Key Engineering Materials Vols. 554-557

Abstract: Simulation of the dry reinforcement preforming, first step of the Resin Transfer Moulding process, become necessary to determine the feasibility of the forming process, compute the fiber directions in the final composite component, and optimize process parameters during this step. Contrary to geometrical approaches, based on fishnet algorithms, finite element methods can take into account the actual physical parameters, the real boundary conditions and the mechanical behavior of the textile reinforcement. The fabric can be modeled either as continuum media with specific material behavior [5, 6], or using discrete structural elements to describe the textile structure at the mesoscopic scale. A semi-discrete approach, which is a compromise between the above continuous and discrete approaches, is also used for simulation. A discrete approach for the simulation of the preforming of dry woven reinforcement has been proposed and presented in a previous paper. This modelling is based on a “unit cell” formulated with elastic isotropic shells coupled to axial connectors. The connectors, which replace bars or beams largely studied in other discrete approaches, reinforce the structure in the yarn directions and naturally capture the specific anisotropic behavior of fabric. Shell elements are used to take into account the in-plane shear stiffness and to manage contact phenomena with the punch and die. The linear characteristic of the connectors, has been extended to a non linear behaviour in the present paper to better account for fabric undulation. Using this numerical model, we propose, in this work to study the effect of process parameters on the woven fabric deformation during the performing step. The emphasis will be placed on the analysis of the influence of the blank holder pressure on the shear angle distribution.

Abstract: The simulation of the draping process of dry textiles allows one to predict the occurrence of folds and the local fibre orientations and fibre positions after draping. In this paper the experiments to determine the mechanical material properties of textile structures are discussed. The mechanical material parameters are used as input for the drape simulation on the macro-scale. The numeric material models can be validated by comparing the numeric results with the experimental draping results of a drapeability test with standardized geometries. The further developed drapeability test to validate the material models for textile structures will be presented.

Abstract: Presenting interesting aspects such as a high strength-to-weight ratio, Carbon Fibre Reinforced Plastic components are frequently used in the aerospace industry. The forming step, which conforms the reinforcement to a specific geometry, is a sensitive phase of the manufacturing process. In order to detect the occurrence of defects prior to any trial, forming methods are often simulated via finite element software. The presented work will detail the simulation validation of a double curved helicopter frame forming out of a unidirectional carbon fibre pre-impregnated material (M21E, Hexcel®). The finite element model was based on an explicit approach at a macroscopic level and developed via the commercially available software Visual-Crash PAM (ESI®) [1]. The validation was carried out on six different preforms. Measurements of the top layers were performed by an enhanced version of a 4D measuring system, originally developed for non-woven fabric [2], able to make reproducible photographic and height measurements (Fig. 1). Experimental results were then compared to simulated ones. Due to material specificities, the photo quality reached for non-crimp fabrics could not be achieved [2]. After hardware and software modifications, measurements and analyses were eventually successfully completed. The validation of the simulation reached an accuracy of 1° to 3° depending on the geometrical features of the preform (Fig. 2).

Abstract: Due to advancements made in 3D weaving process [1] and, in order to develop 3D textile structure as reinforcement of composite material for aeronautic application, a good prediction of the geometry and the mechanical properties of the 3D woven unit cell is required. Due to the complexity of these textile architectures, realistic geometric representations [2] of fabrics are often difficult to obtain especially for 3D woven fabrics, but these descriptions are necessary to define meshes for finite element computation [3]. At present, existing tools which model and define, early at a mesoscopic scale [4], the architecture of 3D fabrics don’t take into account the influence of the manufacturing process on the shape modification of the textile structure. Some numerical model exists for the braiding process [5] and the knitting process [6], but not yet for the weaving process. During the manufacturing process, fibres are subjected to significant deformations due to loads from the component of the loom or from the friction with the others fibres. These significant deformations lead to mechanical strength losses of the fabric. A numerical model of the different steps of the weaving process could predict these significant deformations and their influence on the geometry of the textile architecture. Thus, the objective of the NUMTISS project is to develop a numerical model of the deformation of the yarn during the weaving process. For the numerical modelling of the weaving process developed in finite element method, we considered all loom elements like rigid solid, and we will make the assumption that yarns are transverse isotropic elastic materials. Simulations of the process for a plain weave, a twill 2-2 and a satin 8 fabric have already been performed, as well as the simulation of orthogonal warp interlock structures. Then, to understand the kinematic motions of weaving process, the tracking of some strategic elements on the industrial weaving loom (reed, heddles, rapier,..) have been carried out. The tracking obtained from the video of the high speed camera will help us to define the numerical model of the weaving kinematic closer to reality. Correlations between numerical results and specific structures in glass fibres produced on the loom will be presented. The influence of each step of the manufacturing process on the characteristics of the textile structure could be analyzed [1]X. Chen, L. W. Taylor, L. J.Tsai. ”An overview on fabrication of three-dimensional woven textile preforms for composites”. Textile Research Journal, 2011, 81(9) 932–944 [2] SV Lomov, G Perie, DS Ivanov, I Verpoest and D Marsal. “Modeling three-dimensional fabrics and three-dimensional reinforced composites: challenges and solutions”. Textile Research Journal, 2011, 81(1) 28–41 [3] E. De Luycker, F. Morestin, P. Boisse, D. Marsal. « Simulation of 3D interlock composite performing”. Composite Structures, Volume 88, Issue 4, May 2009, Pages 615-623. [4] M. Ansar, W. Xinwei, Z. Chouwei. “Modeling strategies of 3D woven composites: A review”. Composite Structures 93 (2011) 1947–1963. [5] A. K. Pickett, J. Sirtautas, et A. Erber. « Braiding simulation and prediction of mechanical properties”. Applied Composite Materials, 2009. [6] M. Duhovic, D. Bhattacharyya. “Simulating the deformation mechanisms of knitted fabric composites”. Composites Part A : Applied Science and Manufacturing, 2006.

