Simulations and Experiments on Low-Pressure Permeation of Fabrics: Part I—3D Modeling of Unbalanced Fabric

Abstract

Conformation of fabrics to complex molds during composite processing induces significant fabric deformation and local shear, which in turn alter the processability of these preforms from their unsheared, flat configuration. The present work (Parts I and II) establishes that although composite mold processing can often be described generally by a percolation flow assumption (e.g., Darcy or Poiseuille flow), changes in microarchitecture of fabric in shear result in markedly different flow fronts. We reiterate our earlier finding that use of a transformed Darcy law (i.e., mathematical transformation of tensor of undeformed permeability, to the sheared configuration) does not accurately predict permeability for sheared fabrics. In essence, the effect of the change in microarchitecture of the fabric is not captured by mathematical transformation of the tensorial permeability. Wealso point out the deficiencies of semi-empirical approaches in determining sheared fabric permeability. We then develop a 3D fabric model, which is used to quantify the effects of nesting and changes in gap architecture with shear angle. We show that nesting produces gaps in molds in commonly-used permeability experiments which easily exceed single-layer fabric thicknesses when more than a few layers are used, but that this condition is easily detectable in an experiment (i.e., Poiseuille flow between topmost layer and mold top is easily detected). We also show that shear angle (in our case,. = 0°, 15°, and 30°) produces little difference in nesting, though it significantly alters fabric microstructure and the sizes and shapes of intralayer gaps. In Part II of this paper, we use this fabric model to predict fabric permeability. Our work suggests that departure from the more traditional approach of generation of a large suite of data from permeation experiments to determine manufacturability of preforms, in favor of computational simulation of fabric geometries, is well-justified.