An integrated and systematic characterization methodology for vacumm bag only prepregs

Official URL: https://risc01.sabanciuniv.edu/record=b2486444_(Table of contents)

Composite materials have attracted widespread acceptance aerospace industry in the last decades as it offers high rigidity and outstanding strength/weight ratio while enabling the considerable reductions in the manufacturing and operational costs. The autoclave processes have been widely preferred for the primary and secondary aerospace applications as such requirements as high safety, high quality, and reproducibility of the industry. However, producing larger composite parts through low-cost processes has led the industry to out of autoclave processes. Hence, VBO prepregs were developed for out-of-autoclave processes to deliver high-performance composites, which can compete with the autoclave-quality composites in many aspects, such as low-void content and successful impregnation. This is possible through an appropriate combination of fiber bed architecture and resin system, and process parameters. However, composites produced through VBO prepregs are much more susceptible to voids since the maximum consolidation pressure is limited to atmospheric pressure. To address these issues, VBO prepregs were designed to incorporate relatively permeable air channels and exhibit higher initial fiber volume fractions in comparison with the autoclave prepreg systems. Nevertheless, the physical properties of the prepreg system are still the leading parameters in the evolution of VBO prepreg microstructures during the curing process as the incomplete impregnation before gelation directly controls the amount of voids, which degrades the mechanical performance of the final part. Therefore, there is a vital need for a systematic and overarching characterization methodology. This thesis aims to establish a methodology that systematically characterizes such factors as material properties and constitutive behavior to strengthen the VBO prepreg process model’s accuracy and reliability. To achieve this goal, a systematic was developed to characterize the main properties of the resin and prepreg system. This approach was also implemented to characterize a commercial VBO prepreg system’s properties and processing parameters. Hence, the cure-dependent properties of the resin system were characterized by several sets of semi-empirical phenomenological models. Then, the first fiber architecture, void-content change, the fiber volume fraction of the prepreg system were investigated through sets of x-ray microtomography scans of the prepreg laminates. The initial permeability behavior of the porous media was modeled through a CFD analysis. Finally, the specific heat capacity and thermal conductivity of the constituents were investigated by a series of experiments and numerical studies. This approach can be accepted as a fundamental cornerstone of establishing a process design that integrates characterization, modeling, optimization, and verification approaches to deliver high-performance composites through OoA techniques