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This study assesses the applicability of the Weibull stress for the assessment of the crack-tip plastic constraint effect as well as the mixed mode I and II loading effect on brittle fracture resistance of a ferritic steel. Brittle fracture toughness under different crack-tip plastic constraint conditions and different loading mode conditions was respectively obtained by conducting 3-point bend (3 PB) test for single edge cracked specimens with different crack depth subjected to mode I load, and 4-point shear (4 PS) test for a single edge cracked specimen with deep crack depth subjected to mixed mode I and II load. By using the results of 3 PB tests, the critical Weibull stresses distribution independent of crack-tip plastic constraint was identified, and the critical Weibull stresses obtained by 4 PS tests provided significantly smaller distribution than that for 3 PB specimen. The mixed mode loading provides the different combined stress field around the crack-tip from that under mode I loading, where the 4 PS specimen showed the lower distribution of σ2/σ1 and σ3/σ1 than the 3 PB specimens. The lower principal stress ratio to be enlarge the fracture driving force was found to have no or much less influence to linear energy release rate for micro-crack, that was employed in the conventional derivation of the Weibull stress. Consequently, the reason why the conventional Weibull stress could not evaluate the effect of loading mode on fracture resistance could be that the linear energy release rate could not exactly take into account the local fracture driving force enlarged due to the combined stress field under mixed mode loading.

The small punch (SP) creep test using a TEM disk-type specimen (ϕ3 × t0.25 mm) was applied to the fusion zone (FZ), the coarsened grain heat-affected zone (CGHAZ), the fine grain HAZ (FGHAZ), the tempered HAZ (THAZ) and the base metal (BM), respectively, which were removed from the joint of 8Cr–2W–VTa steel welded by an electron beam welding, in order to investigate the creep properties of such local regimes. Experimental results revealed that the SP creep rupture of the FGHAZ was accompanied by creep void formation even if the rupture time was relatively short and its creep rupture strength was inferior to those of the other regimes. This reduction in rupture strength seemed to be attributable to the microstructural changes during the welding thermal cycles, which were represented by the annihilation of clear lath martensitic structure and the change in precipitate distribution.

The small-punch (SP) test and the SP creep test were carried out by using a further miniaturized plate-type specimen (3 mm diameter × 0.25 mm thick). Those tests were applied to the electron-beam welded joint of reduced-activation ferritic steel for evaluating the distribution of high-temperature strength in the joint. The experimental results revealed that the distribution of the maximum load measured by the SP test at 873 K was very similar to that obtained from Vickers hardness, and the fusion zone was relatively devoid of high-temperature ductility. In addition, the ratio of the load of SP creep test to the stress of standard uniaxial creep test was calculated so that both the creep rupture curves were overlapped to convert the results of SP creep test into those of standard test. As a result, the ratio was determined to be 0.43 for the base metal and this load-stress conversion coefficient is in good agreement with that obtained from finite element analysis.