Julie Teuwen
Delft University of Technology
Publications
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Toughening of Epoxy Systems with Interpenetrating Polymer Network (IPN): A Review
Journal PaperThe Influence of Thermal Contact Resistance on the Thermal History in Laser Assisted Fiber Placement
Conference PaperTeaching Aerospace Structures and Materials to the World–Analysis of the edX MOOC Introduction to Aerospace Structures and Materials
Conference PaperOn differences and similarities between static and continuous ultrasonic welding of thermoplastic composites
Journal PaperThe rain erosion of wind turbine blades is caused by raindrop impacts on the leading edge and is an engineering challenge for the wind industry. This erosion damage due to rainfall is directly related to the raindrop impact energy. Therefore, using an energetic approach, three different variables, namely the total kinetic energy, the kinetic power and the kinetic energy per impact, to characterise the erosion capacity of the raindrop impacts have been calculated using actual meteorological data from the Royal Netherlands Meteorological Institute (KNMI) of the last 25 years. These erosive variables need to be used as a joined set of variables to monitor, predict and prevent the rain erosion damage on the leading edge blades. Moreover, this work has also analysed the effect of different velocity contributions, the log time of data, type of rain and period of data to be considered. Finally, as the main findings, the wind speed is the main contribution to the erosive variables and the intensity of the rainfall, as well as the frequency in log meteorological data, are also significant factors affecting the three erosive variables.
Leading edge erosion of wind turbine blades: Effects of blade surface curvature on rain droplet impingement kinematics
Conference PaperLeading edge erosion of wind turbine blades: Effects of blade surface curvature on rain droplet impingement kinematics
Journal PaperIntimate Contact Development During Laser Assisted Fiber Placement: Microstructure and Effect of Process Parameters
Journal PaperCure-induced residual stresses for warpage reduction in thermoset laminates
Journal PaperContinuous ultrasonic welding of thermoplastic composites: Enhancing the weld uniformity by changing the energy director
Journal PaperA fully coupled thermo-mechanical analysis for the minimisation of spring-in and process time in ultra-thick components for wind turbine blades
Journal PaperThe present paper investigates the generation of cure induced residual stresses during the cure stage of the Vacuum Assisted Resin Transfer Moulding (VARTM) process for the fabrication of ultra-thick components (i.e. 105 mm) for wind turbine blades manufacturing (i.e. root insert). The viscous-elastic material characterisation of the Airstone 780E epoxy resin mixed with the 785H Hardener has been undertaken and the corresponding coupled thermo-mechanical simulation has been implemented using a commercial FE solver. The finding points out that the level of residual stresses generated during the cure stage leads to a spring-in of about 1.3 cm when Manufacturer Recommended Cure Cycle is applied and that improvements in both spring-in and cure time can be obtained by applying different cure cycles.
The impetus for higher performance, robustness and efficiency in the aerospace, automotive and energy industries has been reflected in more stringent requirements which the composite manufacturing industry needs to comply with. The process design challenges associated with this are significant and can be only partially met by integration of simulation in the design loop. The implementation of numerical optimisation tools is therefore necessary. The development of methodologies linking predictive simulation tools with numerical optimisation techniques is pivotal to identify and therefore develop optimal design conditions that allow full exploitation of the efficiency opportunities in composite manufacturing. Numerical and experimental results concerning the optimisation techniques and methodologies implemented in literature to address the optimisation of thermoset composite manufacturing processes are presented and analysed in this study.
Investigation on the melting of the weld interface in continuous ultrasonic welding of thermoplastic composites
Conference PaperEffects of Process Parameters on Intimate Contact Development in Laser Assisted Fiber Placement
Conference PaperThe paper deals with the influence of the convection coefficient and laminate thickness on multi-objective op-timisation of the vacuum assisted resin transfer moulding cure stage for the manufacturing of wind turbinecomponents. An epoxy resin system widely used in the wind turbine industry has been chemically characterisedand the correspondentfinite element implementation validated. The optimisation methodology developed linksthefinite element solution with a genetic algorithm and identifies a set of optimal cure cycles for a range ofthicknesses (10–100 mm) able to minimise cure time (tcure) and the maximum degree of cure gradient developedthrough thickness (Δαmax) during the cure stage as a measure of quality of the product. The results highlight that,by adding convection coefficient as design parameter of the process, significant benefits could be obtained wheninsulation is applied at the vacuum bag side for all thicknesses.
