【作者】 倪永中；

【导师】 徐鸿；

【作者基本信息】 华北电力大学（北京） ， 热能工程， 2010， 博士

【摘要】 随着工业技术的发展,在现代常规电站、核电站及大型化工等行业中,工作介质的温度和压力大为提高,以超超临界汽轮机为例,其蒸汽的温度达到了600℃,压力达到了25MPa,在如此高的参数下长期运行,材料不可避免的要发生疲劳和蠕变破坏行为,当这两种行为同时发生时,产生一种具有更大破坏性的力学行为：棘轮变形。由于其平均应力不为零,导致在循环过程中累积塑性应变逐步增加,一直到材料破坏。在常规电站和核电站等行业中,压力容器的强度设计均要求进行棘轮效应分析。能否进一步提高高温材料的力学性能是限制超超临界汽轮机进一步提高参数的原因之一。材料力学性能的研究的主要内容之一是材料本构关系的研究,一个好的本构关系对于材料的寿命评估具有重要意义。目前大部分统一粘塑性本构模型(如Ohno-Wang、Chaboche等)在低应力区进行应力保持时,其粘性应力σ_v迅速衰减为零,导致塑性应变速率为零,因此不能对蠕变行为进行合理的描述。针对这一现象,提出在本构方程的塑性应变的演变项中加入Norton蠕变准则,蠕变项通过影响运动硬化和随动硬化实现与疲劳项的耦合。由于Norton方程只能描述蠕变的第一和第二阶段,体现不了第三阶段的蠕变加速阶段,根据勒梅特提出了应变等价原理,利用等效应力的概念引入蠕变损伤变量,并把等效应力应用到整个统一粘塑性本构方程中,在引入蠕变及蠕变损伤变量后,模型能够较为合理的描述棘轮与蠕变的交互作用。提出粒子群算法和有限元模块相结合的本构方程参数优化方法,使用面向对象的C++语言开发了有限元计算模块,与面向过程的Fortran语言相比,程序具有很好的扩展性能。提出的优化方法可以直接以实验数据作为输入得到模型参数,与传统的参数确定方法相比可以大大的减少工作量。针对有限元实现过程中的两个主要技术难点：隐式应力积分过程、一致性刚度矩阵推导,实现了耦合蠕变及损伤变量后的应力积分过程和刚度矩阵的表达式,并成功的嵌入到有限元软件ANSYS中。通过对内压管道受轴向循环载荷实验的模拟,结果表明,新的本构方程在描述棘轮蠕变交互作用方面的能力有所提高。更多还原

【Abstract】 With the development of industrial technology, higher steam temperature and higher press are required in power plants, nuclear power plants and chemical industry. For example, in ultra-supercritical steam turbine, the temperature has reached 600℃and the pressure has reached 25MPa. Running at such a high parameters, the failure of metal material would inevitable occur because of the fatigue and the creep behavior. When these two kinds of behavior take place simultaneity, ratchetting deformation is produced which is a greater destructive behavior. For its non-zero mean stress, the accumulated plastic strain is continuous increased until the structure was demaged. In conventional power plants and nuclear power plants, the ratchetting analysis is required to be considered on strength design of pressure vessels. the mechanical properties of high-temperature materials is one of the reasons which limit the parameters of ultra-supercritical steam turbine to be enhanced. The research of the material constitutive model is a improtant part of the field of mechanical properties. A good constitutive model has great significance for life assessment.At present, for most unified viscoplastic constitutive model (such as the Ohno-Wang, Chaboche, etc.),when the stress is keepped in low zone, the viscous stress will fall rapidly to zero which result the plastic strain rate fall to zero, so this model have no reasonable description of the creep behavior. To this phenomenon, the Norton creep criteria was introduced to plastic strain of constitutive model, and creep was coupled with fatigue by hardening equations.Norton creep equation can only describe the first and second stage, but not the third stage. According to strain equivalence principle proposed by Lemaitre, the creep damage variable was introduce to the constitutive model, and the equivalent stress was applied to the unified viscoplastic constitutive equation. The new model can have more reasonable description of ratchetting and creep interaction.The parameter optimization method using particle swarm algorithm and finite element was Proposed. The finite element analysis module was developed using object-oriented C++ language, compared with some FEA software coded with Fortran language, the program has a good scalability. The optimization method can directly obtaine parameter by using experimental data as input data, which can reduce much workload compared with the traditional method of parameter determination.The porcess of finite element implementation has two main technical difficulties which are the implicit stress integration and the consistent stiffness matrix’s derivation,which were realized in this paper with the creep criteria and demage variable coupled, and new model was successfully embedded into the the finite element software ANSYS. Pipe samples subjected internal pressure and axial cyclic loading were simulated, results show that the new constitutive has a good agrement with experiment data in description the interaction of ratcheting creep.更多还原