Fatigue assessment for seismic loads considering material degradation due to stress corrosion cracking

This study was aimed at showing a fatigue damage assessment procedure for seismic loads considering material degradation, specifically stress corrosion cracking (SCC), caused during long term operation of nuclear power plants. To superpose the damage due to cyclic loading and SCC, crack growth analysis was applied to the fatigue damage assessment, although this assessment is conventionally performed using the usage factor. To show applicability of the crack growth prediction to seismic loads, fully-reversed stress- or strain-controlled crack growth tests were conducted using plate specimens made of Type 316 stainless steel. A relatively large load amplitude was applied to simulate a fast growth rate, which was 0.32 mm/cycle for the maximum rate case. It was shown that obtained crack growth rates correlated well with the strain intensity factor range. Furthermore, the effect of crack closure was corrected by using the effective strain intensity factor range. Fatigue life estimated by the crack growth prediction with an initial depth of 50 μm correlated well with the fatigue life obtained by low-cycle and extremely low-cycle fatigue tests, for which the maximum strain range was 12%. Finally, a fatigue damage assessment procedure considering the effect of SCC was presented, in which the initial crack depth was determined by SCC initiation and growth predictions.