Liquid Resin Infusion Process Research Articles

A Constructal-Theory-Based Methodology to Determine the Configuration of Empty Channels Used in the Resin Impregnation of a Square Porous Plate

Liquid composite molding techniques are largely used to produce pieces such as truck cabins or wind turbine blades. The liquid resin infusion processes use a network of injection channels to improve the resin flow through a porous-reinforced medium. The present numerical study predicts the positioning of empty channels by applying constructal theory to an idealized problem. The channels’ position and size were not predefined but instead constructed (made to grow) from an elemental channel. Two strategies were tested for channel growth: each new elemental channel was placed next to the region with the lowest or highest resistance to resin flow. The geometric configuration of the channels was constructed using a control function instead of using pre-defined shapes. The conservation of mass and momentum and an additional transport equation for the resin volume fraction were solved using the finite volume method. The volume of the fluid model was used for the treatment of the multiphase flow (air + resin). The growth of an empty channel with the lowest resistance strategy led to a decrease in the injection time and waste of resin. The size (resolution) of the elemental channel also affected the performance indicators and geometric configuration of the injection channels.

Liquid Resin Infusion Process Validation through Fiber Optic Sensor Technology.

In the proposed work, a fiber-optic-based sensor network was employed for the monitoring of the liquid resin infusion process. The item under test was a panel composed by a skin and four stringers, sensorized in such a way that both the temperature and the resin arrival could be monitored. The network was arranged with 18 Fiber Bragg Gratings (FBGs) working as temperature sensors and 22 fiber optic probes with a modified front-end in order to detect the resin presence. After an in-depth study to find a better solution to install the sensors without affecting the measurements, the system was investigated using a commercial Micron Optics at 0.5 Hz, with a passive split-box connected in order to be able to sense all the sensors simultaneously. The obtained results in terms of resin arrival detection at different locations and the relative temperature trend allowed us to validate an infusion process numerical model, giving us better understanding of what the actual resin flow was and the time needed to dry preform filling during the infusion process.

Optimization of liquid resin infusion process (LRI) using finite element simulation software

Infusion processes as LRI (Liquid Resin Infusion) can lead to a significant cost reduction but are not yet so widespread in aeronautical applications. Some of its advantages are: no need of autoclave, lower costs of materials, and the possibility of fabricating integrated structures that reduce the number of joints. However this process still presents some drawbacks such as the need to apply the "Test-Error" method or availability of extensive experience in the design of structures. The risk and cost associated with the infusion process is high, so the industry tends to use more conservative designs or adopt other less competitive alternative technologies and materials. For this reason, a simulation tool is essential to optimize the LRI process. In this work, the optimization and validation of a demonstrator of a wing box of an aircraft has been carried out using the PAM-COMPOSITES simulation software. Firstly, a geometric study and discretization of model has been carried out, secondly, the parameters that govern the process have been defined, and finally an analysis and validation of results have been carried out by means of a experimental infusion of the demonstrator. The obtained results have allowed us to control manufacturing times, improve injection and vacuum ports, and predict dry zones and front flow velocity. Simulations have allowed us to optimise the LRI process of the demonstrator, thus giving continuity to other studies on the viability of the infusion processes in primary structures in the aeronautical sector.

Challenges of complex monitoring of the curing parameters in coupons for LRI manufacturing

In the aerospace industry, Liquid Resin Infusion (LRI) is gaining more and more importance as an out-of-autoclave alternative manufacturing technique to traditional pre-impregnated (prepreg) fabrics. The research in this field has been focused on understanding the cure and the process parameters of these materials, aiming to optimize the manufacturing process and reduce costs. A major problem derived from these technologies is the distortions induced by LRI process, affecting to composite parts due to non-uniform distribution of residual stresses. Such distortions can lead to non-uniform parts with shape distortions, which is a critical issue when trying to assembly with other parts due to mismatches in shape, leading to the rejection of such components. In this context, ELADINE project aims to understand and quantify the key manufacturing parameters that cause shape distortions on composite coupons (such as spring-in of curved parts) using an integrated numerical-experimental approach. The manufacturing process will be accurately monitored through Fiber Optic Sensors (FOS) and Dielectric sensors (DC) to understand how the process variables affect the distortion phenomena. The monitored data will feed a simulation tool for spring-in prediction for large integral composite wing structures. This article covers the preliminary results of cure monitoring and process parameters of thermoset composites implementing monitoring strategies for manufactured coupons by LRI.

