Abstract
The authers have been attempting to establish a proper method for predicting fatigue lives of structures which have been subjected to material degradation.
For this purpose, the present paper describes a statistical investigation on the effects of material degradation on the crack initiation life and on the crack growth rate in a 2¼ Cr-1Mo (SCMV4) steel. The steel studied in this paper has been used in a plant having operated for over 60000 hours and subjected to material degradation due to temper embrittlement. The results obtained for the degraded steel were discussed in comparison with those for the recovered SCMV4 steel.
The crack initiation life and crack growth rate in the degraded SCMV4 steel and in the recovered SCMV4 steel followed the log-normal distribution. The crack initiation life was shorter in the degraded steel than in the recovered one at a high probability. The crack growth rate da/dN was higher in the degraded steel at higher stress-intensity-factor-range ΔK levels at a high probability. At lower ΔK levels, on the other hand, the mean of da/dN was a little lower in the degraded steel; nevertheless, a larger variation in da/dN implies that da/dN can become higher in the degraded steel.
A Monte Carlo simulation was made on the basis of the data obtained in the present experiments in order to determine the P-S-N diagrams for the degraded and the recovered SCMV4 steels. The obtained P-S-N diagrams statistically revealed that the fatigue life of the degraded steel was shortened at higher stress levels and that its variation became larger at lower stress levels. The larger variation in the life of the degraded steel is attributed to shorter fatigue life in the degraded steel than in the recovered steel at a considerably high probability, although the difference of the mean fatigue lives of the two steels became smaller at lower stress levels.
