Key Engineering Materials Vols. 651-653

Abstract: The results of in-plane shear tests performed on 5-hardness satin woven carbon/PPS thermoplastic prepregs are described. The experimental analyses are based on bias-extension tests performed in an environmental chamber. The results are given for different temperatures on both side of the melting point. This range of temperature is those of the part during a thermoforming process. In another hand it is shown that second-gradient energy terms allow for an effective prediction of the onset of internal shear boundary layers which are transition zones between two different shear deformation modes. The existence of these boundary layers cannot be described by a simple first-gradient model.

Abstract: Process induced residual stresses may play an important role under service loading conditions for fiber reinforced composite. They may initiate premature cracks and alter the internal stress level. Therefore, the developed numerical models have to be validated with the experimental observations. In the present work, the formation of the residual stresses/strains are captured from experimental measurements and numerical models. An epoxy/steel based sample configuration is considered which creates an in-plane biaxial stress state during curing of the resin. A hole drilling process with a diameter of 5 mm is subsequently applied to the specimen and the released strains after drilling are measured using the Digital Image Correlation (DIC) technique. The material characterization of the utilized epoxy material is obtained from the experimental tests such as differential scanning calorimetry (DSC) for the curing behavior, dynamic mechanical analysis (DMA) for the elastic modulus evolution during the process and a thermo-mechanical analysis (TMA) for the coefficient of thermal expansion (CTE) and curing shrinkage. A numerical process model is also developed by taking the constitutive material models, i.e. cure kinetics, elastic modulus, CTE, chemical shrinkage, etc. together with the drilling process using the finite element method. The measured and predicted in-plane residual strain states are compared for the epoxy/metal biaxial stress specimen.

Abstract: During forming, defects can occur and have to be taken into account because they can significantly affect the mechanical performance of the part. This experimental study shows the type and number of defects is a function of the punch geometry, the process parameters, the orientation of the fabric with respect to the punch and the inter-ply friction. Inter-ply friction has a huge effect on the quality of the preform when inter-ply sliding occurs. This inter-ply Friction leads to several overhanging yarn shocks that generate high tangential forces, which inhibit the relative sliding of plies.

Abstract: CFRTP prepreg laminates thermoforming (Continuous Fibre Reinforcements and Thermoplastic Resin) is a fast composite manufacturing process. Furthermore the thermoplastic matrix is favourable to recycling. The development of a thermoforming process is complex and expensive to achieve by trial/error. A simulation approach for thermoforming prepregs thermoplastic is presented. This model is based on a continuous approach. A hyperelastic behaviour is associated with dry reinforcements. The hyperelastic potential is built from the contribution of three principal deformation modes that are supposed to be independent. A nonlinear viscoelastic model based on the generalization of simple rheological models is associated with the in-plane shear mode. The finite element simulation of a thermoforming example using this model is presented.

Abstract: In the preforming process, the textile is draped into the geometry of the structural part and afterwards consolidated with resin via injection. For preforming processes non-crimp fabrics (NCFs) have become increasingly popular for cost effective applications. For the realization of automated draping of two-dimensional textiles into three-dimensional complex geometries during the preforming process there is a high advantage for the use of tailored textiles compared to textiles with uniform material properties. Large flat surfaces require a high bending stiffness and low shear stiffness due to high structural stability of the textile and small radii of curvature require a low bending stiffness due to good drapeability of the textile. The bending and the shear stiffness of NCFs with a given layup can be influenced by the manufacturing parameters of the knitting yarn. With tailored NCFs it is possible to adapt the manufacturing parameters of the knitting yarn locally in the production direction to improve drapeability and handling of the textile in the preforming process. To use the high potential of tailored NCFs, the development of the new textile structure has to go hand in hand with the characterization and with the simulation of the draping process. An experimental approach and a modelling approach using a kinematic drape algorithm have been developed to define the local stitching parameters for tailored NCFs dependent on the geometry of the component part.