Experimental and Numerical Study of Vacuum Resin Infusion of Stiffened Carbon Fiber Reinforced Panels.

Liquid resin infusion processes are becoming attractive for aeronautic applications as an alternative to conventional autoclave-based processes. They still present several challenges, which can be faced only with an accurate simulation able to optimize the process parameters and to replace traditional time-consuming trial-and-error procedures. This paper presents an experimentally validated model to simulate the resin infusion process of an aeronautical component by accounting for the anisotropic permeability of the reinforcement and the chemophysical and rheological changes in the crosslinking resin. The input parameters of the model have been experimentally determined. The experimental work has been devoted to the study of the curing kinetics and chemorheological behavior of the thermosetting epoxy matrix and to the determination of both the in-plane and out-of-plane permeability of two carbon fiber preforms using an ultrasonic-based method, recently developed by the authors. The numerical simulation of the resin infusion process involved the modeling of the resin flow through the reinforcement, the heat exchange in the part and within the mold, and the crosslinking reaction of the resin. The time necessary to fill the component has been measured by an optical fiber-based equipment and compared with the simulation results.

Development of a Flexible Dielectric Sensor for Flow Monitoring of the Liquid Resin Infusion Process.

The analysis and design of a novel flexible dielectric sensor, which can be integrated into a composite materials manufacturing process to measure the resin frontal flow, is presented in this paper. The proposed sensor consists of two parallel line electrodes and a ground plane covered by a dielectric material. The analytical description and the electrostatic modelling were considered for the design of the sensor and to enhance the understanding of the response of the sensor to the resin impregnation of a carbon fabric during the infusion phase. The optimization of the sensor’s response and the increase of its sensitivity with regards to the geometric characteristics and the materials used were the main objectives of this study. An experimental set-up for the vacuum infusion process which includes the proposed sensor was used to measure the capacitance and validate the derived resin flow against visual measurements. The results indicate that the sensor can provide information on the resin frontal flow within 2% accuracy against visual measurements, which make this technology promising for monitoring the liquid resin infusion processes.

Finite Element Simulation of Low Velocity Impact Damage on an Aeronautical Carbon Composite Structure

Low velocity barely visible impact damage (BVID) in laminated carbon composite structures has a major importance for aeronautical industries. This contribution leads with the development of finite element models to simulate the initiation and the propagation of internal damage inside a carbon composite structure due by a low velocity impact. Composite plates made from liquid resin infusion process (LRI) have been subjected to low energy impacts (around 25 J) using a drop weight machine. In the experimental procedure, the internal damage is evaluated using an infrared thermographic camera while the indentation depth of the face is measured by optical measurement technique. In a first time we developed a robust model using homogenised shells based on degenerated tri-dimensional brick elements and in a second time we decided to modelize the whole stacking sequence of homogeneous layers and cohesive interlaminar interfaces in order to compare and validate the obtained results. Both layer and interface damage initiation and propagation models based on the Hashin and the Benzeggagh-Kenane criteria have been used for the numerical simulations. Comparison of numerical results and experiments has shown the accuracy of the proposed models.

