熱疲労荷重に対する簡易弾塑性補正(Ke係数)に関する一考察
A study on simplified elastic-plastic correction (Ke factor) for thermal loading

釜谷　昌幸 （Masayuki Kamaya） 朝田　誠治 （Seiji Asada）

要約
機器設計において簡易弾塑性補正係数K_eは，疲労評価に必要な弾塑性ひずみを弾性解析から推定するために用いられる．K_e係数は種々の規格で用いられているが，その値はそれぞれの規格の考え方に基づいて設定されている．近年，米国機械学会（ASME）の設計規格では，熱荷重に対してK_eの値を小さくするよう変更した．これは，変位制御荷重下での弾塑性ひずみは限定的であるという仮定に基づいている．本稿では，JSME規格に変位制御荷重相当のK_e係数を導入する議論を進めるための基本的な考察をまとめた．まず，変位制御荷重相当のK_e係数の考え方を整理するとともに，各規格のK_e係数を比較した．そして，直管モデルを対象に，熱荷重に対するK_e係数を計算した．その結果，ASMEの熱荷重用のK_e係数は，軸方向に拘束を加えた場合や変動周期が短い場合にのみ有効であることが示された．そして，これらの状況を踏まえて熱荷重に対しするK_e係数を新たに提案した．最後に，熱荷重下でのK_eの特徴をまとめるとともに，日本機械学会の設計・建設規格のK_e係数のあり方についての考察を示した．

キーワード    熱応力，疲労，簡易弾塑性補正，構造設計

Abstract
The simplified elastic-plastic correction factor K_e is quoted in fatigue damage assessments for component design to predict elastic-plastic strain from elastic strain without performing elastic-plastic analyses. The K_e 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 K_e factor for thermal loading. By assuming elastic-plastic strain could not become large for thermal loading, a reduced K_e value was applied when the stress was caused by thermal expansion loading. This paper discusses the applicability of K_e for thermal loading to the design code by the Japan Society of Mechanical Engineers (JSME). First, the K_e factor for thermal loading and its background are reviewed, and then, K_e 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 K_e factor prescribed in the ASME and JSME codes. The K_e values obtained by FEA become larger than those of the revised K_e of the ASME. It is shown that K_e value depends on the boundary condition. The K_e 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 K_e 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 K_e factor is prescribed in the JSME design code.

Keywords   thermal stress, fatigue, simplified elastic-plastic correction, component design

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