•A strain based damage model is proposed to study the strain life behavior.•Effect of strain ratio on the strain life curve is captured adequately.•CDM-XFEM framework is developed to perform the LCF ...crack growth simulations.•Concept of two definitions of stress triaxiality is employed to mimic the experimental observations.•Developed framework predicts the remaining fatigue life easily and efficiently.
In this paper, a strain-based damage model is developed to account the effect of different strain ratios on the fatigue failure. Thereafter, continuum damage mechanics and extended finite element method based framework is proposed to simulate the low cycle fatigue crack growth simulations. The proposed damage model is imposed in the developed framework to evaluate the fatigue life of cracked specimen. Chaboche mixed hardening model is implemented to determine the stress–strain accurately in the cracked domain. The acquired stress-strains are incorporated in the proposed damage model to estimate the remaining fatigue life. The computed strain life curves at different strain ratios as well as crack growth curves are compared with the experimental results available in literature. The simulated results are found in good agreement with the experimental observations. This work ensures that the continuum damage mechanics based damage law is an excellent and readily applicable method to acquire the effect of strain ratios on the strain life. This work also provides comprehensive and significant modeling insights for computing the low cycle fatigue crack growth efficiently.
Liu–Murakami creep damage model is improved to predict the creep life of various cracked specimens. The modified creep damage law is implemented in the framework of extended finite element method ...(XFEM) for performing elasto-plastic creep crack growth simulations. Experiments show that the crack tip constraints vary from component to component which leads to variation in crack growth rates. A stress triaxiality function is introduced in the modified Liu–Murakami damage model to address the variation in crack growth rates. Moreover, a new definition of stress triaxiality (ratio of a linear combination of maximum principal stress and hydrostatic stress to von Mises stress) is proposed based on Leckie and Hayhurst failure criterion. The new definition of stress triaxiality is a key parameter in the prediction of time to failure. The modified Liu–Murakami creep damage model is used for the creep crack growth (CCG) simulations of several specimens under different loading conditions. Parametric studies are also performed to study the influence of various parameters on the CCG. Moreover, a combined framework of continuum damage mechanics and XFEM is used to predict the CCG life of a turbine blade. This work establishes that the modified Liu–Murakami creep damage law accurately predicts the creep life of cracked components under different constraint conditions.
•A continuum damage mechanics (CDM) and XFEM based methodology is developed for high cycle fatigue crack growth simulations.•A new damage model is proposed for the evaluation of fatigue crack growth ...life.•A new criterion is proposed based on the damage evolution to identify the appropriate definition of stress triaxiality.•A non-local CDM approach is implemented to reduce the mesh sensitivity.•The present methodology is found quite successful for fatigue crack growth simulations.
In this paper, we have developed a continuum damage mechanics (CDM) based methodology for high cycle fatigue crack growth simulations. A fatigue damage law is proposed and implemented in the framework of extended finite element method (XFEM). A new criterion is proposed based on damage evolution to identify the appropriate definition of stress triaxiality for acquiring the constraint effect on the stress state correctly. Few mesh regularization schemes are also employed for reducing the mesh sensitivity in the results. Simulations are performed on fracture specimens of different materials subjected to constant amplitude fatigue loading. The fatigue life of a turbine disc is also predicted under constant amplitude loading. The results obtained from present methodology (CDM and XFEM) are found in good agreement with the published experimental results. These simulations highlight that the continuum damage mechanics is a simple and effective tool to perform crack growth simulations under high cycle fatigue conditions.
•Fatigue crack growth experiments conducted on different geometries show that the constraints affect the FCG significantly.•A CDM and XFEM based improved methodology is proposed to capture the effect ...of crack tip constraint.•A triaxiality parameter modifies triaxiality definition to capture the constraint effects.•A damage based criterion is developed to find the correct state of stress.•Proposed CDM-XFEM methodology efficiently predicts the fatigue life of cracked components.
An improved continuum damage mechanics (CDM) and extended finite element method (XFEM) methodology is proposed to consider the effect of crack tip constraints on the fatigue crack growth (FCG) behavior. Several FCG experiments are conducted on compact tension (CT), single edge notch bending (SENB) and middle tension (MT) specimens prepared from modified 9Cr-1Mo steel. Combined CDM-XFEM methodology is improved to capture the crack tip constraints effect. Experimental and simulated FCG lives are compared to each other and found in good agreement. This analysis shows that the triaxiality parameter is a key parameter for estimating the crack tip constraints correctly.
In the present work, elasto-plastic creep crack growth simulations are performed using continuum damage mechanics and extended finite element method. Liu–Murakami creep damage model and explicit time ...integration scheme are used to evaluate the creep strain and damage variable for various materials at different temperatures. Compact tension and C-shaped tension specimens are selected for the simulation of crack growth analysis. For damage evaluation, both local and nonlocal approaches are employed. The accuracy of the extended finite element method solutions is checked by comparing with experimental results and finite element solutions. These results show that the extended finite element method requires a much coarser mesh to effectively model crack propagation. It is also shown that mesh independent results can be achieved by using nonlocal implementation.
