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The effective design of channels in dry tape preforms is crucial for achieving desired preform permeability for successful resin injection for composites manufacturing using Automated Fiber Placement (AFP) process. Achieving target gaps and their locations in the AFP layup is extremely challenging. This work investigates the correlation between the spatial variability of the preforms and the in-plane permeability using an X-ray Computed Tomography (XCT) based characterization framework. The tomographic images of two different dry carbon tape preforms with different tape widths were used to generate realistic and XCT based stochastic models to be used for numerical permeability predictions. The variability in the tape placement by the robotic head and its effect on preform permeability was also examined through stochastic geometric modeling of the laid preform. A benchmark transient permeability measurement set-up was utilized to obtain experimental in-plane preform permeability through 2D radial mold filling. The in-plane numerical permeability values showed significant scatter, with a coefficient of variance of 75%–130%, which deviated from the experimental measurements by approximately one order of magnitude. These findings strongly re-affirm that the experimental permeability measurement technique based on transient mold filling of dry fiber AFP preforms is complex however, the XCT based stochastic modeling technique is an effective way to estimate the permeability of dry fiber AFP preforms virtually. [ABSTRACT FROM AUTHOR]

Part I of this two-part paper presented the framework to integrate the simulation of resin flow and stress development during the manufacturing process of composites. In the current paper, the integrated approach developed in Part I for isotropic materials is extended to the case of transversely isotropic materials. Various numerical examples are considered in which the results obtained from the integrated approach are compared to those generated from the previously established models for processing induced stress development. These comparisons serve to elucidate the importance of accounting for the spatial and temporal variations in the resin volume fraction during processing and its effect on stress development. Such effects cannot be investigated in a non-integrated simulation environment where volume fraction variations due to resin flow have to be mapped sequentially from the flow simulation onto the next phase of the process simulation which treats the resin as an elastic or viscoelastic solid. [ABSTRACT FROM AUTHOR]

Process modelling of thermoset matrix composites is typically divided into two distinct and sequential steps: (i) flow-compaction analysis before gelation of the resin when it behaves in a fluid-like manner, and (ii) thermomechanical analysis after gelation when appreciable modulus and thereby stress development occurs. This two-part paper presents a novel approach to integrate the simulation of resin flow and stress development seamlessly during the processing of composites. Part I lays the theoretical foundation for the simpler case of isotropic materials while Part II extends the methodology to the case of transversely isotropic materials. The formulation is implemented in a 2D plane strain finite element code written in MATLAB. Relevant numerical examples are presented to demonstrate both aspects of flow-compaction and stress development throughout the curing process of thermoset matrix composite materials. The effects of resin flow on the development and the final values of residual stresses are investigated. [ABSTRACT FROM AUTHOR]

This study analyses the influence of the random filament arrangement in fibre bundles on the resin flow behaviour. Transverse steady-state resin flow that occurs behind a liquid resin flow front was simulated numerically through statistically equivalent micro-structures at high-fibre volume fractions, Vf > 0.6, as observed in fibre bundles. The need of applying a minimum gap distance between neighbouring filaments was overcome by automated local mesh refinement. The derived permeability values showed significant scatter. Convergence of these values was determined at a ratio of flow length to filament radius greater than 20 for all three analysed fibre volume fractions. Mean permeabilities were between 6 and 10 times lower than those predicted for a hexagonal fibre array. A statistical model is proposed, which is able to predict the scatter of observed permeabilities based on simple micro-structural descriptors. [ABSTRACT FROM AUTHOR]

Resin infusion (RI) process has been widely used for manufacturing composite parts. The variation of preform thickness brings great difficulty to the three-dimensional simulation of the filling stage. To accurately simulate the preform thickness change and resin flow during resin infusion, precise preform compaction models and dynamically changing geometry models need to be adopted. At present, resin flow is usually considered as two-dimensional and simple compaction models are employed to simplify the simulation, which degrades the prediction accuracy seriously. In this paper, general equations to describe the resin flow in the changing thickness cavity are developed, and the viscoelastic model is adopted which can fully express the dynamic characteristics of the preform compaction. To avoid solving the coupled resin flow/preform deformation equations directly, the volume of fluid method and the dynamic mesh model are employed to implement the tracking of the flow front and updating of cavity geometry model. The resin storage and release induced by porosity variations are adjusted by a master-slave element method to ensure mass conservation. Two simulation examples are carried out to demonstrate the capability of the above approach. The applicability of the approach on arbitrary complex domains and sequential injection strategy is also verified. [ABSTRACT FROM AUTHOR]

