Silicon carbide composites are attractive as structural materials in fusion environments because of their low activation, high operating temperature and strength. However, at present they lack the fracture toughness of metals and, thus, are relatively vulnerable to cyclic loads.
The physical properties of composites are generally anisotropic. The ARIES study [2] calculated several material properties of a 2-D SiC/SiC composite, using the CLASS code [9] to convert the layer properties and configurations into equivalent orthotropic properties. These computations yielded nearly isotropic properties; thus, in this study we assumed an isotropic material. The values used are shown in Table 1. Note that SiC composites have an unusually high modulus, which is detrimental to withstanding thermal loads, but a low thermal expansion coefficient that somewhat compensates.
At present, no SiC/SiC fatigue data is available.
However, Holmes [10] performed fatigue tests on
unirradiated SiC fiber/SiN
matrix composites.
His results should be indicative of the SiC/SiC fatigue strength
and are used here to estimate the impact of high-cycle fatigue
on the design. In Holmes' study, the specimens were subjected to
tensile fatigue, that is, their mean stress was greater than their
alternating stress. To allow comparison to the (fully-reversed)
vanadium data, we have extrapolated the data points by the
Goodman relation [11] to an alternating stress at zero mean with an
equivalent fatigue life. This data is shown in Figure 2.
The allowable alternating stress for 40,000 cycle life is 65 MPa.
Note this is approximately of the vanadium alloy's value.