Key Engineering Materials Vols. 504-506

Abstract: Traditional ply-based and zone-based models are limited in their ability to account for the fiber directions resulting from the forming of fabric-reinforced composite wind turbine blades. Compounding the problem is the presence of defects such as resin-rich pockets of the polymer matrix due to out-of-plane and in-plane waves resulting from the manufacturing process. As a result, blades are typically overdesigned, unnecessarily increasing weight and material costs. In the current research, a methodology is presented for simulating the manufacturing process for fabric-reinforced composite wind turbine blades using ABAQUS/Explicit. The methodology captures the evolution of the yarn directions during the forming process thereby allowing for a map of the fiber orientations throughout the blade. A hybrid approach using conventional beam and shell elements is used to model the various fabric layers. Using experimental shear, tensile, bending, and friction data to characterize the mechanical behavior of the fabric layers, the model captures in-plane yarn waviness and changes in the in-plane yarn orientations as they conform to the shape of the mold, as well as out-of-plane wave defects as a result of the manufacturing process. Subsequently, after the fabric layers have been laid into the mold and the final yarn orientations are known, the structural stiffness of the blade resulting from the resin-infused fabrics can be calculated. The methodology can thereby link the resulting bending and torsional stiffnesses of the blade back to the manufacturing process. This paper discusses the methodology for determining the material properties of the beam and shell elements in their final orientations in the cured composite to predict the structural stiffness of a wind turbine blade.

Abstract: Abstract. Composites are processed by a variety of forming techniques at both preforming and consolidation stages; ranging from hand draping, diaphragm forming, vacuum infusion to Resin Transfer Molding. During these processes, individual fabric or prepreg layers are subjected to inplane tension and shear, inter-ply shear, transverse compression and out-of-plane bending forces. These forming forces are translated into individual tow-level forces leading to tow deformations. Each tow is subjected to tension, transverse compaction (in the plane of the fabric due to shear and normal to the fabric plane due to consolidation force), bending and torsion. The resulting tow geometry and local fibre volume fractions (within a tow) would have a significant impact on resin flow as well as mechanical properties of the composite. In this paper, we present computational as well as experimental approaches to predicting tow deformations, when subjected to various loading conditions. The test rigs, shown in figure 1, can measure stress-strain behaviour of a tow in bending, torsion and transverse compression respectively. Figure shows buckling of carbon tow – bending stiffness can be computed from the post-buckling behavior. Torsional moments at monotonically increased twist angle were measured using a very sensitive torque sensor. An anvil, nearly same size as a tow, is used to compress a tow (under controlled axial tension) and the cross-sectional shape is computed from the flattened image (recorded using a high resolution camera). A mechanics-based model has been developed to predict tow-scale deformations under transverse compression, tension, bending and torsion modes of deformation. Individual fibres in a tow are modeled as ‘3D elastica’ and a simple inter-fibre friction model has been incorporated. Initially developed for twisted fibre bundles, the elastic-based model works reasonably well for untwisted fibre tows (by assuming an extremely small twist level for convergence). Full paper will present comparison between experimental and theoretical results.

Abstract: In order to simulate 3D interlock composite reinforcement behavior in forming processes like Resin Transfer Molding (RTM), it is necessary to predict yarns positions in the fabric during the preforming stage of the process. The present paper deals about thick 3D interlock fabric forming simulation using a specific hexahedral semi-discrete finite elements simulation tool : Plast4. Using the virtual work principle, we distinguish the virtual internal work due to tensions in yarns from other internal virtual works. The part of material stiffness relative to yarns tension is described as "first order stiffness" by a 3D discrete beam model. The rest of the rigidities - like transverse compression, shear strains or friction between yarns - are depicted by a continuous quad-based discretization designated in our work as "second order stiffness". A combination of this "first order" discrete model and a continuous orthotropic hyperelastic "second order" material formulation will enables us to simulate interlock preforming process. Jointly to the simulation work, we also had to specify and perform experimental testing identification of material's parameters. Thoses parameters concern both parts of the model. A bilinear tension approach for the yarns discrete modelization and an orthotropic continuous material for the "second order" part.

