Cyclic debonding of adhesive joints

Bonded lap joints were manufactured and tested under static and fatigue loading. Specimens were designed to fail in the bondline, and all fatigue tests included monitoring the crack growth to failure. Test specimens included aluminum details joined by two different adhesives. Specimens also included titanium and boron-epoxy details joined by an epoxy laminating resin. Additonal program variables included bondline thickness, adherend and spice plate thickness, specimen width, and specimen fabrication procedure. Adhesive aging was found to be generally detrimental to the lives of most of the specimens bonded with one adhesive system. Adhesive material was found to have a major influence on debond rate. Co-cured titanium/boron-epoxy specimens were found to resist debonding better than specimens fabricated with a sequential cure. Splice plate thickness and test section width were found to have little effect on debond rate. The data also suggested the existence of an optimum bondline thickness

Microbial burden prediction model for unmanned planetary spacecraft

The technical development of a computer program for predicting microbial burden on unmanned planetary spacecraft is outlined. The discussion includes the derivation of the basic analytical equations, the selection of a method for handling several random variables, the macrologic of the computer programs and the validation and verification of the model. The prediction model was developed to (1) supplement the biological assays of a spacecraft by simulating the microbial accretion during periods when assays are not taken; (2) minimize the necessity for a large number of microbiological assays; and (3) predict the microbial loading on a lander immediately prior to sterilization and other non-lander equipment prior to launch. It is shown that these purposes not only were achieved but also that the prediction results compare favorably to the estimates derived from the direct assays. The computer program can be applied not only as a prediction instrument but also as a management and control tool. The basic logic of the model is shown to have possible applicability to other sequential flow processes, such as food processing

Centrifugally Obtained Artificial Gravity

Centrifugally obtained artificial gravity effects on space station crew performanc

The Production of Ti44 and Co60 in Supernova

The production of the radioactive isotopes $^{44}$Ti and $^{60}$Co in all types of supernovae is examined and compared to observational constraints including Galactic $\gamma$--ray surveys, measurements of the diffuse 511 keV radiation, $\gamma$--ray observations of Cas A, the late time light curve of SN 1987A, and isotopic anomalies found in silicon carbide grains in meteorites. The (revised) line flux from $^{44}$Ti decay in the Cas A supernova remnant reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper bound expected from our models. The necessary concurrent ejection of $^{56}$Ni would also imply that Cas A was a brighter supernova than previously thought unless extinction in the intervening matter was very large. Thus, if confirmed, the reported amount of $^{44}$Ti in Cas A provides very interesting constraints on both the supernova environment and its mechanism. The abundances of $^{44}$Ti and $^{60}$Co ejected by Type II supernovae are such that gamma-radiation from $^{44}$Ti decay SN 1987A could be detected by a future generation of gamma-ray telescopes and that the decay of $^{60}$Co might provide an interesting contribution to the late time light curve of SN 1987A and other Type II supernovae. To produce the solar $^{44}$Ca abundance and satisfy all the observational constraints, nature may prefer at least the occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia supernovae. Depending on the escape fraction of positrons due to $^{56}$Co made in all kinds of Type Ia supernovae, a significant fraction of the steady state diffuse 511 keV emission may arise from the annihilation of positrons produced during the decay of $^{44}$Ti to $^{44}$Ca. The Ca and Ti isotopic anomalies in pre-solar grains confirm the production of $^{44}$Ti in supernovae and thatComment: 27 pages including 7 figures. uuencoded, compressed, postscript. in press Ap

Is Hyperconjugation Responsible For The Gauche Effect In 1-Fluoropropane And Other 2-Substituted-1-Fluoroethanes?

The energies and geometries of a series of 2-substituted-1-fluoroethanes were computed at the MP2/6-311++G**(6D)//MP2/6-31+G* level of theory for both the maxima and minima of the rotation about the C-C bond. The results did not support the predictions of a hyperconjugative model, that both 1,2-difluoroethane and 1-chloro-2-fluoroethane would strongly prefer a gauche conformation, and that 1-fluoro-2-silylethane would strongly prefer an anti conformation. The existence of competing electrostatic interactions between the fluorine and the substituents at C-2 was indicated by the detailed geometries of the gauche conformers and by the calculated sensitivity of the gauche-anti energy differences to the presence of a polar solvent. However, Fourier analyses of the torsional potential energies were wholly consistent with hyperconjugative electron donation into the C-F sigma* orbital contributing to the conformational preferences of these 1-fluoroethanes. Fourier analyses also showed that hyperconjugation contributes to the small variations in C-C and C-F bond lengths and in fluorine atomic charges that were computed. The torsional potential energies, variations in geometry and atomic charge, and sensitivity to solvent were all in accord with the expected ranking of hyperconjugative electron donating ability of bonds to carbon, C-Si \u3e C-H \u3e C-C \u3e C-Cl \u3e C-F

Self-sterilization of bodies during outer planet entry

A body encountering the atmosphere of an outer planet is subjected to heat loads which could result in high temperature conditions that render terrestrial organisms on or within the body nonviable. To determine whether an irregularly shaped entering body, consisting of several different materials, would be sterilized during inadvertent entry at high velocity, the thermal response of a typical outer planet spacecraft instrument was studied. The results indicate that the Teflon insulated cable and electronic circuit boards may not experience sterilizing temperatures during a Jupiter, Saturn, or Titan entry. Another conclusion of the study is that small plastic particles entering Saturn from outer space have wider survival corridors than do those at Jupiter

A computer program for the calculation of the flow field in supersonic mixed-compression inlets at angle of attack using the three-dimensional method of characteristics with discrete shock wave fitting

The calculation procedure is based on the method of characteristics for steady three-dimensional flow. The bow shock wave and the internal shock wave system were computed using a discrete shock wave fitting procedure. The general structure of the computer program is discussed, and a brief description of each subroutine is given. All program input parameters are defined, and a brief discussion on interpretation of the output is provided. A number of sample cases, complete with data deck listings, are presented

Nuclear Aspects of Nucleosynthesis in Massive Stars

Preliminary results of a new set of stellar evolution and nucleosynthesis calculations for massive stars are presented. These results were obtained with an extended reaction network up to Bi. The discussion focuses on the importance of nuclear rates in pre- and post-explosive nucleosynthesis. The need for further experiments to study specific reactions and nuclear properties (optical alpha+nucleus potentials) is emphasized.Comment: 6 pages, 2 figures; invited talk, to appear in the Proceedings of the Int. Conf. "Structure of the Nucleus at the Dawn of the Century", May 2000, Bologna, Ital

Nucleosynthesis in massive stars revisited

We have performed the first calculations to follow the evolution of all stable nuclei and their radioactive progenitors in a finely-zoned stellar model computed from the onset of central hydrogen burning through explosion as a Type II supernova. Calculations were done for 15, 20, and 25 solar masses Pop I stars using the most recently available set of experimental and theoretical nuclear data, revised opacity tables, and taking into account mass loss due to stellar winds. Here results are presented for one 15 solar masses model.Comment: 4 pages, 1 figure; needs espcrc1.sty; talk at "Nuclei in the Cosmos 2000", Aarhus, Denmark, June 2000; will appear in Nucl. Phys.