A Methodology for Assessing Fatigue Crack Growth Reliability of Railroad Tank Cars

A methodology for assessing structural reliability of railroad tank cars undergoing fatigue crack growth is being developed. A three-degree-of freedom (3-dof) surface crack is employed to account for an asymmetric stress field existing at a fatigue critical location in tank car structures. A fatigue life prediction program using the Walker equation to account for stress ratio effects is developed for the 3-dof surface crack model. A component reliability problem is formulated as a limit state function and solved using a first-order reliability method along with importance sampling method within the commercial reliability software, STRUREL. The methodology is demonstrated using illustrative railroad tank car examples. The results show that: (a) failure probability, Pf, increases as tank car mileage increases; (b) welding residual stresses in the tank structure can significantly increase Pf; (c) uncertainty in the stress amplitude also has a significant effect on Pf; and (d) the correlation between initial crack depth and initial crack aspect ratio is an important parameter.