X-ray observation of micro-failures in granular piles approaching an   avalanche
  An X-ray imaging technique is used to probe the stability of 3-dimensional granular packs in a slowly rotating drum. Well before the surface reaches the avalanche angle, we observe intermittent plastic events associated with collective rearrangements of the grains located in the vicinity of the free surface. The energy released by these discrete events grows as the system approaches the avalanche threshold. By testing various preparation methods, we show that the pre-avalanche dynamics is not solely controlled by the difference between the free surface inclination and the avalanche angle. As a consequence, the measure of the pre-avalanche dynamics is unlikely to serve as a tool for predicting macroscopic avalanches. 
Alexandre Kabla (APPLIED Maths), Georges Debregeas (LPMC), Jean-Marc   Di Meglio (APPLIED Maths, MSC), Tim Senden (APPLIED M...