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Code Design and Evaluation for Cyclic Loading – ASME Sections III and VIII

“Code Design and Evaluation for Cyclic Loading – Sections III and VIII”¬†William J. O’Donnell, Chapter 39, ASME Companion Guide to the Boiler and Pressure Vessel Code, Vol. 2, K.R. Rao, Editor, Third Edition, 2006

Keywords: fatigue; pressure vessels; piping; fatigue strength; cyclic loading; flow induced vibrations; environmental effects; SDB-63

Fatigue is one of the most frequent causes of failure in pressure vessels and piping components. Fatigue strength is sensitive to design details such as stress raisers, and to a myriad of material and fabrication issues, including welding imperfections. Fatigue is also sensitive to often unforeseen operating conditions such as flow induced vibrations, high cycle thermal mixing, thermal striations, and environmental effects. What is somewhat surprising is the large number of fatigue failures which are directly related to poorly chosen design and fabrication details. The ASME Code, and other International Codes and Standards have not been successful in preventing the use of design and fabrication details that are inappropriate for cyclic service. The ASME Code was one of the first Codes and Standards to treat design for fatigue life explicitly.

The failure of metals from fatigue appears to be first documented by Albert in 1838 [1]. Fatigue has long been a major consideration in the design of rotating machinery and aircraft, but the number of cycles for such applications is usually in the millions, and the fatigue stresses are generally not substantially over yield. However, pressure vessels and piping tend to operate in the low cycle regime, where local stresses are far in excess of yield. Useful methods of analyzing fatigue in the low cycle regime were first developed by Langer [2]-[4], Coffin [5] and Manson [6] in the 1950’s and 1960’s.

The fatigue design life evaluation procedures in Section III of the ASME Boiler and Pressure Vessel Code were originally developed in the Naval Nuclear Program. W.J. (Bill) O’Donnell worked with B.F. (Bernie) Langer, W.E. (Bill) Cooper and James (Jim) Farr in the late 1950’s and early 1960’s on the initial formulation of this technology in the Tentative Structural Design Basis for Reactor Pressure Vessels and Directly Associated Components, which became known as “SDB-63.” Section III of the ASME Code “Vessels in Nuclear Service” was the first to include specific Code rules to prevent low cycle fatigue failure. Its first edition was published in 1963; Section VIII, Division 2, “Alternate Rules for Pressure Vessels” followed in 1968. Section VIII, Division 1 of the code still does not include explicit fatigue design life evaluation methods.

The chief difference between high cycle and low cycle fatigue is that the former involves little or no plastic action, whereas in the latter, only those strains in excess of the yield strain can produce failure in a few thousand cycles. In the plastic region large changes in strain can be produced by small changes in stress. Fatigue damage in the plastic region is caused by plastic stain ranges. Therefore, fatigue curves for use in this region should be based on tests in which strain, rather than stress is the controlled variable. As a matter of convenience, the strain values used in the tests are stresses and stresses calculated on the assumption of elastic behavior.