Abstract: NCFs (Non-Crimp Fabrics) infused with epoxy resin are popular in the design of wind turbine blades and other complex systems due to their ability to conform to complex shapes. Past work in the development of a combination beam-shell modeling approach to simulate the forming of NCF composites has been demonstrated to capture the change in the orientations of the yarns during a forming process. The structural performance of these manufactured blades is often analyzed using finite element simulations that consider the material properties of the fibers and of the resin based on the rule of mixtures and orthotropic shells where the model is sectioned into zones that account for changes in the material properties due to variations in the orientations of the lamina and number of layers. With the availability of the beam-shell model, the use of zones can be removed if the individual contributions of the yarns (beam elements) and resin (shell elements) can be characterized and the orientations of the yarns resulting from a forming simulation can be used to account for the variations in the material properties of the composite throughout the blade. This research uses a combination of static flexure tests and impact modal tests to ascertain the material properties of the fibers and resin in a unidirectional and biaxial non-crimp fabric laminate plates. The material properties are used in a finite element model of the plate and the model is analyzed in flexure and in a free-free modal configuration to compare to experimental results. Two different approaches are used in the commercially available software Abaqus to model the plate. One approach uses a combination of beam and shell elements to represent the fibers and the resin, respectively. The other approach uses orthotropic shell elements to capture the unbalanced behavior of the fiber/resin composite. The beam/shell modeling approach better represents the overall behavior of a single-layer plate and can be extended to consider multiple plies.

Abstract: A hybrid finite element discrete mesoscopic approach is used to model the forming of composite parts using a unidirectional glass prepreg non-crimp fabric (NCF). The tensile behavior of the fabric is represented using 1-D beam elements, and the shearing behavior is captured using 2-D shell elements into an ABAQUS/Explicit finite element model via a user-defined material subroutine. The forming of a hemisphere is simulated using a finite element model of the fabric, and the results are compared to a thermostamped part as a demonstration of the capabilities of the used methodology. Forming simulations using a double-dome geometry, which has been used in an international benchmarking program, were then performed with the validated finite element model to explore the ability of the unidirectional fabric to accommodate the presence of interlaminate cabling.

Abstract: When a continuum approach is considered for textile reinforcements, the internal loads are described by a stress tensor. The mechanical behaviour of the textile material is very much dependent on the fiber directions, and the frames defined from warp and weft directions are preferred to write the stress components. The exterior loads in these frames permit to define tensile and pure shear states. Nevertheless these frames are generally not orthogonal. The relationships between the exterior loads and the different stress components are analyzed in the present paper, and, in particular, the relationship between direct stress components and longitudinal loads on one hand, and shear stress components and transversal loads on the other hand. When dealing with textile materials, the exterior loads in the direction of the fibres and transverse to the fibres define the pure tensile and pure shear state. It will be shown that the covariant stress component matrix is diagonal in a pure tensile loading and that the first mixed direct stress components are equal to zero in a pure shear loading. In these cases, the direct relationship between the stresses and the loadings are given. This will be applied to the cases of the picture frame test, the biaxial tensile test or of a combined tension-shear test.

Abstract: High performance composites are used in commercial applications in a steadily growing degree. This increase of advanced materials is accomponied with the development of fully automated fabrication processes. It aims to drive down the time and costs of the production while ensuring a high quality of the product. This can achieved by considering the process of hot press forming with continuous fiber reinforced thermoplastics. The development of the process is, however, accompanied with a few difficulties, which require more research. For example, composite materials with different architectures, lay-ups, and constituents, show large differences in formability. This research examines the effect of friction on the formability of thermoplastic composites. Both experiments and simulations were conducted. Demonstrator products have been press-formed from laminates with different materials and architectures (UD-carbon PEEK, UD-carbon-PEI, 8hs-glass PPS, 5hs-carbon PEEK and UD-glass PPS), to investigate their effects on formability. Creating a doubly curved shape from a flat laminate requires at least three deformation mechanisms, namely in-plane shear, bending and inter-ply slippage This paper focuses on the sliding mechanism and the corresponding friction. In order to quantify the amount of sliding in the press-formed product, a dot pattern has been applied to both surfaces of the laminate. The slip between the outer plies can be analyzed by means of photogrammetry. Besides, the friction coefficient of each material is measured in a special designed friction test set-up. It can be seen that the composite formability is directly linked to its friction properties. FE simulations of the press-form process will be performed based on the measured material properties, to demonstrate the influence of the materials friction coefficient.

Abstract: By combining carbon woven fabric with thermoplastics grains, a thermo-stamping process is proposed for forming parts with complex double curvatures in one step, to implement the affordable application of fiber reinforced composites in high volume merchandises such as automotive industry. In the proposed thermo-stamping process, laminated carbon woven fabrics with thermoplastic grains are heated, and then transferred rapidly to a preheated mould for thermo-stamping, and cooled down to form the carbon fiber reinforced composite part. Various thermoplastics such as PP, PA6 and ABS are used as matrix material in the composite part. Experimental results including shear angle distribution in the fabric, deformed boundary profile of fabric with different original fiber orientation and forming defects are presented. It is demonstrated that high quality parts can be obtained with the proposed forming process, and defects are controllable. By using the proposed process and laminated structures, it is feasible to implement the high-volume and low-cost manufacturing of fiber reinforced composite parts.