Fatigue Evaluation of Welded Joints
In welded structures, fatigue failures typically occur at the welded joints. Stress concentrations and small flaws in the welds are key parameters in determining the fatigue life of weldments. Traditionally, fatigue analysis of welded joints has been performed by attempting to determine the peak stress at the welded joint and then entering an appropriate “S-N curve” to estimate the fatigue life of the joint. The determination of the peak stress is required because the S-N curves are based on smooth bar specimens that do not include the effect of any stress concentrations.
There are two methods to estimate or account for the peak stress in the welded joint. One method is to assign a fatigue strength reduction factor (FSRF). The FSRF correlates the fatigue behavior of a specimen with a notch stress to a specimen without a notch stress and thus accounts for the stress concentration effects of the notch in the fatigue evaluation. That is, the FSRF is an attempt to reconcile data from smooth bar specimens with real-life fatigue performance. The second method is to directly determine the peak stress at the weld joint using finite element analysis (FEA). Both methods have notable disadvantages.
Because smooth bar specimens are prepared so as to be free of imperfections, fatigue testing of smooth bar specimens includes both the crack initiation phase and the crack propagation phase, and, in highcycle fatigue (> about 10,000 cycles), the crack initiation phase typically defines the majority of the fatigue life of the specimen. The introduction of the FSRF implicitly assumes that this single parameter is adequate to correlate or scale smooth bar specimen data to welded joint data for these two distinctly different regimes – crack initiation and crack propagation.
Moreover, the FSRF is not a universal or fundamental parameter. It should be viewed as an empirically determined value that accounts for stress concentrations and other factors that reduce the fatigue strength of a welded joint. It applies to a specific welded joint configuration and loading conditions. Consequently, the determination of the FSRF for a specific situation is somewhat subjective and different analysts may choose different values for the same problem. The determination of the peak stress at a welded joint using FEA is also problematic.
The predicted peak stress is known to be sensitive to mesh refinement, choice of element, and loading mode. In addition, small mechanical and metallurgical flaws that are present in most weldments, but that are too small to detect with commonly used NDE methods, can be sites for initiation of cracks in the high-cycle fatigue regime.
Approximately 15 years ago, researchers began developing a new approach to predicting the fatigue behavior of welded joints called the “Structural Stress Method” or SSM. One key aspect of the SSM is that weld design curves are based on fatigue testing of a large set of welded specimens, not on results from smooth bar specimens. This is a critical difference because it is now known that welds do not behave like base metals and that, in fact, base metal strength is not a major factor in weld behavior. Because welds typically include imperfections, the effects of imperfections on the fatigue life are directly included in the database. This improves the prediction of fatigue life because the crack initiation phase is minimized and most of the fatigue life is due to crack propagation, which better reflects real-world performance.
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