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Co-curing composites facesheets to produce sandwich structures involves heating a prepreg-core assembly in a vacuum bag and compacting it by an external pressure until the prepreg cures and bonds to the core. During this process, vacuum pressure can cause atmospheric air in the honeycomb core to bubble through uncured prepreg facesheets and impact material quality, such as excess resin bleed and void growth in either the facesheet or the bondline. In this paper, we propose a multiphase flow model to predict core gas pressure throughout the process. Fully impregnated prepreg facesheets only become permeable to gas as liquid resin desaturates pores in the fabric weave. To solve for the core gas pressure, resin and gas transport equations are solved simultaneously. The permeability of each phase is a function of saturation–defined by the Brooks-Corey model to be a power law function of the capillary pressure. Deviating from a single phase (gas only) model, using dimensionless analysis we identify four material properties which control the outcome: (i) an intrinsic permeability, K i, which only depends on the fabric architecture and is invariant to all process variables (time, temperature, pressure), (ii) the ratio of fluid viscosities, μ w/ μ g, where resin viscosity varies strongly with temperature and degree of cure, (iii) a bubbling (capillary) pressure, P b, which defines the minimum pressure needed to penetrate the largest pore, and (iv) a pore size distribution parameter, λ, which defines the percentage of pores which may desaturate given the evolving capillary pressure. Changes in observed gas permeability due to time and temperature are explained by the evolving desaturation of facesheet resin and the known relationship to resin viscosity. By applying this model, a reduced set to material characterization is required and complex process cycles can quickly be interrogated. With our experiments we demonstrate that one single value for K i and one single value for λ can be used to predict the gas pressure decay (time dependent) at different temperatures, confirming that the intrinsic permeability is constant for the fiber network and changes in the relative permeability to air are due to the decrease in viscosity only. The limitations of this model are presented and discussed.

Powder technology has been used to coat fibers with polymeric particles to manufacture composites. The material parameters such as particle size and fiber diameter and process parameters such as the particle charge and the particle velocity will influence the powder coating process. In this work, we present an experimental setup in which we characterize the coating of micron size particles on a fiber and examine the role of process parameters on the coating density. Monodispersed Polystyrene (PS) particles (2.8 ± 0.4 μm in diameter) were used to coat a carbon fiber (CF) 7 μm in diameter. A model setup was designed and fabricated in which the PS particles were charged with an ozone charging system, while the CF was maintained at the grounded state at all times. The setup consisted of two parts namely, a cyclone and a coating chamber. The distance between the PS particles was measured and the density of coating was examined using a microscope as a function of the processing parameters used in the setup. The measured average distance between the PS particles on the CF was used in our developed numerical model to estimate the friction coefficient between the charged PS particle and the grounded CF and the fiber volume fraction. The proposed methodology can be extended to any particle size, particle charge, and fiber diameter to estimate the friction coefficient and the fiber volume fraction at the microscale level.

Carbon–carbon composites (CCCs) are a unique form of carbon fiber-reinforced materials that exhibit excellent thermomechanical properties under extreme environmental conditions. Due to the need for the retention of mechanical properties at temperatures exceeding 2000 °C, CCCs have been utilized in heat shields, rocket nozzles, aircraft brakes, and leading edge material in hypersonic vehicles. In order to expand the applicability of CCCs, the fabrication process must be modified such that there is a reduction in cost or processing time. It is hypothesized that maximizing the permeability of the composite, during processing, will grant the largest reduction in the fabrication time as it leads to a larger volume of pores filled during re-densification. This review attempts to capture the various parameters that have led to increased permeability, as well as outlining process modifications that have demonstrated influence over the carbonized microstructure. In addition, this review seeks to differentiate itself by systematically outlining research advances that have been made in each step of the fabrication process. In doing so, scientific gaps that exist can be expounded upon while simultaneously summarizing what is necessary to advance the field.

