7 Fatigue failure is the phenomenon leading to fracture under repeated or fluctuating stresses that are less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of fluctuating stress. There are three stages of fatigue failure: initiation, propagation, and final fracture. The location of the initiation is at a stress concentration [6-9]. It is believed that stress concentration in the subject case is attributed mainly to the acute or sharp angle zone of 16" pipe/20" header assembly. In other words, such acute or sharp angle worked as site for initiation of fatigue damage at weld toe on outer surface of 16" pipe/20" header assembly, just beside its circumferential weld line. The current piping system is subjected to more than just static forces. They can have severe vibrations induced mainly due to several start up operations. Vibration can be also induced due to temperature change. Since the piping system is operated in an interrupted or irregular mode, it is subjected to low stress high cycles fatigue. Under such repeated conditions, fatigue crack could be initiated at the highest stress concentration zone (acute angle zone). After the original fatigue crack is formed, it becomes an extremely sharp stress concentration that tends to drive the crack ever deeper into the metal with each repeating of the stress. The local stress at the tip of the crack is extremely high because of the sharp “notch,” and with each crack opening, the depth of the crack advances by one “striation”. Fatigue striation is not clear at the fracture initiation zone due to low stress cycles. As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area, it eventually weakens the material so greatly that final, complete fracture occurs. Conclusions and Recommendations Based on the results obtained in this investigation, it can be concluded that the subject premature failure is attributed mainly to fatigue damage. High stress concentration at the acute or sharp angle of 16" pipe/20" header assembly and low stress high frequency vibration due to mainly several start upshut down operations, both have shortens the lifetime of the spool. Fatigue failure is the phenomenon leading to fracture under repeated or fluctuating stresses that are less than the tensile strength of the material. The initiation site of fatigue failure is minute, never extending for more than two to five grains around the origin. The location of the initiation is at a stress concentration. It is believed that the acute or sharp angle of 16" pipe/20" header assembly that acts as local stress raiser played a remarkable role in initiation of the fatigue damage on the outer surface, just beside circumferential weld. It is obvious that the fracture initiated at the outer surface and propagated across the thickness in two opposite directions. As the propagation of the fatigue crack continues, gradually reducing the crosssectional area, it eventually weakens the material so greatly that final, complete fracture occurs. As a preventive measure, the other six connections of 16" pipe/20" header were subjected to visual and dye penetrant tests where absence of external surface cracks was confirmed. In order to minimize the possibility of such failure in future, the design of 16" pipe to 20" header connection was modified, where a compensation plate fitted was used to minimize stress concentration at this connection zone. Besides, circumferential welds were made with a better quality even it had no direct relation with the failure.
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