|Reliability Analysis of Corrosion-Accelerated Fatigue Crack Growth|
Fatigue crack growth in a tank car may be accelerated by the wall thinning effect due to tank car general corrosion. The recently developed reliability analysis methodology for tank car fatigue crack growth is used for reliability analysis of corrosion-accelerated fatigue crack growth by introducing a time-dependent tank wall thickness. The acceleration of fatigue crack growth is due to a thickness reduction caused by the corrosion, corresponding to a scenario in which fatigue crack growth occurs on one (outer or inner) surface of the tank, while corrosion damage occurs on the opposite surface of the tank, i.e. no coupling in mechanisms between crack growth and corrosion.
The lower-left figure shows a schematic for the corrosion-accelerated fatigue crack growth scenario, where the time-varying tank wall thickness is denoted as B(t). To introduce the thickness reduction caused by corrosion into fatigue crack growth process, it is necessary to relate corrosion time with the fatigue load spectrum. Tank car usage can vary greatly, for example, from a few thousand miles a year to about 20,000 miles a year. For illustration purposes, a consensus tank car usage of 14,000 miles per year is considered. Therefore, one year of corrosion time corresponds to a fatigue load spectrum representing 14,000 miles of travel distance.
The lower-right figure shows failure probability as a function of corrosion initiation time, ti, for the corrosion-accelerated fatigue crack growth with Cr as a random variable as given in the figure. ti is assumed to be a deterministic parameter. The result is for a required fatigue life corresponding to about 28.5 years of service, or 40 passes of tank-car load spectrum. The trends show that (a) a longer corrosion initiation time means a longer lasting protection of the tank from corrosion, thus a lower failure probability; (b) at a given ti, a higher corrosion rate corresponds to a higher failure probability, which is most pronounced at ti=0, and disappears at ti=28.5, at which the failure probability is equal to that for the case of no corrosion.