A numerical model that can accurately predict the electrostatic forces (attractive and repulsive) between charged polymer particles in contact with a grounded carbon fiber considering circumferential interactions is developed. The results obtained are used to predict the particle-to-particle equilibrium circumferential spacing as a function of the associated friction coefficient that exists between the charged particles and the carbon fiber surface. A parametric study is conducted to study the effects of particle charge levels and particle sizes (R2/R1 = 1, 2, and 5) in the presence of the grounded carbon fiber on the electrostatic forces as a function of particle angular spacing along the fiber circumference. The parametric results are useful in the design of electrostatic powder impregnation processes for composites where the fiber volume fraction is determined by the diameter of particle size, particle charge, and equilibrium spacing governed by electrostatic forces and particle/fiber surface frictional interactions.

One can compute the final deformation of a known geometry under specific boundary conditions using the constitutive laws of mechanics that describe their stress strain behavior. In such cases the initial geometry is known, and all operators mapping the deformation are defined on the reference domain. However, there are situations in which the final configuration of a deformation might be known but not the initial. The inverse formulation allows one to determine the initial geometry of a domain, given its final deformation state, the material behavior law and a set of boundary conditions. In the present work we propose a method to reconstruct the mesoscale geometry of a textile based on its mechanical response during compaction. To do so, stress boundary conditions are acquired by means of a pressure-sensitive film. By adopting an appropriate material law, the thickness and width information of the yarns are deduced from the pressure field experienced by the compacted textile. Unlike 3D scanning techniques such as μ -CT, the proposed method can be applied on any domain size, allowing long-range variability to be captured. To the best of the authors' knowledge, there are no previous works that use a pressure-sensitive film on a large domain to capture the input data for a shape reconstruction. This example application serves as a demonstration of a methodology which could be applied to other classes of materials.

Manufacturing of large composite structures is labor-intensive and challenging. Traditional processing methods can be replaced by automated approaches such as Automated Tape Placement (ATP). The focal point of this effort is on thin-ply thermoset prepreg materials which have shown better resistance to damage tolerance under applied stress when compared to standard ply-thickness materials in recent times. However, composites processed by thin-ply materials require more placement iterations to achieve the same net volume of coverage as the standard ply-thickness materials. This can increase the probability of placement defects such as gaps and overlaps, which may degrade the mechanical performance of the composite. Modifications to the existing ATP system are made to address the issues of placement defects, which resulted in improved accuracy for thin ply placement. Unidirectional and quasi-isotropic panels are fabricated from 12’’ wide unidirectional prepreg sheet with the conventional hand layup (CHL) method for the baseline property comparison with panels fabricated by the ATP process using a ¼’’ wide slit tape. This paper investigates the effects of processing defects and material variability on the mechanical properties such as tensile, Open hole tension and Bearing tests of thin ply composites. In addition, the microstructure of the fabricated thin-ply composite panels is analysed using ultrasonic C-scans and confocal microscopy to examine the processing and material quality.

Connected vehicle technologies present new opportunities to minimize energy consumption in vehicles. Much of the prior work has focused on the impact of vehicle-to-infrastructure (V2I) communication on energy savings. Here, we analyze the impact of connectivity on both energy savings and battery degradation reduction for an electric bus. Dynamic programming is employed to optimize the velocity profile of a single electric bus passing through a traffic intersection under a V2I regime wherein the intersection timing of the traffic light is known a priori. The optimization seeks to minimize the overall cost function consisting of the electric energy consumption and battery degradation. Simulations have been conducted for both the connected vehicle case and the baseline case without connectivity. Three different battery sizes were investigated, and the results indicate that connectivity confers the greatest cost savings for the smallest battery. An overall cost savings of 27% can be realized by using connectivity to reduce both energy consumption and battery degradation.

