material and the fiber properties are considered to be independent to temperature changes.

The laminate properties were estimated using the micromechanics methods. The

micromechanics methods made a good agreement with the semi-empirical solutions. The

transverse properties estimated from the micromechanics model was used to verify the

transverse isotropy of the composite laminates. The hexagonal unit cell satisfies the

transverse isotropy better than the square unit cell. Therefore, the hexagonal unit cell is

more realistic and better model for the micromechanics methods.

 The micromechanics method yields detailed micro-stress distribution in the fiber,

matrix and the interface between the fiber and matrix phases. These micro-stresses can be

used to predict possible microcrack propagation of the composite at various temperatures

with or without external loads. When a unidirectional laminate is subjected to free

boundary conditions, the laminate stresses are zero from the stress-strain relation.

However, the microstresses in fiber and matrix phases exist since the coefficient of

thermal expansion of constituents are different. When the unidirectional laminate is

subjected to cryogenic temperature, the thermal contraction between fiber and matrix

phases causes the microstresses. The microstress results are compared with the material

strength to predict the possible microcrack propagation. The method was used to analyze

the micro-stresses in the liquid hydrogen composite tank. When the composite tank is

subjected to cryogenic temperature without external loads, the micro-stresses did not

exceed the material strength of the constituents. When the internal pressure of the

composite tank reaches 10KPa at cryogenic temperature, the stresses in the matrix phase

seem to exceed the tensile strength of the matrix material indicating micro-cracking is a