In the 1950’s Markl et al. conducted numerous fatigue test of welded piping components that were later used to develop the ASME B31 and other piping design codes. To analyze low cycle fatigue data, Markl proposed an extrapolation method to obtain the pseudo-elastic stress for fatigue data analysis. In this paper, this extrapolation method is revisited to figure out its underlining mechanism, application scope and limitation. Two sets of fatigue data representing 4 point bending and cantilever bending condition are analyzed to support the study. A structural strain method is then proposed as an improvement and generalization of Markl’s approach, which can correlate large amount weldment fatigue data in both low-cycle and high-cycle regime by defining an equivalent structural strain parameter.

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