Global behaviour of a composite stiffened panel in buckling. Part 1: Numerical modelling

The present study analyses an aircraft composite fuselage structure manufactured by the Liquid Resin Infusion (LRI) process and subjected to a compressive load. LRI is based on the moulding of high performance composite parts by infusing liquid resin on dry fibres instead of prepreg fabrics or Resin Transfer Moulding (RTM). Actual industrial projects face composite integrated structure issues as a number of structures (stiffeners, …) are more and more integrated onto the skins of aircraft fuselage. A representative panel of a composite fuselage to be tested in buckling is studied numerically.     

Global behaviour of a composite stiffened panel in buckling. Part 2: Experimental investigation

The present study analyses an aircraft composite fuselage structure manufactured by the Liquid Resin Infusion (LRI) process and subjected to a compressive load. LRI is based on the moulding of high performance composite parts by infusing liquid resin on dry fibres instead of prepreg fabrics or Resin Transfer Moulding (RTM). Actual industrial projects face composite integrated structure issues as a number of structures (stiffeners, …) are more and more integrated onto the skins of aircraft fuselage.A post-buckling test of a composite fuselage representative panel is set up, from numerical results available in previous works. Two stereo Digital Image Correlation (DIC) systems are positioned on each side of the panel, that are aimed at correlating numerical and experimental out-of-plane displacements (corresponding to the skin local buckling displacements of the panel). First, the experimental approach and the test facility are presented. A post-mortem failure analysis is then performed with the help of Non-Destructive Techniques (NDT). X-ray Computed Tomography (CT) measurements and ultrasonic testing (US) techniques are able to explain the failure mechanisms that occured during this post-buckling test. Numerical results are validated by the experimental results.     

Influence of fabrics’ design parameters on the morphology and 3D permeability tensor of quasi-unidirectional non-crimp fabrics

This research addresses the effects of quasi-UD non-crimp fabric (NCF) design parameters on the fabric architecture and on the permeability tensor. These fabrics are designed for the Liquid Resin Infusion (LRI) of large and thick composite parts. Three fabrics’ parameters intended to bring a flow enhancement to the NCF are investigated: the stitch spacing, the stitch pattern and the weft tow lineal weight. Image analysis is undertaken to characterize the morphology of non-crimp fabric composite. A new continuous permeability measurement method based on compressive tests is proposed to relate the permeability of the quasi-UD NCF to the design parameters during the infusion process. The latter are proven to influence significantly both the fabric architecture and the permeability tensor coefficients.     

Numerical simulations for class A surface finish in resin transfer moulding process

Simulation tools for Liquid Composite Moulding (LCM) processes are a key to predict and solve manufacturing issues. Despite the fact that numerical process analyses are commonly used to predict mould filling, resin cure and exothermic temperatures, more comprehensive computational tools are still required. Resin additives such as low profile additives (LPA) show a significant impact on process performance and part quality. In this work, mould pre-heating experiments were compared to numerical predictions using commercial simulation software. Non-isothermal simulations were then carried out and the predicted flow and degree-of-cure evolution were compared to experiments. Finally, a volume change model, previously developed, was implemented in this work to calculate mould pressure increases in RTM of resins with four different LPA contents (0%, 5%, 10% and 40%). The predictions were compared to the results from the mould pressure transducers in the mould cavity. Simulation results matched closely with the experimental results. Pressure evolution of low profile resins was found to be very sensitive to the model parameters.     

A comprehensive filling and tooling force analysis for rigid mould LCM processes

A comprehensive tooling force analysis is presented for rigid tool Liquid Composite Moulding (LCM) processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM). This has been implemented within SimLCM, a generic LCM filling simulation under development at the University of Auckland. The simulation has been verified against existing analytic and semi-analytic solutions, considering fill times and clamping force due to reinforcement compaction. Industrial application is demonstrated through consideration of a fireman’s helmet, which has demonstrated the complex evolution of both local and global tooling forces during RTM and I/CM cycles. Resultant forces are computed in the closing and lateral directions, having practical benefits for design of moulds and supporting equipment. The evolution of tooling forces has been shown to be sensitive to the accuracy of the applied fibre reinforcement compaction model, which is used to predict normal and tangential stresses exerted on mould surfaces.     

Simulating the effect of temperature elevation on clamping force requirements during rigid-tool Liquid Composite Moulding processes

Moulds used for rigid-tool Liquid Composite Moulding (LCM) processes, namely Resin Transfer Moulding (RTM) and Compression RTM, are often subjected to large internal forces which originate due to resin injection and from the compaction of fibre reinforcements. Appropriate clamping equipment (e.g. press or perimeter clamps) is necessary to equilibrate these forces. An optimal selection (or design) of such clamping equipment calls for an accurate prediction of the tooling forces generated. This work aims to introduce a comprehensive numerical scheme which addresses this issue, including the case of non-isothermal mould filling. A hybrid Finite Element/Finite Difference (FE/FD) methodology is utilised for solving the coupled flow/energy/species equations. A new fibre compaction model, developed in order to reduce computational complexity while maintaining solution accuracy, is implemented into the simulation algorithm. The force predictions obtained for a planar axisymmetric part reveal that the chosen combination of mould and resin temperatures, together with other process variables, plays a crucial role in allowing fast fill times while keeping setup costs low.     

