4,909 research outputs found
Quasi-static and Dynamic Behavior of Additively Manufactured Metallic Lattice Cylinders
Lattice structures have tailorable mechanical properties which allows them to
exhibit superior mechanical properties (per unit weight) beyond what is
achievable through natural materials. In this paper, quasi-static and dynamic
behavior of additively manufactured stainless steel lattice cylinders is
studied. Cylindrical samples with internal lattice structure are fabricated by
a laser powder bed fusion system. Equivalent hollow cylindrical samples with
the same length, outer diameter, and mass (larger wall thickness) are also
fabricated. Split Hopkinson bar is used to study the behavior of the specimens
under high strain rate loading. It is observed that lattice cylinders reduce
the transmitted wave amplitude up to about 21% compared to their equivalent
hollow cylinders. However, the lower transmitted wave energy in lattice
cylinders comes at the expense of a greater reduction in their stiffness, when
compared to their equivalent hollow cylinder. In addition, it is observed that
increasing the loading rate by five orders of magnitude leads to up to about
36% increase in the peak force that the lattice cylinder can carry, which is
attributed to strain rate hardening effect in the bulk stainless steel
material. Finite element simulations of the specimens under dynamic loads are
performed to study the effect of strain rate hardening, thermal softening, and
the failure mode on dynamic behavior of the specimens. Numerical results are
compared with experimental data and good qualitative agreement is observed.Comment: 20th Biennial Conference of the APS Topical Group on Shock
Compression of Condensed Matte
Constraining the Minimum Mass of High-Redshift Galaxies and Their Contribution to the Ionization State of the IGM
We model the latest HST WFPC3/IR observations of > 100 galaxies at redshifts
z=7-8 in terms of a hierarchical galaxy formation model with starburst
activity. Our model provides a distribution of UV luminosities per dark matter
halo of a given mass and a natural explanation for the fraction of halos
hosting galaxies. The observed luminosity function is best fit with a minimum
halo mass per galaxy of 10^{9.4+0.3-0.9} Msun, corresponding to a virial
temperature of 10^{4.9+0.2-0.7} K. Extrapolating to faint, undetected galaxies,
the total production rate of ionizing radiation depends critically on this
minimum mass. Future measurements with JWST should determine whether the entire
galaxy population can comfortably account for the UV background required to
keep the intergalactic medium ionized.Comment: 9 pages, 6 figures, submitted to ApJ, comments welcom
Lower Bounds on Implementing Robust and Resilient Mediators
We consider games that have (k,t)-robust equilibria when played with a
mediator, where an equilibrium is (k,t)-robust if it tolerates deviations by
coalitions of size up to k and deviations by up to players with unknown
utilities. We prove lower bounds that match upper bounds on the ability to
implement such mediators using cheap talk (that is, just allowing communication
among the players). The bounds depend on (a) the relationship between k, t, and
n, the total number of players in the system; (b) whether players know the
exact utilities of other players; (c) whether there are broadcast channels or
just point-to-point channels; (d) whether cryptography is available; and (e)
whether the game has a k+t$ players, guarantees that every player gets a
worse outcome than they do with the equilibrium strategy
- …