Experimental studies and field experience related to the behaviour of pipeline steels and pipeline girth welds containing surface breaking (or embedded) flaws demonstrate that failure is governed by plastic collapse (ductile failure). A number of methods are available for assessing failure by plastic collapse. The user of these methods assumes that ductile failure occurs without any flaw extension. Hence, the failure conditions are predicted by the flow stress of the material surrounding the flaw and geometrical factors ('initial' flaw size, wall thickness and 'are length' or pipe circumference). Moreover, pipeline literature and fitness-for-purpose standards depart from the assumption that the application of the published plastic collapse assessments yield safe predictions. That is, the pipeline industry ignores the effect of ductile tearing on the calculated plastic collapse stress. This might be an acceptable assumption for pipeline steels of moderate strength. Curved wide plate tests on pipe materials containing surface breaking flaws have shown that considerable ductile tearing might occur during loading. However, the effect of ductile tearing is normally ignored in plastic collapse based flaw assessments. The information presented in this paper shows that, if a minimum margin of safety has to be achieved, this assumption may lead to unconservative plastic collapse predictions. It is concluded that the effect of ductile on plastic collapse predictions merits further study.
Effects of stable ductile crack growth on plastic collapse defect assessments
Einfluss einer stabilen Verformungsrissausbreitung auf die Beurteilung der plastischen Zusammenbruchdefekte
2000
21 Seiten, 12 Bilder, 1 Tabelle, 16 Quellen
Aufsatz (Konferenz)
Englisch
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