A methodology to reduce variability during vacuum infusion with optimized design of distribution media

Seemann Composites Resin Infusion Molding Process (SCRIMP) is a widely used version of Vacuum Assisted Resin Transfer Molding (VARTM) in which a highly permeable layer (distribution media) is placed on top of the dry preform to distribute the resin with very low flow resistance to reduce the filling and hence the manufacturing time. The flow patterns during filling may vary from part to part due to the variability associated with the material, part geometry, and layup of the assembly, which may result in race-tracking channels. The process is considered as reliable and robust only if the resin completely saturates the preform despite changing filling patterns caused by flow disturbances.The resin flow pattern can be manipulated with a tailored distribution media layout as it does impact the flow patterns significantly. The continuous distribution media layout over the entire part surface works well for very simple geometries with no to little potential for race-tracking along the edges. In this study we address complex cases, which require placement of an insert within the assembly, which will introduce race-tracking along its edges, and hence uniform placement of distribution media over the entire top surface will fail to yield a void free part. We introduce a methodology using a predictive tool to design an optimal shape of distribution media, which accounts for the flow variability introduced due to race-tracking along the edges of the inserts. This iterative approach quickly converges to provide the placement of distribution media on selective areas of the preform surface that ensures complete filling of the preform despite the variability. This approach has been validated with an experimental example and will help mitigate risk involved in manufacturing complex composites components with Liquid Molding.     

Holistic and Consistent Design Process for Hollow Structures Based on Braided Textiles and RTM

The present paper elaborates a holistic and consistent design process for 2D braided composites in conjunction with Resin Transfer Moulding (RTM). These technologies allow a cost-effective production of composites due to their high degree of automation. Literature can be found that deals with specific tasks of the respective technologies but there is no work available that embraces the complete process chain. Therefore, an overall design process is developed within the present paper. It is based on a correlated conduction of sub-design processes for the braided preform, RTM-injection, mandrel plus mould and manufacturing. For each sub-process both, individual tasks and reasonable methods to accomplish them are presented. The information flow within the design process is specified and interdependences are illustrated. Composite designers will be equipped with an efficient set of tools because the respective methods regard the complexity of the part. The design process is applied for a demonstrator in a case study. The individual sub-design processes are accomplished exemplarily to judge about the feasibility of the presented work. For validation reasons, predicted braiding angles and fibre volume fractions are compared with measured ones and a filling and curing simulation based on PAM-RTM is checked against mould filling studies. Tool concepts for a RTM mould and mandrels that realise undercuts are tested. The individual process parameters for manufacturing are derived from previous design steps. Furthermore, the compatibility of the chosen fibre and matrix system is investigated based on pictures of a scanning electron microscope (SEM). The annual production volume of the demonstrator part is estimated based on these findings.     

Prediction and experimental verification of normal stress distributions on mould tools during Liquid Composite Moulding

Moulds for Liquid Composite Moulding (LCM) processes such as Resin Transfer Moulding and Compression RTM must withstand significant forces generated by resin injection and preform compaction. Prediction of tooling forces will allow optimisation of setup costs and time, and optimal selection of peripheral equipment (such as presses). A generic LCM simulation (SimLCM) is being developed with the capability to predict clamping forces and stress distributions acting on mould tools. Both mixed-elastic and viscoelastic reinforcement compaction models are implemented within SimLCM. A series of novel rigid tool LCM experiments were undertaken using a flat plate part geometry, and are compared to results generated by SimLCM. In general, predictions are very good, with the viscoelastic model providing significant improvement over the mixed-elastic model during phases involving stress relaxation within the reinforcement. Novel aspects of this work include measurement and prediction of spatial normal stress distributions and time dependent stress relaxation behaviour of reinforcements.     

Bisphenol A based polyester binder as an effective interlaminar toughener

Bisphenol A based thermoplastic polyesters are commonly used in the industry as binders, or tackifiers, to produce cost-saving preforms in Liquid Composite Moulding processes such as Vacuum Assisted Resin Transfer Moulding (VARTM). However, it is often reported that the presence of these polyesters has a detrimental effect on the mechanical properties of the resulting composite laminates. In contrast, this study shows that interlaminar toughness can be increased without negatively affecting other properties by coating the reinforcing plies with a bisphenol A based thermoplastic polyester if some precautions are taken in mind.The polyester was added to an epoxy resin in order to study its effect on the thermophysical properties and fracture toughness of the bulk epoxy. The polyester molecules acted as a plasticizer for the epoxy resin when the polyester was added in low amounts. This increased the bulk fracture toughness of the epoxy resin by 30%. Polyester modified glass/epoxy laminates were produced and tested for Mode I interlaminar fracture toughness and flexural properties. The increased toughness of the epoxy matrix led to a 60% increased Mode I interlaminar fracture toughness of the laminates, without negatively affecting flexural stiffness and strength of the laminates.     

