Combined crystal plasticity and grain boundary modeling of creep in ferritic-martensitic steels: II. The effect of stress and temperature on engineering and microstructural properties

Abstract

This paper describes a series of physically-based crystal plasticity finite element method (CPFEM) simulations of long-term creep and creep rupture of Grade 91 steel. It is Part 2 of a two part series of papers. Part 1 describes the simulation framework; this part focuses on specific simulations and on how the predicted long-term creep properties of Grade 91 compare to the assumptions used in current high temperature design practices. This work extends the model developed in Part 1 to look at creep properties at different temperatures, principal stresses, and multiaxial stress states. The simulations show that empirically extrapolating creep rupture stresses from short-term experimental data may substantially over predict the actual long-term creep properties of Grade 91. Additionally, the CPFEM calculations predict a transition from notch strengthening creep behavior for high values of maximum principal stress and moderate notch severity to notch weakening behavior for low principal stresses and more severe notches. The latter regime better categorizes conditions in engineering components designed for long term elevated temperature use, which implies Grade 91 may be a notch weakening material in actual service. This would have a significant impact on high temperature design practices, though confirmatory test data on long-life, low stress notched specimens is difficult to obtain. Finally, one advantage of the physically-based modeling approach adopted here is that the simulation results also elucidate the microstructural mechanisms causing the macroscopic trends in engineering properties predicted by the simulations. This paper shows that the detailed micromechanical mechanisms predicted by the CPFEM simulations can be abstracted with a simple micromechanical model that can be used to both explain the detailed results and make improved predictions of engineering properties from experimental data.

Identify the influence of microstructure on mesoscale creep and fatigue damage

“…For instance recent experiments performed at the Advanced Photon Source show Identify the influence of microstructure on mesoscale creep and fatigue damage September 2020 that not all the favorably oriented grain boundaries will experience cavitation. This is in contrast with the baseline results of the utilized physically-based micro-mechanical model [4]. The most probable cause of this behavior is the homogeneity of grain boundary properties in the model.…”

“…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. This chapter summarizes key features.…”

“…The current work builds on a deterministic crystal plasticity finite element method (CPFEM) model for creep deformation and rupture in Grade 91 [3,4] developed through past work in the Advanced Reactor Technologies (ART) program. This past work developed the model to make deterministic predictions for long-term creep in Grade 91.…”

Initial development and verification of a primary load design method based on elastic-perfectly plastic analysis

“…Grade 91 steel (c.f. [16], [17]) demonstrates this effective stress measure best fits the multiaxial failure of Grade 91 under creep conditions, including very long rupture time and triaxial stress simulations that cannot be implemented with experimental tests. This work will appear in an upcoming publication.…”

Survey of Modeling and Simulation Techniques for Advanced Manufacturing Technologies Volume II – Predicting Material Performance from Material Microstructure

“…A purely empirical model, for example the classical Larson-Miller correlation [1] might miss a mechanism shift that occurs at relatively the low stresses found in operating components but rarely tested because of the correspondingly long rupture times. By contrast, a microstructural model can capture this mechanism transition and lead to better predictions for long-term properties [2]. A similar concept applies to other material properties where direct test data is sparse.…”