For fast and complete impregnation in Liquid Composite Molding, knowledge about the permeability of the fibrous reinforcement is required. While development of experimental methods continues, a parallel benchmark effort to numerically characterize permeability is being pursued. The approach was to send out the images of a real fibrous microstructure to a number of participants, in order for them to apply their methods for virtual permeability prediction. Via resin transfer molding a plate was manufactured, using the glass woven fabric Hexcel 01102 (295 g/m²) at a fiber volume content of 54% and a thermoset resin. From this plate, a specimen was scanned using a 3D x-ray microscope at a scan size of 1000 x 1000 x 1000 μm³ and a resolution of 0.521 μm³ per voxel. The sample extracted for the simulations with a size of 523 x 65 x 507 μm³ contains about 400 fibers of a single tow. It revealed a variation of filament diameters between 7.5-9.3 μm and a fiber volume content in average of 56.46% with a variation of 54 - 59% in the individual 2D-slices transverse to the fiber direction. The image segmentation was performed by 2D-slices, to which a Hough transform was applied to detect fiber centers and cross-sections. Then fiber paths were tracked through-out the slices by the closest neighbor algorithm. Finally, fiber paths were smoothened by means of the local regression using weighted linear least squares and a 1st degree polynomial model. The participants received a stack of 973 segmented (binary) 2D-images and a corresponding segmented 3D volume raw-file. They were asked to calculate the full permeability tensor components and fill out a detailed questionnaire including questions e.g. on applied flow models and conditions, numerical discretization and approximation methods, fluid properties etc. The received results scatter considerably over two orders of magnitude, although the participants were provided an already segmented image structure, thus eliminating from the beginning a significant source of variation that could have come from image processing. Model size, meshing and many other sources of variation were identified, allowing further specification of the guidelines for the next step.

Honeycomb sandwich structures are co-cured to bond partially cured thermoset prepreg facesheets with an adhesive layer to both sides of the honeycomb core under a pre-defined pressure and temperature cycle in an autoclave. High dependency of the co-cure process on the materials and process parameters makes it susceptible to defect such as poorly consolidated facesheet and highly porous bondline which can cause premature failure of the structure. The temperature and pressure in the autoclave and pressure in the vacuum bag are the parameters that describe the cure cycle of the process. In this work, an optimization of the process cycle for the co-cure process of sandwich structures that maximizes the fiber volume fraction within the prepreg and reduces the porosity is presented. The objective function is constructed to reflect the quality of both the facesheet consolidation and bond-line porosity. The Surrogate Optimization Algorithm is employed to find the cure cycle resulting in the highest facesheet consolidation level and the lowest porosity within the bond-line.

The presence of blockages in the flow field (FF) of proton exchange membrane fuel cells (PEMFCs) increases the mass transport of reactants toward the catalyst layer regions and improves cel...

Resin transfer molding and vacuum-assisted resin transfer molding are two of the most commonly used liquid composite molding processes. For resin transfer molding, mold filling simulations can predict the resin flow patterns and location of voids and dry spots which has proven useful in designing the mold and injection locations for composite parts. To simulate vacuum-assisted resin transfer molding, even though coupled models are successful in predicting flow patterns and thickness distribution, the input requires fabric compaction characterization in addition to permeability characterization. Moreover, due to the coupled nature of flow and fabric compaction, the simulation is computationally expensive precluding the possibility to optimize the flow design for reliable production. In this work, we present an alternative approach to characterize and use an “effective” permeability in the resin transfer molding solver to simulate resin flow in vacuum-assisted resin transfer molding. This decoupled method is very efficient and provides reasonable results. The deviations in mold filling times between experiments and simulations for the resin transfer molding process with E-glass CSM and carbon 5HS were 4.7% and 1.0%, respectively, while for the vacuum-assisted resin transfer molding case using “effective permeability value” with E-glass CSM and carbon 5HS fabrics were 11.1% and 12.3%, respectively, which validates the approach presented.