Assessment of Liquid Resin Infusion Impregnation Quality Using Scanning Electron Microscopy

Fibre reinforced composite materials are increasingly becoming the popular choice of materials for designers in many fields such as aircraft manufacturing. As well as the accompanying high costs, one of their main limitations is the difficulty of making shaped parts such as those of aircraft and UAVs. To overcome this problem, various composite manufacturing techniques have been developed including the vacuum resin infusion (VRI) process and its many derivatives. In this work, a cross ply glass fibre reinforced plastic (GFRP) composite laminate was manufactured using liquid resin infusion (LRI) process which is a derivative of VRI. Specimens cut from the manufactured laminate are tested under static tensile loading and then investigated for resin impregnation quality using two types of state of the art Field Emission Scanning Electron Microscopes (FESEM). Images taken showed some plies with well intralaminar (inter fibre within single ply) impregnation whereas in some locations - within mainly the internal plies - there was very little impregnation. The implication is that unidirectional (UD) laminated when stacked in a cross ply form present another LRI process factor that is not widely investigated. Consequently, infusing UD cross ply composite laminates may present more challenges compared to infusing woven fabrics which are helped by the gaps between the fibres weave and weft.

A coupled structural and flow approach for numerical simulation of the light resin transfer moulding process. II: Fabric permeability and compaction characterisation, model results and a 6-kW wind turbine blade case study

This is the second paper in a two-part study into the numerical simulation of the light resin transfer moulding (LRTM) process. It focuses on the development of empirically derived models for the permeability and compaction behaviour of Unifilo during light resin transfer moulding. A detailed review of relevant literature allowed identification of important material parameters, and appropriate testing procedures were devised accordingly. Previously published approaches were used to construct light resin transfer moulding specific permeability and compaction models. A material response for each stage of the process was quantified and then inputted into the light resin transfer moulding simulation developed in the Part I of this study. Results were verified using data from an empirical testbed, and the completed light resin transfer moulding model was then applied to a 6-kW wind turbine blade case study to evaluate different infusion strategies and minimise fill time. Finally, it was used to compare the light resin transfer moulding, resin transfer moulding and liquid resin infusion processes for the manufacture of this part.

New methodology to determine the compressibility curve in a RIFT process

In liquid resin infusion processes, the compression phase and the resin flow are important stages that influence the quality of the obtained parts. In this study, an experimental device is presented to measure thickness and pressure variations in order to obtain an experimental compressibility curve that can be implemented in a software dedicated to numerical simulations. To perform these experiments no typical compression testing machine is required and there is no test fluid. The aim of this work is to present a new methodology in order to obtain an empirical relation to determine the compressibility as a power law function. In order to evaluate the reliability of the proposed methodology, the obtained experimental curve is compared with another curve achieved performing standard compaction test.

Liquid Resin Infusion process monitoring with superimposed Fibre Bragg Grating sensor

This paper reports on the instrumentation and monitoring of a fibreglass/epoxy composite manufacturing process using an optical fibre sensor based on a Bragg grating. The sensor is made of superimposed Long Period Grating (LPG) and Fibre Bragg Grating (FBG), both written on the same fibre zone. This sensor gives very accurate results and good spatial resolution due to its ability to discriminate intrinsically between thermal and kinematic effects. The sensor was initially tested during the curing of an epoxy resin alone in order to assess temperature and strain measurements. It is shown that these two factors can be well discriminated and quantified. The paper then presents results for monitoring Liquid Resin Infusion (LRI), with the FBG/LPG sensor embedded in a composite part. Dielectric analysis was also performed. Results and analyses of the thermal cycle of curing and induced residual strains are discussed.

