Developments in Ceramic Materials Research

198
M. A. Sheik
Figure 25. Warp and Fill Fibre Tow geometry with the HITCO composite Unit Cell developed from the
optical micrograph with fibre-volume fraction Vf =50 %.
In designating an RVE Unit Cell with fibre tows that exhibit orthotropic thermal
properties, the regions created with ‘Loft’ and ‘Sweep’ function have been assigned special
polar coordinate systems. These have been necessary in order to dictate the fibre orientation
and their directional property bias as shown superimposed upon a single fill fibre tow in
Figure 26. The assignment of the local polar coordinate system requires selection of a discrete
volume or region. This region is then assigned a unique local coordinate system, rectangular
or polar, which ever is applicable. To obtain distinct regions, it was imperative to divide
bigger regions of the fibre tow into smaller ones from places where the curvature sign was
changing.
This can be seen in Figure 25 where partitioning sections with lines are visible in both
warp and fill fibre tows that run parallel to the lenticular area outline, used for the fibre tow
generation with the ‘sweep’ function. After separating these unique regions, different local
coordinate systems are created with relevant selection of coordinates for centers of curvature
and assigned to each curved section. The concern of the various regions thermally interacting
with each other that had arisen in the DLR-XT Unit Cell does not arise here as in the present
case of HITCO material, the RVE Unit Cell has been generated as a continuous volume.
Therefore, there is no need for any interaction property definition within itself. The need for
these interactions may arise when multiple RVE Unit Cells are brought together to form a
larger section of the composite lamina or during the stacking to form the prototype laminate.
Moreover, the HITCO composite laminate has around 8 to 12 laminae stacked upon each
other in various positions with respect to specific features of the weave pattern such as the
location of fibre tow crossover points. For through-thickness interaction between the laminae,
again, care is needed while assigning thermal properties at the common ‘tied’ surfaces i.e.,
between master and slave surfaces defined with proper thermal interaction properties.
An important aspect of an FE analysis has always been the selection and discretisation
density of elements for bringing in the governing equation within the mesh for solution.
Having the luxury to selectively ‘mesh’ individual regions, with various element types
available, makes the meshing process very thorough and interesting, but at the same time,
certain constraints always restrict selection from all available options. The 3D tetrahedral,