Abstract: This paper presents a methodology for extending the use of the beam-shell forming model to predict the structural properties of the composite part. After the forming simulation has been performed, the material definition will be changed such that the beam elements will represent the fiber reinforcements and the shell elements will represent the resin. The methodology behind the entire approach will be demonstrated using a stitched uniaxial glass fabric. The methodology for characterizing the fabric behavior will be discussed. After the part has been formed, it will be infused with resin. The methodology for characterizing the composite behavior will be introduced. The finite element model will be compared with experimental data to validate the methodology.

Abstract: Carbon fiber reinforced composites (CFRCs) have been used in various high-end industries due to their outstanding specific mechanical properties. Recently, carbon nanotube (CNT)-grafted carbon fibers (CFs) made via direct growth has emerged as an advanced and hierarchical reinforcement that can improve the reinforcing effect of CFs in CFRCs. On the other hand, CF reinforced thermoplastic composites (CFRTPs) have attracted much attention because of their quick and mass production capability, e.g., which is important for automotive part manufacturing. Here, we report on the manufacture of CFRTPs using CNT-grafted CFs and their mechanical properties. First, the interfacial shear strength of CNT-grafted CFs with thermoplastic resins was characterized to demonstrate improved interfacial properties due to the CNTs grafted on CFs. Then, the composites were manufactured in two ways; polymer nanoparticles and in-situ polymerization. Polymer nanoparticles were used to improve the interfacial properties due to their small size and good mechanical locking with CF surfaces. In-situ polymerization was also used to manufacture CFRTPs, i.e., monomers with catalyst were transferred into CNT-grafted CF fabric preform using vacuum assisted resin transfer molding and then polymerized into solid matrix. This in-situ polymerization enabled the manufacture of CNT-grafted CF thermoplastic composites by overcoming the difficulties of filling the surface of CNT-grafted CFs with thermoplastic polymers. Finally, the mechanical, thermal, electrical, and damping properties of CNT-grafted CF thermoplastic composites were characterized and compared with their thermoset composites.

Abstract: This paper presents the investigation of a thermoplastic cross-ply sheet for use in manufacturing small arms protective helmets. The material system contains four unidirectional layers oriented in a [0°/90°/0°/90°] configuration. This advanced composite is wholly thermoplastic, consisting of ultrahigh molecular weight polyethylene (UHMWPE) fibers within a polyurethane matrix. Due to the polymeric nature of the constituent materials, the mechanical behavior of the composite system will have a dependence on forming temperature. The shear characterization of the prepreg and the investigation of the factors influencing the representative shear stiffness including sample geometry, strain rate, conditioning and temperature are discussed.

Abstract: The macro-scale and meso/macro-scale simulations for press forming of CFRP sheet using ABAQUS S/W were both implemented to study the influence fiber orientation on the formability of CFRP sheet. The Hashin damage criterion was used to predict the fiber failure in the forming process. The properties of plain woven fiber fabric were obtained from the tensile test and bias extension test. The forming experiments with rectangular punch were carried out to validate the numerical results.

Abstract: This paper presents the development of a novel omega-shaped resin transfer molding (RTM) tool, which is especially designed to host different types of sensors and to avoid common problems of RTM (e.g. uneven heating, low tool durability, deflection). Permeability measurements were executed in order to get real permeability measurements for numerical mold filling simulations. Three different kinds of flow behaviors (isotropic, orthotropic and anisotropic) were considered as filling patterns and the flow front predictions. Due to the U-shaped composite part design, the mold curvature effects on the flow front propagation caused by the increased fiber volume content in these areas were also taken into account. The tool was designed with a heating ability using purified liquid water guided to a channel circuit within both top and bottom halves of the tool. Deflection and heat transfer simulations were performed with the finite element method (FEM). All three executed simulations (filling, heat transfer and deflection) were used as a guideline for the final mold design.