The quality of composite parts manufactured by resin transfer molding is sensitive to material and process variations during the preform impregnation. In order to improve the process robustness in two-dimensional injection cases, this work proposes a fast method for tracking and controlling the resin flow through the preform, using only a small number of pressure sensors embedded in the mold. The approach combines pressure signals and flow modeling in a quick algorithm that returns on-line estimations of the flow front profiles. Virtual and real injection tests demonstrated the accuracy of the methodology and provided information for designing the sensing system. Moreover, the investigation proved the feasibility of a simple and effective procedure of flow rate control. Based on a computer-controlled pressure regulator acting on the inlet gate, a feedback mechanism was implemented in the system and allowed keeping the flow front velocity within a target range of values. Such a control procedure may be used to limit the formation of voids and enhance the final part quality. [ABSTRACT FROM AUTHOR]

Two-dimensional finite-element method solution of the post-filling stage of vacuum infusion was studied based on mass conservation in an infinitesimal control volume. First, resin pressure distribution at the instant of mold filling was calculated and then used as the initial condition for the transient post-filling stage. Explicit time-marching algorithm was used for the evolution of resin pressure and part thickness, and its stability was ensured by selecting the time step adaptively. Finite-element method solution was verified analytically for one-dimensional case and numerically for two-dimensional cases using global mass conservation. The time that it took for the settlement of pressure and thickness was investigated to compare the effectiveness of different resin-bleeding scenarios where different number and locations of gates were used. It was shown that the settlement time increased exponentially as the dimensions of the mold increased, which proved that process simulation fed with correctly designed material characterization can replace tedious trial-and-error search of control actions to reduce the settlement time and variation in part thickness. [ABSTRACT FROM AUTHOR]

Liquid resin infusion (LRI) processes are promising manufacturing routes to produce large, thick, or complex structural parts. They are based on the resin flow induced, across its thickness, by a pressure applied onto a preform/resin stacking. However, both thickness and fiber volume fraction of the final piece are not well controlled since they result from complex mechanisms which drive the transient mechanical equilibrium leading to the final geometrical configuration. In order to optimize both design and manufacturing parameters, but also to monitor the LRI process, an isothermal numerical model has been developed which describes the mechanical interaction between the deformations of the porous medium and the resin flow during infusion.1,2 With this numerical model, it is possible to investigate the LRI process of classical industrial part shapes. To validate the numerical model, first in 2D, and to improve the knowledge of the LRI process, this study details a comparison between numerical simulations and an experimental study of a plate infusion test carried out by LRI process under industrial conditions. From the numerical prediction, the filling time, the resin mass and the thickness of the preform can be determined. On another hand, the resin flow and the preform response can be monitored by experimental methods during the filling stage. One key issue of this research study is to highlight the changes in major process parameters during the resin infusion stage, such as the temperature of the preform and resin, and the variations of both thickness and fiber volume fraction of the preform. Moreover, this numerical/experimental approach is the best way to improve our knowledge on the resin infusion processes, and finally, to develop simulation tools for the design of advanced composite parts. [ABSTRACT FROM PUBLISHER]

The simulation of composite manufacturing processes is a great aid to obtaining efficient production and high-quality parts. The mold and process design must allow for fast filling times as well as dry-spot free parts. Besides an accurate simulation of the resin flow through the reinforcement, the compaction response of the preform is also needed. The stress response of the textile to compaction has an influence on the local and global forces exerted on tooling. The numerical prediction of the clamping force helps to trade-off fast production times against affordable machinery. This article describes the accurate simulation of force controlled Resin Transfer Molding (RTM) and Compression RTM, and compares results of simulations with experimental data. A parametric study is performed in order to minimize the simulation time without compromising the accuracy of the results. The controlled force algorithms have been implemented within SimLCM, a code under development at the University of Auckland to address the liquid composite molding (LCM) family of manufacturing processes. With these new tools, the trade-off between production process time and equipment cost can be considered, and optimal process design solutions found. [ABSTRACT FROM AUTHOR]

