Effect of fiber distributions on the mechanical performance of CMC materials: Virtual manufacturing and testing approach

Ceramic matrix composites (CMCs) exhibit superior thermal stability in an elevated temperature environment and thus are considered as a promising candidate material for gas turbine applications in the field of power generation industry. CMCs also have a higher fracture resistance than conventional technical ceramics owing to the coated ceramic fibers embedded in the matrix. However, the complex heterogeneous microstructure results in complicated damage and failure behavior at the fiber length scale, which appears non-linear stress-strain response at the macro length scale. When a crack is initiated in the matrix phase, the crack grows very rapidly since the ceramic matrix is a brittle material. However, the rapid crack propagation is restrained when the crack tip reaches the ductile coating interface. This fracture process occurring inside the CMC material results in a highly complicated post-peak response and makes its fracture toughness comparable to that of metals. The post-peak response is greatly influenced by local topology of fibers situated inside the composite material and thus the fracture toughness of a CMC may vary locally due to the irregular distribution of fibers. In the present study, the effect of fiber distributions on the post-peak response and the corresponding mechanical performance of a CMC material is closely investigated by utilizing representative volume elements (RVEs) with various fiber distributions. Two-dimensional square RVE enclosing randomly distributed circular fibers with coating layers is considered to represent the microstructure of a long-fiber-reinforced CMC material. Random, yet realistic distribution of fibers is achieved through the virtual force dispersion (VFD) algorithm. The present VFD algorithm arranges fibers with coating layers after the fibers are randomly seeded into a square RVE. Fibers should be rearranged after the random seed, since overlapped regions between fibers are unavoidable during the initial distribution process. The VFD algorithm assumes that fibers are connected through virtual springs, which provide repulsive forces between them. The VFD algorithm finds an equilibrium state in which the springs are completely relaxed and there exists no repulsive force in the system. In this manner, various RVEs with different fiber distributions are easily created for the next step of analysis. Please click Additional Files below to see the full abstract

Process Simulation and Mechanical Analysis of High Temperature Resistance Composite Materials

