N. D. Ngo

Microscale permeability predictions of porous fibrous media

A good understanding of woven fiber preform permeabilities is critical in the design and optimization of the composite molding processes encountered in resin transfer molding (RTM); yet these issues remain unresolved in the literature. Many have attempted to address permeability predictions for flat undeformed fiber preform, but few have investigated permeability variations for complex geometries of porous fibrous media. In this study, the objectives are to: (i) provide a brief review of existing methods for the prediction of the fiber mat permeability; (ii) postulate a more realistic representation of a unit cell to account for such fabric structures as crimp, tow spacing and the like; and (iii) apply computational approximations to predict effective permeabilities for use in modeling of structural composites manufacturing processes. The Stokes equation is used to model the flow in the inter-tow region of the unit cell, and in the intra-tow region, the Brinkmans equation is used. Initial permeability calculations are performed for a three-dimensional unit cell model representative of the PET-61 woven fabric composite. The results show good agreement with experimental data published in the literature.

Recent Developments Encompassing Non-Isothermal/Isothermal Liquid Composite Molding Process Modeling/Analysis: Physically Accurate, Computationally Effective, and Affordable Simulations and Validations

The mathematical and associated computational modeling and analysis of mold filling, heat transfer, and polymerization reaction kinetics in Resin Transfer Molding (RTM) are quite complex and not only require accurate computational approaches to capture the process physics during the simulations, but also must permit complex geometric configurations to be effectively analyzed. The process simulations at a macroscopic level require the representative macroscopic constitutive behavior which can be predicted from a microscopic analysis of the representative volume element (RVE) of the fiber preform configurations. This is first presented here for purposes of illustration in reference to determination of the preform flow permeabilities. Next, an effective integrated micro/macro approach and developments including a viable flow solution modeling and analysis methodology with emphasis on providing improved physical accuracy of solutions and computational advantages are described for the transient flow progression inside a mold cavity filled with a fiber preform under isothennal and non-isothermal flow conditions. The improved physical accuracy and the overall effectiveness of the new computational developments for realistic process modeling simulations are first demonstrated for isothermal conditions. Subsequently, the new integrated flow/ thermal methodology and developments are extended for non-isothermal conditions. The highly advective nature of the non-isothermal conditions involving thermal and polymerization reactions also require special numerical considerations and stabilization techniques and are also addressed here. Finally, validations and comparisons are presented with available analytical and experimental results whenever feasible. Emphasis is also placed upon demonstrations for practical engineering problems.

THREE-DIMENSIONAL RESIN TRANSFER MOLDING: ISOTHERMAL PROCESS MODELING AND EXPLICIT TRACKING OF MOVING FRONTS FOR THICK GEOMETRICALLY COMPLEX COMPOSITES MANUFACTURING APPLICATIONS - PART 1

In resin transfer molding (RTM) process modeling, current practices involved in the simulation of resin impregnation through porous media have been generally restricted to two-dimensional formulations based on Darcys law for flow through thin cavities due to the increased computational demand and stringent stability restrictions of the traditionally employed explicit finite element-control volume (FE-CV) type approaches. The presence of multiple fiber layers in thick composites, or the notion of introducing impermeable inserts inside the fiber bundles to serve as protective armor, causes the resin impregnation to be a three-dimensional flow. This paper describes full three-dimensional simulations based on an explicit FE-CV technique to assess the practicality and suitability of the approach. Though viable, the technique treats the transient mold filling problem as a series of quasi-steady state problems. Additionally, an effective alternate form and discretization of the field variables based on a flux-b...