The size of the individual powerplant components on board a fuel cell/battery hybrid vehicle affects the power management strategy which determines both the fuel economy and the durability of the fuel cell and the battery, and thus the average lifetime cost of the vehicle. Cost is one of the major barriers to the commercialization of fuel cell vehicles, therefore it is important to study how the sizing configuration affects overall vehicle cost. In this paper, degradation models for the fuel cell and the battery on board a fuel cell/battery hybrid bus are incorporated into the power management system to extend their lifetimes. Different sizing configurations were studied and the results reveal that the optimal size with highest lifetime and lowest average cost is highly dependent on the drive cycle. The vehicle equipped with a small fuel cell stack serving as a range extender will fail earlier and consume more fuel under drive cycles with high average power demand resulting in higher overall cost. However, the same configuration gives optimal results under a standard bus cycle with lower average power demand. At the other end of the spectrum, a fuel cell-dominant bus does not guarantee longer lifetime since the fuel cell operates mostly under low-load conditions which correspond to higher potentials reducing lifetime. Such a configuration also incurs a higher initial capital cost of the fuel cell stack resulting in a high average cost. The best configuration is a battery-dominated system with moderately-sized fuel cell stack which achieves the longest lifetime combined with the lowest average running cost throughout the lifetime of the vehicle.

International audience; In this work we develop a void filling and void motion dynamics model using volatile pressure and squeeze flow during tape placement process. The void motion and filling are simulated using a non-local model where their presence is reflected in the global macroscale behavior. Local pressure gradients during compression do play a critical role in void dynamics, and hence the need for a non-local model. Deriving a non-local model accounting for all the void motion and dynamics entails a prohibitive number of degrees of freedom, leading to unrealistic computation times with classical solution techniques. Hence, Proper Generalized Decomposition – PGD – is used to solve the aforementioned model. In fact, PGD circumvents the curse of dimensionality by using separated representation of the space coordinates. For example, a 2D problem can be solved as a sequence of 1D problems to find the 2D solution. The non-local model solution sheds light on the fundamental of the void dynamics including their pressure variation, motion and closure mechanisms. Finally, a post treatment of the transient compression of the voids is used to derive conclusions regarding the physics of the void dynamics.

A comprehensive review of experimental and analytical studies since 1990 on the variability of permeability induced by fibrous media (reinforcement) distortions across the composite part manufacturing chain is presented. The review covers variability in as-supplied dry reinforcements, which include tow waviness, tow size and tow shape variations, interconnectedness of the pore space inside the tows (tortuosity), and changes during production, handling or storage, affected by the reinforcement deformation (non-uniform shearing, stretching and compression) and nesting of the layers in the laminate. The review clearly indicates that the interdependencies among these parameters is one of the root causes of the spatial variation of the permeability of the preform. This has spurred recent work to quantify flow front and filling time variabilities and development of optimal injection strategies using stochastic flow simulations. State-of-the-art experimental and theoretical approaches and modelling of this stochastic behaviour within numerical frameworks for the design of composite manufacturing processes are introduced and future outlook on the role of such variability at different scales is forecasted.

Increasing applications of carbon fiber reinforced polymers exploiting its electrical properties demand a good understanding of the electrical conduction mechanisms of carbon fiber reinforced polymer. Resin-rich interface, which is not uncommon to exist between composite laminae, not only affect the mechanical properties, but also the electrical conduction behavior. This study focuses on the impact of resin-rich layer on the through-thickness resistivity of carbon fiber reinforced polymer. Electrical characterizations are carried out on dry fiber tow systems as well as cured composites. Through-thickness resistivity changes of dry fibers with the sizing are compared against fibers without the sizing layer, and cured composites with added resin-rich layer against the composite laminates without the resin-rich layer. A localized Joule heating theory is proposed to explain the difference in the electrical responses. The theoretical and experimental investigations should prove useful for the development of quantitative models with Joule heating to describe electrical resistivity behavior of carbon fiber reinforced polymer.

In the past, various analytical and semi-analytical permeability models have been implemented for process modeling of flow through single scale fabric with reasonable success. However, characterization of permeability for a dual scale fabric is relatively more complex requiring specification of permeability at tow and fabric length scales. In the current work, a semi-analytical model for transverse tow permeability is developed taking into consideration the details of microstructural filament arrangements within a single tow by conducting detailed scanning electron microscopy (SEM) of a cured tow cross-section. The current analysis indicates that by explicitly accounting for said local fiber volume fraction and its distribution, the transverse tow permeability predictions improve when compared to existing analytical results. In addition, a new experimental methodology for transverse tow permeability characterization has been developed and presented. The predictions considering locally varying fiber volume fractions were found to match well with the experimentally evaluated tow permeability.

