Abstract
Effect of stress ratio on the crack extension rate da/dN was examined for an iso-tropic nuclear graphite IG-11, using tapered double cantilever beam specimens. Loads were applied to the specimens cyclically at a loading rate of 251 N/s in the load range where the stress ratio R range of 0∼0.8. Here R is defined as the ratio of the minimum stress intensity factor K min to the maximum stress intensity factor K max. The rate da/dN was measured optically using a comparator. Conclusions derived as 1. For a given value of R, crack extension rate da/dN can be expressed as da/dN = C(ΔK)n. here, ΔK is a stress intensity factor range, i.e. ΔK = Kmax − Kmin , and C and n are constants and strongly dependent on R value respectively.2. Threshold stress intensity factor range ΔKth which was defined as the point below which no crack extension more than 10 μm was observed after loadings of 105 cycles was related with R-values as ΔKth /ΔKtho = (1 − R)0.89, (ΔKtho = ΔKth at R = 0).
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