Abstract
In the nuclear industry, some methods for calculating the time of crack initiation from pre-existing defects are needed. For this purpose, a correlation is checked between T i (initiation time) or N i (initiation cycles) and local parameters at a characteristic finite distance d from the crack-tip ( α d criterion) (D. Moulin, B. Drubay and D. Acker, PVP-Vol. 223 (1992), Pressure Vessel Fracture, Fatigue and Life Management, ASME, 1992.) For fatigue tests, relevance of σ d criterion is observed when using finite element code for calculating axial stress range Δσ d. For creep tests, experimental points corroborate σ d criterion when no stress relaxation at the crack tip is supposed. Finite element code with plane strain or plane stress conditions gives a large value of axial stress at 50 μm from the crack tip. Crack initiation prediction is then conservative. Further, a remarkable T i -C h ∗ correlation was observed for creep tests. Both criterions are compared. For creep-fatigue tests, crack initiation prediction is conservative, but comparison of σ d criterion with other approaches is still in progress. Fracture surface examinations show that both creep and creep-fatigue specimens revealed the same intergranular surface aspect, whereas pure fatigue specimens presented transgranular cracking.
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