Crack initiation on CT25 specimens under creep and creep-fatigue conditions

The behavior of crack-like defects in components operating at high temperature where creep is significant must be under control. A practical engineering and validated method for calculating the time of crack initiation from preexisting defects is needed. An important test program named AMORFIS has been performed at CEA Saclay on 316L(N) CT specimens, in order to develop and validate a methodology for prediction of crack initiation in 316L(N) austenitic steel components operating at high temperature under monotonic loading or cyclic loading with or without holdtime. Moulin et. al. have proposed a simplified method (noted σd method) which relies on the evaluation of stress-strain history on a small distance d (d = 0.05mm) close to the crack front, and material characteristics (stress to rupture curves and fatigue curves) that are available in nuclear codes like ASME code cases or RCC-MR. Continuous fatigue, creep, and creep-fatigue tests results at 600 C and 650 C are presented. Tests have been performed on machined and no sharp fatigue precracked specimens and compared for some loadings with test results on the same but sharp fatigue precracked CT specimens. The step by step procedure of the σd method is described for fatigue, creep and creep-fatigue conditions. A good correlation is obtained by comparing the time of initiation predicted by σd method and the initiation time obtained experimentally