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Pressure vessels are usually designed for only a few thousand stress cycles and seldom for more than 105 cycles. For fatigue analysis to be practical for most of the ASME Code applications, a concept has been developed different from the practice used in machine design for high-cycle fatigue in rotating machinery. The concept uses fatigue design curves developed for the different materials. The curves account for most of the factors that reduce fatigue life as shown by a comparison with fatigue data generated with laboratory specimens. Because most pressure vessels are of welded fabrication, the fatigue design curves have full corrections for tensile mean stress in the applicable region to account for tensile residual stresses. For corrosive conditions, the fatigue design curves may require further corrections. A modified concept of strain range partitioning has been developed for the analysis of the creep and fatigue interaction for high-temperature design. The concept uses the strain difference range to calculate creep and fatigue damage, which is similar to the shear stress failure used in lower temperature design.
The use of the fatigue design curves is demonstrated with the evaluation for fatigue life of two fossil-fuel boiler components. One of these components illustrates the difference in use of the fatigue design curves when creep is a further consideration.
fatigue, low cycle fatigue, elevated temperature fatigue, strain range partitioning, pressure vessel design, ASME Boiler and Pressure Vessel Code
Staff Engineer, Engineering Development Fossil Power Systems, Combustion Engineering, Inc., Windsor, Conn.