Relationships Between LRI Process Parameters and Impact and Post-Impact Behaviour of Stitched and Unstitched NCF Laminates

The general context of the development of out-of-autoclave processes in the aeronautics industry raises the question of the possible links between these new processes and impact behaviour. In this study, a Taguchi table was used in a design of experiment approach to establish possible links. The study focused on the liquid resin infusion process applied to laminates made with stitched or unstitched quadri-axial carbon Non-Crimp Fabric (NCF). On the basis of previous studies and an analysis of the literature, five process parameters were selected (stitching, curing temperature, preform position, number of highly porous media, vacuum level). The impact energy was set at 35 J in order to obtain enough residual dent depth. The parameters analysed during and after impact were: maximum displacement of the impactor, energy absorbed, permanent indentation depth, and delaminated surface. Then, compression after impact tests were performed and the corresponding average stress was measured. The interactions found by statistical analysis show a very high sensitivity to stitching, which was, of course, expected. A very significant influence of curing temperature and a significant influence of preform position were also found on the permanent indentation depth and a physical explanation is provided. Globally, it was demonstrated that the resin infusion process itself did not influence the impact behaviour.

Numerical and experimental analyses of resin infusion manufacturing processes of composite materials

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.

Monitoring the resin infusion manufacturing process under industrial environment using distributed sensors

A novel direct approach to detect the resin flow front during the Liquid Resin Infusion process under industrial environment is proposed. To detect the resin front accurately and verify the results, which are deduced from indirect micro-thermocouples measurements, optical fiber sensors based on Fresnel reflection are utilized. It is expected that the results derived from both techniques will lead to an improvement of our understanding of the resin flow and in particular prove that micro-thermocouples can be used as sensors as routine technique under our experimental conditions. Moreover, comparisons with numerical simulations are carried out and experimental and simulated mold filling times are successfully compared.

Warpage of Flat Composite Part Induced by Liquid Resin Infusion (LRI) Processes

Composite materials manufacturing of high performance part require high cure temperature in a multi-material environment. It results multi-scale product-process-resources interactions, which are responsible for low dimensional fidelity between the mould and the part shape. Because of the different implementation by LRI processes, warpage cannot be account only for tool-part interaction like by prepreg-autoclave processes. An interaction between the part and the moulding resources are characterized, which is in majority responsible of the part warpage. Experimental setups were developed to investigate the main mechanisms of deformation present by LRI processes. The deformations due to the part-moulding resources interaction have been experimentally isolated and the influences of main driver parameters have been quantified. A simple mechanical model based on the thermal and cure behaviours of the part and the resources are proposed to predict the warpage. The model correlates with an acceptable accuracy the experimental warpage of different part lengths and part thick.

Liquid Resin Infusion process monitoring with embedded superimposed long period and short period Bragg grating sensor.

We propose here the monitoring of the Liquid Resin Infusion (LRI) process for using a superimposed long period (LPG) and short period (FBG) Bragg grating sensor. Monitoring of such a process is usually made measuring simultaneously temperature and strain by the use of an electro-optical device (FBG-Thermocouple). It has been shown that an applied solicitation is measured by a wavelength shift with a different sensitivity for LPG and FBG; thus strain and temperature influences can be determined separately by measuring corresponding wavelength shifts. The reported configuration is based on the use of these two Bragg gratings types written in the same fibre section, which allows us to discriminate the contributions of the temperature and strain. The sensor is embedded in a composite specimen manufactured by LRI process for monitoring in real time and simultaneously the applied temperature and strain.

Characterization of Liquid Resin Infusion (LRI) filling by fringe pattern projection and in situ thermocouples

Prepregs are not alone suited to the manufacturing of new types of aircraft structural parts that tend to be thicker and have more complex shapes. Direct processes called Liquid Composites Moulding (LCM), like Resin Transfer Moulding (RTM) or Resin Infusion Process (LRI: Liquid Resin Infusion, RFI: Resin Film Infusion) are now available. Particular attention is paid here to LRI process that looks very promising. In order to optimize both the design and manufacturing parameters in LRI processes, a general model to analyze the isothermal fluid flow through highly compressible porous media such as fibrous preforms has been recently proposed [1–3]. To validate the model and to improve the knowledge of the LRI process, an experimental approach is proposed. Two different measurement techniques, micro-thermocouple sensors and fringe projection technique, have been used to characterize the process. It appears that results derived from both techniques are in agreement and support the assumption that the resin flow occurs mainly transversely to the preform plane.