This text is a guide how to solve problems in which viscoelasticity is present using existing commercial computational codes. The book gives information on codes structure and use, data preparation ...and output interpretation and verification. The first part of the book introduces the reader to the subject, and to provide the models, equations and notation to be used in the computational applications. The second part shows the most important Computational techniques: Finite elements formulation, Boundary elements formulation, and presents the solutions of Viscoelastic problems with Abaqus.
•Incorporation of incremental viscoelastic formulation into the generalized FVDAM theory.•Incremental procedure based on the concept of state variables to evaluate the viscoelastic strains.•The use ...of material parameters obtained from a creep test and the possibility to model different unit cell’s geometries.•Evaluation of both homogenized creep and relaxation functions.•The generalized FVDAM theory presents much better results for the local stress fields than its 0th order version.
In the last decades, new generations of advanced materials have been designed and manufactured for specific applications. The micromechanics plays an important role in the development of heterogeneous materials, enabling efficient analyses of composite materials with complex geometries, circumventing the traditional trial-and-error approach, producing substantial cost savings. The unit cell problem to the analysis of periodic heterogeneous media can be solved by the well-established 0th order version of the finite-volume theory, named finite-volume direct averaging micromechanics (FVDAM) theory. This standard version of the FVDAM theory employs an incomplete second-order displacement field within individual subvolumes of a discretized analysis domain together with a surface-averaging framework, which does not enforce displacement or traction continuity in a point-wise manner. This, in turn, produces interfacial interpenetrations and non-traction stress discontinuities, thereby demanding very refined meshes in order to produce good interfacial conformability and pointwise stress continuity between adjacent subvolumes. To overcome these shortcomings, a generalized FVDAM theory has been proposed to enable analysis of periodic heterogeneous materials in the finite-deformation domain. The generalization is based on a higher-order displacement field representation and on the definition of elasticity-based surface-averaged kinematic and static variables related through a local stiffness matrix. Herein, we specialize the generalized FVDAM theory to the infinitesimal analysis of periodic materials with viscoelastic phases, where a total or secant formulation is employed, with the viscoelastic strains evaluated incrementally using an algorithm based on the concept of state variables. The generalized or 2nd order version considerably improves interfacial conformability and pointwise traction and non-traction stress continuity between adjacent subvolumes in comparison with the 0th order version, but with a higher computational cost. Furthermore, for the same mesh discretization, these two versions provide comparable macroscopic response. Considering these features, the 0th order version is recommended to evaluate the effective elastic properties and the homogenized creep and relaxation functions, while the 2nd order version is more efficient in the evaluation of the microscopic displacement and stress fields.
Computational aspects of the recently developed parametric finite-volume theory for functionally graded materials are critically examined through comparison with the finite-element method for two ...problems which highlight the finite-volume theory’s structural and microstructural modeling capabilities. The parametric version of the theory is based on a mapping of a reference square subvolume onto a quadrilateral subvolume in the actual discretized microstructure, which is also used to construct local stiffness matrices of quadrilateral subvolumes employed in the analysis. This formulation significantly advances the capability and utility of the finite-volume theory, enabling modeling of curved boundaries of functionally graded structural components, as well as inclusions employed for grading purposes, without the disadvantage of stress concentrations at the corners of rectangular subvolumes used in the standard version. Convergence of the results for local stress fields with mesh refinement to the exact analytical results and the corresponding execution times demonstrate that the parametric finite-volume theory is a very competitive alternative to the finite-element method for the considered class of problems.
This contribution is motivated by the recent IJSS article titled “Formulation of the High-Fidelity Generalized Method of Cells with Arbitrary Cell Geometry for Refined Micromechanics and Damage in ...Composites” by Haj-Ali and Aboudi. In the present discussion, we critically compare these authors’ approach with the Parametric Finite-Volume Direct Averaging Micromechanics (FVDAM) theory developed in a sequence of papers dating to 2004. We show that the extension proposed by Haj-Ali and Aboudi follows the framework of the parametric FVDAM theory, and not the original HFGMC model as claimed. Just as importantly, we demonstrate that the proposed extension is fundamentally flawed as it violates accepted principles which every mechanics theory must satisfy, namely correct reducibility and coordinate frame indifference. Direct comparison of predictions generated by the parametric FVDAM theory, the proposed extension called HFGMC with arbitrary cell geometry and finite-element method demonstrates no need for the extension carried out in the specific manner proposed by Haj-Ali and Aboudi, which in fact produces inferior results.
This article presents a new numerical model for the analysis of structures of heterogeneous materials with linear viscoelastic constituents. The model is based on the recently developed parametric ...finite-volume theory that has produced a paradigm shift in the finite-volume theory's development. This parametric formulation is here extended to model linear viscoelastic behavior. The present model employs the state variables approach for the computation of the time-dependent strains. Several examples, including both homogeneous and heterogeneous situations, are analyzed. Comparison between the numerical and analytical results shows the excellent performance of the proposed model.
A major failure mechanism in thermal barrier coatings involves delamination of the ceramic top coat from the substrate, which is accompanied and/or aided by transverse cracks that initiate at the ...outer surface. Techniques to mitigate these failure modes include grading the transition region between the ceramic top coat and the metallic bond coat by gradually varying the content of the two phases. This concept is explored herein for a graded cylinder subjected to transient thermal cyclic loading, that simulates a thermal shock durability test, using the parametric finite-volume theory for functionally graded materials. Previous investigation into the transient response of a three-layer cylinder under the considered thermal shock loading revealed two potential failure modes, one of which was a direct result of transient effects. Herein, examination of average phase-level stress fields indicates that grading influences the initiation of delamination and potential radial cracking through redistribution of the radial and hoop stress components in the thermal coating's graded region upon rapid heating. The large hoop stress reversal responsible for radial crack initiation at the outer surface, however, is not reduced upon rapid cooling, pointing to the importance of accurately modeling transient effects in thermal shock testing as well as crack-growth management through grading.
