Combined crystal plasticity and grain boundary modeling of creep in ferritic-martensitic steels: I. Theory and implementation

Sham

2020

This report describes an initial Crystal Plasticity Finite Element Method (CPFEM) model for cyclic plasticity and damage in Grade 91 steel. The objective of this work is to develop a framework for modeling creep-fatigue interaction in Grade 91 steel to better predict the onset of damage in high temperature microreactor components. Many microreactor concepts envision low operating pressures but relatively high thermal stresses. Under these conditions, creep-fatigue will likely be the dominant design failure mechanism. Physically based models, like the one under development here, could lead to a better understanding of creep-fatigue mechanisms and the effect of stress multiaxiality and hold time on creep-fatigue damage. In turn, this could lead to more efficient microreactor component designs.

Section: Large Deformation Rve Strain Decompositionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Initial microstructural model for creep-fatigue damage in Grade 91 steel

Sham

2020

“…NEML is a framework developed at Argonne National Laboratory and is compatible with MOOSE [11]. Following the example of Nassif et al [3] our model uses isotropic elasticity to describe the elastic behavior of the grain bulk, a crystal plasticity based model to incorporate deformation caused by dislocation glide, and an isotropic plasticity-based model to incorporate diffusion creep. References [3,4] describe this model in detail.…”

Section: The Prior Austenite Grain Modelmentioning

confidence: 99%

“…The grain boundary cavitation model is an improvement from the one described by Nassif et al [3]. This model was initially conceived by Sham and Needleman [19] and later extended by Van Der Giessen et al [20] to higher triaxiality regimes.…”

Section: Grain Boundary Cavitation Modelmentioning

confidence: 99%

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Identify the influence of microstructure on mesoscale creep and fatigue damage

2020

This report describes the development of a microstructural model that can quantify the uncertainty in the observed rupture life of Grade 91 steel. The model is microstructural, meaning it relates microstructural characteristics of the material to the resulting material response. As such, one of the uses of this model is to identify the key microstructural parameters controlling the development of damage in Grade 91 operating at elevated temperatures. The report describes two veins of work: improvements to the crystal plasticity model required to run the uncertainty quantification analysis and the results of that UQ analysis. For creep, the model identifies the grain boundary diffusivity as the critical parameter controlling the rupture life of the material. The report demonstrates that a reasonable microstructural distribution of grain boundary diffusivity can account for the observed macroscale variation in rupture life at fixed temperature and load.

Accurate Effective Stress Measures: Predicting Creep Life for 3D Stresses Using 2D and 1D Creep Rupture Simulations and Data

Integr Mater Manuf Innov

Parks

et al. 2021

Self Cite

Operating structural components experience complex loading conditions resulting in 3D stress states. Current design practice estimates multiaxial creep rupture life by mapping a general state of stress to a uniaxial creep rupture correlation using effective stress measures. The data supporting the development of effective stress measures are nearly always only uniaxial and biaxial, as 3D creep rupture tests are not widely available. This limitation means current effective stress measures must extrapolate from 2D to 3D stress states, potentially introducing extrapolation error. In this work, we use a physics-based, crystal plasticity finite element model to simulate uniaxial, biaxial, and triaxial creep rupture. We use the virtual dataset to assess the accuracy of current and novel effective stress measures in extrapolating from 2D to 3D stresses and also explore how the predictive accuracy of the effective stress measures might change if experimental 3D rupture data was available. We confirm these conclusions, based on simulation data, against multiaxial creep rupture experimental data for several materials, drawn from the literature. The results of the virtual experiments show that calibrating effective stress measures using triaxial test data would significantly improve accuracy and that some effective stress measures are more accurate than others, particularly for highly triaxial stress states. Results obtained using experimental data confirm the numerical findings and suggest that a unified effective stress measure should include an explicit dependence on the first stress invariant, the maximum tensile principal stress, and the von Mises stress.