|Welding Residual Stresses|
A reliable DTA depends upon, among other things, a reliable stress analysis accounting for both the applied loads and residual stresses. Railroad tank cars are constructed by joining various structural components (shells, plates and beams) using fusion welding. The primary residual stresses are due to the welding process. Since fatigue crack growth and fracture of tank car structures have been shown to initiate primarily at welds, especially those welds that connect the tank to the under-frame structure (e.g., the stub sill region), a basic understanding of how fatigue life is affected by welding residual stresses is needed. Using the high flux isotope reactor instrument at Oak Ridge National Laboratory, the three-dimensional, through-thickness residual stress field was measured along a transverse section of curved plates made of TC-128B steel welded under conditions similar to those used in tank car construction.
The upper half of the lower-left figure shows one surface of a thin, polished, transverse cross-section of the weld region after etching with a 1-2% solution of nital. As shown in the figure, the first-pass weld was made on the interior surface of the curved section, penetrating to a depth of approximately 60% of the thickness. The second-pass weld penetrated both the material to be joined and the first-pass weld; the weld nuggets overlapped by approximately 2.5 mm. The welding residual strains were measured at the 85 positions shown in the lower half of the lower-left figure. The lower-right figure shows the principal residual stresses along the line of N=9.1 mm.