Thermally induced interfacial stress states between fiber and matrix at cryogenic temperature were studied using three-dimensional finite element based micromechanics. Mismatch of the coefficient of thermal expansion between fiber and matrix, and mismatch of coefficient of thermal expansion between plies with different fiber orientation were considered. In order to approximate irregular fiber distributions and to model irregular fiber arrangements, various types of unit cells, which can represent nonuniformity, were constructed and from the results the worst case of fiber distributions that can have serious stress states were suggested. Since it is difficult to measure the fiber transverse coefficient of thermal expansion at the micro scale, there is an uncertainty problem for stress analysis. In order to investigate the effect of error in assumed fiber transverse coefficient of thermal expansion on thermally induced interfacial stresses, systematic studies were carried out. In this paper, the effect of measurement errors on the local stress states will be studied. Also, in order to determine fiber transverse CTE values from lamina properties, a back calculation method is used for various composite systems.

Fiber pushout test models are developed for a fiber-matrix-composite with a nonhomogeneous interphase. Using design of experiments, the effects of geometry, loading and material parameters on critical parameters of the pushout test such as the load-displacement curve and maximum interfacial shear and normal stresses are studied. The sensitivity analysis shows that initial load displacement curve is dependent only on the indenter type and not on parameters such as fiber volume fraction, interphase type, thickness of interphase, and boundary conditions. In contrast, interfacial shear stresses are not sensitive to indenter type, while the interfacial radial stresses are mainly sensitive to fiber volume fraction and the boundary conditions.