User Program study with A. Humphreys of North Carolina State University
| The HTML’s newly
automated large specimen x-ray stress mapping facility was used by
North Carolina State University in a project whose goal is to
understand the changes in residual stress during fatigue life of socket
and butt-welded joints of 304L stainless steel (Fig 5.6). The project
involved the characterization of the "as welded" joints and the same
specimens after fully-reversed displacement-controlled loading cycles. A butt-weld and several socket-weld specimens were characterized for near surface (x-ray) residual stresses as a function of distance from the weld before and after the controlled loading cycles. Hoop and axial stress measurements were obtained in the region near the toe of the weld and at several distances from the weld. Replicate measurements showed that the residual stresses could be reliably reproduced for any position along the length or around the tube (Fig 5.6). The as-welded stresses showed a consistent pattern of compressive residual stresses as a function of distance from the weld metal with the maximum displaced from the circumferential weld. A correlation with weld geometry and order of weld beads was clearly detected. The extent of change of residual stress had previously been assumed in predicting life of these reactor components. These results will be valuable to many industries including those using boilers and nuclear reactors that contain 304L tubing. For example, nuclear reactor design methodologies assume that the residual stresses "shakedown" or relax to near zero after a couple cycles in the plastic range. NCSU studies suggest that shakedown is only partial and there are residual stresses remaining after the low cycle fatigue event. The existence of residual stresses possibly may explain a mysterious premature fatigue failure of piping components. However, measurement of the hoop and axial stress at the same locations after “shakedown” cyclic fatigue revealed that the residual stress state was substantially changed and the residual stress level reduced to near zero. |
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Figure 5.6 - Ms. Abigail Humphreys (left) changing the
x-ray collimator for her work on 304L tubes. Typical reproducibility
(right) of residual stresses with distance from the weld line and at
several locations around the tube before “shakedown”. |
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Acknowledgments
URL: http://www.html.ornl.gov/duc/tisic.html