing.

Abstracts:Thermally conductive poly (2-ethylhexyl acrylate) (P2EHA)/functionalized graphene/h-boron nitride composites materials were fabricated by colloid blending and self-assembly approach. Here, self-assembly technology has been used to achieve the facile surface modification of hexagonal boron nitride (h-BN) microplatelets by forming a functionalized graphene (f-G) sheets on its surface via π-π interaction. The polar functionality on the graphene surface allowed the permeation of the polymer matrix through the secondary interaction between them, increasing the content of this hybridized filler in composite. The effective and successful fabrication of f-G@h-BN microplates have been confirmed by SEM, Raman spectroscopy, XRD, and TGA investigations. The self-alignment 2D stacked fillers result in higher in-plane thermal conductivities and excellent electrical insulation of the composites. In addition to the good adhesive properties, the procedure is environment-friendly, easy operation, and potential for the practical application in large scale.

Abstracts:Flexible fiber-reinforced laminated composite adhesive combining with functionalized-graphene (f-G) layer and hexagonal boron nitride (h-BN) layer was prepared by colloid-blending and self-assembly technology with the assistance of the secondary force and hydrophilic difference. In this system, poly (2-ethylhexyl acrylate) (P2EHA) as the polymer matrix linked the layer of functionalized-graphene and another layer of hexagonal boron nitride like the cross-linker or adhesive via self-assembly technology. Lewis acid-base (δ⁺−δ⁻) interaction and π-π stacking improved the compatibility between the filler and the polymer matrix. The effective and successful fabrication of flexible f-G/h-BN laminated composite adhesive has been confirmed by SEM, Raman spectroscopy, and XRD investigations. The oriented stacking and laminated structure resulted in much higher in-plane thermal conductivities (∼4.20W/mK) and insulation in the direction through the plane and good adhesive properties. The procedure was environmental friendly, easy operation, and potential for the practical application in industry.

Abstracts:This paper proposes a novel functionally graded layer composite structure (FGLCS) for use in hypersonic vehicles. It consists of a passive thermal protection layer made from ultra-high temperature ceramic composites, an interface layer of high temperature insulating material and an active thermal protection layer of alloy. The transient thermal stress alleviation characteristics of an FGLCS plate and an FGLCS strut under aerodynamic heating conditions were studied using a finite element method. Compared to a traditional ultra-high temperature ceramic (UHTC) plate and strut, FGLCS can greatly reduce the thermal stress level of structures, the stress relief can be up to 84% in a case study considered in this paper. In addition, the long-term and reusable performance of a prepared FGLCS strut, with much lower thermal stress, was completely validated for the first time by an oxy-propane testing system in a combustion environment.

Abstracts:Graphite-like carbon nitride (g-C3N4) was innovatively modified by diethylphosphinic acid through hydrogen bonding; g-C3N4/organic aluminum diethylhypophosphites (g-C3N4/DAHPi) hybrid (denoted as CDAHPi) was synthesized by salification reactions, and subsequently incorporated into PS matrix to prepare composites through melt blending method. Thermal data showed that g-C3N4 could protect DAHPi from external heat, leading to improved thermal stability of DAHPi. Moreover, it was found that the introduction of the hybrid reduced the values of heat release rate (HRR) and total heat release (THR) of the composites. Compared with those of pure PS, the HRR and THR of PS/CDAPHi4 decreasing by 43% and 21% respectively were observed at loadings as low as 4.0wt%. The PS/CDAHPi exhibited an additional advantage in suppression of pyrolysis gas production in comparison with neat PS. In addition, the additive showed higher interfacial adhesion with PS.

Abstracts:It was found that metallic oxide nanoparticles may positively influence insulation properties of polymers working under high electric field. Herewith, four kinds of surface-modified silica nanoparticles were employed to fabricate epoxy nanocomposites. The surface properties of nanoparticles were characterized by Fourier-transform infrared spectroscopy, thermogravimetric analysis and contact angles. The effects of surface modification, filler fraction and test temperature on volume resistivity (ρ v) of epoxy nanocomposites were studied. It was found that, at different test temperatures, the long-alkyl-modified nanoparticles resulted in higher ρ v values of epoxy and less ρ v sensitivity to temperature, compared to the short-alkyl-modified and hydroxyl-modified ones. The surface polarity of nanoparticles was found to correlate with the ρ v values well. The surface modification may cause two possible mechanisms that affect the ρ v values of the epoxy: (i) to offer the nanoparticles different levels of water absorption; (ii) to change the Maxwell-Wagner-Sillars polarization behaviors of the composites.

Abstracts:Blends of difunctional epoxy monomer with a 4,4′-diaminodiphenylsulfone hardener and poly(ε-caprolactone) (PCL) were used as a self-healing matrix in woven glass fibre-reinforced polymer composites (FRPs). FRPs with these blends (containing 0, 25 and 37vol% of PCL in the blend) were manufactured through Vacuum Assisted Resin Infusion Moulding at high temperature and the matrix, resulting from polymerization induced phase separation, consisted of interconnected epoxy particles embedded in PCL. With 25vol% PCL in the matrix, similar storage modulus and interfacial shear strength as compared to unmodified systems have been observed, however toughness was decreased by 40%. Up to 45% toughness recovery and over 100% stiffness recovery were observed over several cycles when the blend matrix composite samples were re-tested after a thermal cycle at 150°C for 30 min. These composites can thus provide efficient crack healing, but remain more sensitive to initial crack propagation due to confinement of the thermoplastic phase.