4.11.20.4.1 Coefficient of thermal expansion

Significant differences have been observed between the unstressed CTE and stressed CTE, as illustrated in Figure 58. Compressive creep strain was found
to increase the CTE, and tensile creep strain to decrease the CTE.

The changes in CTE caused by irradiation creep have similarities to those caused by the applica­tion of stress on unirradiated graphite. Figure 59(a) shows the changes in CTE in irradiated, crept specimens plotted as a function of creep strain92 and Figure 59(b) gives the changes in CTE in unirradi­ated graphite due to stress.93

At room temperature, the average CTE of an isotropic graphite with no porosity should be the average of the crystallite CTEs, that is, the crystallite CTEs are -27.0 x 10-6K-1 and -1.0 x 10-6K-1 in the V and У directions respectively, giving an average

of 8.0 x lO^KT1. From Figure 59(a), which is for Gilsocarbon with an unirradiated CTE of

4.0 x 10~6K~1,itisinterestingto note that the increase in CTE in compression is approaching that value.