CRACK INITIATION UNDER CREEP AND CREEP-FATIGUE ON CT SPECIMENS OF AN AUSTENITIC STAINLESS-STEEL

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 (sigma(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 sigma(d) criterion is observed when using finite element code for calculating axial stress range Delta sigma(d). For creep tests, experimental points corroborate sigma(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 mu m from the crack tip. Crack initiation prediction is then conservative. Further, a remarkable T-i-C-h* tests. Both criterions are compared. For creep-fatigue tests, crack initiation prediction is conservative, but comparison of sigma(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.