Process modeling of composites by resin transfer molding: Sensitivity analysis for non-isothermal considerations

Purpose – The purpose of the present paper is to describe the modeling, analysis and simulations for the resin transfer molding (RTM), manufacturing process with particular emphasis on the sensitivity analysis for non‐isothermal applications.Design/methodology/approach – For the manufacturing of advanced composites via RTM, besides the tracking of the resin flow fronts through a porous fiber perform, the heat transfer and the resin cure kinetics play an important role. The computational modeling is coupled multi‐disciplinary problem of flow‐thermal‐cure. The paper describes the so‐called continuous sensitivity formulation via the finite element method for this multi‐disciplinary problem for process modeling of composites manufactured by RTM to predict, analyze and optimize the manufacturing process.Findings – Illustrative numerical examples are presented for two sample problems which include examination of sensitivity parameters for the case of material and geometric properties, and boundary conditions in...

Process modeling of composites by resin transfer molding: practical applications of sensitivity analysis for isothermal considerations

The resin transfer molding process for composites manufacturing consists of either of two considerations, namely, the fluid flow analysis through a porous fiber preform where the location of the flow front is of fundamental importance, and the combined flow/heat transfer/cure analysis. In this paper, the continuous sensitivity formulations are developed for the process modeling of composites manufactured by RTM to predict, analyze, and optimize the manufacturing process. Attention is focused here on developments for isothermal flow simulations, and various illustrative examples are presented for sensitivity analysis of practical applications which help serve as a design tool in the process modeling stages.

Computational developments for simulation based design: Multi-scale physics and flow/thermal/cure/stress modeling, analysis, and validation for advanced manufacturing of composites with complex microstructures

SummaryIn the process modeling and manufacturing of large geometrically complex lightweight structural components comprising of fiber-reinforced composite materials with complex microstructures by Resin Transfer Molding (RTM), a polymer resin is injected into a mold cavity filled with porous fibrous preforms. The over-all success of the manufacturing process depends on the complete impregnation of the fiber preform by the polymer resin, prevention of polymer gelation during filling, and subsequent avoidance of dry spots. Since the RTM process involves the injection of a cold resin into a heated mold, the associated multi-physics encompasses a moving boundary value problem in conjunction with the multi-disciplinary and multi-scale study of flow/thermal/cure and the subsequent prediction of residual stresses in side the mold cavity. Although experimental validations are indispensable, routine manufacture of large complex structural geometries can only be enhanced via computational simulations; thus, eliminating costly trial runs and helping designers in the set-up of the manufacturing process.This manuscript describes an in-depth study of the mathematical and computational developments towards formulating an effective simulation-based design methodology using the finite element method. The present methodology is well suited for applications to practical engineering structural components encountered in the manufacture of complex RTM type lightweight composites, and encompasses both thick and thin shell-type composites with the following distinguishing features: (i) an implicit pure finite element computational methodology to track the fluid flow fronts with illustrations first to isothermal situations to overcome the deficiencies of traditional explicit type methods while permitting standard mesh generators to be employed in a straightforward manner: (ii) a methodology for predicting the effective constitutive model thermophysical properties, namely, the permeability tensor of the fiber preform microstructures in both virgin and manufactured states, the conductivity tensor, and the elasticity tensor; (iii) extension of the implicit pure finite element methodology to non-isothermal situations with and without influence of thermal dispersion to accurately capture the physics of the RTM process; (iv) stabilizing features to reduce oscillatory solution behavior typically encountered in the numerical analysis of these classes of problems: and (v) as a first step, preliminary investigations towards the prediction of residual stresses induced in the manufacturing process during post-cure cool-down.The underlying theory and formulations detailing the relevant volume averaging and homogenization techniques are first outlined for the multi-scale problem. Then the implicit pure finite element methodology, followed by the models for permeability prediction, is presented and compared for the case of isothermal mold filling. Applications of the pure finite element method is next extended to non-isothermal situations to accurately capture the flow/thermal/cure effects and the physics of the RTM process. Subsequently, a preliminary attempt is made to integrate the developments with the problem of thermoelasticity for residual stress prediction during post-cure cool-down. Where applicable, extensive validations of numerical results are made with analytical solutions and/or available experimental data. From these comparisons, relevant conclusions are drawn about the effectiveness of the present developments and their subsequent application to large-scale practical analysis of fiber-reinforced composite structures. Finally, some future directions relevant to the present study encompassing the multi-physics and multi-scale aspects of fibrous preforms with complex microstructures for use in lightweight composites are outlined.

