Laminate thickness and resin pressure evolution during axisymmetric liquid composite moulding with flexible tooling

This paper presents experimental observations from the filling and post-filling stages of 1D axisymmetric Resin Infusion (VARTM) and RTM Light. A series of experiments have been performed to investigate the influence of mould flexural stiffness and fill mode on fluid pressure, cavity thickness, filling stage time, and post-filling stage time. Observations are also made on the effect of those parameters on the repeatability of nominally identical experiments. This paper helps identify the circumstances where a RTM simulation would be sufficiently accurate for an RTM Light process, and consequently where a full flexible tooling simulation is necessary.     

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.     

Fabrication,process simulation and testing of a thick CFRP component using the RTM process

The present article introduces the case of a CFRP con-rod beam, and describes many aspects regarding its production with the Resin Transfer Moulding (RTM) process.The objective was to find the best process parameters of the injection and curing stages in order to manufacture the 20 mm thick CFRP part. The results are analysed in terms of the aesthetic aspect, the porosity and the mechanical properties of the final component.For the resin injection stage, results obtained from production experiences are presented, which have been performed with different set-ups, and simulations of the resin flow are used to analyse them. The results show that the resin flow during injection could be rather unpredictable, probably because of the fibre rearrangement and race tracking effects. Improvements in terms of aesthetic aspect and porosity of the part could be achieved by a process which included final compaction of the cavity by means of compressed air.Regarding the curing stage, the article presents the simulation results of a curing cycle, and it’s validation through DSC analysis of specimens obtained from the finished component.Finally, results of tensile mechanical tests are provided, performed on finished components produced by RTM and compared to others produced with the method of hand lay-up of pre-impregnated plies and curing in autoclave (Prepreg + Autoclave). The results confirm that it is possible to achieve components through RTM with comparable mechanical performance to those produced with the Prepreg + Autoclave process.     

A fast method for the generation of boundary conditions for thermal autoclave simulation

Manufacturing process simulation enables the evaluation and improvement of autoclave mold concepts early in the design phase. To achieve a high part quality at low cycle times, the thermal behavior of the autoclave mold can be investigated by means of simulations. Most challenging for such a simulation is the generation of necessary boundary conditions. Heat-up and temperature distribution in an autoclave mold are governed by flow phenomena, tooling material and shape, position within the autoclave, and the chosen autoclave cycle. This paper identifies and summarizes the most important factors influencing mold heat-up and how they can be introduced into a thermal simulation. Thermal measurements are used to quantify the impact of the various parameters. Finally, the gained knowledge is applied to develop a semi-empirical approach for boundary condition estimation that enables a simple and fast thermal simulation of the autoclave curing process with reasonably high accuracy for tooling optimization.     

Liquid crystalline texture and hydrogen bond on the thermal conductivities of intrinsic thermal conductive polymer films

Polymer-dispersed liquid crystal(PDLC)films comprising polyvinyl alcohol(PVA)and liquid crystal monomer(LCM)were successfully obtained by the method of solution casting&thermal compress-ing.LCM was distributed orderly in PVA matrix by hydrogen bond interaction,to form PVA-LCM interpenetrating-layered networks.When the mass fraction of LCM was up to 35 wt％,the corresponding in-plane thermal conductivity coefficient(λ∥)of PDLC film was significantly increased to 1.41 W m-1 K-1,about 10.8 times that of neat PVA(0.13 W m-1 K-1).High intrinsic λ//values of PDLC films were mainly attributed to the formed microscopic-ordered structures from ordered stacking of LCM,ordered arrangement of PVA chains,and their hydrogen bond interaction.This work would offer a new way to design and prepare novel intrinsic high thermal conductive polymers.     

