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•Part II demonstrates the validation and effectiveness of the integrated model.•Detailed investigations and discussions are presented based on numerical simulations.•Model prediction ...for void nucleation/growth agreed with the experimental results.•Loading and temperature effects were clarified by comprehensive simulations.•Polycrystalline morphology factors’ effects on creep performance were also clarified.
Part II of the paper presents comprehensive investigations aimed at demonstrating the validation and effectiveness of the proposed integrated model for simulating Coble creep deformation and void nucleation/growth in polycrystalline solids for material design. Quantitative comparisons between the model’s predictions of void area fraction in a 2D cross-section and experimental results using Alloy 201, a commercially pure nickel, demonstrated a high degree of agreement. Furthermore, the numerical simulation results align with theoretical equations, affirming the model’s validity and its capacity to represent complex phenomena accurately. Although traditional laboratory testing for pure Coble creep is challenging due to constraints such as time and equipment limitations, the proposed model offers a distinct advantage, allowing numerical simulation of Coble creep in materials with arbitrary polycrystalline morphologies under various environmental conditions. Leveraging this capability, the study conducted comprehensive numerical simulations to quantitatively explore the effects of environmental and polycrystalline morphology factors on Coble creep deformation and void nucleation/growth in polycrystalline solids. These aspects, which have not been comprehensively addressed in existing studies, were thoroughly investigated. The findings presented here establish a novel foundation for designing heat-resistant materials applicable across diverse equipment scenarios.
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•A model integrating Coble creep deformation and void nucleation/growth is proposed.•Part I provides a comprehensive theoretical framework and numerical modelling.•The model is based ...on the RVE approach simulating a 3D polycrystalline structure.•The deformation stage has two sub-models: grain boundary migration and diffusion.•The void nucleation/growth stage is driven by the results from the deform. stage.
This study proposes an integrated model for simulating Coble creep deformation and void nucleation/growth in a 3D polycrystalline solid. Part I of the paper provides a theoretical framework for the proposed model. A representative volume element approach is employed to predict the effects of 3D polycrystalline morphology. The model comprises two distinct but interconnected stages: deformation and void nucleation/growth. The deformation stage of the proposed model comprises two sub-models: grain boundary (GB) migration and GB diffusion. The void nucleation/growth stage is composed of three consecutive calculations: void nucleation, void growth, and postprocessing. The process simulated in the void nucleation/growth stage is driven by relative GB velocity of the respective GBFs and diffusional fluxes between adjacent GBFs, which are provided from the deformation stage. The void nucleation rate is quantified using the relative GB velocity, and the initial void size is determined based on the stability condition for void existence derived from Helmholtz free energy. The void growth rate is evaluated by the atomic diffusion on the GBF and the surface of each void.
•Three different analysis methodologies are compared for failure analysis of fir-tree joints.•Liu-Murakami creep damage parameters for Inconel 718 at 620 °C are determined for the first time.•FE ...efficient modeling strategy is suggested for predicting stress and creep in blade-disk attachment.•Simple FE models are efficient for damage tolerance analysis of blade-disk attachment.
Due to several physical and computational difficulties in Finite Element (FE) analysis of contact in fir-tree blade-disk attachments of gas turbines, there has always been a great demand for stress, fatigue and creep life assessment of fir-tree joints without modeling physical contact. However, reliability of these models is not trivial. To meet these challenges, in the current paper, three different analysis methodologies were examined for creep life assessment in a turbine engine disk. This is a significant problem for designers and operators of gas turbines. Two simple models were developed with removal of the blade from the FE domain as well as a detailed 3D model containing a blade and it’s contact details. Numerical results were compared with available experimental stress measurement. Creep material behavior models were also developed and parameters for the Liu-Murakami damage model together with the Norton creep law were determined for Inconel 718 at 620 °C. A creep subroutine was written and creep analyses of these joints were performed for typical service temperature of 620 °C and the maximum time period of 100,000 h. FE results were compared with fracture mode of service induced cracks. For the different load cases ranging from 3000 rpm to 9000 rpm, it is demonstrated that methodology based on simple models are very efficient. The equivalent pressure model always underestimates the value of peak stress whereas the virtual link model provided more realistic results. The results are discussed according to the different failure criteria for the fir-tree joints life assessment. Results presented in the paper are valuable for failure analysis of these critical components. Moreover, the stress distribution obtained from the simple realistic model can be used for the life assessment codes applicable to other types of failure mechanism.
Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high-strength or high-manufacturing-cost metals such as Ti-6Al-4V widely applied in aeronautical industry ...components with high material waste or complex geometry. However, one of the main challenges of AM parts is the variability in fatigue properties. In this study, standard cyclic fatigue and monotonic tensile testing specimens were fabricated by SLM and subsequently heat treated using the standard heat treatment (HT) or hot isostatic pressing (HIP) methods. All the specimens were post-treated to relieve the residual stress and subsequently machined to the same surface finishing. These specimens were tested in the low-cycle fatigue (LCF) regime. The effects of post-process methods on the failure mechanisms were observed using scanning electron microscopy (SEM) and optical microscopy (OM) characterization methods. While the tensile test results showed that specimens with different post-process treatment methods have similar tensile strength, the LCF test revealed that no significant difference exists between HT and HIP specimens. Based on the results, critical factors influencing the LCF properties are discussed. Furthermore, a microstructure-based multistage fatigue model was employed to predict the LCF life. The results show good agreement with the experiment.
