ABSTRACTALEX FRENCH. Modeling the Thermal-Mechanical Effects of Phase Change Materials in Lightweight Optics. (Under the direction of Dr. WESLEY WILLIAMS) This thesis investigates the effects of adding a phase change material (PCM) such as paraffin wax to a lightweight optical component, specifically a mirror for directed energy applications. The goal of adding phase change material to the lightweight mirror is to reduce the temperature and deformation in the mirror under thermal loading, which would increase the system's optical performance. Lumped capacitance simulations performed in MATLAB Simscape showed that adding PCM to the optic reduced the maximum temperature by 58.6% (53.7°C). Subsequent thermal finite element analysis simulations performed in SolidWorks showed the expected relationship between areal density and the maximum temperature of the optic (i.e., lower areal density optics tend to have higher maximum temperatures and deformation), but also showed that the PCM’s heat of fusion significantly contributed to lowering maximum temperatures achieved during multiple energy pulses. Thermal-mechanical finite element analysis additionally revealed that adding PCM to the optic reduced the mirror face deformation by 42.3% (2.0 μm) and the average temperature by 19.3% (7.9°C) after multiple energy pulses. The findings of this study could also be applied to other systems that require thermal management, such as renewable energy thermal storage and aerospace components. Ultimately, the results of this study suggest that embedding PCM into the backing of lightweight optics results in increased thermal capacity within the lightweight mirror allowing for increased dimensional stability which will increase the effectiveness by improving the quality of the beam on target due to the reduced distortion across the optic face.