Date of Award

9-2025

Document Type

Thesis

Degree Name

Master of Science in Aeronautical Engineering

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD

Abstract

Advanced aerospace systems require structural materials that can perform reliably in high-temperature environments for long durations. The performance of standard polymer matrix composites (PMCs) in these conditions is often limited by their susceptibility to time-dependent deformation, such as creep. The objective of this research is to characterize the high-temperature creep behavior of a novel unitized material system comprising a polymer matrix composite (PMC) and a ceramic matrix composite (CMC) co-cured together. The PMC part consists of the polyimide matrix reinforced with laminated carbon fibers woven in an eight harness satin weave (8HSW). The CMC part consists of a zirconia-based ceramic matrix reinforced with laminated 8HSW quartz fiber fabric. The unitized composite had a ±45° fiber orientation. Specimens of the unitized PMC/CMC composite were cut from two different panels A and B. Unitized specimens from panel A had a considerably thicker PMC portion than those from panel B. Creep performance of a ±45° PMC with the same constituents as the PMC portion of the unitized material system was also characterized. To evaluate the fitness of the PMC/CMC and PMC for service in aerospace thermal protection systems, namely components designed to contain high-temperature environments, creep tests were performed under temperature conditions mimicking the actual service conditions. In all elevated-temperature experiments, one side of the test specimen (the CMC side in a unitized composite) was at 329°C, while the other side (the PMC side in a unitized composite) remained open to laboratory air. Tensile stress-strain behavior of the PMC/CMC and of the PMC were evaluated and tensile properties measured. Tensile creep tests were conducted with creep stresses ranging from 47 to 85 % of the ultimate tensile strength (UTS). Creep runout was set to 100 h hours. All specimens that survived 100 h of creep without failure were subsequently tested in tensile to failure at elevated temperature to determine retained tensile strength and stiffness. The unitized PMC/CMC A material system demonstrated significantly superior creep performance, reaching 100-h creep runout at stress levels up to 75 % UTS with low accumulated creep strain, whereas the PMC exhibited lower creep lifetimes and much higher creep strains. The unitized PMC/CMC A showed better creep performance than the PMC/CMC B. Post-test examination of the specimens under an optical microscope suggests that in the case of the PMC/CMC A, the PMC portion dominates the creep deformation response, while the CMC portion provides adequate thermal protection.

AFIT Designator

AFIT-ENY-MS-25-S-001

Comments

An embargo was observed for posting this dissertation on AFIT Scholar.
Approved for Public Release, Distribution A: Distribution Unlimited. PA Case Number 88ABW-2025-0715

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