Modeling of coupled dual-scale flow–deformation processes in composites manufacturing

The present contribution is a part of the work towards a framework for holistic modeling of composites manufacturing. Here we focus our attention onto the particular problem of coupled dual-scale deformation–flow process such as the one arising in RTM, Vacuum Assisted Resin Infusion (VARI) and Vacuum Bag Only (VBO) prepregs. The formulation considers coupling effects between macro-scale preform processes and meso-scale ply processes as well as coupling effects between the solid and fluid phases. The framework comprises a nonlinear compressible fiber network saturated with incompressible fluid phase. Internal variables are introduced in terms of solid compressibility to describe the irreversible mesoscopic infiltration and reversible preform compaction processes. As a main result a coupled displacement–pressure, geometrically nonlinear, finite element simulation tool is developed. The paper is concluded with a numerical example, where a relaxation–compression test of a planar fluid filled VBO preform at globally un-drained and partly drained conditions is considered.     

Numerical simulation of metal matrix composites and polymer matrix composites processing by infiltration: a review

The injection of a liquid metal through a fibrous preform is one of the techniques used to manufacture metal matrix composites (MMCs). The flow of metal through fibrous preform is a problem of fluid mechanics in porous medium. Numerical simulations of this process were developed in particular for non-isothermal infiltrations which take into account the phenomena of phase change. In addition, numerical models were developed to predict the appearance of defects in the end product and to study the evolution of the deformation of the fibrous preform during metal infiltration. After pointing out the analogous numerical studies devoted to the Resin Transfer Moulding (RTM) process, we give a progress report on the models developed to date for MMCs.     

Out-of-autoclave cure cycle study of a resin film infusion process using in situ process monitoring

Liquid resin infusion (LRI) of textile tailored reinforcements (TRs) is increasingly applied in new processing technologies for manufacturing carbon fibre composites. This work presents a cure cycle study of an out-of-autoclave toughened resin film infusion (RFI) process as part of the examination of an alternative manufacturing process for composites. To successfully produce laminates using resin film infusion in combination with a fast-curing process, the flow behaviour of the selected resin material under changed processing conditions was investigated. The effect of processing parameters, specifically heating rates and dwell times, on resin viscosity and laminate infiltration was evaluated through experimental work and supported by in situ process monitoring. A DC-resistance sensor system was applied to track the change in resin viscosity during cure. Results showed that cure cycles with a relatively short dwell time and higher heating rate compared to an autoclave cure led to enhanced flow properties of the toughened resin system. High quality laminates, comparable to autoclave panels, were manufactured with vacuum pressure only by modifying the original vacuum bagging arrangement.     

Measurements of normal stress distributions experienced by rigid liquid composite moulding tools

Mould tools used for LCM processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) must withstand local forces due to compaction of the fibre reinforcement, and due to resin pressure generated within the laminate. A series of RTM and I/CM experiments have been carried out, with the focus placed on measurement of normal stress distributions exerted on the mould surface. In addition, total mould clamping force and injection gate pressure histories have been recorded. I/CM experiments using force-controlled secondary compaction were also undertaken, and compared to the velocity-controlled cases. Observed fluid pressure fields showed good agreement with theory, namely a logarithmic distribution during fluid injection and a quadratic distribution during the compression driven filling phase of I/CM. Significant spatial variation in normal stress due to reinforcement compaction was observed. The influence of the fluid pressure on the total stress experienced by the mould was observed to be a function of both the fibre volume fraction of the part and the applied injection pressure, the latter being more pronounced at lower part volume fractions.     

Experimental validation of post-filling flow in vacuum assisted resin transfer molding processes

In Liquid Composite Molding (LCM) processes with compliant tool, such as Vacuum Assisted Resin Transfer Molding Process (VARTM), resin flow continues even after the inlet is closed due to the preform deformation and pressure gradient developed during infusion. The resin flow and thickness changes continue until the resin pressure becomes uniform or the resin gels. This post-filling behavior is important as it will determine the final thickness and fiber volume fraction distribution in the cured composite. In this paper, a previously proposed one dimensional coupled flow and deformation process model has been compared with the experimental data in which the resin pressure and part thickness at various locations during the post-filling stage is recorded. Two different post-infusion scenarios are examined in order to determine their impact on the final part fiber volume fraction and thickness. The effects of different venting arrangements are demonstrated. The model predictions compare favorably with the experimental data, with the minor discrepancies arising due to the variability of material properties.     

