High-toughness graphite/epoxy composite material experiment

This experiment was designed to measure the effect of near-earth space exposure on three mechanical properties of specially toughened 5208/T300 graphite/epoxy composite materials. The properties measured are elastic modulus, strength, and fracture toughness. Six toughness specimens and nine tensile specimens were mounted on an external frame during the 5.8-year orbit of the Long Duration Exposure Facility (LDEF). Three identical sets of specimens were manufactured at the outset: the flight set, a zero-time non-flight set, and a total-time non-flight set

Manufacture of fiber-epoxy test specimens: Including associated jigs and instrumentation

Experimental work on the manufacture and strength of graphite-epoxy composites is considered. The correct data and thus a true assessment of the strength properties based on a proper and scientifically modeled test specimen with engineered design, construction, and manufacture has led to claims of a very broad spread in optimized values. Such behavior is in the main due to inadequate control during manufacture of test specimen, improper curing, and uneven scatter in the fiber orientation. The graphite fibers are strong but brittle. Even with various epoxy matrices and volume fraction, the fracture toughness is still relatively low. Graphite-epoxy prepreg tape was investigated as a sandwich construction with intermittent interlaminar bonding between the laminates in order to produce high strength, high fracture toughness composites. The quality and control of manufacture of the multilaminate test specimen blanks was emphasized. The dimensions, orientation and cure must be meticulous in order to produce the desired mix

Toughened graphite-epoxy composites exposed in near-Earth orbit for 5.8 years

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77202/1/AIAA-26612-927.pd

The contribution of nitrate respiration to the energy budget of the symbiotic clam Lucinoma aequizonata: a calorimetric study

Heat production and nitrate respiration rates were measured simultaneously in the gill tissue of Lucinoma aequizonata. This marine bivalve contains chemoautotrophic, intracellular, bacterial symbionts in its gill tissue. The symbionts show constitutive anaerobic respiration, using nitrate instead of oxygen as a terminal electron acceptor. An immediate increase in heat production was observed after the addition of nitrate to the perfusion medium of the calorimeter and this was accompanied by the appearance of nitrite in the effluent sea water. The nitrate-stimulated heat output was similar under aerobic and anaerobic conditions, which is consistent with the constitutive nature of nitrate respiration. The amount of heat released was dependent on the concentration of nitrate in the perfusion medium. At nitrate concentrations between 0.5 and 5 mmol l-1, the total heat production was increased over twofold relative to unstimulated baseline values. A mean (±s.e.m.) experimental enthalpy of -130±22.6 kJ mol-1 nitrite (N=13) was measured for this concentration range

Nitrate respiration in chemoautotrophic symbionts of the bivalve Lucinoma aequizonata

Chemoautotrophic bacteria live symbiotically in gills of Lucinoma aequizonata, an infaunal clam inhabiting an oxygen-poor environment. These intracellular symbionts respire nitrate, i.e. they use nitrate instead of oxygen as a terminal electron acceptor in the respiratory chain. Nitrate is only reduced to nitrite and not further to nitrogen gas. Nitrate is respired by the symbionts under fully aerobic conditions at the same rate as under anaerobic conditions. The bacterial symbionts contain a nitrate reductase that is associated with the membrane-containing fraction of the symbiont cell and that is sensitive to respiratory inhibitors; both features are consistent with the respiratory role of this enzyme. A review of nitrate reductase in chemoautotrophic syrnbionts suggests that nitrate respiration may be common among these symbioses. Symbiont nitrate reductase may be an ecologically important factor permitting the survival of animal hosts in oxygen-poor environments

Increased fracture toughness of graphite-epoxy composites through intermittent interlaminar bonding

Intermittent interlaminar bonding, which can lead to a large increase in the fracture surface area, was achieved through the introduction of thin perforated Mylar between the layers of a multi-layer continuous-filament graphite-epoxy composite. For the best optimum condition included in this study, fracture toughness was increased from about 100 kJ/sq m for untreated specimens to an average of about 500 kJ/sq m, while tensile strength dropped from 500 MPa to 400 MPa, and elastic modulus remained the same at about 75 GPa. An approximate analysis is presented to explain the observed improvement in toughness

The rate of thermal diffusion in mixtures of carbon dioxide and hydrogen

An equation is derived for the time rate of change of composition of a binary gas mixture of initially uniform composition by thermal diffusion in an apparatus consisting of two chambers, one above the other, connected by a straight tube. The upper chamber is held at a given constant high temperature and the lower chamber at a given constant lower temperature. Using the thermal conductivity method of analysis the rate of change of composition in the lower chamber has been measured for carbon dioxide-hydrogen mixtures for various compositions, pressures and higher temperatures. From the experimental results and the derived equation, values are determined for the thermal diffusion coefficient, the ordinary or concentration diffusion coefficient and the temperature exponent for these coefficients.</p

Closed-system programmed-temperature pyrolysis on n-octadecane: Implications for the conversion of oil to gas

The aim of this work is to determine the molecular and stable carbon isotope compositions of pyrolysates from normal octadecane and, then, to study methane generation kinetic and carbon isotope fractionation of gaseous hydrocarbons generated from n-octadecane cracking. Pyrolyses were carried out in an anhydrous closed system (gold tubes) under a constant pressure of 50 MPa at heating rates of 20°C/h and 2°C/h. Results show that secondary cracking of pyrolysates from n-octadecane largely contributes to the amount of methane generation, much more than primary cracking of noctadecane. Cracking and polymerization at relatively low temperatures and disproportionation reactions leading to light hydrocarbons and polyaromatic hydrocarbons at high temperatures are probable causes for the carbon isotope reversal of gaseous hydrocarbons that is commonly observed in pyrolysis experiments. This study of methane generation kinetics suggests that n-alkane hydrocarbon cracking begins to generate methane at 170°C but a great deal would be generated at 200°C in sedimentary basins. Keywords: n-octadecane, pyrolysis experiment, gaseous hydrocarbon, stable carbon isotope, kinetic in crude oils. It is widely accepted that the thermal evolution of oils is controlled by the kinetics of cracking reactions, which is generally modeled by the Arrhenius equation The history of experiments designed to understand oil cracking is a long one, including experiments on whole rocks and kerogens (Behar et a

Metabolism of nitrogen and sulfur in ectosymbiotic bacteria of marine nematodes (Nematoda, Stilbonematinae)

Nematodes of the family Stilbonematinae are known for their highly specific association with ectosymbiotic bacteria. These worms are members of the meiofauna in marine, sulfide-rich sediments, where they migrate around the redox boundary layer. In this study, bacterial ectosymbionts of 2 species of marine nematodes, Stilbonema sp. and Laxus oneistus, were shown to be capable of the respiratory reduction of nitrate and nitrite (denitrification). The use of these alternative electron acceptors to oxygen by the bacteria allows the animals to migrate into the deeper, anoxic sediments, where they can exploit the sulfide-rich patches of the deeper sediment layers. The accumulation of thiols (sulfide, thiosulfate, sulfate and glutathione) in body tissues of the worms was determined following incubation in the presence of various electron donors (sulfide, thiosulfate) and acceptors (nitrate). In their chemoautotrophic metabolic potential, the ectosymbionts of the 2 nematode species were found to resemble the phylogenetically related, intracellular symbionts of macrofaunal hosts of deep-sea hydrothermal vents and other sulfide-rich habitats