A thermodynamically-based viscoelastic-viscoplastic model for the high temperature cyclic behaviour of 9–12% Cr steels
A thermodynamically-based viscoelastic-viscoplastic model for the high temperature cyclic behaviour of 9–12% Cr steels
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DOI：
10.1016/j.ijplas.2018.03.015
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发表时间：
2018-04
影响因子：
9.8
通讯作者：
A. Benaarbia;J. Rouse;Wei Sun
中科院分区：
文献类型：
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作者：
A. Benaarbia;J. Rouse;Wei Sun
在许多工程应用中，提高对材料的长期速率相关行为的理解是至关重要的。如果不了解这一点，就很难在满足可持续性和负责任的资源管理要求的同时，确保工厂安全有效地运行。在目前的工作中，提出了一个基于几何的本构模型，以捕捉率敏感性，应力松弛和加速循环软化过程中观察到的P91钢在高温（600° C）的循环变形。该模型是在不可逆过程热力学和广义标准材料形式主义的框架内开发的，从而提供了粘弹性（在截然不同的时间尺度上的半可恢复应变积累）和粘塑性（在应力阈值以上观察到的不可逆应变）的一个基于物理基础的耦合。后一部分结合了双曲正弦功率流规则与各向同性和运动硬化的非线性等温循环演化。该模型的适用性，以各种机械载荷（例如，循环拉伸-压缩试验，疲劳松弛试验，滞回试验）进行验证，通过设计一个启发式优化程序的基础上的非线性最小二乘函数加上Levenberg-Marquardt算法。优化过程仅通过无滞后型经验数据（其中在波形中的各个点处引入长期负载保持时段）来通知（通过初始材料参数估计和目标函数评估的估计）。通过考虑不同加载（应变）速率（0.1%）的实验波形来确定和验证速率依赖性。s-1，0.01%。s-1和0.001%。s-1）和范围（0.25%，0.4%和0.5%），以突出疲劳和松弛过程中涉及的大多数变形机制。通过比较预测和实验观察到的材料响应，它表明在目前的工作中，粘弹-粘塑性应变分解有能力捕获的加速循环软化和解耦的应力松弛行为（低于和高于屈服）的P91钢在高温下。
Improving the understanding of the long term rate dependent behaviour of materials is of critical importance in many engineering applications. Without this understanding, it is potentially difficult to ensure safe and effective plant operation while simultaneously satisfying requirements for sustainability and responsible resource management. In the present work, a thermodynamically-based constitutive model is proposed to capture the rate sensitivity, the stress relaxation and the accelerated cyclic softening observed during cyclic deformation of a P91 steel at an elevated temperature (600° C). The model is developed within the framework of Thermodynamics of Irreversible Processes and Generalized Standard Materials formalism, thereby offering a thermodynamically grounded coupling of both viscoelasticity (semi-recoverable strain accumulation at vastly different time scales) and viscoplasticity (irreversible strain observed above the stress threshold). The later part combines a hyperbolic sine-power flow rule with non-linear isothermal cyclic evolution of isotropic and kinematic hardening. The applicability of the model to various mechanical loadings (eg, cyclic tensile-compression tests, fatigue-relaxation tests, anhysteretic tests) is validated by designing a heuristic optimisation program based on a nonlinear least-squares function coupled with the Levenberg-Marquardt algorithm. The optimisation procedure is informed (through the estimation of initial material parameter estimates and objective function evaluation) by anhysteretic type experiential data only (wherein long term load hold periods are introduced at various points in the waveform). Rate dependency is determined and validated by considering experimental waveforms with different loading (strain) rates (0.1%. s− 1, 0.01%. s− 1 and 0.001%. s− 1) and ranges (0.25%, 0.4% and 0.5%) to highlight most of the deformation mechanisms involved during the fatigue and relaxation processes. By comparing predicted and experimentally observed material responses, it is demonstrated in the present work that the viscoelastic-viscoplastic strain decomposition has the ability to capture the accelerated cyclic softening and the uncoupled stress relaxation behaviour (below and above yielding) for a P91 steel at elevated temperatures.