Developments in the wind industry reveal intricate engineering challenges, one of them being the erosion on the leading edge of the wind turbine blades. In this review work, the main issues for the wind industry in the experimentation with respect to erosion are examined. After a historical and general overview of erosion, this review focuses on the rain erosion on the leading edge of the wind turbine blades giving prominence to (1) the rain simulations, (2) experimental erosion facilities, and (3) variables to characterise erosion. These three factors have to be improved to establish a research field enabling the prediction of erosion behaviour and providing useful information about how the rainfall affects the leading edge of the wind turbine blades. Moreover, these improvements in the experimentation of the erosion would be a first step to understand and predict the erosion damage of the wind turbine blades. Finally, this review work also will help to cope with experimental investigations and results in the rain erosion on the leading edge with a deeper critical thinking for future researchers.
The paper addresses the multi-objective optimization of the cure process of a Vacuum Assisted Resin Transfer Molding for components ranging from 40 to 100 mm thickness and aims to investigate the effect of thickness on the identification and quantification of a set of optimal cure profiles that minimize temperature overshoot and process time. Optimal cure solutions are sought among three dwells temperature profiles and are compared to the manufacturer’s recommended cure cycle (MRCC). The methodology successfully approximates the efficient fronts for the three different cases under study (40, 70 and 100mm) and points out the efficiency opportunity available compared to MRCC. In the case of 70 and 100 mm thick component temperature overshoot reductions of about 75% are achievable and 67% reduction in process time. The results also suggest a change in the objectives’ landscape for the higher thicknesses in the vertical region of the Pareto.
Continuous ultrasonic welding (CUW) is an innovative high-speed joining method for thermoplastic composites. Currently, thin flat energy directors (EDs) are used to focus the heat generation at the weld line. The resulting fracture surfaces exhibit large areas of intact ED, resulting in a non-uniform weld, and significantly lowering the strength. The goal of this study is to improve the weld uniformity of continuous ultrasonically welded joints. In the first part of this paper we found that a 0.20 mm-thick woven mesh ED significantly improved the weld uniformity and strength in comparison to a 0.08 mm thick flat ED. The second part the paper focuses on understanding why the mesh gives this improved weld uniformity by analyzing the feedback data from the welder and by performing a microscopy analysis of the weld line at different moments during the static welding process. It was found that at the beginning of the welding process the mesh filaments expand within the open areas of the mesh while flattening; the mesh is being pre-formed in between the adherends. This pre-forming most likely created a good uniform intimate contact between the ED and adherends, which most likely resulted in a uniform heat generation and therefore created a uniform weld line. Because energy directing meshes make it possible to create uniform weld lines, they are expected to play an important role in the future for the continuous ultrasonic welding of thermoplastic composites.
Adhesive joining of carbon fibre reinforced polymer (CFRP) is cumbersome due to the careful surface preparation required and multiple validation steps to certify adhesion quality. Further these joints are often supplemented by mechanical fastenings add weight whilst also localising bearing stress. As an alternative technique, CFRP parts can be functionalized with thermoplastic surfaces during manufacture to enable cost-effective welding of composite structures. In the process of manufacturing the CFRP, curing an epoxy resin in the presence of the functionalising thermoplastic polymer can lead to local dissolution of the latter in the epoxy, followed by a reaction-induced phase separation. This results in a thermosetting-thermoplastic interphase featuring gradient concentrations and a multiphase morphology, which promotes load transfer between the thermosetting matrix and the thermoplastic joint. The aim of the work presented in this paper was to investigate interphase formation between high-Tg epoxy and polyetherimide (PEI) at different curing temperatures. The morphology was characterised using scanning electron microscopy and the composition of the interphase was quantified through Raman spectroscopy. The curing experiments indicated that temperature has a significant effect on the interphase morphology and led to two different biphasic morphologies which generally increased in size with increasing curing temperature. This suggests that the size of the gradient interphase can be tailored through the curing process, which is as a fundamental step in optimising the structural performance of welded joints with PEI-functionalized epoxy-based CFRPs.
