Coupled Heating/Forming Optimization of Knitted Reinforced Composites

Abstract The feasibility of knitted fabric reinforcement for highly flexible composites has been investigated for the thermoforming process. The composite sheets were made through compression molding before being shaped. We used thermoplastic elastomers as matrices: Thermoplastic Elastomers and Thermoplastic Olefins. The knit reinforcement was provided by jersey knitted fabrics of polyester fibers. We first introduced the fundamentals involved in the study. The manufacturing is presented through compression molding and thermoforming. The latter is a two-step process: IR heating and plug/pressure assisted deformations. For the IR heating phase, several material properties have been characterized: the emissivity of matrices, absorption, reflection and transmission of radiations in the composite structure have been studied. We particularly paid attention to the reflection on the composite surfaces. The non-reflected or useful radiations leading to the heating are quantified and simulated for three emitter-composite configurations. It has been found that the emitter temperatures and the angle of incidence have significant roles in the IR heating phase. Thermal properties such as calorific capacity and thermal conductivity of the composites were also presented. Thermograms were carried out with an IR camera. Equipment and Thermogram acquisitions were both presented. Optimization of emitters was performed for a three emitter system. The objective function method has been illustrated.Regarding mechanical purposes, the characterizations of the matrices, reinforcements and flexible composites have been carried out. The studied loadings were uniaxial traction, pure shear and biaxial inflation. For the uniaxial extension, both the reinforcement and the composite were found highly anisotropic regarding the orientation of the loading toward the coursewise of the fabric. The resulting strains and stresses to rupture are also found anisotropic. However, for pure shear loading we observed isotropic behavior. Biaxial deformations have been studied; the stress-strain curves are closer to the ones from pure shear loading than from uniaxial traction. The stress and strains of the inflated disks were deduced from measurement on the deformed contours. A routine for contour extraction is presented. We pointed out that unreinforced matrices are strongly subjected to sudden polymer properties in biaxial deformation at certain temperatures. The stress-strain curves are affected by the resulting jumps in mechanical properties. On the other hand, the composites do not show those gaps in stress; the reinforcement rules the deformations. The thicknesses of inflated disks were also measured, fabric reinforcement is found to provide a better thickness repartition. It would be a major improvement for thermoformed good production. In order to predict the forming parameters (temperature, pressure, maximum deformation before rupture...), we introduced several hyperelastic models. They were used to simulate the stress-strain curves of the reinforced and non-reinforced elastomers for uniaxial traction, pure shear and biaxial loadings. Some material constants had been expressed and used as input for finite element simulations. Simulations have been introduced using first a direct stiffness method for a mass-spring assembly, then finite elements were presented and illustrated for the three studied deformations. Hyperelastic models were used. The fabric was simulated using one ANSYSTM code based on hexagonal elements. Pertinent results have been found for uniaxial and pure shear deformations. Regarding flexible composites, a fast method has been proposed based on cloth simulation technique.