Developments in Ceramic Materials Research

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M. A. Sheik
3.2.3. Matrix Cracks (Class C Porosity)
This class of porosity is composed of cracks, embedded in the matrix, which surround the
tow in planes perpendicular to the tow axes. C matrix cracks run circumferentially around the
tows. The generation of the matrix cracks occurs during cooling of the composite due to
tensile stresses generated in the matrix arising from the difference in the values of the linear
coefficients of thermal expansion for the matrix and the fibre parallel to its axis. Class C
porosity is schematically given in Figure 7. Figure 8(b) is a section taken in the Y-Z plane
which cuts through the tow centres running in the Y direction. This is shown in Figure 10(a).
Likewise, Figure 8(c) is a section taken in the X-Z plane which cuts through the silicon
carbide matrix encapsulating the tows running in the X-direction. Hence it can be seen that
the Class C matrix cracks run circumferentially around the tows as shown schematically in
Figure 10(b). In addition, Figure 9 shows a SEM primary electron image of Class C defects in
a plane normal to the X direction.
Figure 9. Class C porosity (matrix cracks): SEM primary electron image of a plane normal to the X
direction. Region (a) has fibres running normal to the plane; region (b) is silicon carbide matrix; and
region (c) has fibres running parallel to the plane. The matrix region (b) surrounds the fibre tow (c).
Two Class C periodic matrix cracks are shown in region (b) which are circumferential to the in-plane
tow (c).
Figure 10(a). Two orthogonal tows sectioned normal to the X-axis in a Y-Z plane cutting through the
centre of the lower tow.