Laser assisted automated tape or fiber placement (LATP/LAFP) with in-situ consolidation is a promising technique for manufacturing large structures, eliminating the limitations of autoclave curing. Currently, 2-D models are mostly preferred for the thermal analysis of the process. A 3-D, transient thermal finite element model is developed to analyze the effect of the alignment of the heat source with the tape laying direction and is compared with a model imitating a 2-D analysis space. This aspect of the process has not been considered in the literature so far. Effects of this aspect on temperature history and intimate contact evolution are presented.
A 1-D through-the-thickness transient heat transfer model is built to simulate the curing process of thick-walled glass-fibre-reinforced anionic polyamide-6 (APA-6) composites. The temperature and the degree of polymerisation through the thickness of the composite are calculated and compared to the experimentally obtained results. The kinetic models describing the polymerisation behaviour of APA-6 are implemented in the model. The kinetic model not taking into account the convection in the polymerisation process shows the best results. It is found that the predicted temperature profiles agree well with the experimental data.
Semi-adiabatic temperature measurements are recorded and used to define semi-empirical equations for the simulation and prediction of the anionic polyamide-6 (APA-6) reaction kinetics. The resin mixture used has a long infusion window before the reaction starts. The prediction of the induction time and its corresponding initial temperature of reaction is explored. By means of this semi-empirical approach and an optimised fitting procedure, the reaction kinetics of APA-6 can successfully be described. The adiabatic polymerisation can be predicted on the basis of an autocatalytic Kamal-Sourour model for thermoset resins, and the crystallisation can be described using the isothermal crystallisation model. The reaction kinetics of APA-6 is successfully described by introducing a new kinetic equation providing a more predictable way of describing the kinetics of this autocatalytic system. The resin mixture used has a long infusion window before reaction starts. Equations are defined for the prediction of the infusion window, i.e., induction time and the corresponding initial temperature of reaction.
In previous research, the potential of glass fibre reinforced anionic polyamide-6 (APA-6) composites for use in wind turbine blades was proven. Based on polymer properties, viscosity, processing time, costs and recyclability, APA-6 composites are considered the most suitable reactive thermoplastic material candidate. However, more research is needed to mature the knowledge of the APA-6 material and its processing which can be achieved by understanding the effect of the individual steps in the manufacturing process and by studying the material behaviour in more detail. First of all, an experimental study on the effect of the individual steps in the manufacturing and post-processing process was performed to increase the homogeneity of the composites and identify the most important processing parameters. Secondly, semi-empirical models for the prediction of the reaction kinetics and rheology were built to better estimate the infusion time, start of reaction and the behaviour of the material. These models were then used to predict the heat build-up due the exothermic reaction in thick-walled composites. Based on the models for the reaction kinetics and rheology and the knowledge build from the experimental investigation, it is thought that an optimisation of the whole manufacturing process for a specific product is feasible and that the material behaviour within that process can be well predicted.