Department of Mechanical EngineeringHigh-temperature resistance composite materials are rapidly replacing conventional metal alloys used for thermal protection systems (TPS) of spacecraft and missiles exiting and/or reentering the atmosphere. Recently, South Korea succeeded in launching Nuri, the first domestically developed space rocket. Ongoing space programs of S. Korea include the commercialization of the Nuri technologies as well as the development of the next-generation space launch vehicles and the spaceships for exploring the moon and deep space. The Korea government is also actively developing a long-range reentry missile after the ballistic-missile range limits was abolished. These rockets and missiles are subjected to extremely high temperature and pressure when they pass through the atmosphere and thus typically designed with TPS to protect internal devices and human pilots. High-temperature resistant yet lightweight TPS materials are preferred in order to reduce a gross launch payload. Ceramic or carbon-based composite materials are much lighter than metals but also excellent thermal insulators with exceptional dimensional stability at elevated temperature. The ceramic and carbon-based composite materials are often denoted as ceramic matrix composites (CMCs) and carbon-carbon (CC) composites, respectively. CMCs consist of ceramic fibers embedded in a ceramic matrix. The carbon matrix of CC composites is reinforced with carbon fibers. The most typical manufacturing methods of the CMCs and CC composites is a chemical vapor infiltration (CVI) process. In the CVI process, a porous fibrous preform is commonly used as the initial skeleton of a composite. The preform is placed in a CVI reactor, and the reactor is then pressurized and heated before a precursor gas is supplied. When this gas chemically reacts at the pore surfaces inside the preform, a pyrolytic carbon or ceramic layer is deposited onto the preform surfaces. The deposition process slowly changes the precursor into the matrix, filling the empty spaces of the preform. Although the CVI process is a seemingly only viable method to produce a large-scale product, porosity in the final product is not completely avoidable because infiltrated fibers may barricade the path of the precursor gas into internal voids. The porosity is considered defects and the sources of the degradation of mechanical properties. In the present PhD study, comprehensive numerical analysis has been performed from the CVI process simulation to the micro- and meso-scale mechanical analysis of the composite materials. Firstly, the mechanisms of porosity formation are examined by developing a physico-chemical model. The effects of the porosity on the mechanical performances are investigated using a microscale and mesoscale composite models. In the very first part, a fully three-dimensional (3D) physicochemical CVI model is developed to simulate an isothermal CVI process for fabricating bulk carbon-carbon composites using methane as a precursor gas and a multi-layered preform consisting of a non-crimp fabric and felt. The flow inside the CVI reactor was modeled using the Navier-Stokes equation, coupled with the convection-diffusion equation, to simulate the dispersive behaviors of the reactive gases inside the porous preform. The interactive molecular diffusion of methane (CH4), ethylene (C2H4), acetylene (C2H2), and benzene (C6H6) were modeled by considering the multi-step hydrocarbon reactions between the species. The hydrocarbon concentration changes, resulting from the carbon deposition on the preform surface, were computed to predict the evolution of the preform density and porosity. The current surface area of the preform was then determined based on the current porosity. The numerical results for the average preform density agreed well with the experimental data. In addition, the present model can provide detailed simulations of the temporal and spatial evolution of the preform density that cannot be experimentally observed. The effectiveness and utility of the developed model could benefit the design of CVI reactors and processes and minimize the need for test runs when processing conditions change. In the second part, the results obtained in the micromechanical analysis were passed into a meso-scale thick 3D woven textile composite (T3DWC) model, which was a candidate of TPS for a reentry missile. Finite element analysis is performed to virtually measure homogenized thermal and mechanical properties. For the measurements over a wide range of temperature, temperature-dependent thermal and mechanical properties of constituents are considered. A two-step homogenization approach is adopted here. The first-step homogenization is carried out at a tow level using an analytical homogenization scheme as well as the micromechanical analysis in the second part. Fiber tows are homogenized and assigned with effective elastic and thermal properties. The solid tows are then implemented into a representative volume element considering the unique in-plane periodic fiber architecture of the thick composite material. Due to the unique in-plane periodicity, conventional periodic boundary conditions for thermal and mechanical loading conditions are reformulated. Anisotropic thermal conductivity of T3DWC is obtained from the second-step homogenization based on virtual thermal tests performed at ambient to elevated temperatures. In the third part, the micromechanical behavior of the CVI-produced porous composites materials is studied. Especially, microstructural fracture behavior of a ceramic matrix composite (CMC) with nonuniformly distributed fibers is examined. A comprehensive numerical analysis package to study the effect of nonuniform fiber dimensions and locations on the microstructural fracture behavior is developed. The package starts with an optimization algorithm for generating representative volume element (RVE) models that are statistically equivalent to experimental measurements. Experimentally measured statistical data are used as constraints while the optimization algorithm is running. Virtual springs are utilized between any adjacent fibers to nonuniformly distribute the coated fibers in the RVE model. The virtual spring with the optimization algorithm can efficiently generate multiple RVEs that are statistically identical to each other. Smeared crack approach (SCA) is implemented to consider the fracture behavior of the CMC material in a mesh-objective manner. The RVEs are subjected to tension as well as the shear loading conditions. SCA is capable of predicting different fracture patterns, uniquely defined by not only the fiber arrangement but also the specific loading type. In addition, global stress-strain curves show that the microstructural fracture behavior of the RVEs is highly dependent on the fiber distributions.ope

Endoplasmic Reticulum Stress and Insulin Biosynthesis: A Review

Insulin resistance and pancreatic beta cell dysfunction are major contributors to the pathogenesis of diabetes. Various conditions play a role in the pathogenesis of pancreatic beta cell dysfunction and are correlated with endoplasmic reticulum (ER) stress. Pancreatic beta cells are susceptible to ER stress. Many studies have shown that increased ER stress induces pancreatic beta cell dysfunction and diabetes mellitus using genetic models of ER stress and by various stimuli. There are many reports indicating that ER stress plays an important role in the impairment of insulin biosynthesis, suggesting that reduction of ER stress could be a therapeutic target for diabetes. In this paper, we reviewed the relationship between ER stress and diabetes and how ER stress controls insulin biosynthesis