Optimization of fuel cell/battery hybrid vehicle systems has primarily focused on reducing fuel consumption. However, it is also necessary to focus on fuel cell and battery durability as inadequate lifespan is still a major barrier to the commercialization of fuel cell vehicles. Here, we introduce a power management strategy which concurrently accounts for fuel consumption as well as fuel cell and battery degradation. Fuel cell degradation is quantified using a simplified electrochemical model which provides an analytical solution for the decay of the electrochemical surface area (ECSA) in the fuel cell by accounting for the performance loss due to transient power load, start/stop cycles, idling and high power load. The results show that the performance loss based on remaining ECSA matches well with test data in the literature. A validated empirical model is used to relate Lithium-ion battery capacity decay to C-rate. Simulations are then conducted using a typical bus drive cycle to optimize the fuel cell/battery hybrid system. We demonstrate that including these degradation models in the objective function can effectively extend the lifetime of the fuel cell at the expense of higher battery capacity decay resulting in a lower average running cost over the lifetime of the vehicle.

A numerical methodology is proposed to predict void content and evolution during autoclave processing of thermoset prepregs. Starting with the initial prepreg void content, the void evolution model implements mechanisms for void compaction under the effect of the applied pressure, including Ideal Gas law compaction, and squeeze flow for single curvature geometries. Pressure variability in the prepreg stack due to interactions between applied autoclave pressure and anisotropic material response are considered and implemented. A parametric study is conducted to investigate the role of material anisotropy, initial void content, and applied autoclave pressure on void evolution during consolidation of prepregs on a tool with single curvatures. The ability of the model to predict pressure gradient through the thickness of the laminate and its impact on void evolution is discussed.

International audience; This paper reports the results of an international benchmark exercise on the measurement of fibre bed compaction behaviour. The aim was to identify aspects of the test method critical to obtain reliable results and to arrive at a recommended test procedure for fibre bed compaction measurements. A glass fibre 2/2 twill weave and a biaxial (±45°) glass fibre non-crimp fabric (NCF) were tested in dry and wet conditions. All participants used the same testing procedure but were allowed to use the testing frame, the fixture and sample geometry of their choice. The results showed a large scatter in the maximum compaction stress between participants at the given target thickness, with coefficients of variation ranging from 38 % to 58 %. Statistical analysis of data indicated that wetting of the specimen significantly affected the scatter in results for the woven fabric, but not for the NCF. This is related to the fibre mobility in the architectures in both fabrics. As isolating the effect of other test parameters on the results was not possible, no statistically significant effect of other test parameters could be proven. The high sensitivity of the recorded compaction pressure near the minimum specimen thickness to changes in specimen thickness suggests that small uncertainties in thickness can result in large variations in the maximum value of the compaction stress. Hence, it is suspected that the thickness measurement technique used may have an effect on the scatter.

Numerical models are developed to examine fiber suspension flows through axisymmetric geometries, such as a circular pipe, a center-gated disk and a die exit. The fiber orientation micro-structure is fully described by using the entire probability distribution function (PDF) in 3D instead of the second and fourth moments of the PDF, which introduce errors due to the closure approximation when using orientation tensor descriptions. A Newtonian suspending fluid is considered for the constitutive relationship and simulations are performed with and without coupling the flow and fiber orientation. Results are compared with numerical simulations obtained by using the standard evolution equation for the second-order orientation tensor and the IBOF closure approximation. It is found that the IBOF closure approximation does a relatively good job of matching the exact orientation results given by the probability distribution function. For the three explored flows, the difference between the predicted orientation results using the coupled and decoupled approaches is not significant. There is a noticeable difference in the velocity field solutions, specifically at the die exit where the coupled approach increases the die swell ratio when the fibers enter the inlet with random orientation state.

