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Title:Thermal regulation of vascularized fiber-reinforced polymer matrix composites
Author(s):Coppola, Anthony M.
Director of Research:White, Scott R.
Doctoral Committee Chair(s):White, Scott R.
Doctoral Committee Member(s):Sottos, Nancy R.; Geubelle, Philippe H.; Lambros, John
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Composite Materials
Thermal Regulation
Active Cooling
Carbon fiber
Glass Fiber
Vascular Materials
Abstract:Fiber reinforced polymer matrix composites (PMC) possess outstanding structural properties, including high strength and stiffness, low weight, long fatigue life, and excellent manufacturability of complex geometries. However, their application is still limited where high temperature service is required, including during high-speed flight, in engine exhaust systems, or as support for electronics. The polymer matrix suffers reduced structural performance, cracking, creep, mass loss, and even thermal degradation at elevated temperatures. Furthermore, most polymer matrices are flammable and combustion may occur if temperatures rise high enough. Maximum service temperature is typically set by glass transition temperature (Tg) and for typical aerospace grade epoxies are approximately 150-200 °C, while even the best high temperature structural polymers are typically below 300 °C. Thermal regulation is required to use PMCs outside of their service temperature. Internal vascular networks provide a platform for thermal regulation, allowing for on-demand, adaptive heat removal and preservation of material properties. This dissertation uses the established Vaporization of Sacrificial Components (VaSC) technique for manufacture of the vascular network. In this method, sacrificial templates composed of a thermoplastic polymer and catalyst are embedded in the fiber preform prior to PMC manufacture, and then removed by vaporization during an elevated temperature post-cure after matrix solidification. Vascular networks enable a vast array of multifunctionality in PMCs, including self-healing, electromagnetic modulation, damage sensing, and of course, thermal regulation. However, without a thorough understanding of the effect of the vascular networks on structural performance real world application will never be realized. Here, the tensile properties and damage evolution of vascularized PMCs are compared to non-vascularized PMCs. Results show that reductions in performance are associated with misalignment of the fiber architecture. Vascular architectures that only remove matrix without altering the structural fibers showed no detectable effect. This dissertation explores two methods for thermal regulation: active cooling and transpirational autonomic cooling. During active cooling, a fluid is pumped through the vascular network to transfer heat from the solid to the fluid and out of the PMC. This method requires external powering of the pumping and a method for removing heat from the fluid outside the PMC. As an alternative, a new method for thermal regulation of PMCs, transpirational autonomic cooling, is developed utilizing evaporative cooling and capillary pumping to eliminate the need for these external systems. This system is inspired by transpiration in plants and is completely self-powered and adaptive to changing heat loads. Active cooling enables safe operation of PMCs even under very high thermal loading. Active cooling of incident heat fluxes up to 300 kW/m^2 is achieved in a bilayer shape memory alloy (SMA)-PMC hybrid. In this composite, both the SMA and PMC are actively cooled. Temperatures in both layers are maintained below the threshold for irreversible damage. Results show that while most of the cooling occurs in the higher thermal conductivity SMA, cooling in the PMC further reduces temperature even under the same total flow rate by dispersing the coolant through more channels. Two studies are conducted to assess the enhancement to thermomechanical performance in PMCs under active cooling. While active cooling is known to reduce temperature, no prior research has investigated the effect on mechanical performance. During active cooling, large thermal gradients are developed and in many cases a portion of a PMC is above the Tg, while the rest is below. Flexural testing in a environmental chamber demonstrates up to 90% stiffness retention in a 325 °C environment using active cooling, compared to complete structural deterioration of the non-cooled PMCs. Under these conditions, the outer surfaces are above Tg while the interior is maintained below. Survivability under sustained compressive loading and heating on one side also demonstrates tremendous improvements using active cooling. Actively cooled PMCs survive more than 30 minutes under 200 MPa (~25% of room temperature strength) and 60 kW/m^2. In stark contrast, non-cooled PMCs fail in just 1 minute under the same loading and fail in 10 minutes at just 10 kW/m^2. As engineered systems become more complex there is an increasing need for multifunctional materials that autonomically adapt to changing conditions without external sensing, control, or powering. Transpirational autonomic cooling achieves these goals by using capillary pumping to replenish evaporated water in a microporous protective layer manufactured onto the PMC. Applied heat is transformed to mechanical energy for delivering cooling fluid in the form of capillary pumping making the approach inherently self-powered and adaptive. Evaporation absorbs a large amount of heat and caps the temperature within the PMC near the saturation temperature (boiling point) of the water. The system is autonomically adaptable to changing thermal conditions and adjusts flow rate depending on the incident heat flux. The temperature of a PMC utilizing TAC is maintained below 100° C up to a heat flux of 30 kW/m^2. In contrast, the temperature of a non-cooled PMC exceeds 100° C at about 5 kW/m^2 and reaches 240° C at a 16 kW/m^2.
Issue Date:2015-10-26
Rights Information:Copyright 2015 Anthony M. Coppola
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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