Yizhuo Gu

Numerical Simulation of Flow and Compaction During the Cure of Laminated Composites

The finite element formulation is developed to solve the flow and compaction problems to simulate the resin flow and laminate compaction during the hot-pressing process of fiber-reinforced composite laminates. The numerical flow-compaction model is based on the effective stress formulation, the continuity equations, and Darcy’s flow theory. Two methods are used to calculate the variation of the laminate thickness, the simulated results validate the relationship of fiber motion and resin flow velocity. Together with the post-processor software, the visual laminate deforming process is realized through the node moving. The comparison between the predicted and experimental data shows that the program is reliable to predict the laminate thickness and the final fiber volume fraction distribution in the thickness direction.

Influence of oxidation and distribution of carbon nanotube on mechanical properties of buckypaper/epoxy composites:

In order to elucidate the effects of carbon nanotube surface modification and carbon nanotube distribution on mechanical properties of the buckypaper composites, two oxidation processes were developed to prepare functionalized buckypapers. The tube geometries, the entanglement, and stacking state of the carbon nanotubes were characterized by scanning electron microscope, transmission electron microscope, and atom force microscopy. Surface morphology and volume density of the buckypapers indicate that denser and homogeneous carbon nanotube skeletons are achieved by acidification on carbon nanotube powders, while only slight change can be observed from the oxidized buckypaper. The contact angles suggest that both oxidation and carbon nanotube distribution can affect the wettability of buckypapers with epoxy at 120°C, in which homogeneous and dense carbon nanotube skeleton exhibits relatively low wettability. The tensile moduli and strengths of the functionalized buckypaper composites are increased by 20–37% and 42–68%, respectively, with respect to the pristine buckypaper composite. Further analysis demonstrates that the mechanical property improvement is caused by dense carbon nanotube network and strong adhesion of oxidized carbon nanotube to the matrix. Moreover, dynamic mechanical analysis shows that the order of magnitude of storage moduli is consistent with that of the tensile Young’s moduli for different buckypaper composites.

Macro and Micro-interfacial properties of carbon fiber reinforced epoxy resin composite under hygrothermal treatments

Short beam shear test and single-fiber fragmentation test with an energy-based model were employed to investigate the changes of interfacial properties of carbon fiber/epoxy resin composites under hygrothermal treatments, including 70℃ water immersion, boiling water immersion and re-drying. Scanning electron microscope pictures of shear fracture section in short beam shear and polarized photos at first fiber break in single-fiber fragmentation test were used to analyze the failure mode at interphase region before and after water aging. Furthermore, the similarities and differences between macro- and micro-interfacial properties were compared for two kinds of carbon fiber composites. The experimental results show the consistency of interfacial hygrothermal resistance based on the results from short beam shear and single-fiber fragmentation test. The resin tensile modulus, interlaminar shear strength from short beam shear and interfacial fracture energy from single-fiber fragmentation test all decrease after water aging and recovery to certain extents in re-drying treatment. The failure mode is different for different treated samples, especially showing irreversible change after boiling water immersion. Moreover, the obvious differences between the retention rates of interfacial fracture energy and those of interlaminar shear strength indicate the discrepancy between the micro-interfacial property and the macro-interfacial property on hygrothermal resistance.

Measurement and Analysis on in-Plane and Through-Thickness air Permeation of Fiber/Resin Prepreg

Nowadays, zero-bleeding process and out-of-autoclave process are prevailing for aircraft composite structures. For these cases, the prepreg property such as air permeability becomes a crucial element for entrapped air venting off from prepreg stack during cure process. Therefore, the measurement of air permeability is important for the understanding and optimization of autoclave and vacuum-bag processes to obtain void-free composite parts. In this paper, an air permeation measuring system was established to test in-plane and through-thickness air permeabilities of prepreg stack, and the effects of compacting pressure and temperature on the air permeability were investigated. Furthermore, the influences of air permeability of prepreg stack on the void characteristics inside the cured laminates processed by autoclave and vacuum-bag processes were analyzed. The results indicate that the proposed air permeation measuring systems can quantitatively measure the in-plane and through-thickness gas permeabilities of prepreg stack. The compacting pressure and temperature have important effects on air permeability. For the same prepreg system, the in-plane air permeability is two orders of magnitudes higher than the through-thickness one. Finally, void defects are sensitive to the air permeability for vacuum-bag process, especially in the case of thick laminates. It provides an effective way to evaluate the quality of the prepreg for control of void defect and an important guideline for prepreg manufacturers.

Strong, flexible and thermal-resistant CNT/polyarylacetylene nanocomposite films

This paper reports a new nanocomposite made from a combination of high char yield polyarylacetylene (PAA) resin and ductile carbon nanotube (CNT) film. Benefiting from a big specific surface area and the excellent mechanical properties of the CNT film, the resultant nanocomposite conquers the shrinkage and crack defects of neat PAA. This nanocomposite has great flexibility and tensile strength, with moduli of 303 ± 38 MPa and 22 ± 2 GPa respectively. The through-thickness thermal conductivity of the CNT film/PAA composite reaches 1.15 W (mK)−1, seven times higher than that of pristine CNT film, and the electric conductivity increases to 700 S cm−1. Meanwhile, a lock-up effect of PAA on the CNT network ensures good stability of the structure. TG testing demonstrates that in comparison with a CNT film/epoxy composite, the CNT film/PAA composite has a significantly high decomposition temperature with a char yield of up to 90.7%. After carbonization at 900 °C for 0.5 h, the nanocomposite retains over 66% of its tensile strength.

