Fatigue Crack Growth (FCG) Analysis

The FCG is a complicated process that is sensitive to numerous factors, including material properties, microstructure, environment, load spectrum, crack size (small crack behavior) and crack orientation. The current state of the art for predicting FCG considers the effects of various factors by using empirically determined material properties and parameters for a given situation. Therefore, to accurately predict FCG for a given situation, it is important to use the material properties and parameters determined for the given condition. A review of the literature at the time of the work showed that a very limited amount of material property data useful in FCG analysis exists for tank wall materials (e.g., A516-70, TC128-B, and A515 for older tank cars). There was no FCG data available in the literature for TC128-B. Several sources of fatigue data for A516-70 were found in the literature. It is also noted that no data exists to consider load sequence effects under spectrum loading for these steels. To predict FCG under variable R-ratio tank car load spectrum by using the available constant R-ratio Paris equation, a “primary R-ratio” concept is proposed in order to have a closer match of the actual R-ratios with appropriate Paris equation parameters. The primary R-ratio at a given potential fatigue critical location (PFCL) is the R-ratio range of the load steps that contributes most to the fatigue damage in the spectrum. The primary R-ratio range will be either (a) low R-ratio range (L-R): 0<R<0.2, (b) medium R-ratio range (M-R): 0.2<R<0.5, or (c) high R-ratio range (H-R): R>0.5.

FCG analyses of the seventeen identified PFCLs identified that the PFCL designated as N1 is the most critical. Thus, location N1 controls the safety of the tank shell, provided that equal size and shape of initial cracks exist at each PFCL. The fatigue critical location (FCL), N1, is located on the external surface of the head at the weld of the front sill pad, as shown in the lower-left figure, along with the crack parameters. The lower-right figure compares inspection intervals predicted for A515 steel tank car of different initial crack cases with and without welding residual stresses. It can be seen that the tensile residual stresses can significantly reduce the inspection intervals.

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