7,098 research outputs found
Recommended from our members
Manufacturing Metallic Parts with Designed Mesostructure via Three-Dimensional Printing of Metal Oxide Powder
Cellular materials, metallic bodies with gaseous voids, are a promising class of materials that offer
high strength accompanied by a relatively low mass. In this paper, the authors investigate the use of ThreeDimensional Printing (3DP) to manufacture metallic cellular materials by selectively printing binder into a
bed of metal oxide ceramic powder. The resulting green part undergoes a thermal chemical post-process in
order to convert it to metal. As a result of their investigation, the authors are able to create cellular
materials made of maraging steel that feature wall sizes as small as 400 µm and angled trusses and channels
that are 1 mm in diameter.Mechanical Engineerin
Human Powered Vehicle Frame Design, Analysis, Manufacturing, and Testing
The University of Akron Human Powered Vehicle Design Team began this academic year with a mission to create a practical, lightweight, safe, and efficient human powered vehicle. The main objectives of ZC18 were inspired by competition in the 2018 Human Powered Vehicle Challenge at ASME E-Fest East. In addition to adhering to the rules of this competition, scoring high on the submitted reports, and competing well in the racing events, the team set a goal to assist in furthering knowledge related to the topic of human powered vehicles.
ZC18 was constructed nearly completely in the University of Akron’s Design Center with research, design, analysis, testing, and assembly done solely by undergraduate engineering students. These students volunteered approximately 50-60 total man hours per week during the 2017-2018 academic year, while utilizing the university’s resources to manufacture a competitive vehicle for the 2018 season.
ZC18 is a recumbent tricycle consisting of a 6061-T6 aluminum alloy frame and a carbon fiber fairing. This vehicle is structurally sound and maintains a team required factor of safety of 2.0. ZC18 utilizes an indirect linkage style steering system, hydraulic brakes, a triangular shaped Rollover Protection System (RPS), a versatile drivetrain for both speed and endurance events, and a carbon fiber fairing that provides a drag reduction and protection from the elements. By encompassing a three-point harness, lights, a mirror, and a variety of other safety equipment, ZC18 is built not only for the Human Powered Vehicle Challenge, but also as a method of transportation for a daily commute
Recommended from our members
Examining Variability in the Mechanical Properties of Parts Manufactured via PolyJet Direct 3D Printing
In Objet’s PolyJet process, part layers are created by selectively inkjetting photopolymers
onto a build substrate and then cured with ultraviolet lamps. With an eye towards using PolyJet
as a manufacturing process to fabricate end-use products, the authors examine the sensitivity of
part material properties to variation in process parameters. Specifically, a design of experiments
is conducted using a full-factorial design to analyze the effects of three parameters on the
specimens’ tensile strength and tensile modulus: the in-build plane part orientation (X-Y), the
out-of-build plane part orientation (Z), and the distance between specimens. Results show that
part spacing has the largest effect on the tensile strength, but the three parameters produced no
statistically significant effects on the tensile modulus. Orienting specimens in XZ orientation
with minimal part spacing resulted in the highest tensile strength and modulus. Whereas,
orienting specimens in the YZ orientation at the farthest part spacing led to the lowest
mechanical properties.Mechanical Engineerin
Galaxy Morphology - Halo Gas Connections
We studied a sample of 38 intermediate redshift MgII absorption-selected
galaxies using (1) Keck/HIRES and VLT/UVES quasar spectra to measure the halo
gas kinematics from MgII absorption profiles and (2) HST/WFPC-2 images to study
the absorbing galaxy morphologies. We have searched for correlations between
quantified gas absorption properties, and host galaxy impact parameters,
inclinations, position angles, and quantified morphological parameters. We
report a 3.2-sigma correlation between asymmetric perturbations in the host
galaxy morphology and the MgII absorption equivalent width. We suggest that
this correlation may indicate a connection between past merging and/or
interaction events in MgII absorption-selected galaxies and the velocity
dispersion and quantity of gas surrounding these galaxies.Comment: 6 pages; 3 figures; contributed talk for IAU 199: Probing Galaxies
through Quasar Absorption Line
The Radio Evolution of SN 2001gd
We present the results of observations of the radio emission from Supernova
2001gd in NGC 5033 from 2002 February 8 through 2006 September 25. The data
were obtained using the Very Large Array at wavelengths of 1.3 cm (22.4 GHz), 2
cm (14.9 GHz), 3.6 cm (8.4 GHz), 6 cm (4.9 GHz), and 20 cm (1.5 GHz), with one
upper limit at 90 cm (0.3 GHz). In addition, one detection has been provided by
the Giant Metrewave Radio Telescope at 21 cm (1.4 GHz). SN 2001gd was
discovered in the optical well past maximum light, so that it was not possible
to obtain many of the early radio "turn-on" measurements which are important
for estimating the local circumstellar medium (CSM) properties. Only at 20 cm
were turn-on data available. However, our analysis and fitting of the radio
light curves, and the assumption that the Type IIb SN 2001gd resembles the much
better studied Type IIb SN 1993J, enables us to describe the radio evolution as
being very regular through day ~550 and consistent with a nonthermal-emitting
model with a thermal absorbing CSM. The presence of synchrotron-self absorption
(SSA) at early times is implied by the data, but determination of the exact
relationship between the SSA component from the emitting region and the
free-free absorption component from the CSM is not possible as there are
insufficient early measurements to distinguish between models. After day ~550,
the radio emission exhibits a dramatically steeper decline rate which, assuming
similarity to SN 1993J, can be described as an exponential decrease with an
e-folding time of 500 days. We interpret this abrupt change in the radio flux
density decline rate as implying a transition of the shock front into a more
tenuous region of circumstellar material. A similar change in radio evolution
has been seen earlier in other SNe such as SN 1988Z, SN 1980K, and SN 1993J.Comment: 3 tables, 2 figures, To appear in the Astrophysical Journa
Forest structure, stand composition, and climate-growth response in montane forests of Jiuzhaigou National Nature Reserve, China.
