Developpement et analyse du procede d'injection flexible pour la mise en forme de pieces composites fortement courbees

Thermosetting polymers reinforced by continuous fibers possess a widely recognized potential for the design of advanced primary structures. Over the last decades, these composite materials have notably replaced metallic materials in many aeronautical applications. The use of such high performance composites is however limited in the automotive industry. Because of the relatively long cycle times required, existing processing techniques are indeed not suited for high production volume. The main objective of this thesis is to identify the key processing parameters in order to formulate appropriate recommendations for future implementation with real industrial components.;Firstly, an experimental setup has been designed to process a rectangular stair-shaped composite panel. This step required adopting an adequate sealing strategy and selecting an appropriate material for the flexible membrane. Furthermore, a preforming procedure was specially devised to prepare semi-rigid fibrous preforms suited for the desired geometry. The developed methodology was applied to produce a first series of parts out of vinyl ester resin and multiaxial glass fiber fabric. Longitudinal cross-sections of the fabricated specimens were then inspected visually and treated by image analysis to assess the quality of the processing. Results show that the process provides a uniform consolidation of the flat sections along any direction. However, manufacturing defects such as thickness variations and resin rich zones are created in the corners of the structure. This behavior is caused by the specific deformation of the membrane and of the fiber bed in the curved areas. The initial geometry of the preform is a key parameter having a strong impact on the quality of the final part. It is also suggested that the magnitude of the different defects can be reduced by modifying the geometry of the preforming tool.;The second part of the project investigated further the mechanisms that generate preform induced manufacturing defects. During this study, new manufacturing experiments were conducted with a quasi unidirectional glass fiber fabric to control precisely the architecture of the preform. Experimental observations of the processing quality in the corners of the specimens were compared with the predictions made by a simplified numerical model. To develop this simulation tool, the deformation of the fiber bed was first analyzed during the different stages of the production cycle in the specific case of planar geometry. Such analysis allowed proposing a transverse compaction model that takes into account the effect of the thermosetting binder on the mechanical behavior of the preform. This model was then implemented in a finite element software to simulate the fibers deformation from initial preforming to final processing of the composite. A simple characterization test was developed to analyze the corner preforming behavior of the fabric and build the initial geometry of the finite element model. Overall, experimental observations and numerical simulations are in good agreement and show the major influence of the preforming stage on the quality of the final product. In particular, it is shown that the preforming conditions (i.e., geometry of the preforming tool and preforming pressure) have to be selected according to the targeted fiber fraction to produce a defect-free stair-shaped component.;The last part of the work studied the residual distortion of the specimens processed during the second manufacturing program. A procedure was devised to measure the thermoelastic spring-in of the curved structure by recording the change in shape of the sample during a simple cool down experiment. The experimental results were compared with the predictions made by different modeling approaches to investigate further the influence of the layup quality. This study shows that manufacturing defects located at the corner of the composite can impact the thermoelastic component of distortion caused by differential thermal expansion. Firstly, thickness variations affect locally the coefficients of thermal expansion of the composite by changing the fiber volume fraction in the curved areas. Secondly, resin rich zones create stress concentration in the structure when the temperature varies because of their important coefficient of thermal expansion. Results also suggest that the impact of processing defects can be different for the shrinkage-induced distortion. It is also noted that fiber stresses due to consolidation of the preform in the curved regions can act as an additional cause of distortion. (Abstract shortened by UMI.)...