Abstract of Dissertation Presented to the Graduate School
 of the University of Florida in Partial Fulfillment of the
 Requirements for the Degree of Doctor of Philosophy

 MICROMECHANICS, FRACTURE MECHANICS AND GAS PERMEABILITY OF
 COMPOSITE LAMINATES FOR CRYOGENIC STORAGE SYSTEMS
 By

 Sukjoo Choi

 May 2005

Chair: Bhavani Sankar
Major Department: Mechanical and Aerospace Engineering

 A micromechanics method is developed to investigate microcrack propagation in a

liquid hydrogen composite tank at cryogenic temperature. The unit cell is modeled using

square and hexagonal shapes depends on fiber and matrix layout from microscopic

images of composite laminates. Periodic boundary conditions are applied to the unit cell.

The temperature dependent properties are taken into account in the analysis. The laminate

properties estimated by the micromechanics method are compared with empirical

solutions using constituent properties. The micro stresses in the fiber and matrix phases

based on boundary conditions in laminate level are calculated to predict the formation of

microcracks in the matrix. The method is applied to an actual liquid hydrogen storage

system. The analysis predicts micro stresses in the matrix phase are large enough to cause

microcracks in the composite.

 Stress singularity of a transverse crack normal to a ply-interface is investigated to

predict the fracture behavior at cryogenic conditions using analytical and finite element