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This work presents results of numerical simulations to investigate the effect of different void percentages on composite materials' Coefficient of Thermal Expansion (CTE) and local stress fields. A random distribution of voids is considered within the Representative Volume Element (RVE) matrix, and different types of microstructures are considered, including square-packed and randomly distributed fibers. The use of a higher-order beam model within the framework of Carrera Unified Formulation (CUF) leads to a Component-Wise (CW) approach, resulting in an accurate, 3D description of the cross-section although using a 1D formulation. Numerical results for different fiber volume fractions and void concentration percentages demonstrate the agreement of the computed effective coefficients of thermal expansion of the present micromechanical thermoelastic model with references from the literature. The local stress fields are affected by voids, with higher effects over the matrix. Furthermore, higher void fractions lead to higher variability of stress peaks. [ABSTRACT FROM AUTHOR]

The current study is focused on understanding the role of different void removal mechanisms in VBO processing of advanced thermoplastic composites. For this purpose, two commercially available Carbon/Poly-Ether-Ketone-Ketone (C/PEKK) tape materials were evaluated, distinct in morphologies, such as surface roughness and fiber-matrix distribution, and physical characteristics, mainly the presence of volatiles. The VBO consolidation results proved that the void reduction and removal mechanisms varied depending on the tape material. However, restricting the void reduction mechanism to dissolution and diffusion alone. Depending on the tape material, a significant difference in the consolidation dwell time was observed to achieve <1% void content parts. Thus, indicating that despite the tapes having the same polymer matrix, they differ in their diffusion behavior. The difference in the times required for consolidation may be due to the following. Firstly, the diffusion coefficients may be different for the two tapes. Although the matrix material in both tapes is PEKK, the exact formulation is unknown. Secondly, the volume of gases, which comprises entrapped air and the volatiles that evaporate during the process, that need to be removed maybe be different. [ABSTRACT FROM AUTHOR]

As an emerging composite curing process, the microwave curing process has been widely concerned by many research teams due to its several major advantages over conventional conductive heating when used to cure carbon fiber reinforced polymer composites, especially in speed of processing. However, many studies have shown that the quality and properties of microwave cured-composites have been reduced compared with the autoclave process, which limits the universal application of the technology in the field of composites manufacturing. In this study, in order to improve the quality of microwave cured-composites and reduce the influence of pressure on the forming process, a series of laminates have been prepared at different heating rates by introducing vibration treatment into the microwave curing process. The short-beam three-point bending test, optical digital microscopy, scanning electron microscope were used to analyze laminates from micro- and macro-scale point of view. The results showed that the introduction of vibration treatment could effectively reduce the internal defects and improve the mechanical properties of the microwave cured-laminates, which provided a new method for high-quality, low-defects microwave curing of composites. [ABSTRACT FROM AUTHOR]

The effect of voids on the state of water absorbed in epoxy has been examined using differential scanning calorimetric analysis for a range of void contents (0 to 50%). For void-free specimens, water absorbed is classified as bound water (nonfreezing). Analysis of the medium and high void content specimens revealed water absorbed in three states: free water, freezable bound water, and nonfreezable bound water. The differential scanning calorimetric results show that the proportions of free water and freezable bound water increase with increasing void content, while the content of nonfreezable bound water decreased. Moisture-induced swelling decreased with increasing void content. The swelling is attributed to the content of nonfreezable bound water as determined by differential scanning calorimetric analysis. [ABSTRACT FROM AUTHOR]

In this paper, additive manufacturing technology has been used in processing carbon nanofiber (CNF) reinforced thermoplastic composites through fused deposition modeling (FDM). The effects of nanofiber concentrations, nozzle geometries on void contents, and tensile properties of FDM printed nanocomposites have been studied. The contact angles and void geometries have been characterized as a function of bead spreading orientations. Such measurements were carried out using micrographs of optical and scanning electron microscopy (SEM) examinations. The dispersion and orientation of nanofiber in polylactic acid (PLA) matrix have been studied using transmission electron microscopy observations. Finally, the fracture surfaces of both neat PLA and CNF/PLA nanocomposites have been investigated in order to understand the failure mechanism. The results show improved modulus, strength, and strain up to elastic limit in CNF/PLA nanocomposites in comparison to neat PLA matrix. The square nozzle geometry shows increased tensile strength and reduced void geometry in comparison to circular shaped nozzle. Three types of voids such as inter-bead, intra-bead, and interfacial bead were observed with total voids ranging from 24% to 30% with circular nozzle. The level of void content is reduced to about 7% for square nozzle. Contact angles and bead orientation play an important role in forming void contents. CNF/PLA nanocomposites exhibit decreased contact angle and higher void contents in comparison to neat PLA resin. Transmission electron microscopy study of CNF/PLA nanocomposites show uniform dispersion of carbon nanofibers with aligned orientation in both composite filament and three-dimensional printed specimen. Fractured surfaces of CNF/PLA nanocomposites show fiber pullout mechanism and comparatively coarse surface topography indicating higher fractured energy than neat polylactic acid resin. [ABSTRACT FROM AUTHOR]

