THERMAL GRADIENT ON HYBRID COMPOSITE PROPELLANT TANK MATERIALS AT CRYOGENIC TEMPERATURES

摘要

Cryogenic tanks are devices that are commonly used to store extremely low temperature fluids, usually in their liquid state. Cryogenic fuel tanks carry cryogenic propellants such as liquid oxygen, liquid methane or liquid hydrogen, at subfreezing temperatures in its condensed form in order to generate highly combustible liquids. This type of tank is exposed to an extremely cold temperature in its interior and to ambient temperature on its external surface resulting in large temperature gradient across the thickness of the wall. In this paper, hybrid textile composites with carbon and Kevlar fabric are explored as means to reduce the influence of thermal gradient in order to enhance the material performance when cryogenic propellant fuels are stored in spacecraft applications. Previous initial studies of tensile and flexural tests have indicated that carbon and Kevlar textile composites are suitable materials for cryogenic temperatures. The pristine mechanical properties of carbon composites changed within a maximum of 3-4% after initial cryogenic exposure during the fueling stage, while 17% for Kevlar composites. Computational models of hybrid carbon-Kevlar composites were subjected to cryogenic temperature (77 K) to investigate the effect of exposure for extended periods and to aid in the design of optimum layups for the same. Six optimal combinations were selected that resulted in low interface stresses and lower number of peak stresses through the thickness of the laminate. These layups were deduced to perform better compared to other layups due to lesser susceptibility to delamination type failure upon cryogenic exposure. Experimental investigation of the chosen hybrid composites has revealed few optimum combinations for use in tanks. As a next step, computational analysis of cryogenic exposure to only one surface of hybrid composites was performed to simulate the composite wall containing the liquid fuel. Based on the suggestions from the computational models, experiments to determine optimum designs of the composite wall were conducted. An ABS plastic insulating holder was computationally designed and 3D printed to hold the specimens such that only one surface is exposed to LN_2. A total of eight composite layups were exposed to liquid nitrogen using the plastic holder to study their response to thermal gradient cryogenic exposure. Based on the results obtained computationally and supported by experiments, optimum hybrid layups of composites to sustain cryogenic exposure were determined.