Thick-section composite parts are difficult to manufacture using thermosetting resins due to their exothermic curing reaction. If processing is not carefully controlled, the build-up of heat can lead to warpage or material degradation. This risk can be reduced or removed with the use of a low-exotherm resin system. Material and process models are presented which describe vacuum-bag-only processing of thick-section composites using a novel, low-exotherm epoxy powder. One-dimensional resin flow and heat transfer models are presented which govern the fabric impregnation and temperature evolution, respectively. A semi-empirical equation is presented which describes the sintering of the epoxy powder. The models are coupled via laminate thickness change, which is determined for a simplified ply microstructure. The resulting system of equations are discretised and solved numerically using a finite difference code. A case study is performed on a 100-ply laminate, and the advantages and disadvantages of using epoxy powders are discussed.

Fiber washout during high-injection-pressure resin transfer molding (HIPRTM) is an unwanted emergent defect. This article addresses the mechanisms that cause fiber washout during manufacturing of composite parts by HIPRTM. The key focus of this work is on one-dimensional flow experiments using HIPRTM process in which in-plane drag force due to high resin pressures is responsible for the fiber washout. An experimental parametric study is conducted to investigate the influence of the fabric architecture, number of plies, and injection pressure to identify controllable process and material factors to prevent fiber washout. The results show that as the ratio of tangential force between the mold/ply and the drag force increases, one would expect fiber washout to occur and either cause fiber waviness or form wrinkles and dry spots. The fiber washout distances measured from the experiments were used to validate the model developed in our previous work. The numerical simulations by LIMS qualitatively agreed with the experimentally measured behavior. It is concluded that the formation of dry spot is due to the contribution of two main factors: (a) very significant permeability reduction in the wrinkled region and (b) some race-tracking. © 2019 Society of Plastics Engineers

A new process to align short fibers into a preform for composite applications has been developed. The short fibers are dispersed in a fluid at a high dilution ratio. During fluid extraction, the fibers decelerate with respect to the fluid velocity and are subjected to significant forces that provide the mechanism to align the fibers. In this study, a model that describes the motion and deformation of a flexible elastic fiber in low Reynolds number flow is formulated. The governing equations are described and forces acting on the fiber are estimated. The numerical implementation of the model is described, and its numerical performance is evaluated. Several validation checks are performed to determine if the model is capturing the experimentally observed phenomena and if the estimated parameters provide reasonable quantitative agreement with experimental observations. The model is then applied to several simple examples such as the evaluation of bending stress (moment) and the associated fiber deformation (buckling behavior) versus the results observed in the process. The effect of fiber boundary condition is demonstrated. It is shown that the model can describe the buckling-like slow folding of the misaligned fiber. Finally, the model utility is demonstrated by a simple parametric study which explores the influence of process parameters on the fiber alignment process.

In this article, we provide a general description of a random stack of solid particles. We consider fine particles of any shape distributed evenly on a flat surface. The angular distribution of the particles is arbitrary. In the absence of constraints imposed on the medium, the volume fraction is expressed analytically according to the parameters of the problem. We thus define a generalized strain measurement of the medium taking into account the internal reorganizations of the microstructure which can then be used in analytical mechanical models to predict their response. The approach is verified digitally by generating digital stacks of fibers, disks and rectangular particles.

During the automated tape placement process, gaps between adjacent tapes are created due to the limitation of machine accuracy, especially at higher speeds. These gaps are undesirable as they cause porosity, thickness variation, and fiber waviness, introducing defects in the composite part. Applying pressure and temperature to part can assist in partially or completely filling these gaps. For thermosetting resins, such processing is usually required for the material to reach complete consolidation. Modeling the tape deformation as squeeze flow of fibrous suspension in the liquid matrix is usually deployed to determine gap filling and tape deformation. However, existence of resin rich zones in the micrographs of fabricated parts suggests that there may be a second flow process involved which is the percolation of matrix through the fiber bed into the remaining empty space of the gaps. This paper extends the previous work that modeled the cross-ply deformation and squeeze flow as the primary mechanisms in the gap filling process during consolidation of thermosetting composites. Based on our experimental results, this work adds the percolation flow to the model to predict (1) the spread of fiber/matrix suspension into the gap, (2) the resin percolation out of the fiber bed into the gap and (3) the deformation of the cross (bridging) ply or plies. The governing equations are formulated, and scaling analysis is performed to estimate the effect of tape thickness on percolation flow confirming that this mechanism is more significant when thin tapes are employed. Fixed mesh numerical scheme is used to evaluate the deformation and flow progression of both the resin percolation flow and the squeeze flow of fiber/resin suspension during the consolidation process. A parametric study is conducted to evaluate the effect of model parameters. Finally, the predicted percolated resin flow along with squeeze flow advancement is compared with experimental results.

