Investigation of rock fragmentation during rockfalls and rock avalanches via 3-D discrete element analyses

This paper investigates the characteristics of dynamic rock fragmentation and its influence on the postfailure fragment trajectory. A series of numerical simulations by discrete element method (DEM) were performed for a simple rock block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. The rock block is modeled as an assembly of bonded spherical particles with fragmentation arising from bond breakages. Bond strength and stiffness were calibrated against available experimental data. We analyzed how dynamic fragmentation occurs at impact, together with the generated fragment size distributions and consequently their runout for different slope topographies. It emerges that after impact, the vertical momentum of the granular system decreases sharply to nil, while the horizontal momentum increases suddenly and then decreases. The sudden boost of horizontal momentum can effectively facilitate the transport of fragments along the bottom floor. The rock fragmentation intensity is associated with the input energy and increases quickly with the slope angle. Gentle slopes normally lead to long spreading distance and large fragments, while steep slopes lead to high momentum boosts and impact forces, with efficient rock fragmentation and fine deposits. The fragment size decreases, while the fracture stress and fragment number both increase with the impact loading strain rate, supporting the experimental observations. The fragment size distributions can be well fitted by the Weibull's distribution function.Wile

Starquake-Induced Glitches in Pulsars

The neutron star crust is rigid material floating on a neutron-proton liquid core. As the star's spin rate slows, the changing stellar shape stresses the crust and causes fractures. These starquakes may trigger pulsar glitches as well as the jumps in spin-down rate that are observed to persist after some glitches. Earlier studies found that starquakes in spinning-down neutron stars push matter toward the magnetic poles, causing temporary misalignment of the star's spin and angular momentum. After the star relaxes to a new equilibrium orientation, the magnetic poles are closer to the equator, and the magnetic braking torque is increased. The magnitude and sign of the predicted torque changes are in agreement with the observed persistent spin-down offsets. Here we examine the relaxation processes by which the new equilibrium orientation is reached. We find that the neutron superfluid in the inner crust slows as the star's spin realigns with the angular momentum, causing the crust to spin more rapidly. For plausible parameters the time scale and the magnitude of the crust's spin up agree with the giant glitches in the Vela and other pulsars

Physics of Neutron Star Crusts

The physics of neutron star crusts is vast, involving many different research fields, from nuclear and condensed matter physics to general relativity. This review summarizes the progress, which has been achieved over the last few years, in modeling neutron star crusts, both at the microscopic and macroscopic levels. The confrontation of these theoretical models with observations is also briefly discussed.Comment: 182 pages, published version available at <http://www.livingreviews.org/lrr-2008-10