Resin infusion between double flexible tooling: prototype development

The Resin Infusion between Double Flexible Tooling (RIDFT) technique is a novel two-stage process, which incorporates resin infusion and wetting with vacuum forming. The flow front of the infused resin is two-dimensional and avoids flow complexities prevalent in the three-dimensional flow seen in other liquid composite molding techniques. It employs a one-sided mold, which provides obvious cost benefits when compared with resin transfer molding. On-going prototype development of the RIDFT process has yielded positive results. Composite laminates with good surface quality, micro structural characteristics, and mechanical properties have been repeatedly produced with cost savings of 24% when compared with SCRIMP. This paper describes the RIDFT process, outlining its merits and presenting its challenges, whilst identifying potential benefits to industry. Current work being undertaken include the refining of production parameters, the construction of a larger prototype to examine the full extent of its suitability for the manufacture of large composite components and the incorporation of the UV curing technique to reduce the cycle time in the manufacture of large structures.     

Capillary effects on flax fibers – Modification and characterization of the wetting dynamics

Introducing bio-based composites has now become an opportunity of development for industry. Accordingly, Liquid Composite Moulding (LCM) processes are increasingly used for manufacturing those composites, mainly in the transportation industry, since they are considered as effective and low cost routes to manufacture bio-based composites fitting high quality requirements, even for parts with complex shape. However observations of a large amount of voids in bio-based composites call for an improved understanding of the local wetting phenomena that occur during impregnation of the natural reinforcements. The purpose of the present work is to study the influence of flax fiber surface chemistry on the local wetting dynamics. Flax reinforcements were submitted to a thermal treatment to modify the chemical composition of fiber surface. In order to analyze the fiber’s wetting behavior, some methods for measuring apparent static contact angles and surface energy were firstly validated on solids of defined geometry, and subsequently applied to untreated and treated flax fibers. The Owens–Wendt’s approach was used to determine both components of apparent surface energy, indicating polar and dispersive interactions in materials. Subsequently dynamic tests were carried out on both types of chopped flax fibers in order to evaluate apparent advancing dynamic contact angles. Considerations about morphological effects have also been included. Finally bio-based composite plates reinforced with untreated and treated flax quasi-UD were simultaneously fabricated by LCM process, and observation of the porosities highlighted some benefits of using treated flax fibers.     

Resin flow modeling in compliant porous media: an efficient approach for liquid composite molding

In Liquid Composite Molding (LCM) processes such as Resin Transfer Molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM), complete saturation of reinforcement with resin during the injection step is necessary. In RTM, reinforcement experiences no deformation during infusion but for other methods reinforcement thickness changes during the injection. To model resin flow in compliant media, RTM flow simulation software is routinely used. It has been successful in predicting flow patterns if appropriate “effective” permeability is used. The proper approach requires new implementation that couples the deformation and pressure field which is computationally more demanding. Our work describes a computationally efficient methodology to add corrections into RTM simulation environment to account for deformation. This approach is verified with known solutions and experimental validation. The simulation is applied to a complex geometry which demonstrates better computational performance and confirms that the “effective” permeability may be used to model flow in complex geometries.     

Radial flow permeability measurement. Part B: Application

This paper deals with permeability measurement in the context of Resin Transfer Moulding (RTM). A new approach to two-dimensional radial flow permeability measurement with constant inlet pressure is proposed in part A of this paper. In this second part experimental studies are performed to validate the new approach. The new approach is shown to accurately predict the orientation of the tensor axes. It was demonstrated how the convergence chart can be used to check whether the experimental results follow the underlying assumptions of the model. Examples are given of acceptable and unacceptable measurements. Experimental design is introduced to investigate the variation of measured permeability between individual experimental runs. It was concluded that most of the observed variation was the result of flow front measurement sensors used in the experiment. Finally the new approach is compared with current methods. Differences are discussed. It is shown that the new approach extends the capabilities of current methods.     

Radial flow permeability measurement. Part A: Theory

This paper deals with permeability measurement in the context of Resin Transfer Moulding (RTM). A new approach to two-dimensional radial flow permeability measurement with constant inlet pressure is proposed. It allows principal permeability to be measured even if the experimental axes are not aligned with the principal direction. This part of the paper looks at the underlying theory of the new approach while part B reports on validation experiments. Formulae are derived which allow to calculate principal permeability and the orientation of the principal axes from flow front measurements in three directions. Transient effects of the developing flow front caused by the circular inlet are discussed and its influence on the measured permeability is illustrated. Numerical studies are performed which show that the shape of the flow front is dependent only on the size of the inlet diameter and the degree of anisotropy. This leads to the development of a formula for estimating the minimum required mould size for permeability measurement.