| Polymeric composite materials have been widely utilized to take advantage of their large specific strength and stiffness. These characteristics have made them attractive for use in aerospace applications as containment structures for cryogenic fluids. However, the anisotropic, heterogeneous, and viscoelastic nature of polymeric composite materials creates a unique set of challenges for the storage of cryogenic fluids. Mismatches in thermal expansion between the fibers and the matrix in these materials result in the generation of thermal stresses at low temperatures that can ultimately cause failure in the form of microcracks that propagate throughout the material, degrading performance. This work examined the phenomenological and theoretical aspects of microcrack formation in carbon fiber/epoxy composite materials at low temperatures. Microcrack formation was initially investigated using an experimental approach. The role of the fiber and matrix type in microcracking was studied, along with the interaction between the fibers and the matrix and the effects of nanoparticle matrix modification. It was found that the fiber and matrix type had significant effects on microcrack formation, with decreased fiber moduli, decreased matrix coefficient of thermal expansion, nanoparticle modification, rubber toughening, and increased adhesion all corresponding to reduced microcracking. These improvements were achieved by reducing the thermal stresses in the materials studied and increasing the failure resistance of the laminates. It was found that the processing conditions of a laminate, namely the cure temperature, had a direct impact on microcracking by changing the stress free temperature of a material and the corresponding thermal stresses at cryogenic temperatures. The analysis of the variables that contributed to microcrack formation was used to develop a stress-based compound beam model to predict the onset temperature for microcracking in composite materials. This model was unique in that it accounted for the variation in material properties at low temperatures. Collectively, this work examined the phenomena behind the cryogenic microcracking of composite materials and applied this information to develop a predictive model for low temperature failure of composite materials from thermal stress generation. |
