The Effect of Micro-Structure Variations on the Permeability of Preform Materials

Most efforts at modelling the permeability of preform materials are based on unit cell approaches in which it is assumed that the medium consists of a regular array of structures of uniform size, shape and position. However, the microstructure of real materials almost always has some degree of dimensional fluctuation about a mean. These variations from uniformity have an effect on the overall permeability, which in general cannot be adequately accounted for in model calculations by defining unit cells based on average geometric properties. In this study, a method for incorporating the effect of variations in microstructure into model-based calculations of the permeability of an idealized preform structure is developed. The method is used to model the permeability of a unidirectional material. It is shown that the inherent variation in the material microstructure leads to large deviations in the “local” values of the permeability. These variations are quantified using statistical analysis to obtain an expected value and standard deviation for the permeability. Use of the method leads to a consistent comparison of experimentally measured and numerically computed permeability values.