Oxidation of Carbon Fibers in a Cracked Ceramic Matrix Composite Modeled as a Function of Temperature

The oxidation model simulates the oxidation of the reinforcing carbon fibers within a ceramic matrix composite material containing as-fabricated microcracks. The physics-based oxidation model uses theoretically and experimentally determined variables as input for the model. The model simulates the ingress of oxygen through microcracks into a two-dimensional plane within the composite material. Model input includes temperature, oxygen concentration, the reaction rate constant, the diffusion coefficient, and the crack opening width as a function of the mechanical and thermal loads. The model is run in an iterative process for a two-dimensional grid system in which oxygen diffuses through the porous and cracked regions of the material and reacts with carbon in short time steps. The model allows the local oxygen concentrations and carbon volumes from the edge to the interior of the composite to be determined over time. Oxidation damage predicted by the model was compared with that observed from microstructural analysis of experimentally tested composite material to validate the model for two temperatures of interest. When the model is run for low-temperature conditions, the kinetics are reaction controlled. Carbon and oxygen reactions occur relatively slowly. Therefore, oxygen can bypass the carbon near the outer edge and diffuse into the interior so that it saturates the entire composite at relatively high concentrations. The kinetics are limited by the reaction rate between carbon and oxygen. This results in an interior that has high local concentrations of oxygen and a similar amount of consumed carbon throughout the cross section. When the model is run for high-temperature conditions, the kinetics are diffusion controlled. Carbon and oxygen reactions occur very quickly. The carbon consumes oxygen as soon as it is supplied. The kinetics are limited by the relatively slow rate at which oxygen is supplied in comparison to the relatively fast rate at which carbon and oxygen reactions occur. This results in a sharp gradient in oxygen concentration from the edge where it is supplied to the nearest source of carbon, which is where the oxygen is quickly consumed. A moving reaction front is seen in which the outlaying carbon is consumed before the next inner layer of carbon begins to react

Oxygen Diffusion and Reaction Kinetics in Continuous Fiber Ceramic Matrix Composites

Previous stressed oxidation tests of C/SiC composites at elevated temperatures (350 C to 1500 C) and sustained stresses (69 MPa and 172 MPa) have led to the development of a finite difference cracked matrix model. The times to failure in the samples suggest oxidation occurred in two kinetic regimes defined by the rate controlling mechanisms (i.e. diffusion controlled and reaction controlled kinetics). Microstructural analysis revealed preferential oxidation along as-fabricated, matrix microcracks and also suggested two regimes of oxidation kinetics dependent on the oxidation temperature. Based on experimental results, observation, and theory, a finite difference model was developed. The model simulates the diffusion of oxygen into a matrix crack bridged by carbon fibers. The model facilitates the study of the relative importance of temperature, the reaction rate constant, and the diffusion coefficient on the overall oxidation kinetics

Stressed Oxidation of C/SiC Composites

Constant load, stressed oxidation testing was performed on T-300 C/SiC composites with a SiC seal coat. Test conditions included temperatures ranging from 350 C to 1500 C at stresses of 69 MPa and 172 MPa (10 and 25 ksi). The coupon subjected to stressed oxidation at 550 C/69 MPa for 25 hours had a room temperature residual strength one-half that of the as-received coupons. The coupon tested at the higher stress and all coupons tested at higher temperatures failed in less than 25 hr. Microstructural analysis of the fracture surfaces, using SEM (scanning electron microscopy), revealed the formation of reduced cross-sectional fibers with pointed tips. Analysis of composite cross-sections show pathways for oxygen ingress. The discussion will focus on fiber/matrix interphase oxidation and debonding as well as the formation and implications of the fiber tip morphology

An Estimation of Periconceptional Under-reporting of Dietary Energy Intake

Background The purpose of this cross-sectional study was to examine periconceptional misreporting of energy intake (EI) using the Willet food frequency questionnaire (WFFQ). Methods Women were recruited in the first trimester. Women completed a semi-quantitative WFFQ. Maternal body composition was measured using eight-electrode bioelectrical impedance analysis. Under-reporters were those whose ratio of EI to their calculated basal metabolic rate fell below the calculated plausible threshold for their physical activity category. Results The mean age was 30.1+5.3 years (n ¼ 524). The mean body mass index (BMI) was 25.4+5.6 kg/m2 , and 16.6% were obese (BMI 30.0 kg/m2 ). Under-reported EI was observed in 122 women (23.3%) with no over-reporters in the sample. Under-reporters were younger (P , 0.001), less likely to have a normal BMI (P ¼ 0.002) and more likely to be obese (P , 0.001) than plausible reporters. Under-reporters had higher percentage of body-fat and lower percentage of body fat-free mass (P , 0.001), were more likely to be at risk of relative deprivation (P ¼ 0.001) and reported a higher percentage of EI from carbohydrate (P ¼ 0.02) than plausible reporters. Conclusions Observed differences between under-reporters and plausible reporters suggest that the exclusion of these under-reporters represents an important potential source of bias in obesity research among women in the periconceptional period