Negative permittivity behavior of aligned carbon nanotube films

This paper reports a negative permittivity of aligned carbon nanotube (CNT) film materials by mechanical stretching. A CNT laminate with five layers of aligned CNT film exhibited unique negative permittivity, which was measured with waveguide method. Based on the microscopic morphology of aligned CNTs, a structure model from classical periodic array was introduced to describe the CNT arrangement and investigate the negative permittivity behavior of aligned CNT film. The influences of films electrical conductivity, radius of CNT bundle, and lattice constant of aligned CNT assembly on real part of permittivity were discussed. Both experimental and simulation results indicate that the negative permittivity of aligned CNT film is closely related to its stretching ratio. The real part of permittivity drops below zero when stretching ratio reaches a certain level, which is about 15% stretching ratio in our experiment, and then it keeps decreasing with increasing stretching ratio.

Effect of processing temperature on the micro- and macro-interfacial properties of carbon fiber/epoxy composites

Three different temperature schemes were applied on carbon fiber/epoxy composite to elucidate the effect on interfacial shear strength (IFSS) and inter-laminar shear strength (ILSS). It showed that carbon fiber/epoxy IFSS was significantly influenced by the processing temperature, while ILSS was only slightly changed. Moreover, the mechanical properties revealed no necessary relationship between the micro- and macro-interfacial strengths with the properties of epoxy matrix. Among all the temperature schemes, Pro2 (the one-platform curing scheme with relatively rapid heating rate) produced highest IFSS and ILSS. Fourier transform infrared spectroscopy analysis demonstrated that the sizing agent can chemically react itself and also react with epoxy resin at temperature 180 °C. The resin rheological data showed that different temperature schemes can considerably impact diffusion behavior of the resin molecules. Hence, the highest interfacial strengths for Pro2 scheme were ascribed to large extent of chemical reactions and good inter-diffusion between components, at the interface region.

Resin Pressure and Resin Flow Inside Tapered Laminates During Zero-Bleeding and Bleeding Processes

Resin pressure greatly influences resin flow, void defect, and fiber distribution inside the laminates, especially inside the composite with complex geometry. Thus, obtaining a fundamental understanding on the resin pressure inside laminates with complex shapes can provide an important guidance to control these qualities. This article is mainly focused on the resin pressure inside the composite laminates with tapered thickness and its effect on the compaction of fiber bed. The resin pressure variations inside carbon fiber fabric/epoxy resin prepreg stacks during zero-bleeding and bleeding processes were investigated by means of an on-line monitoring method. It is demonstrated that significant differences are found between the two cases. The resin pressure inside tapered stack under zero-bleeding condition evenly distributes throughout the whole cycle and leads to even distribution of fiber content. In the case of bleeding condition, the application of bleeder materials results in through-thickness and in-plane resin pressure gradients, which provide driving forces for resin flow, and it directly results in different level of fiber compaction at different regions. These results provide a basic understanding on the resin pressure characteristic inside tapered laminate during autoclave process.

Hybrid effect of carbon nanotube film and ultrathin carbon fiber prepreg composites

This paper successfully interlaced floating catalyst chemical vapor deposition-grown carbon nanotube film and ultrathin carbon fiber prepreg to achieve strong and flexible carbon nanotube/carbon fiber hybrid composites with high carbon nanotube loading. Epoxidation was also introduced to improve interlaminar interfacial bonding. It was found that pristine carbon nanotube film/carbon fiber interply hybrid composite (carbon fiber/carbon nanotube/carbon fiber) showed sudden and brittle failure, while epoxidation caused a gradual failure behavior. Hybrid effect analysis suggested that the improved tensile performance and synergistic effect of epoxidized carbon nanotube film/carbon fiber hybrid composite were attributed to good load transfer and suppressed delamination induced by improved interfacial bonding. In addition carbon fiber/carbon nanotube/carbon fiber manifested excellent damping capacity with the maximum loss factor of 0.13. The in-plane electrical conductivity of composite with global carbon nanotube content of 21 wt% increased to the same order of magnitude as carbon nanotube film composite. The excellent mechanical, damping, and electrical properties demonstrated great potential for both structural and multifunctional applications of the resultant hybrid composites.

Improvement in skin-core adhesion of multiwalled carbon nanotubes modified carbon fiber prepreg/Nomex honeycomb sandwich composites

An experimental investigation on the sandwich composites composed of the carbon fiber face sheets and Nomex honeycomb core has been carried out in this study. Multiwalled carbon nanotubes were added into the matrix of prepreg for converting the traditional pure resin adhesive fillet to composites. Resin viscosity was measured to evaluate the effect of the additive amount of multiwalled carbon nanotubes on the rheological properties. The size of adhesive fillet was obtained from the optical microscopy to assess the forming quality. Climbing drum peel test and edgewise compression test were employed for the mechanical assessment. The results showed that the addition of multiwalled carbon nanotubes reinforcement to epoxy resin in the prepreg was very effective in improving the skin–core adhesion. The peel load and peel energy release rate as well as the edgewise compressive strength and edgewise compressive modulus of the sandwich composites varied with different magnitudes due to the additive amount of multiwalled carbon nanotubes. Reinforcing mechanism of the adhesive fillet with multiwalled carbon nanotubes reinforcement was discussed on the basis of the fractographic observations by scanning electron microscopy.