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•Creep-fatigue-oxidation crack growth behaviour is modelled based on continuum damage mechanics.•Polycrystalline microstructure adjacent the crack tip is simulated using Voronoi ...tessellation.•Inter/transgranular cracking modes are predicted using the multiscale finite element models.•A close agreement in both trend and magnitude between predictions and experimental data is observed.
Material degradation and crack growth affect the residual service life of high temperature components used in power plants and aerospace industry. In this study, we present a predictive multiscale continuum damage model to account for the effects of creep, fatigue and oxidation on the damage evolution and crack growth behaviour of 316H stainless steel at 650 °C. The model is used in finite element predictions of mode I crack growth which explicitly considers the polycrystalline microstructure near the crack tip of a compact-tension specimen loaded in creep and fatigue at high temperature. The predicted crack growth rates are found to be in good quantitative agreement with measurements and with the bounds predicted by the approximate multiaxial ductility model. We discuss the relative importance of creep, fatigue and oxidation on the damage evolution in different loading regimes.
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•The influence of 3D polycrystalline morphology on Coble creep deformation was modelled quantitatively for the first time.•The representative volume element model was incorporated the ...grain growths through grain boundary migration and diffusion.•The proposed model accurately reproduced the effects of grain size, applied stress, and temperature.•Numeric simulations allowed evaluation of the influence of polycrystalline morphology.•This model is suitable for designing new materials with high-temperature creep resistance.
To evaluate creep in materials under actual-use conditions, it is necessary to consider the polycrystalline morphology. Herein, we propose the first model for quantitatively predicting the influence of 3D polycrystalline morphology on Coble creep deformation. This model was developed using a representative volume element (RVE). Two grain growth mechanisms, namely, grain boundary migration and grain boundary diffusion, were formulated considering the 3D data structure of a polycrystal and then superposed to simulate RVE deformation. The model was validated by comparison with the theoretical solution for Coble creep deformation under uniaxial loading conditions. The proposed model accurately reproduced the influences of grain size, applied stress, and temperature, thus demonstrating its validity. In addition, the influence of polycrystalline morphology was investigated by performing systematic numerical simulations. The model directly provided quantitative predictions of the performance of materials with arbitrary polycrystalline morphologies under any loading or temperature condition. Consequently, the proposed RVE model has the potential to serve as a basis for designing novel materials with resistance against high-temperature creep.
Functionally graded materials (FGMs) are high temperature-resistant materials that can simultaneously maintain metallic tenacity and anti-corrosive properties. Nevertheless, using FGMs during a ...multi-year service life at ultrahigh temperatures is crucial. Hence, the time-dependent creep response of variable-thickness rotating disks made of FGM is investigated. Four different disk profiles of linear, concave, convex, and uniform are considered. The material's creep properties are defined by the Bailey-Norton creep law. Loading is a rotation-based mechanical body force and a uniform temperature throughout the disk. Simultaneous solution of equilibrium, stress-strain, and strain-displacement equations yields a non-homogenous differential equation containing variable and time-dependent coefficients. In an attempt to optimize the computation cost, Bat and Fish algorithms were used to optimize the initial strain presumptions. Semi-analytical solution of this differential equation gives radial and circumferential stress histories and displacement histories. To confirm the solution method, initial thermo-elastic radial stress, and the effective stress history are validated with the existing literature; there is a good agreement between the results. In addition, the finite element software ABAQUS was used to model the FGM disk thermo-elastic behavior, and the result was compared with the semi-analytical solution results. This study emphasizes the significance of accounting for creep effects in the design of FGM rotating disks, as remarkable changes in their displacements and stresses occur over time. This study emphasizes the significance of accounting for creep effects in the design of FGM rotating disks, as notable changes in their displacements and stresses occur over time.
This paper proposes a method to simulate creep failure using finite element damage analysis. The creep damage model is based on the creep ductility exhaustion concept, and incremental damage is ...defined by the ratio of incremental creep strain and multi-axial creep ductility. A simple linear damage summation rule is applied and, when accumulated damage becomes unity, element stresses are reduced to zero to simulate progressive crack growth. For validation, simulated results are compared with experimental data for a compact tension specimen of 316H at 550
°C. Effects of the mesh size and scatter in uniaxial ductility are also investigated.
Virtual methods of predicting creep crack growth (CCG), using finite element analysis (FE), are implemented in a compact tension specimen, C(T). The material examined is an austenitic type 316H ...stainless steel at 550
°C, which exhibits power-law creep–ductile behaviour. A local damage-based approach is used to predict crack propagation and the CCG rate data are correlated with the
C
∗ parameter. Two-dimensional elastic–plastic–creep analyses are performed under plane stress and plane strain conditions. Finite element CCG rate predictions are compared to experimental data and to the NSW and modified NSW (NSW–MOD) CCG models’ solutions, which are based on ductility exhaustion arguments. An alternative version of the NSW–MOD model is presented for direct comparison with the FE implementation. The FE predictions are found to be in agreement with the appropriate analytical solutions, and follow the trends of the experimental data at high
C
∗ values. Accelerated cracking behaviour is observed experimentally at low
C
∗ values, which is consistent with the standard plane strain NSW–MOD prediction. The FE model may be developed to predict this accelerated cracking at low
C
∗ values so that the trends between CCG rates at high and low
C
∗ values may be determined.
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