Steam generator (SG) tubes in a nuclear power plant can undergo rapid changes in pressure and temperature during an accident; thus, an accurate model to predict short-term creep damage is essential. The theta (θ) projection method has been widely used for modeling creep-strain behavior under constant stress. However, many creep test data are obtained under constant load, so creep rupture behavior under a constant load cannot be accurately simulated due to the different stress conditions. This paper proposes a novel methodology to obtain the creep curve under constant stress using a modified θ projection method that considers the increase in true stress during creep deformation in a constant-load creep test. The methodology is validated using finite element analysis, and the limitations of the methodology are also discussed. The paper also proposes a creep-strain model for alloy 690 as an SG material and a novel creep hardening rule we call the damage-fraction hardening rule. The creep hardening rule is applied to evaluate the creep rupture behavior of SG tubes. The results of this study show its great potential to evaluate the rupture behavior of an SG tube governed by creep deformation.

核电站中的蒸汽发生器 (SG) 管在事故期间会经历压力和温度的快速变化；因此，预测短期蠕变损伤的准确模型至关重要。theta ( θ ) 投影方法已广泛用于模拟恒定应力下的蠕变应变行为。然而，许多蠕变试验数据是在恒载下获得的，因此由于应力条件不同，无法准确模拟恒载下的蠕变断裂行为。本文提出了一种使用修正的θ获得恒定应力下的蠕变曲线的新方法在恒载荷蠕变试验中考虑蠕变变形过程中真实应力增加的投影法。使用有限元分析验证了该方法，并讨论了该方法的局限性。本文还提出了合金 690 作为 SG 材料的蠕变应变模型和一种新的蠕变硬化规则，我们称之为损伤-分数硬化规则。应用蠕变硬化规则来评估 SG 管的蠕变断裂行为。这项研究的结果表明，它在评估受蠕变变形控制的 SG 管的破裂行为方面具有巨大的潜力。