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 crac k initiation from pre-existing defects are needed. For this purpose, a correlation is checked between T-i (initiation time) or N-i (initiati on cycles) and local parameters at a characteristic finite distance d from the crack-tip (sigma(d) criterion) (D. Moulin, B. Drubay and D. A cker, PVP-Vol. 223 (1992), Pressure Vessel Fracture, Fatigue and Life Management, ASME, 1992.) For fatigue tests, relevance of sigma(d) crit erion is observed when using finite element code for calculating axial stress range Delta sigma(d). For creep tests, experimental points cor roborate sigma(d) criterion when no stress relaxation at the crack tip is supposed. Finite element code with plane strain or plane stress co nditions gives a large value of axial stress at 50 mu m from the crack tip. Crack initiation prediction is then conservative. Further, a rem arkable T-i-C-h tests. Both criterions are compared. For creep-fatigu e tests, crack initiation prediction is conservative, but comparison o f sigma(d) criterion with other approaches is still in progress. Fract ure surface examinations show that both creep and creep-fatigue specim ens revealed the same intergranular surface aspect, whereas pure fatig ue specimens presented transgranular cracking.