A study on simplified elastic-plastic correction (Ke factor) for thermal loading

Abstract

The simplified elastic-plastic correction factor Ke is quoted in fatigue damage assessments for component design to predict elastic-plastic strain from elastic strain without performing elastic-plastic analyses. The Ke factor has been widely used in various design codes, although its value has been determined according to individual policy to achieve conservative assessments. Recently, the American Society of Mechanical Engineers (ASME) modified the Ke factor for thermal loading. By assuming elastic-plastic strain could not become large for thermal loading, a reduced Ke value was applied when the stress was caused by thermal expansion loading. This paper discusses the applicability of Ke for thermal loading to the design code by the Japan Society of Mechanical Engineers (JSME). First, the Ke factor for thermal loading and its background are reviewed, and then, Ke values from several codes are compared. Second, finite element analyses (FEA) for a straight pipe subjected to a fluid temperature transient are conducted to obtain values for comparison of the Ke factor prescribed in the ASME and JSME codes. The Ke values obtained by FEA become larger than those of the revised Ke of the ASME. It is shown that Ke value depends on the boundary condition. The Ke factor of ASME is valid only when the axial displacement is fully constrained or frequency of the fluid temperature fluctuation is fast enough. Third, based on detailed investigations, conservative Ke value for thermal loading is proposed. Finally, the characteristics of the change in the elastic-plastic strain for thermal loading are summarized and discussion is made about how the Ke factor is prescribed in the JSME design code.

Keywords

• thermal stress
• fatigue
• simplified elastic-plastic correction
• component design