基于累积模型的高温构件蠕变疲劳寿命评估

S. Linn, C. Kontermann, M. Oechsner
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引用次数: 2

摘要

交变温度会在厚壁部件(如涡轮转子或外壳)中引起热机械应力,从而导致疲劳和叠加蠕变。随后,它们的受热表面会发生损伤。当前火电厂普遍存在冷、暖、热多启动的运行工况,以应对火电厂为保证电网稳定而产生的电力需求的高波动性,因此寿命评估方法成为研究的重点。工程师们正试图估计在役部件的剩余寿命,发电厂的操作员要求制定策略,以最大限度地减少计算材料损坏,同时在改变需求的情况下提供最大的灵活性和最短的响应时间。在本构模型中,用于寿命评估的累积模型早在几十年前就被引入,并在适用标准中得到了部分考虑。这种基于损害累积的模型易于应用,但被认为要么非常不精确,要么非常保守,而保守性反映了大安全边际的必要性。本文总结了几种适用于提高基于简单时间分数规则的模型预测质量的措施。所提出的模型是基于等温和非等温条件下的滞回线的综合,考虑生命周期内循环软化或硬化的概念,处理内部背应力,平均应变或应力的概念,以及考虑蠕变-疲劳相互作用的概念。后者基于所谓的过渡时间概念,其中停留时间期间的蠕变损伤部分归因于疲劳损伤部分,而疲劳损伤部分又由停留时间实验的疲劳寿命曲线确定。此外,该模型还包含了瞬态有限元计算的后处理和处理多轴加载条件的概念。由于采用过渡时间概念提出的方法的要点早在10多年前就已公布,因此所列出的修改提高了日常工程使用的效益。验证实验提供了模型在可接受的不确定性下预测能力的证据。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Aspects of Creep Fatigue Lifetime Assessment for High Temperature Components With Accumulative Model
Alternating temperatures induce thermomechanical stresses in thick-walled components such as turbine rotors or housings, which can lead to fatigue and superimposed creep. Subsequently, damage can occur at their heated surfaces. Under the nowadays prevailing operating conditions of power plants with multiple cold, warm and hot starts as reaction to the high volatility of electric demand from fossil fired power plants for ensuring grid stability, methods for lifetime assessment are coming more into the focus of investigations and research. Engineers are trying to estimate the residual lifetimes of in-service components and operators of power plants ask for strategies to minimize the calculative material damage while simultaneously providing a maximum flexibility with shortest response times on altered demands. Among constitutive models, which are not subject of this paper, accumulative models for lifetime assessment were introduced several decades ago and are partially considered in applicable standards. Such models based on a damage accumulation are easy to apply but they are considered to be either very imprecise or very conservative, while the conservatism reflects the necessity of large safety margins. This paper summarize a few measures, which are suitable to improve the predictive quality of models based on a simple time-fraction rule. The proposed model is based on a synthesis of hysteresis loops for isothermal and non-isothermal conditions, concepts for consideration of cyclic softening or hardening during lifetime, concepts for dealing with internal back stresses, mean strains or stresses, and for accounting of creep-fatigue interaction. The latter is based on a so-called transition time concept, where the creep damage during dwell times partially attributes to the portion of fatigue damage, which in turn is determined from fatigue life curves for dwell time experiments. In addition, the model comprises a concept for the post-processing of transient FEM calculations and dealing with multiaxial loading conditions. Since the essentials of the proposed method with the transition time concept were published more than 10 years ago, the listed modifications improve the benefit for daily engineering usage. Validation experiments provide evidence of the models predicting capabilities with acceptable uncertainty.
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