Numerical prediction of saturation in dual scale fibrous reinforcements during Liquid Composite Molding

This paper presents a fractional flow model based on two-phase flow, resin and air, through a porous medium to simulate numerically Liquid Composites Molding (LCM) processes. It allows predicting the formation, transport and compression of voids in the modeling of LCM. The equations are derived by combining Darcy’s law and mass conservation for each phase (resin/air). In the model, the relative permeability and capillary pressure depend on saturation. The resin is incompressible and the air slightly compressible. Introducing some simplifications, the fractional flow model consists of a saturation equation coupled with a pressure/velocity equation including the effects of air solubility and compressibility. The introduction of air compressibility in the pressure equation allows for the numerical prediction of the experimental behavior at low constant resin injection flow rate. A good agreement was obtained between the numerical prediction of saturation in a glass fiber reinforcement and the experimental observations during the filling of a test mold by Resin Transfer Molding (RTM).     

Development of a measuring system for on-line in situ monitoring of composite materials manufacturing

A unique portable measuring system using an impedance spectroscopy method with a self-adapting frequency of measurement is introduced. The system is intended for the on-line in situ monitoring of composite materials curing under industrial conditions. The capabilities of the developed system are demonstrated through the results obtained from on-line in situ measurements of unreinforced thermosetting resin, as well as of composites under real manufacturing conditions. Observations are supported by the results of other established methods for determining the degree of curing: temperature-modulated differential scanning calorimetry (MDSC), Fourier transform infrared spectroscopy (FT-IR) and broadband dielectric spectroscopy (BDS). Compressive and bending tests were also carried out on manufactured composites removed at different stages of the post-curing phase. Due to the self-adapting frequency, the system has enhanced sensitivity in the post-cure phase when the diffusion-controlled reactions proceed and, therefore, is suitable also for the analysis of hard post-cure samples.     

Evaluation of eddy current testing for quality assurance and process monitoring of automated fiber placement

Standards in energy and cost efficiency are higher the ever especially in the aerospace industry. While structures made from carbon-fiber reinforced plastics (CFRP) show significant advantages in regards to specific strength and lightweight design, further improvements in their production processes are essential in order for CFRP to be competitive in the future. The authors present eddy current (EC) testing as a means for quality assurance (QA) and process monitoring for CFRP parts produced by automatic fiber placement (AFP), which is one the most prevalent production methods in aerospace industry. Eddy current testing shows the potential for highly automated process monitoring that can reduce error correction and cycle time in AFP.     

Process-induced strains in RTM processing of polyurethane/carbon composites

The development of residual strains and stresses is critical to manufacture composite structures with the required dimensional stability and mechanical performance. This work uses Fiber Bragg Grating (FBG) sensors to monitor strain build-up in carbon fiber composites with a polyurethane (PU) matrix designed for high production volume applications. The PU matrix presents an initially low viscosity combined with a fast cure reaction, which makes it adequate to very short processing cycles. FBG sensors were incorporated into PU-matrix composites manufactured by vacuum assisted resin transfer molding (VARTM). The measured strains were compared with those obtained with different benchmark epoxy-matrix composites and with those obtained through micromechanical finite element simulations. Results showed that most of the residual strains were built-up during cool-down from the post-curing temperature and that stresses in the PU-matrix composites were comparable to those obtained for epoxies with similar T_g.     

Three-dimensional reconstruction of resin flow using capacitance sensor data assimilation during a liquid composite molding process: A numerical study

Liquid composite molding (LCM) is a method to manufacture fiber-reinforced composites, where dry fabric reinforcement is impregnated with a resin in a molding apparatus. However, the inherent process variability changes resin flow patterns during mold filling, which in turn may cause void formation. We propose a method to reconstruct three-dimensional resin flow in LCM, without embedding sensors into the composite structure. Capacitance measured from pairs of electrodes on molding tools and the stochastic simulation of resin flow during an LCM process are integrated by a sequential data assimilation method based on the ensemble Kalman filter; then, three-dimensional resin flow and permeability distribution are estimated simultaneously. The applicability of this method is investigated by numerical experiments, characterized by different spatial distributions of permeability. We confirmed that changes in resin flow caused by spatial permeability variations could be captured and the spatial distribution of permeability could be estimated by the proposed method.     