Low coefficient of thermal expansion (CTE) prepreg is used, instead of the conventional high-CTE material like acrylic adhesive, as a bonding sheet to improve the reliability of rigid-flex printed circuit (RFPC). No-flow polyimide prepreg is then developed to fulfill the requirement of minimizing resin flow. Thermal characterization of no-flow polyimide prepreg reveals the resin behavior when a heat load is applied. Differential scanning calorimetry analysis reveals the process window when the resin starts to melt and then cure. Rheological tests further help us to observe the change in viscosity when the temperature rises as well as the resin flow effects at different heating rates (HRs). Moreover, a study of design of experiment (DOE) reveals that the HR, pressure and conformal material will have a significant impact on the resin flow of the no-flow polyimide prepreg in RFPC lamination. The combination of the resin behavior and DOE result provides the necessary information to achieve an optimized RFPC lamination process with minimized resin flow. [ABSTRACT FROM AUTHOR]

Vacuum-assisted resin transfer molding (VARTM) processes are increasingly used in manufacturing scale-up composite applications. Accordingly, gravity can significantly influence the flow behavior in tall-structure composite manufacturing processes. The present study developed a closed-form analytic solution incorporating gravity effects with the equivalent parameter approach in order to predict the resin flow behavior in a tall structure resin infusion. A hybrid model was used that consists of thin distribution media and a fibrous preform. An analytic solution was developed and validated with experiments as well as numerical methods in terms of the resin flow front shape, lag length, and the flow front location with time for the horizontal, upward, and downward infusion cases. The flow front locations were monitored using time domain reflectometry sensors embedded in the fabric layers. The lag length was constant in the horizontal infusion case, but decreased and increased in the upward and downward infusion cases, respectively, as resin progressed. The downward infusion case showed the fastest fill time, which corresponds to the previous results using a homogenous model. However, the flow became unstable at a certain location where the local flow front speeds increased suddenly along the distribution media. These phenomena were not observed in a model with homogenous media but were observed in a model with hybrid media, only in the downward infusion cases. The analytic solution also identified the stable flow conditions with the mold angles. The results obtained in the present study can be used to estimate stable process conditions in designing VARTM processes for manufacturing large-scale tall composite structures. [ABSTRACT FROM AUTHOR]

The variation of permeability with the flow rate has been investigated. This variation is demonstrated by conducting specially designed experiments. The results show that permeability increases significantly as the flow rate increases. The implication of this observation is that the permeability measured under some given conditions is not always applicable to the actual molding situation. Furthermore, the permeability is not constant over both the space and time domains during the filling process. In order to take such dependence into account, a permeability model and an estimation technique for determining the coefficients of the model are proposed. [ABSTRACT FROM AUTHOR]

The penetration of the polymeric melt into the fine interstices (typically of the order of a few microns or less) between the fibers of a thermoplastic prepreg depends upon the shape, volume fraction and geometry of packing of the fibers, the rheology of the polymeric melt and the interfacial surface tension. A model for preimpregnation would help determine optimal processing variables. In this paper the methods currently used to prepare thermoplastic prepregs are briefly reviewed and a preimpregnation model (which is generic to most processes involving melt impregnation) for describing the flow of resin into fiber-tows is developed. Such flows are usually modelled using Darcy's Law, but the approach of this paper represents an improvement in that empirical parameters (like permeability) are not required and detailed flow profiles can also be calculated. The effect of different geometries of packing on processing variables (fiber spacing, impregnation time, pressure drop) and flow characteristics (velocity, planar elongation and shear rates) is calculated. The fiber spacing (which depends upon the fiber volume fraction and geom etry of packing) has been found to be a critical parameter. The degree of shear-thinning of the polymeric melt on the pressure drop during flow through reinforcement is found to be very significant. Numerical values for the various flow and processing variables are calculated based upon experimental data reported in the patent literature. The value of the model is that it lets one predict the processing parameters required to completely impreg nate the tow and whether effective impregnation is feasible for a given polymer- reinforcement system. [ABSTRACT FROM PUBLISHER]