Stress Rupture Behavior of Silicon Carbide Coated, Low Modulus Carbon/Carbon Composites

The disadvantages of carbon-carbon composites, in addition to the oxidation problem, are low thermal expansion, expensive fabrication procedures, and poor off axis properties. The background of carbon-carbon composites, their fabrication, oxidation, oxidation protection and mechanical testing in flexure are discussed

Metal alloy resistivity measurements at very low temperatures

High speed, automated system accurately measures to approximately one percent in three minutes. System identifies materials having constant thermal or electric conductivity, predicts new material properties, develops alloys in accordance with desired specifications, and develops nondestructive devices for measuring precipitation hardening

Observation of Single Transits in Supercooled Monatomic Liquids

A transit is the motion of a system from one many-particle potential energy valley to another. We report the observation of transits in molecular dynamics (MD) calculations of supercooled liquid argon and sodium. Each transit is a correlated simultaneous shift in the equilibrium positions of a small local group of particles, as revealed in the fluctuating graphs of the particle coordinates versus time. This is the first reported direct observation of transit motion in a monatomic liquid in thermal equilibrium. We found transits involving 2 to 11 particles, having mean shift in equilibrium position on the order of 0.4 R_1 in argon and 0.25 R_1 in sodium, where R_1 is the nearest neighbor distance. The time it takes for a transit to occur is approximately one mean vibrational period, confirming that transits are fast.Comment: 19 pages, 8 figure

Three-dimensional structure of basal body triplet revealed by electron cryo-tomography.

Basal bodies and centrioles play central roles in microtubule (MT)-organizing centres within many eukaryotes. They share a barrel-shaped cylindrical structure composed of nine MT triplet blades. Here, we report the structure of the basal body triplet at 33 Å resolution obtained by electron cryo-tomography and 3D subtomogram averaging. By fitting the atomic structure of tubulin into the EM density, we built a pseudo-atomic model of the tubulin protofilaments at the core of the triplet. The 3D density map reveals additional densities that represent non-tubulin proteins attached to the triplet, including a large inner circular structure in the basal body lumen, which functions as a scaffold to stabilize the entire basal body barrel. We found clear longitudinal structural variations along the basal body, suggesting a sequential and coordinated assembly mechanism. We propose a model in which δ-tubulin and other components participate in the assembly of the basal body

The transition of ground-based space environmental effects testing to the space environment

The goal of the space flight program at the Center for Commercial Development of Space (CCDS)--Materials for Space Structures is to provide environmentally stable structural materials to support the continued humanization and commercialization of the space frontier. Information on environmental stability will be obtained through space exposure, evaluation, documentation, and subsequent return to the supplier of the candidate material for internal investigation. This program provides engineering and scientific service to space systems development firms and also exposes CCDS development candidate materials to space environments representative of in-flight conditions. The maintenance of a technological edge in space for NASA suggests the immediate search for space materials that maintain their structural integrity and remain environmentally stable. The materials being considered for long-lived space structures are complex, high strength/weight ratio composites. In order for these new candidate materials to qualify for use in space structures, they must undergo strenuous testing to determine their reliability and stability when subjected to the space environment. Ultraviolet radiation, atomic oxygen, debris/micrometeoroids, charged particles radiation, and thermal fatigue all influence the design of space structural materials. The investigation of these environmental interactions is the key purpose of this center. Some of the topics discussed with respect to the above information include: the Space Transportation System, mission planning, spaceborne experiments, and space flight payloads

Gravitational macrosegregation in binary Pb-Sn alloy ingots

A space shuttle experiment employing the General Purpose (Rocket) Furnace (GPF) in its isothermal mode of operation is manifested on MSL-3, circa 1989. The central aim of this experiment is to investigate the effect of reduced gravity levels on the segregation behavior in a slowly, and isothermally, cooled sample of a binary Pb-15 wt% Sn alloy. This experiment should be able to simulate, in a small laboratory sample, some aspects of the segragation phenomena occurring in large industrial ingots. Ground-based experiments conducted in the single-cavity simulator of the GPF, in support of the microgravity experiment are described in detail. The results of the MSFC experiments are compared with other related experiments conducted at Case Western Reserve University (CWRS), wherein the isothermal constraints were relaxed. The isothermally processed samples indicate a small and gradual increase in fraction eutectic, and a corresponding increase in tin content, from the bottom to the top of the ingot. The radial variations are minimal near the ingot bottom, but there are large radial variations in the top half. In the CWRU experiments, more severe segregations, including segregation defects known as freckles. Follow up experiments employing the GPF without the isothermal constraints, or other suitably modified space shuttle hardware are suggested

Electromagnetic Scattering from Relativistic Bound States

The quasipotential formalism for elastic scattering from relativistic bound states is formulated based on the instant constraint in the Breit frame. The quasipotential electromagnetic current is derived from Mandelstam's five-point kernel and obeys a two-body Ward identity. Breit-frame wave functions are obtained directly by solving integral equations with nonzero total three-momentum, thus accomplishing a dynamical boost. Calculations of electron-deuteron elastic form factors illustrate the importance of the dynamical boost versus kinematic boosts of the rest frame wave functions.Comment: RevTeX 3.0 manuscript, 9 pages. UU-file is a single PostScript file of the manuscript including figures. U. MD PP #93-17

Root System Characteristics of Two Soybean Isolines Undergoing Water Stress Condition

Environmental stress may have a differential influence on root development of soybean [Glycine max (L.) Merr.] isolines which vary in pubescence density. Root length density and root dry matter distribution as a function of depth and distance from the row were determined for two isolines of \u27Harosoy\u27 soybean in association with an experiment designed to evaluate the influence of epidermal pubescence on root development, water use, and photosynthetic characteristics of the two isolines. The isolines, Harosoy normal (HN) and Harosoy dense (HD), differed in the density of trichomes on the epidermal surfaces of leaves, stems, and pods. The study was conducted at the Univ. of Nebraska Field Laboratory at Mead, Nebr., during the 1980 growing season. Root samples were collected 47 (full bloom) and 78 (beginning seed) days after planting. Until the first sampling, soil water content was high at all depths, and roots were concentrated in the surface 0.15-111 layer, especially under the row. Eighty percent of the roots were found within the upper 0.30 m. By 78 days after planting and after 30 days of drought, root length density was greatest at the 0.90 to 1.20-m layer; 80% of the roots pared to were found within the 0 to 1.2-m layer; and uniform lateral distribution was observed. Harosoy dense pubescence isoline tended to have a greater root density, to explore deeper into the soil, and to extract more soil water during the drought than did the normal pubescence isoliie. However, the rate of water extraction (per unit root length) was greater for the HN isoline

A farm creaming experiment

A short time ago we were asked whether any injustice would result to either creamery-man or patron, if the latter should set his evening’s milk, skim it in the morning, mix the cream with the morning’s milk and send the mixture to the creamery instead of sending the milk of both evening and morning, as is the usual custom. The most satisfactory way to answer all such queries is by practical demonstrations, and it was determined to conduct an experiment bearing on the point in question. A short time ago we were asked whether any injustice would result to either creamery-man or patron, if the latter should set his evening’s milk, skim it in the morning, mix the cream with the morning’s milk and send the mixture to the creamery instead of sending the milk of both evening and morning, as is the usual custom. The most satisfactory way to answer all such queries is by practical demonstrations, and it was determined to conduct an experiment bearing on the point in question