Energy radiation of moving cracks

The energy radiated by moving cracks in a discrete background is analyzed. The energy flow through a given surface is expressed in terms of a generalized Poynting vector. The velocity of the crack is determined by the radiation by the crack tip. The radiation becomes more isotropic as the crack velocity approaches the instability threshold.Comment: 7 pages, embedded figure

Les possibilités fourragères de Digitaria umfolozi en zone soudanienne : Problèmes posés par l'étude systématique d'une plante fourragère

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Time resolved particle dynamics in granular convection

We present an experimental study of the movement of individual particles in a layer of vertically shaken granular material. High-speed imaging allows us to investigate the motion of beads within one vibration period. This motion consists mainly of vertical jumps, and a global ordered drift. The analysis of the system movement as a whole reveals that the observed bifurcation in the flight time is not adequately described by the Inelastic Bouncing Ball Model. Near the bifurcation point, friction plays an important role, and the branches of the bifurcation do not diverge as the control parameter is increased. By fitting the grains trajectories near the wall it is possible to quantify the effective acceleration acting on them. A comparison of the mass centre flying time and the flying time determined for the grains near the wall exposes the underlying mechanism that causes the downward flow

Dynamics of Crack Propagation in Brittle Materials

An experimental study of the properties of crack propagation in brittle materials is described. Specifically the crack speed, the sound emission and the fracture profile have been measured as a function of the applied stress. We find that the local velocity can be considered as a control parameter which determines the properties of the crack. The steady state velocity turns out to be a function of the applied stress. Several thresholds have been identified. One corresponds to the presence of a weak sound emission associated with velocity oscillations and another one to the onset of the surface roughness associated with velocity oscillations and another one to the onset of the surface roughness associated with a strong sound emission. We also find a strong crosscorrelation between sound and velocity oscillations. The statistical properties of the fracture profile depend on the steady state velocit

Time resolved particle dynamics in granular convection

We present an experimental study of the movement of individual particles in a layer of vertically shaken granular material. High-speed imaging allows us to investigate the motion of beads within one vibration period. This motion consists mainly of vertical jumps, and a global ordered drift. The analysis of the system movement as a whole reveals that the observed bifurcation in the flight time is not adequately described by the Inelastic Bouncing Ball Model. Near the bifurcation point, friction plays an important role, and the branches of the bifurcation do not diverge as the control parameter is increased. By fitting the grains trajectories near the wall it is possible to quantify the effective acceleration acting on them. A comparison of the mass centre flying time and the flying time determined for the grains near the wall exposes the underlying mechanism that causes the downward flow