Developments in Ceramic Materials Research

192
M. A. Sheik
3.4.2. Thermal Analysis of DLR-XT – Strategy and Results
The morphology and the geometric structure of the material has been translated into a
three-dimensional macro unit cell model, as shown in Figure 13. Following that, microscopic
sub-models of porosity types A, B, C, as shown in Figure 14, have been created for steadystate
finite element analyses similar to that performed for the macro unit cell model. Class D
porosity is then introduced in the main unit cell model, where a denuded portion of the central
section, as shown in Figure 11, is created. The results of these analyses in the form of the
degraded fibre tow and matrix thermal properties are listed in Table 5.
Material properties for the fibre tow and the matrix in the macro unit cell are the
prescribed values tabulated in Table 5. Assignment of degraded thermal properties hence
addresses the influence of initial porosities on the thermal behaviour of the material.
From Table 6 an obvious degradation of the overall composite thermal conductivity is
noted during the sequential introduction of each type of porosity. In addition, it is also evident
that Class B porosity (trans-tow cracks) has a dominant effect on the transverse thermal
conductivity, ‘k’ value for the composite whereas Class C porosity (matrix cracks) mainly
affects the longitudinal ‘k’ value. To a certain extent, the latter value is also influenced by
shrinkage de-bonding or Class A2 porosity. These results conclude that Class C porosity is
the primary form of defect that causes the most appreciable reduction of overall spatial
thermal conductivity profile of the CMC when these values are used at the macro unit cell
level.
Finally, the analysis of 3D thermal transport carried at the composite macro Unit Cell
level, in combination with the fourth class of porosity, the Denuded Matrix Regions,
produced the final k values are presented in Table 7.
Table 5. Degradation of thermal conductivity with sequential introduction
of each class of porosity