The Evaluation of CP-001 (a Standardized Herbal Mixture of Houttuynia cordata

In the present study, the effect of CP-001, a standardized herbal mixture of Houttuynia cordata, Rehmannia glutinosa, Betula platyphylla, and Rubus coreanus, on cytochrome P450 (CYP) enzyme-mediated drug metabolism was investigated in vitro to evaluate the potential for herb-drug interactions. CP-001 was tested at concentrations of 1, 3, 10, 30, and 100 μg/mL. A CYP-specific substrate mixture was incubated with CP-001 in human liver microsomes, and the metabolites generated by each CYP-specific metabolic reaction were measured by liquid chromatography-tandem mass spectrometry. CP-001 seemed to slightly inhibit some CYP isozymes, but the IC50 values for all CYP isozymes were greater than 100 μg/mL. Furthermore, CP-001 did not exhibit time-dependent CYP inhibitory activities, indicating that it does not act as a mechanism-based inactivator of CYP enzymes. In conclusion, the effects of CP-001 on CYP isozyme activities were negligible at the concentrations tested. Therefore, the likelihood of herbal mixture-drug interaction is considered minimal

Effects of Nonuniform Fiber Geometries on the Microstructural Fracture Behavior of Ceramic Matrix Composites

Microstructural fracture behavior of a ceramic matrix composite (CMC) with nonuniformly distributed fibers is studied in the presentation. A comprehensive numerical analysis package to study the effect of nonuniform fiber dimensions and locations on the microstructural fracture behavior is developed. The package starts with an optimization algorithm for generating representative volume element (RVE) models that are statistically equivalent to experimental measurements. Experimentally measured statistical data are used as constraints while the optimization algorithm is running. Virtual springs are utilized between any adjacent fibers to nonuniformly distribute the coated fibers in the RVE model. The virtual spring with the optimization algorithm can efficiently generate multiple RVEs that are statistically identical to each other. Smeared crack approach (SCA) is implemented to consider the fracture behavior of the CMC material in a mesh-objective manner. The RVEs are subjected to tension as well as the shear loading conditions. SCA is capable of predicting different fracture patterns, uniquely defined by not only the fiber arrangement but also the specific loading type. In addition, global stress-strain curves show that the microstructural fracture behavior of the RVEs is highly dependent on the fiber distributions

Effects of pre-existing hydrogen to stress triaxiality and damage evolution on ultra high strength steel

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Reduced systemic vascular resistance is the underlying hemodynamic mechanism in nitrate-stimulated vasovagal syncope during head-up tilt-table test

AbstractBackgroundNitroglycerin (NTG) challenge during head-up tilt-table testing (HUTT) is often utilized to determine the etiology of unexplained vascular syncope. However, conflicting results concerning nitrate-induced hemodynamic changes during HUTT have been reported. The purpose of this study was to assess the determinants of presyncopal symptoms during NTG-stimulated HUTT.MethodsWe evaluated 40 patients with suspected vasovagal syncope. Beat-to-beat changes in blood pressure, heart rate (HR), cardiac index (CI), and systemic vascular resistance (SVR) during HUTT were measured with thoracic impedance cardiography and a plethysmographic finger arterial pressure monitoring device.ResultsNone of the 40 patients complained of presyncopal symptoms during passive HUTT. However, after the administration of NTG 28 patients showed presyncopal symptoms (NTG+ group) and the remaining 12 patients did not (NTG– group). HR, CI, and the stroke index did not significantly differ between the two groups, whereas mean arterial pressure and SVR were significantly lower in the NTG+ group.ConclusionsPresyncopal symptoms during NTG-stimulated HUTT are SVR mediated, not cardiac output mediated. This study challenges the conventional idea of a decrease in cardiac output mediated by NTG as the overriding cause of presyncopal symptoms during HUTT

Trichosanthes kirilowii

Trichosanthes kirilowii tuber is a traditional medicine which exhibits various medicinal effects including antidiabetic and anticancer activities in several cancer cells. Recently, it was reported that Cucurbitacin D (CuD) isolated from Trichosanthes kirilowii also induces apoptosis in several cancer cells. Constitutive signal transducer and activator of transcription 3 (STAT3), which is an oncogenic transcription factor, is often observed in many human malignant tumor, including breast cancer. In the present study, we tested whether Trichosanthes kirilowii ethanol extract (TKE) or CuD suppresses cell growth and induces apoptosis through inhibition of STAT3 activity in breast cancer cells. We found that both TKE and CuD suppressed proliferation and induced apoptosis and G2/M cell cycle arrest in MDA-MB-231 breast cancer cells by inhibiting STAT3 phosphorylation. In addition, both TKE and CuD inhibited nuclear translocation and transcriptional activity of STAT3. Taken together, our results indicate that TKE and its derived compound, CuD, could be potent therapeutic agents for breast cancer, blocking tumor cell proliferation and inducing apoptosis through suppression of STAT3 activity