Montane forests of western China provide an opportunity to establish baseline studies for climate change. The region is being impacted by climate change, air pollution, and significant human impacts from tourism. We analyzed forest stand structure and climate-growth relationships from Jiuzhaigou National Nature Reserve in northwestern Sichuan province, along the eastern edge of the Tibetan plateau. We conducted a survey to characterize forest stand diversity and structure in plots occurring between 2050 and 3350 m in elevation. We also evaluated seedling and sapling recruitment and tree-ring data from four conifer species to assess: 1) whether the forest appears in transition toward increased hardwood composition; 2) if conifers appear stressed by recent climate change relative to hardwoods; and 3) how growth of four dominant species responds to recent climate. Our study is complicated by clear evidence of 20(th) century timber extraction. Focusing on regions lacking evidence of logging, we found a diverse suite of conifers (Pinus, Abies, Juniperus, Picea, and Larix) strongly dominate the forest overstory. We found population size structures for most conifer tree species to be consistent with self-replacement and not providing evidence of shifting composition toward hardwoods. Climate-growth analyses indicate increased growth with cool temperatures in summer and fall. Warmer temperatures during the growing season could negatively impact conifer growth, indicating possible seasonal climate water deficit as a constraint on growth. In contrast, however, we found little relationship to seasonal precipitation. Projected warming does not yet have a discernible signal on trends in tree growth rates, but slower growth with warmer growing season climates suggests reduced potential future forest growth
Long Term Radio Monitoring of SN 1993J
We present our observations of the radio emission from supernova (SN) 1993J,
in M 81 (NGC 3031), made with the VLA, from 90 to 0.7 cm, as well as numerous
measurements from other telescopes. The combined data set constitutes probably
the most detailed set of measurements ever established for any SN outside of
the Local Group in any wavelength range. Only SN 1987A in the LMC has been the
subject of such an intensive observational program. The radio emission evolves
regularly in both time and frequency, and the usual interpretation in terms of
shock interaction with a circumstellar medium (CSM) formed by a pre-SN stellar
wind describes the observations rather well considering the complexity of the
phenomenon. However: 1) The 85 - 110 GHz measurements at early times are not
well fitted by the parameterization, unlike the cm wavelength measurements. 2)
At mid-cm wavelengths there is some deviation from the fitted radio light
curves. 3) At a time ~3100 days after shock breakout, the decline rate of the
radio emission steepens without change in the spectral index. This decline is
best described as an exponential decay starting at day 3100 with an e-folding
time of ~1100 days. 4) The best overall fit to all of the data is a model
including both non-thermal synchrotron self-absorption (SSA) and a thermal
free-free absorbing (FFA) components at early times, evolving to a constant
spectral index, optically thin decline rate, until the break in that decline
rate. Moreover, neither a purely SSA nor a purely FFA absorbing models can
provide a fit that simultaneously reproduces the light curves, the spectral
index evolution, and the brightness temperature evolution. 5) The radio and
X-ray light curves exhibit similar behavior and suggest a sudden drop in the SN
progenitor mass-loss rate at ~8000 years prior to shock breakout.Comment: 45 pages, 13 figures, accepted for Ap
Beyond Goldwater-Nichols
This report culminated almost two years of effort at CSIS, which began by developing an approach for both revisiting the Goldwater-Nichols Department of Defense Reorganization Act of 1986 and for addressing issues that were beyond the scope of that landmark legislation
- …