Abstract: The preforming stage of the LCM composite manufacturing processes lead to fibrous reinforcement deformations which may be very large especially for double curvature shapes. Those deformations have significant influence on the second stage of the process, i.e. the injection of the resin. A way to predict accurately the spatial distribution of the permeability tensor consists in simulating for various configurations, the deformed shape of the reinforcement at the scale of the yarns. Mesoscopic scale analyses of textile reinforcements generally consider the yarns as a continuous material despite their fibrous nature. In order to have an accurate simulation tool, it is necessary to build up a constitutive law which accounts for the physical specificities linked to the microstructure of the yarns. Several models exist with reasonable accuracy. The present paper proposes a new approach in the hyperelasticity framework. The proposed model is based on the definition of mathematical invariants linked to the four main deformation modes of the yarn material: tension, compaction, longitudinal shear and transverse shear. The strain energy potential build up with those invariants is identified using classical fabric material tests: uni- and bi-axial tension and compression. The model has been validated on laboratory tests such as bias extension tests and gives promising results.

Abstract: Knitted steel fiber fabrics are used in the fabrication of automotive windshields. To obtain the complex window shapes, the leading technology to produce automotive glass is compression molding at high temperatures. Direct contact between the mold and the glass during the forming process would lead to inadmissible defects and optical distortion of the automotive glass. To ensure the quality of the glass, a soft heat resisting separating layer is used. Knitted steel fiber fabrics are draped over the mould prior to production, shown in Fig. 1, and thus the fabric comes in direct contact with the glass playing a primary role in the quality of the formed windshield.

Abstract: The properties of final composite parts depend on properties of dry preforms often being formed over doubly-curved shapes. In this case the fibrous preforms exhibit intricate large deformations, including shear, tension, and bending modes. Although the bending stiffness of fibrous materials is small, in shaping of preforms, when wrinkling occurs, its influence is important, not negligible and responsible for the wrinkles shape. Because of the structural and mechanical peculiarities, the experimental determination of bending properties of fibrous materials is rather complex, and there is no unique generally adopted test. A set of cantilever tests was chosen to be carried out in this study, in the form of sequence of different loading cases for one material that permits to reveal the eventual non-linear and non-elastic behavior of the material in bending. The tests were realized for glass fabrics with different types of weaving patterns and different areal weights. The effect of these parameters on the bending response is studied. The analysis of the data on bending of fabrics and bending of yarns, extracted from fabrics with the preserved undulated shape, is performed as well. The regions of the deformed specimens characterized by the largest scatter of experimental data are identified and analyzed. Besides, the analytical model based on corrugated plates theory, taking into account the undulated architecture of fabrics, is employed to characterize its bending properties, and to make a future comparison with the test results.

Abstract: Matrices employed in composites parts of aeronautic structures consist of a thermoset / thermoplastic mixture. Thermoplastic is introduced in low concentration in order to improve the mechanical properties, in particular the ones related to choc resistance. However, there are two antagonist mechanisms, the one related to energy that leads to demixing and the one related to entropy that tends to mix. These effects are strongly coupled with the elasticity of thermoplastic, the evolution from a newtonian fluid to a viscoelastic one of thermosets, the presence of reinforcement fibers, … and are nowadays bad understood despite the significant impact that these effects have on the composite microstructure and then on its mechanical properties. This work constitutes a first attempt to understand these complex physics.