The recently developed parametric finite-volume theory for functionally graded materials is employed to investigate the response of a layered cylinder under transient thermal loading that simulates a ...cyclic thermal shock durability test. The results reveal a potential for the occurance of two distinct failure modes that may be activated due to two different stress components reaching critical values during different portions of the thermal cycle at different locations. These are delamination of the ceramic top coat from the bond coat, and radial cracking of the top coat that potentially initiates at the outer surface subjected to concentrated transient thermal load. Steady-state analysis substantially underestimates the magnitude of the radial and hoop stresses and, moreover, does not predict the stress reversals during cooldown that likely initiate radial cracks at the outer surface. The fidelity with which local stress fields are captured provides a convincing evidence that the parametric finite-volume theory is an attractive alternative to the finite-element analysis for this class of problems.
Leishmaniasis is a neglected parasitic disease for which the conventional treatment can be considered inefficient and extremely aggressive, generating several and severe side effects. Therefore, the ...discovery of new drug candidates is important for the improvement in the quality of life of patients. Previously, we reported the promising results of isopentyl caffeate (ICaf) against Leishmania chagasi (agent of visceral leishmaniasis) and Leishmania amazonensis (agent of cutaneous leishmaniasis) promastigotes, displaying IC50 of 1.56 and 1.71 μM, respectively. Herein, we aimed to decipher the mechanisms of anti-Leishmania action of ICaf. Light and scanning electron microscopy assays showed relevant morphological changes in promastigotes when treated with ICaf, including rounding of the parasite body, shortening of the flagellum, blebs on the plasma membrane and cellular aggregation. The parasite mitochondrion was targeted by ICaf, resulting in a significant reduction in its metabolic activity and electric membrane potential followed by an increase in the production of reactive oxygen species, which culminated in the loss of plasma membrane integrity and parasite death. Relevantly, ICaf also had a potent anti-amastigote action. The IC50 values calculated for intracellular amastigotes of L. amazonensis were 3.27, 1.60 and 1.52 μM, while for L. chagasi the values were 2.48, 1.84 and 1.60 μM, respectively, after treating the infected macrophages with ICaf for 24, 48 and 72 h. ICaf was well tolerated by THP-1 macrophages, which gave rise to excellent selectivity indexes considering both Leishmania species. The current results suggest that ICaf may emerge as a chemotherapeutic alternative for the treatment of leishmaniasis.
•Treatment of leishmania with conventional drugs is regarded as inefficient and extremely aggressive.•Isopentyl caffeate was well tolerated by THP-1 macrophages with high selectivity.•Isopentyl caffeate may emerge as a chemotherapeutic treatment alternative.
Herein, we aimed at identifying global transcriptome microRNA (miRNA) changes and miRNA target genes in lung adenocarcinoma. Samples were selected as training (N = 24) and independent validation (N = ...34) sets. Tissues were microdissected to obtain >90% tumor or normal lung cells, subjected to miRNA transcriptome sequencing and TaqMan quantitative PCR validation. We further integrated our data with published miRNA and mRNA expression datasets across 1,491 lung adenocarcinoma and 455 normal lung samples. We identified known and novel, significantly over- and under-expressed (p ≤ 0.01 and FDR≤0.1) miRNAs in lung adenocarcinoma compared to normal lung tissue: let-7a, miR-10a, miR-15b, miR-23b, miR-26a, miR-26b, miR-29a, miR-30e, miR-99a, miR-146b, miR-181b, miR-181c, miR-421, miR-181a, miR-574 and miR-1247. Validated miRNAs included let-7a-2, let-7a-3, miR-15b, miR-21, miR-155 and miR-200b; higher levels of miR-21 expression were associated with lower patient survival (p = 0.042). We identified a regulatory network including miR-15b and miR-155, and transcription factors with prognostic value in lung cancer. Our findings may contribute to the development of treatment strategies in lung adenocarcinoma.
Isopentyl caffeate (ICaf) is a bioactive ester widely distributed in nature. Our patented work has shown promising results of this molecule against Leishmania. However, ICaf shows poor solubility, ...which limits its usage in clinical settings. In this work, we have proposed the development of an inclusion complex of ICaf in β-cyclodextrin (β-CD), with the aim to improve the drug solubility, and thus, its bioavailability. The inclusion complex (ICaf:β-CD) was developed applying three distinct methods, i.e., physical mixture (PM), kneading (KN) or co-evaporation (CO) in different molar proportions (0.25:1, 1:1 and 2:1). Characterization of the complexes was carried out by thermal analysis, Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and molecular docking. The ICaf:β-CD complex in a molar ratio of 1:1 obtained by CO showed the best complexation and, therefore, was selected for further analysis. Solubility assay showed a marked improvement in the ICaf:β-CD (CO, 1:1) solubility profile when compared to the pure ICaf compound. Cell proliferation assay using ICaf:β-CD complex showed an IC
of 3.8 and 2.7 µg/mL against
and
promastigotes, respectively. These results demonstrate the great potential of the inclusion complex to improve the treatment options for visceral and cutaneous leishmaniases.
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