The lattice Boltzmann method (LBM) is used to simulate the growth of a solid deposit on the walls of a circular tube resulting from a gas-to-solid reaction and precip- itation process. This process is of particular interest for the design of reactors for the production of hydrogen by the heterogeneous hydrolysis of steam with Zn vapor in the Zn/ZnO thermochemical cycle. The solid deposit of ZnO product on the tube wall evolves in time according to the temporally- and axially-varying convective-diffusive transport and reaction of Zn vapor with steam on the solid surface. The LBM is well-suited to solving problems with coupled flow, heat and mass transfer in a time-evolving domain. Here, a D2Q9 axisymmetric multiple-relaxation-time (MRT) lattice Boltzmann scheme is used to simulate incompressible fluid transport while a D2Q5 axisymmetric MRT lattice Boltz- mann scheme is used to simulate the convective-diffusive transport of Zn vapor. The model is first validated against several analytical solutions, followed by a parametric study to un- derstand the effect of Reynolds, Schmidt, and Damkohler numbers on the time evolution of the ZnO deposition profile along the tube axis. The axial location of the fastest deposition is found to increase with increasing Peclet number, and decrease with increasing Damkohler number, with no independent effect from the Schmidt number. When the reaction kinetics are assumed to increase along the tube axis due to non-isothermal tube wall temperature, a second peak in the deposition profile can be observed for sufficiently low values of Da/P e.

A micromechanical unit cell model is formulated to investigate the deformation in a unit cell of highly concentrated suspension of long discontinuous aligned fibers. The periodic unit cell is many fiber lengths long to allow for microstructural variations due to random fiber length overlaps and end-to-end gap distances between the fibers. This feature of the unit cell makes it possible to examine the effect of such variabilities on the macroscopic material behavior (effective viscosity) during the forming process. The influence of microstructure variability on viscosity is predicted. The standard deviation of the effective extensional viscosity is significant and increases with material extensional deformation and the variation of the end to end fiber spacing. This could have ramifications on the forming process and the final physical properties of the materials of interest. The predicted dependence of the mean extensional viscosity value on the usual material parameters (fiber aspect ratio, fiber volume fraction) is found to be consistent with the relations predicted by reported models using deterministic periodic unit cells. The behavior of standard deviation of our model follows the same dependence as the mean value. On the other hand, it shows negligible variability of effective longitudinal-transverse shearing viscosity of such materials, regardless of the random perturbations of fiber length overlaps and end to end fiber spacings within the unit cell.

In this study, the influence of type of carbon fiber, sizing amount on the fiber surface and the degree of compaction on the through-thickness electrical resistivity of dry unidirectional carbon fiber tows is investigated to validate the conduction pathways and mechanisms proposed by our previously reported micromechanics electrical resistivity model. An automated experimental setup has been developed and implemented, which measures the electrical resistivity and fiber volume fraction of carbon fiber tows under compression in real time. An extensive experimental study is conducted with five types of commercial PAN-based carbon fibers which vary in fiber diameter, number of fibers in a tow including two unsized fibers and three sized fibers with sizing amount of 0.25% and 1.0% by weight. The fiber volume fraction was increased by compacting the fiber tows using a mechanical testing system (Instron, Norwood, MA). The results show that the fiber sizing and fiber volume fraction impact the through-thickness electrical resistivity of carbon fiber tows. Sized fibers demonstrate 1–2 orders of magnitude higher electrical resistivity than the unsized fibers at lower fiber volume fractions (below 45%), while at higher fiber volume fraction (60%–70%), the electrical resistivity of the two fiber systems tends to be of similar magnitude. Fibers with more sizing (1 wt.%) demonstrated 10 times larger through-thickness resistivity than those with less sizing (0.25 wt.%), indicating the significant impact of fiber sizing on electrical resistivity. The results show good agreement with our micromechanics electrical resistivity model.