Abstract: Liquid Composite Molding (LCM) processes, widely used to manufacture thermosetting matrix composite materials, are characterized by the impregnation of a dry fibrous perform, by the means of injection or infusion of the catalyzed resin. The increasing industrial application of LCM processes is due to the demand for high performances materials and constant quality productions, combined with the need to reduce human intervention and costs due manufacturing inefficiency. The opportune planning of LCM processes results, however, very complex, being the process characterized by non-stationary multiphase flows in a three dimensional porous domain with anisotropic permeability, by the cure reaction, influencing the temperature, the degree of cure, and the viscosity of the processing resin, and by the elastic deformation of the fiber bundle due to the applied pressure, which affects significantly preform properties. Nowadays, process planning and optimization is mainly based on trial and error procedures or on computational simulations. Although the existing simulation packages, developed thanks to the efforts spent by several research groups, led to a better understanding and more effective application of LCM processes, on line monitoring of resin flow is very desirable to account for unpredicted variations of processing conditions. Moreover, an accurate experimental evaluation of fiber preform properties is crucial for a reliable process simulation. In this paper, a dielectric capacitive system has been designed, realized, and applied to monitor the position of the saturated as well as the unsaturated flow fronts and to evaluate in plane bulk permeability and tow permeability of dual scale fibrous porous media, typically used in LCM processes. The used sensors, analysed and optimized by computational simulations, have been embedded into opportunely designed rigid dies. Several preform impregnation tests have been performed. Good agreement has been found between results provided by the used system and data obtained using conventional techniques, evidencing the capability of proposed method for process monitoring, as well as for material properties evaluation.

Abstract: A new and promising approach to the reduction of greenhouse gas emissions is the use of improved lightweight constructions based on multi-material systems comprising sheet metal with local carbon fibre reinforced plastic (CFRP) reinforcements. The CFRP is used to reinforce highly stressed areas and can be aligned to specific load cases. The locally restricted application of CFRP means that the material costs can be effectively reduced by comparison to parts made entirely of CFRP on account of the expensive production process requiring the use of an autoclave. These parts are thus only used in high-priced products. The production of hybrid CFRP steel structures in a mass production process calls for an efficient production technology. Current research work within the scope of a collaborative research project running at the University of Paderborn is concentrating on the development of manufacturing processes for the efficient production of automotive structural components made up of sheet metal blanks with local CFRP patches. The project is focusing especially on basic research into the production of industrial components. The aim of the investigation is to create an efficient and controlled process for producing CFRP reinforced steel structures from semi-finished hybrid steel-CFRP material. This includes tool concepts and an appropriate process design to permit short process times. The basis of an efficient process design is an in-depth knowledge of the material behaviour, and hence a thorough characterisation was performed. Material parameters were determined for both simulation and forming. For this, monotonic tensile, shear and bending tests were conducted using both uncured prepregs and cured CFRP specimens. To achieve an accurate simulation of the forming process, a special material model for carbon fibre prepregs has been developed which also includes the anisotropic material behaviour resulting from fibre orientation, the viscoelastic behaviour caused by the matrix and the hardening effects that prevail during curing. Recent results show good qualitative agreement and will be presented in this paper. In order to control the properties of the hybrid components, four different tool concepts for the prepreg press technology have been developed and tested. The concepts are presented and the results of experimental investigations are discussed in this paper.

Abstract: Simulations of manufacturing processes are of utmost importance in order to check on process feasibility of composites products already during the design phase. In order to benchmark the different software for (thermo)forming simulations of textiles and composites a benchmark geometry was agreed during previous Esaform conferences. Round 2 results have led to the insight that a stronger definition of the benchmark was needed, see [1]. The geometry, referred to as double-dome, combines doubly curved regions with steep walls and small radii. Therefore it may be considered critical with respect to forming behavior. As testing material a Twintex comingled glass/PP both as plain and twill weave woven fabric were chosen [2]. This paper addresses the simulation of the double-dome with the finite-element software Aniform. Shear angles, draw-in and the possible presence of wrinkles will be taken into account and compared to round 2 results of other participants. Additionally, a numerical sensitivity study of material and process parameters will be presented in order to identify major influences on the forming results. The paper concludes with a number of recommendations for further research as well as possible improvements for numerical modeling. [1] Sargent et.al., “Benchmark study of finite element models for simulation the thermostamping of woven-fabric reinforced composites”. Proceedings of the 13th Esaform Conference, Brescia 2010. [2] Cao et.al., “Characterisation of mechanical behaviour of woven fabrics: experimental methods and benchmark results”, Composites Part A: Applied Science and Manufacturing, 2008.