| A variety of aspects ranging from polymer molecular engineering to final composite mechanical properties were discussed in relation to the autoclave laminate lay-up and resin transfer processes. This study utilized new materials including high performance epoxy systems, lightly crosslinked thermosets (LXT), carbon stitched fibrous preforms, semi-crystalline epoxy binder, and toughened epoxy prepreg. Thermal and mechanical property changes of resin matrices at extended cure time were investigated by integrated analysis methodology. The extended cure times improved the crosslinking reactions of epoxy systems. Resin film infusion process, a resin transfer process, was used to manufacture various stitched and non-stitched structural composites. The objective was to understand the possibilities and limitations of the new textile preform utilization for aircraft structural composites. Furthermore, three epoxy systems were also used to investigate the microcracking phenomenon in the particular composite structure, as well as resin infusion processability. Binder interaction with LXT was investigated to replace the stitching fiber with a semi-crystalline epoxy binder for resin transfer processes. Thermal and mechanical characterization of the LXT-binder mixture showed that the binder disrupts the crucial final crosslinking required in developing a tough network structure. Finally, void formation during the autoclave process and the void influences on composite mechanical properties were investigated. Air permeation measurements and optical microscopy identified the importance of interlaminar voids in this prepreg system. Various composite void identification methods like ultrasonic C-scan analysis, density measurements and optical microscopic study were utilized. Autoclave pressure was determined to be a major processing parameter that influenced air-entrapped voids. Voids were found to decrease the composite interlaminar shear strength. Collectively, resin and fibrous preform characteristics and processing conditions were related to the final composite performance. The understanding of processing-structure-property relationship will provide the aircraft composite industry with a science-based knowledge in many composite manufacturing processes. |