Voids are one of the most common defects in fibre-reinforced composite materials. Insightful understandings of the correlation between void morphology and material properties can benefit the desired performance of industrial products. A two-level analysis method for unidirectional fibre-reinforced composites containing voids is developed based on micromechanical modelling. Representative volume cells at different levels are developed based on the description of fibre distribution and void geometry. Finite element analysis was performed on the representative volume cell to determine the effective elastic moduli. Good agreements are found among the finite element analysis results, theoretical results of Halpin–Tsai equation and experimental characterization from literature. Results show that the mechanical properties of composites are significantly affected by void morphology. Larger width–height aspect ratio leads to a less modulus reduction of in-plane modulus, but it produces larger reduction of out-of-plane modulus. Moreover, the finite element analysis results can give good explanation to the discreteness of the experimental characterization. It indicates that typical voids that are formed in the autoclave process have significant impact on the out-of-plane modulus. The study considered the temperature effect on the composite containing voids. Results show that there is no coupling effect between porosity and temperature. [ABSTRACT FROM AUTHOR]

We investigate the unsaturated resin flow behavior in a dual scale porosity preform by observing the pressure distribution and the void content. The experimental data show that the pressure profile in the unsaturated flow is nonlinear with positive curvature whereas that in the saturated flow is linear as expected from the classical Darcy's law. To address this issue, the governing equation for mass conservation is modified by introducing a mass sink term. Eventually, it has been found that the discrepancy between the unsaturated and saturated permeability values comes from a misinterpretation of the pressure gradient at the flow front in the unsaturated permeability measurement method and the permeability for a given preform is a unique value regardless of measurement method or flow condition. Based on this investigation, the ratio of unsaturated permeability to saturated permeability is represented as a dimensionless number in terms of void content. [ABSTRACT FROM AUTHOR]

To give a better understanding of the impact of manufacturing defects and porosity on the strength of composite components, the strength knock-down of a material containing multiple voids or flaws is characterized. The voids are modeled as elliptical holes with equal size but random position and orientation on a 2D square domain, and modeled using a semi-automated Finite Element approach. Edge traction forces apply a uniform nominal shear stress, and a relatively coarse mesh is chosen in order to average out highly localized stresses and reveal trends in stress intensity as porosity is increased. A Weibull analysis of the localized peak stresses for different levels of porosity is undertaken. It was found that although void shape and percentage porosity levels play a role in the strength reduction, the critical factor is the variation of distance between the voids. Void distributions which are more homogeneous exhibit ‘defender hole’ stress shielding characteristics and show significantly less strength reduction. [ABSTRACT FROM PUBLISHER]

High-strength carbon fibers were oxidized by exposure to nitric acid and single-fiber wettability predictions were compared to the actual wettability of multiple fibers in resin. Single-fiber wettability was predicted through contact-angle measurements and surface-energy calculations. Multiple-fiber wettability in resin was evaluated by immersing treated fiber bundles in catalyzed vinyl ester resin, followed by cross-sectional viewing after curing. Fiber cohesion, macrocomposite void content, and transverse tensile strength were also examined as a function of fiber treatment time. Fiber surface energy increased with treatment time, suggesting improved wettability. However, fiber cohesion also increased and composite wetting was found to suffer. Increasing fiber treatment times resulted in larger unwetted areas, higher void content, and declining transverse tensile strength. [ABSTRACT FROM AUTHOR]

The comparative compressive properties of syntactic foam with and without the inclusion of E-glass fibers in the form of chopped strands are reported. The effort pointed to the fact that the fiber-free syntactic foam had a higher compressive strength than the fiber-bearing one whereas as regards the moduli values they did not differ much. The difference in strength is correlated with the amount of voids present in two foams. The scope of the work was further expanded by including scanning electron microscopy for examining the surface features of samples prior to and after compression test. [ABSTRACT FROM PUBLISHER]