| dc.description.abstract | Manufacturing of advanced aerospace-grade structural composites has traditionally utilized autoclaves to impart heat and pressure, in addition to vacuum, to create high-quality, void (defect)- free, reproducible structures. Carbon (micro) fiber reinforced polymer (CFRP) composites, which are pre-impregnated with a thermoset or thermoplastic polymer to form prepreg sheets, are in widespread use via autoclave processing due to their ease of use and high fiber volume fraction. However, autoclaves have high capital costs, and incur high operating costs due to the convective heating and applied pressure. Furthermore, the fixed capacity of an autoclave limits the size and design of composite parts, and the production rate is limited by autoclave availability. As a result, there has been an increasing interest in the development of alternatives, for example, out-of-autoclave (OoA) specially-formulated prepregs that only require heat and vacuum (i.e., pressure is not required). OoA prepreg processing also has drawbacks due to their specialized morphological and chemical formulation for vacuum-only conditions, as well as part quality (especially, composite interlaminar properties) that is below autoclave-processed materials. In light of the limitations described above, this dissertation (1) develops a novel prepreg processing technique, termed 'out-of- oven' (OoO) curing, that conductively cures OoA prepregs via nanoengineered resistive heating; (2) expands the applicability of the OoO process to conventional autoclave-formulated prepregs; and (3) introduces multifunctionality in the form of cure status sensing. Characteristics of the OoO process using a CNT film as a heating element are first examined and compared to those of an oven curing process, focusing on an aerospace-grade OoA-formulated unidirectional aerospace-grade CFRP prepreg system. Thermophysical and mechanical property comparisons suggest that there is no difference in laminates cured via OoO and oven curing as evaluated by void content, degree of cure analysis, short beam shear interlaminar testing, dynamic mechanical analysis, and double-edge notch tensile testing. The OoO process reduces electrical energy consumption by two orders of magnitude (from 13.7 to 0.12 MJ) due to conductive vs. convective heating, under a typical industrial curing condition for a small (60 mm x 50 mm) test panel. Modeling shows that for parts beyond a meter-scale, energy savings will also be at least two orders of magnitude. Moreover, comparative finite element modeling of the OoO and oven curing shows excellent agreement with measured values, including the reduction in electrical energy and instantaneous power consumption. Altogether, these findings show that OoO curing works for OoA prepreg systems, with significant energy savings. Given the results of the first study, the next study effectively removes the need for an autoclave by adapting the OoO process to conventional autoclave-formulated prepreg systems that currently require applied pressure of ~700 kPa in addition to vacuum. This technique entails OoO curing plus insertion of a nanoporous network (NPN, e.g., vertically aligned CNT arrays) into the interlaminar regions of autoclave-formulated composite laminates. Capillary pressure due to the NPN is calculated to be of the same order as the pressure applied in conventional autoclave processing. Results show that capillary-enhanced polymer wetting by the NPN enables sufficient reduction of interlaminar voids to levels commensurate with autoclave-processed composites. Thermophysical property comparisons and short beam shear interlaminar strength testing show that OoO-processed composites with NPN are equivalent to those of autoclave-cured composites, with energy and other savings similar to OoO curing with OoA prepreg in the first study. Conformability of the NPN to the micron-scale topology of the prepreg surface, and continuous vacuum channels created by the NPN, are identified as key factors underlying interlaminar void reduction. Finally, this dissertation introduces a multifunctional aspect of the OoO manufacturing: an in situ cure status monitoring technique utilizing the nanostructured CNT-based heating element of the OoO process. The OoO heating elements are nanoporous and CNT-based, but in this study have different morphology (randomly-oriented or in-plane aligned CNTs) than the NPN (vertically aligned CNTs, A-CNTs). As OoO curing proceeds and the heating element is powered, the adjacent polymer flows into the nanoporous heater via capillary action. Based on cure status sensing experiments and theoretical models, it is found that electrical resistance changes of the heating element correspond to several mechanisms associated with different stages in the cure process, including polymer infiltration into the CNT network that causes the average CNT-CNT junction distance to increase, giving a resistance increase. Later in the manufacturing, as the polymer cross-linking occurs after infiltration into the heating element, chemical cure shrinkage decreases the CNT-CNT junction distance, leading to a decrease in resistance. Thus, the heating element is multifunctional as a cure status sensor, and is found to be highly repeatable, demonstrating a new capability to enhance both quality and productivity of composite manufacturing. OoO curing and related processing techniques introduced here are expected to contribute to the design and manufacturing of next-generation multifunctional composite architectures. These processing techniques have several advantages, including: (1) compatibility with a wide range of composite materials, including OoA- and autoclave-formulated prepregs; (2) removal of size and shape constraints on composite components imposed by the use of a heating vessel; (3) manufacturing cost savings by efficient conductive (as opposed to convective) thermal processing; (4) production improvements via the in situ cure status monitoring by multifunctional heating elements as cure sensors; and (5) the potential for spatial heating control to accommodate structural features such as thick and thin transitions. Future work will expand the techniques to thermoplastics and other high-temperature polymers. The OoO techniques are expected to enable several systems-level production and operational savings, such as accelerated cure cycles, that require further study. Other areas of exploration include on-site composite curing and repair, and leveraging the spatial control of heat flux from the OoO technique into other OoA composite processes, such as resin infusion and resin transfer molding. | en_US |