In recent years, the use of thermoplastic composites (TPC) has increased significantly because of their low cost, fast processing cycles and recyclability. In an effort to provide a manufacturing technique well suited for large TPC parts, the Delft University of Technology has developed an infusion process based on a reactive anionic polyamide-6 (APA-6) resin system. Following recent work where significant differences in mechanical and physical laminate properties with respect to flow direction were identified, the work presented in this paper aims at improving laminate uniformity by testing different mould heating strategies. Through a first study, key laminates properties were evaluated (interlaminar shear strength, degree of crystallinity and degree of conversion) using two different heating methods (conduction and radiation) over three different curing temperatures (160, 170 and 180°C). The results of this first study clearly identified that the radiation method yielded better laminates than the conduction heating method. However, most of the laminates produced still showed non-uniform properties with respect to the flow direction. Laminates cured at 180°C showed better average properties. Based on these results, a second study was conducted using low infusion temperatures (110-150°C) followed by curing at 180°C, aiming at enhancing the uniformity of properties. The best results were achieved with an infusion temperature of 150°C. The degree of crystallinity and interlaminar shear strength clearly exhibited a lower standard deviation when compared to other laminates. This was also validated with comparative C-scan inspections and optical microscopy.
In order to manufacture thicker, larger and more integrated thermoplastic composite parts than currently can be achieved by melt processing, a vacuum infusion process is currently being developed at the Delft University of Technology using a reactive thermoplastic polymer called anionic polyamide-6 (APA-6). In previous studies it was demonstrated that the anionic polyamide-6 (APA-6) resin that is used has excellent mechanical properties. The present study assesses infused thermoplastic composites and focuses on fiber-matrix interactions. Part I of this study focuses on the thermal effects, causes for deactivation of the initiator and the restriction caused by the low in-plane permeability of the fiber textiles on various transport phenomena. It will be shown that addition of pre-heated fibers not only shortens the infusion window, but also influences the matrix properties by reducing the exothermic heat production. In addition, the low in-plane permeability of the fiber textiles influences the infusion time and causes the entrapment of voids. Finally, reactions between the matrix and the fiber surface can lead to deactivation of the initiator and bond formation with the activator. Interfacial bonding, however, is discussed in more detail in Part II of this study, whereas the effect of adding a nucleating agent is discussed in Part III.
One of the advantages of reactive processing of thermoplastic composites is that due to in situ polymerization of a thermoplastic resin, interfacial bond formation occurs at a higher extend that can be achieved with melt processing. In other words, a thermoplastic composite can be obtained with an interface that is typical for thermoset composites. This paper assesses the effect of the interface on the mechanical performance of a reactively processed and a melt processed polyamide-6 composite. It will be shown that a strong interface especially beneficial for reducing the void content and improving the fatigue performance.
To explore the potential of fibre-reinforced thermoplastic anionic polyamide-6 composites for lightweight structures, the effect of the fibre addition needed to be investigated. The reactive processing of neat anionic polyamide-6 (APA-6) has been investigated and it was demonstrated that it exhibited excellent resin properties. The addition of 70% wt of preheated E-glass fibres altered the heat transfer to the resin. Consequently, the overall internally produced heat decreased and needed to be compensated for by a higher external heat to get the same properties as the neat resin. Due to the high thermal conductivity of the preheated fibres also a reduction of the infusion window was observed. Besides the change in properties of the laminates, a temperature gradient in flow direction resulting in different resin properties from inlet to outlet of the composite.
Moisture ingress in honeycomb sandwich structures is an issue that has attracted significant attention from aircraft operators, maintenance, repair and overhaul (MRO) organizations as well as the research community. A particular problem of interest has been rudders on the CF-18, where indications of moisture ingress were found in the composite sandwich structure. Not only does bulk water in honeycomb sandwich components cause weight gain and a change in the dynamic characteristics of components such as the CF-18 rudder, it also affects structural integrity by causing physical and chemical degradation of the matrix and adhesive, and weakened facesheet/core bonds. In this study, an overview of mechanisms for moisture ingress into composite structures and the detrimental effects on structural performance is provided. Based upon non-destructive inspection results, moisture ingress occurrence maps were developed for a large sample size of CF-18 rudders. These maps illustrate a consistent moisture ingress pattern for this structure. Rather than infusing through the honeycomb composite facesheets, moisture ingress mainly occurred via joints where direct water ingress could occur. These maps also provide indications of an important moisture migration path within the sandwich component that was later verified by liquid penetration tests. Based upon this investigation, possible moisture removal strategies including non-invasive and invasive methods are discussed.