Maternal Nutrient Intakes From Food and Drinks Consumed in Early Pregnancy in Ireland

Background: The aim of this observational study was to measure food, macronutrient and micronutrient intakes of women presenting for antenatal care and assess compliance with current nutritional recommendations

Relationship between fasting plasma glucose levels and maternal food group and macronutrient intakes in pregnancy

Aim Increased maternal body mass index (BMI) has been consistently associated with elevated blood glucose levels during pregnancy. Studies to date investigating the relationship between maternal blood glucose levels and dietary intake have shown mixed results. We investigated the association between maternal fasting plasma glucose (FPG) levels and food group and macronutrient intakes in the first trimester of pregnancy, after adjustment for maternal bodyweight. Methods Women were recruited after sonographic confirmation of an ongoing singleton pregnancy in the first trimester. Dietary information was collected using the validated Willett Food Frequency Questionnaire. Maternal height and weight were measured and BMI calculated. Body composition was measured using advanced bioelectrical impedance analysis. FPG levels were obtained for women who were selectively screened with a 75 g oral glucose tolerance test. Results No associations were observed between maternal FPG levels and food group or macronutrient intakes but higher energy and starch intakes were found in obese subjects (P = 0.009 and P = 0.03 respectively). On univariate analysis, higher FPG levels were associated positively with higher maternal bodyweight, BMI, body fat, fat free mass and visceral fat (all P \u3c 0.001). However, on multivariate regression analysis, higher FPG levels remained associated only with maternal BMI \u3e 29.9 kg/m2 (OR 7.4, P = 0.01). Conclusions Our findings indicate that maternal BMI is the key determinant of maternal glycaemia. Interventions which focus on overall energy restriction and especially the limitation of dietary starch to optimise prepregnancy maternal bodyweight are likely to be useful in improving glycaemic control in higher risk pregnancies

Granite–a new telescope for TeV gamma ray astronomy

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87435/2/253_1.pd

Improving explanation of motor disability with diffusion-based graph metrics at onset of the first demyelinating event

BACKGROUND: Conventional magnetic resonance imaging (MRI) does not account for all disability in multiple sclerosis. OBJECTIVE: The objective was to assess the ability of graph metrics from diffusion-based structural connectomes to explain motor function beyond conventional MRI in early demyelinating clinically isolated syndrome (CIS). METHODS: A total of 73 people with CIS underwent conventional MRI, diffusion-weighted imaging and clinical assessment within 3 months from onset. A total of 28 healthy controls underwent MRI. Structural connectomes were produced. Differences between patients and controls were explored; clinical associations were assessed in patients. Linear regression models were compared to establish relevance of graph metrics over conventional MRI. RESULTS: Local efficiency (p = 0.045), clustering (p = 0.034) and transitivity (p = 0.036) were reduced in patients. Higher assortativity was associated with higher Expanded Disability Status Scale (EDSS) (β = 74.9, p = 0.026) scores. Faster timed 25-foot walk (T25FW) was associated with higher assortativity (β = 5.39, p = 0.026), local efficiency (β = 27.1, p = 0.041) and clustering (β = 36.1, p = 0.032) and lower small-worldness (β = -3.27, p = 0.015). Adding graph metrics to conventional MRI improved EDSS (p = 0.045, ΔR2 = 4) and T25FW (p < 0.001, ΔR2 = 13.6) prediction. CONCLUSION: Graph metrics are relevant early in demyelination. They show differences between patients and controls and have relationships with clinical outcomes. Segregation (local efficiency, clustering, transitivity) was particularly relevant. Combining graph metrics with conventional MRI better explained disability

TEV gamma ray emission from the active galactic nucleus Markarian 421

TeV energy gamma ray emission from the blazar Markarian 421 was detected with the Whipple 10 meter imaging Cherenkov telescope initially during the observation period March‐June 1992 at an average photon flux of 1.5×10−11 cm2 s−1. The photon spectrum in the TeV range lies on the extrapolation of the data points observed by EGRET during viewing phase I and indicates that the emission follows an unchanged power law between 100 MeV and 10 TeV with no significant intergalactic absorption. More recent observations from the spring of 1993 confirm the 1992 measurements. Here we give an update on the status of the observations, the analysis for time variability, and the results from the analysis to determine the spectral index for Markarian 421 at TeV energies.The search for TeV emission from other extragalactic sources detected by the EGRET instrument on the Compton Gamma Ray Observatory at GeV energies have so far been negative.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87648/2/597_1.pd

45th Annual Larval Fish Conference & 13th International Larval Biology Symposium San Diego, California 29 August – 1 September, 2022

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