Low-cycle fatigue crack growth considerations in pipe fracture analyses

This paper presents some results from an ORNL cyclic pipe experiment that demonstrated the magnitude of low-cycle fatigue crack growth from simulated seismic loading. The concern with this result is that current pipe flaw evaluation procedures do not consider the cyclic crack growth during a seismic event, i.e. the peak dynamic load is used in a static analysis. This was the main incentive for the analyses developed in this paper. Additional applications of the methodology developed in this paper would be high stress-low cycle fatigue transients that can occur in service. In this paper, we show that the low-cycle fatigue crack growth data from the ORNL pipe test agreed well with the extrapolation of high-cycle fatigue crack growth data from small specimens. The next step was to evaluate the accuracy of several different methods to predict the low-cycle fatigue crack growth. In this evaluation it was found that for low-cycle fatigue LEFM analysis would significantly underpredict the crack growth and EPFM analyses were needed. Three different EPFM analyses were used. The GE/EPRI method was found to be overly conservative, which is consistent with comparisons with past pipe fracture data. The LBB.NRC and LBB.ENG2 J-estimation schemes provided good predictions of the experimental crack growth data when the crack-closure load was taken into account. (C) 1997 Elsevier Science S.A.