In patients with acute kidney injury (AKI), serum creatinine level does not increase until moderate to severe reduction in glomerular filtration rate (GFR) occurs. Thus its use for estimating GFR in ...early AKI delays detection of kidney damage and making important therapeutic decisions. Moreover, serum cystatin C is not affected by gender, age, race, and muscle mass and also does not suffer from lag period for its rise in early AKI. We studied 200 healthy subjects and 130 AKI patients over a period of 2 years at a tertiary care hospital. Serum creatinine and serum cystatin C were studied and analyzed in relevance to early AKI. We found that 56.2% of patients of AKI group had normal levels of serum creatinine in early phase, while all patients had elevated serum cystatin C at same time. Multiple logistic regression analysis revealed cystatin C-based GFR reflecting decline in GFR with worsening AKI in better than creatinine-based GFR. Serum cystatin C is a better marker of renal function in early stages of AKI and is less affected by age, gender, muscle mass, and ethnicity. Its use helps in early therapeutic intervention and possibly favorable outcome.
DNA methyltransferase 3A (DNMT3A) mutations are observed in myeloid malignancies, including myeloproliferative neoplasms (MPN), myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). ...Transplantation studies have elucidated an important role for Dnmt3a in stem cell self-renewal and in myeloid differentiation. Here, we investigated the impact of conditional hematopoietic Dnmt3a loss on disease phenotype in primary mice. Mx1-Cre-mediated Dnmt3a ablation led to the development of a lethal, fully penetrant MPN with myelodysplasia (MDS/MPN) characterized by peripheral cytopenias and by marked, progressive hepatomegaly. We detected expanded stem/progenitor populations in the liver of Dnmt3a-ablated mice. The MDS/MPN induced by Dnmt3a ablation was transplantable, including the marked hepatomegaly. Homing studies showed that Dnmt3a-deleted bone marrow cells preferentially migrated to the liver. Gene expression and DNA methylation analyses of progenitor cell populations identified differential regulation of hematopoietic regulatory pathways, including fetal liver hematopoiesis transcriptional programs. These data demonstrate that Dnmt3a ablation in the hematopoietic system leads to myeloid transformation in vivo, with cell-autonomous aberrant tissue tropism and marked extramedullary hematopoiesis (EMH) with liver involvement. Hence, in addition to the established role of Dnmt3a in regulating self-renewal, Dnmt3a regulates tissue tropism and limits myeloid progenitor expansion in vivo.
In the present study, influence of conventional normalizing (1050 °C) and double austenitization treatment (950–1050 °C) followed by tempering (750 °C) have been investigated to study the fatigue ...threshold, tensile strength and high cycle fatigue behavior of modified 9Cr–1Mo steel at room temperature (24 °C). By designing a double austenitization treatment, a homogeneous tempered martensitic microstructure with improved area fraction of precipitates and small prior austenite grain size can be obtained. As a result, improvement in fatigue threshold and tensile strength is achieved. However, a small decrease in fatigue strength is observed. Fatigue strength is affected due to presence of coarse particles precipitated along the grain boundaries. Fine particles precipitated along the grain boundaries create more barriers for the dislocation motion. As a result, more cycles are required for crack initiation. A damage model based on continuum damage mechanics is coupled with extended finite element method to obtain the fatigue life. The fatigue life predicted using numerical simulation found in good agreement with the experimental results. This study shows the effectiveness of numerical simulations to perform fatigue crack growth analysis using continuum damage mechanics and extended finite element method. Fractographs of tensile sample shows the presence of cracks in radial direction and axial splitting. However, such unique behavior is not obtained in high cycle fatigue samples.
We present a fast high-order scheme for the numerical solution of a volume-surface integro-differential equation. Such equations arise in problems of scattering of time-harmonic acoustic and ...electromagnetic waves by inhomogeneous media with variable density wherein the material properties jump across the medium interface. The method uses a partition of unity to segregate the interior and the boundary regions of the scattering obstacle, enabling us to make use of specially designed quadratures to deal with the material discontinuities in a high-order manner. In particular, the method uses suitable changes of variables to resolve the singularities present in the integrals in conjunction with a decomposition of Green’s function via the addition theorem. To achieve a reduced computational cost, the method employs a Fast Fourier Transform (FFT) based acceleration strategy to compute the integrals over the boundary region. Moreover, the necessary offgrid evaluation of the density and the inter-grid transfer of data is achieved by applying an FFT-based refined-grid interpolation strategy. We validate the performance of the method through multiple scattering simulations. In particular, the numerical experiments demonstrate that the proposed method can handle high-contrast material properties without any adverse effect on the number of GMRES iterations.
The magnetohydrodynamic (MHD) Stokes equations have several applications in the field of biofluid dynamics. In the present study, we propose the staggered finite volume method (S-FVM) for MHD Stokes ...equations and establish its equivalence to a nonconforming finite element approximation. We also theoretically establish the convergence of the proposed S-FVM. The error estimation is carried out in an unstructured grid framework which is known for its flexibility and robustness in dealing with complex domains. The apriori estimate shows that the
-norm of the error for the pressure and velocity components is of order
, the spacial grid size. After validating the numerical performance of the scheme against benchmark test cases, we do numerical simulations for the blood flow through an injured arteriole and analyze the influence of the magnetic force on hemodynamics in the arteriole under an injured condition.
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