An inverse model for optimisation of laser heat flux distributions in an automated laser tape placement process for carbon-fibre/PEEK

High power diode lasers have enabled higher placement rates to be achieved in automated tape placement (ATP) of thermoplastic-based composite materials. Laser ATP heads in published literature employ homogeneous linear or rectangular laser spots, however a variety of solutions are available to produce customised irradiance profiles. No efforts to date have investigated what a more ideal heat flux profile would be for laser ATP in terms of length and distribution. This paper describes a method to determine the required laser heat flux profiles to achieve desired heating zone temperature profiles by means of an inverse thermal model. A bonding model was implemented to assess the performance of various heating zone temperature profiles for placement at 400 mm/s. Short beam strengths from experimental trials (Stokes-Griffin and Compston, 2015) were used to validate the bonding model. A two-stepped heating profile was found to provide a good balance of increased strength with a small increase in power requirement.     

Infrared stress measurements of thermal damage to laser-processed carbon fiber reinforced plastics

Recently, the laser processing of carbon fiber reinforced plastics (CFRPs) has attracted attention owing to the high processing speed and less tool wear. A problem in the laser processing of CFRPs is the lower strength than that of CFRPs processed by machines. This is considered to be due to the heat-affected zone (HAZ) generated during laser processing. In this study, the stress distributions of CFRPs processed by a laser obtained was evaluated by using infrared thermography. X-ray CT images were also obtained, which enabled us to discuss the stress distribution in terms of the HAZ. The stress distribution showed that the area with reduced stress generated in the HAZ which was introduced by laser processing. The region of low stress in the HAZ was visualized by infrared thermography. It is shown that the regions with reduced stress induce the conventionally reported decrease in strength of laser-processed CFRPs.     

Data assimilation through integration of stochastic resin flow simulation with visual observation during vacuum-assisted resin transfer molding: A numerical study

This study investigated data assimilation through integration of visual observation with a stochastic numerical simulation of resin flow during vacuum-assisted resin transfer molding. The data assimilation was performed using the four-dimensional asynchronous ensemble square root filter and a stochastic numerical simulation by means of the Karhunen–Loève expansion of the permeability field. Through numerical experiments of linear flow, it was verified that the estimation accuracy of the resin impregnation behavior improved compared to that when using conventional data assimilation and that the permeability field could be estimated simultaneously, although it is not explicitly related to the observation. We also investigated the applicability of the proposed method to radial-injection VaRTM by varying the model thickness. The proposed method successfully estimated the resin impregnation behavior and permeability field. Additionally, the required condition for the number of ensemble members was clarified.     

Characterization of textile permeability as a function of fiber volume content with a single unidirectional injection experiment

Textile permeability is a fundamental property to describe preform impregnation in Liquid Composite Molding (LCM) processes. It depends on textile architecture and fiber volume content (FVC). Conventional methods to measure in-plane permeability are based on radial or unidirectional injection experiments performed at fixed FVC. A complete characterization involves a series of tests and requires several material samples. This study presents a novel approach to characterize permeability as a function of FVC through a unique unidirectional injection experiment with a preform containing different FVC sections. The same experimental set-up as in conventional unidirectional unsaturated permeability measurements is used with a second pressure transducer embedded in the mold in addition to the one located at the inlet gate. A fast algorithm is developed to exploit the data from the two sensors and automatically derive the permeability distribution without any need of visual flow front observations. The methodology is validated with a random fiber mat and a woven fabric. Results show that accurate permeability characterization can be achieved for both kinds of textiles.     

Finite element simulation and experimental study on mechanical behavior of 3D woven glass fiber composite sandwich panels

The results of finite element simulation followed by an experimental study are presented in order to investigate the mechanical behavior of three-dimensional woven glass-fiber sandwich composites using FE method. Experimental load–displacement curves were obtained for flatwise compressive, edgewise compressive, shear, three-point bending and four-point bending loads on the specimens with three different core thicknesses in two principal directions of the sandwich panels, called warp and weft. A 3D finite element model is employed consisting of glass fabric and surrounding epoxy resin matrix in order to predict the mechanical behavior of such complex structures. Comparison between the finite element predictions and experimental data showed good agreement which implies that the FE simulation can be used instead of time-consuming experimental procedures to study the effect of different parameters on mechanical properties of the 3D woven sandwich composites.     

