Studies on Fatigue Design Margins of Welded Steel Structures

This paper discusses fatigue design margins based on large-sized specimens that are considered to be more closely related to actual components. A fatigue design curve is generally determined by applying an adequate margin to a best-fit curve obtained from laboratory tests using small-sized specimens. However, the reason and the background for the margin setting are unclear and controversial.

In ASME Boiler and Pressure Vessel Codes Section III, for example, a factor of 2 on stress and a factor of 20 on life are applied to the best-fit curves. The conservatism of these factors has been the topic of discussion recently. In addition to a basic method like ASME Section III, new design concepts for welded structures, equivalent structural stress approach, has been proposed recently and adopted to design codes in Europe (EN13445) and the U.S.

(ASME Section VIII Division 2). Thus, the design margins by these three methods were compared. The comparison made it clear that the ASME Sec.

III method gives the most conservative margin, and the lowest value is approximately 2.