The use of constant probability fatigue curves in predicting deaerator vessel failure

摘要

This paper gives an assessment of some of the studies which have attempted to apply statistical distributions, or probability models, to the fatigue crack growth process in an effort to structure fatigue crack growth predictions in a more reliable manner. The fatigue crack growth process is defined in terms of the well known Paris-Erdogan law which successfully predicted the fatigue process at intermediate crack growth rates, da/dtThe following expression was developed to generate constant probability fatigue crack growth curves, viz.:Da/dt = A_1(#DELTA#K)~m(-ln P_i)~#beta#,where da/dt is the crack growth rate, #DELTA#K the stress intensity factor range, P the probability level while m and #beta# are constants.Crack growth in deaerator feedwater storage vessels was taken as a useful practical application to demonstrate the benefits of this probability approach to fatigue crack growth. In this instance the average fatigue crack growth law for low alloy steels at 200 deg C in water was utilised to assess the fatigue crack growth, which initiated from a surface corrosion pit, and extended through the vessel wall.Realistic probability of failure, POF, levels were established for a variety of circumstances and the effects of a delay in an inspection on the increased levels of POF in working deaerators were quantified.Finally, this paper illustrates how probabilistic calculations can be practically incorporated into any decision making process.