Process modeling and implicit tracking of moving fronts for three-dimensional thick composites manufacturing

The emerging trends in Resin Transfer Molding (RTM) process technology for composites manufacturing are its applications in the manufacture of complex thick composite structures. Process modeling simulation tools analyzing the three-dimensional flow of resin impregnating the thick fiber preform are instrumental in process optimizations. The true three-dimensional flow field with multiple flow fronts and the merging and diverging fronts involved in the solution of the time progression of the resin inside a mold cavity pose significant challenges and there is a need for efficient, physically accurate algorithms for process simulation studies. In this paper, a new formulation based on the time dependent conservation of resin mass and a pure finite element method are employed to implicitly solve both for the pressure field and track the flow front progression of the resin inside the mold cavity of three-dimensional thick composites. The numerical developments involving the implicit tracking of resin progression flow fronts provide an accurate representation of the physical problem, do not involve the time step restrictions based on Courant condition as in past explicit finite element-control volume associated formulations. * Graduate Research Assistants t Professor * Research Scientists The discrete time progression of the flow front computed based on the present methodology does not depend on the time step size and the tracked location of the flow front at any discrete time does not depend on the time step size employed to reach that discrete time. The present developments have been validated with simple geometries and then extended to practical applications involving geometrically complex thick composite sections.

Permeability approximations and process modeling of geometrically complex composite structures manufactured by resin transfer molding

A good understanding of woven fiber mat permeability for composite molding processes is critical for realistic process modeling, yet these issues remain unresolved in the literature. Many have attempted to address permeability prediction for flat undeformed fiber preforms, but few have investigated permeability variations for complex three-dimensional molds. Attempts have been made over the years to predict the permeabilities via two schools of thought, namely, microscopic and macroscopic approximations. The objectives of this paper are to: (i) first provide a brief review of existing approximations for predicting these constitutive relations which includes microscopic and macroscopic prediction methods, (ii) postulate a more realistic representation of a unit cell to account for fabric structure such as crimp, tow spacing, and the like and then apply computational approximations for predicting effective permeabilities for use in actual structural composites manufacturing, and (iii) provide an improved understanding of the flow behavior at a 3-D unit cell level and in practical structural 3-D RTM composite parts. Emphasis is placed on techniques requiring little or no empirical data for accurate predictions where the end product should be a local permeability tensor suitable for insertion in mold simulations.

Corona discharge effects on aerosol sampling efficiency

Abstract Experiments were carried out to investigate the effects of probe electrification on the efficiency of the sampling of particles from air moving at relatively high wind speeds ranging from about 10 to 30 m s −1 . The aim was to gain physical insight which might be extended to the case of the sampling of atmospheric aerosol from aircrafts. Polydisperse NaCI aerosol particles with number median diameter of 0.05 μm were sampled using a metallic thin-walled probe aligned with the air stream. Penetration of aspirated particles through tubing of different materials was measured when a potential of −30 kV DC was applied to the probe. Penetration was found to increase from 0.16 to 0.87 as the external air speed increased, and the results were interpreted as indicating that sampling efficiency was influenced by the electrification of the probe itself. Experiments were also performed with nearly monodisperse liquid DOP aerosols with number median diameters from 0.05 to 0.5 μm and the results were used to examine the charge transfer of ions (from the corona discharge which occurred at the sharp-edged probe tip) to the aerosol particles. In this part of the study, negative and positive corona discharges around the sampling inlet were created by applying potentials of up to ± 20 kV to the probe. It was found that, for given potential, the negative corona was much more effective in charging the aerosol, probably due to the presence of unattached free electrons. Such charge transfer process may be attributed to combined field and diffusion charging mechanisms.