The addition of ceria nanoparticles can significantly enhance the durability of sulfonated poly (ether ether ketone) (SPEEK) membranes for fuel cell applications. Here, the fuel cell performance and durability of SPEEK/ceria nanocomposite membranes containing different ceria loadings were investigated. Ceria nanoparticles were synthesized via a microwave-assisted gel-combustion method and were characterized by XRD, FESEM, HRTEM, BET, and pore-structure analysis. The SPEEK/ceria nanocomposite membranes (SPEEK- x wt% CeO2; x = 2.5, 5.0, 7.5, and 10.0) were prepared by the solution-casting method and characterized by XRD, DSC, water uptake, proton conductivity, and oxidative stability. The physical, thermal, and chemical analyses showed that the nanocomposite membrane had improved properties over the pure SPEEK membrane. Proton conductivity measurements before and after a 100 h Fenton test confirmed that the SPEEK nanocomposite membranes had superior chemical stability compared to pure SPEEK membranes. Fuel cell performance was evaluated by obtaining polarization curves at different temperatures, and accelerated stress tests (AST) were conducted by holding the cell at open circuit voltage (OCV) at 90 °C and low humidity (30% RH) for 230 h. The nanocomposite membranes showed a lower degradation rate than the pure SPEEK membrane, and the degradation rate decreased with increasing ceria content. The gas crossover, polarization curves, and ohmic resistance before and after AST showed that the SPEEK nanocomposite membranes were extremely stable. XRF measurements indicated that the ceria nanoparticles were highly resistant to migration and leach-out from the SPEEK/ceria nanocomposite membranes, and FESEM images confirmed that the nanocomposite membranes did not undergo significant thinning. The results indicate that SPEEK/ceria nanocomposite membranes are potentially good candidates for proton exchange membrane fuel cells.

In Vacuum Assisted Resin Transfer Molding (VARTM), fabrics are placed on a tool surface and a Distribution Media (DM) is placed on top to enhance the flow in the in-plane direction. Resin is introduced from one end and a vacuum is applied at the other end to create the pressure gradient needed to impregnate the fabric with resin before curing the resin to fabricate the composite part. Heterogeneity in through the thickness permeability of a woven fabric is one of the causes for the variability in the quality of the final composite part fabricated using the VARTM process. The heterogeneity is caused by the varying sizes of pinholes which are meso-scale empty spaces between woven tows as a result of the weaving process. The pinhole locations and sizes in the fabric govern the void formation behavior during impregnation of the resin into the fabric. The pinholes can be characterized with two parameters, a gamma distribution function parameter α and Moran's I (MI). In this work, manifold embedding methods such as t – Distributed Stochastic Neighborhood Embedding (t_SNE) and Principal Component Analysis (PCA) are used to visually characterize fabrics of interest with the two variables, α and MI, through the reduction of dimensionality. To demonstrate the manifold embedding method, a total of 450 training sample data with ranges of α from 1 to 3 and MI from 0 to 0.5 were used to create a map in three-dimensional space for ease of visualization and characterization. The method is validated with a plain-woven fabric sample in a testing step to show that the two parameters of the fabric are identified with its corresponding α and MI using these machine learning algorithms. Numerical flow simulations were carried out for varying α , MI, and DM permeability, and the results were used to predict final void percentage. The quick online identification of the fabric parameters with machine learning algorithms can instantly provide expected variability in void formation behavior that will be encountered in a VARTM process.