Flow of non-Newtonian liquid polymers through deformed composites reinforcements

The flow of non-Newtonian liquid polymers through fibrous reinforcements is a phenomenon which is often encountered during polymer composites manufacturing. In a previous work, we have proposed from a multiscale theoretical approach a method to model this phenomenon when the polymer can be regarded as a generalised Newtonian fluid [Orgéas et al. J. Non-Newtonian Fluid Mech. 2007; 145]. In this paper, the capability of the method is tested with power-law fluids flowing through deformed plain weave fabrics. For that purpose, the flow problem is firstly analysed at the mesoscale from numerical simulations performed on representative elementary volumes of the fabrics. The influences of both the current deformation of the fabrics and the fluid rheology on the macroscopic flow law are emphasised. Secondly, it is shown that the proposed method allows a nice fit of numerical results.     

Transient gas flow technique for inspection of fiber preforms in resin transfer molding

A transient gas flow method was developed to determine the quality of fibrous preforms in resin transfer molding (RTM) prior to resin injection. The method aims at detecting defects resulting from preform misplacement in the mold, accidental inclusions, preform density variations, race tracking, shearing, etc. Unlike the previously developed method based on steady-state gas flow, the new method allows for the acquisition of continuous time-varying pressure data from multiple ports during a single test. The validity of the method was confirmed by one-dimensional flow experiments.     

In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement

Filtration of nanofillers into porous fabric media is still an issue during the preparation of advanced fiber-reinforced composites. The assessment of resin/multiwall carbon nanotube (MWCNT) flow, MWCNT filtration, and the cure monitoring of glass fiber/carbon nanotube-polyester composites by means of the measurement of the electrical resistance was introduced. The vacuum-assisted resin transfer molding technique was used. The electrical resistances measured over the span of a composite were qualitatively correlated with MWCNT flow and the degree of MWCNT filtration. It was found that while the complexity of the fabrics could likely introduce preferential deposition of MWCNTs, their filtration is mainly affected by their dispersion state in the resin suspension. Relationships among critical parameters such as the lengths and diameters of MWCNTs, the inter- and intra-tow dimensions of glass fabrics, the dispersion level of MWCNTs, and the viscosity of nanocomposite samples are discussed and correlated to the filtration, cure, and flow phenomena. We showed that our method can also serve as an early warning to obviate defects in the resulting composite.     

Numerical/experimental impact events on filament wound composite pressure vessel

Impacts on pressure vessels, produced by winding glass fibre with vinyl ester resin over a polyethylene liner, were numerically and experimentally investigated in the current work.Pressure vessels were experimentally tested under low velocity impact loads. Different locations and incident energies were tested in order to evaluate the induced damage and the capability of the developed numerical model.An advanced 3-D FE model was used for simulating the impact events. It is based on the combined use of interlaminar and intralaminar damage models. Puck and Hashin failure theories were used to evaluate the intralaminar damages (matrix cracking and fibre failure). Cohesive zone theory, by mean of cohesive elements, was used for modelling delamination onset and propagation.The experimental impact curves were accurately predicted by the numerical model for the different impact locations and energies. The overall damages, both intralaminar and interlaminar, were instead slightly over predicted for all the configurations.The model capabilities to simulate the low velocity impact events on the full scale composite structures were proved.     

The effects of heat input on adjacent paths during Automated Fibre Placement

One key aspect of the quality of parts attainable from the thermoset Automated Fibre Placement process is the impact of the heat source. For most industrial applications, an infrared heater is used and suitable process windows are still defined by trial-and-error approaches. Within this study, the need for robust thermal prediction tools is shown as well as the need for thermal management of the process. The influence of the radiation distribution on adjacent paths is presented experimentally and numerically, highlighting the non-uniform temperature distribution of tool and component. A multi-angle flat plate component was laid up, and bulk temperatures measured point-wise by a grid of thermocouples, as well as surface temperatures captured using a thermography camera. Finally a parameterised 3D Finite Element model was developed as a basis for precise prediction of the thermal history during the complete layup process.