This study made an effort to understand the fatigue failure mechanisms of welded piping joints in the low-cycle fatigue range. Experiments are conducted to explore the influence of residual stresses on fatigue life of butt- and socket-welded joints. Detailed strain responses at the weld toe area are recorded. Important observations made from the study are: 1. Both axial and circumferential strains at the weld toe always demonstrate ratcheting with progressive cycles. The ratcheting continues with cycle until the fatigue crack(s) is initiated. 2. Comparison of the welded joint ratcheting responses with those from the straight pipe cyclic bending and material ratcheting experiments indicates that the residual stresses at the welded joints are primary factors in inducing ratcheting. This conclusion is also supported by the symmetric strain responses (no ratcheting) at the mid-pipe length, which is located away from the weld joint and where there are no residual stresses to induce ratcheting. 3. Fatigue cracks in all welded joint experiments always occurred at the location where ratcheting strain is the largest. It indicates that the fatigue life of materials is reduced in the presence of ratcheting. 4. In the weld joint tests conducted, axial strain ratcheting occurs under displacement-controlled cyclic bending. In curvature-controlled cyclic bending experiment on straight pipe, such axial strain ratcheting is not observed. Understanding this newly observed weld failure mechanism may explain the reasons of many unexpected failures. 5. The load responses in the tests demonstrate cyclic softening. The cyclic softening response in the tests is surprising as the pipe and end fixture materials are SS304L and the weld material is SS308L, which are known to be cyclically hardening materials. 6. Socket weld fabricated through quarter-circumferential welding has longer fatigue life than the one with fullcircumferential welding. Also, the socket-weld with quarter-circumferential welding and four weld passes has a relatively longer fatigue life than the socket-weld with three weld passes. This indicates that the weld sequence can be optimized for enhancing fatigue life. More elaborate experimental and analytical programs need to be developed for design implementation of this concept. Further experimental and analytical studies on welded joints are underway through a National Science Foundation grant towards understanding the weld joint ratcheting failure mechanisms.
A fatigue failure mechanism of welded piping joints
2005
7 Seiten, 12 Bilder, 1 Tabelle, 12 Quellen
Aufsatz (Konferenz)
Englisch
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