Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings

TL;DR

This study investigates how 3D granular packings age and recover under tilting, revealing slow reconfiguration of weak contacts and surface activity, with implications for understanding granular stability and failure.

Contribution

It provides experimental evidence of aging and slow restoration of weak-contact configurations in tilted 3D granular packings, combining imaging and acoustic methods.

Findings

Gradual aging of surface activity and weak contacts with tilt cycles

Abrupt reactivation of weak contacts beyond previous maximum tilt angle

Spontaneous restoration of weak contacts after rest periods

Abstract

Granular packings slowly driven towards their instability threshold are studied using a digital imaging technique as well as a nonlinear acoustic method. The former method allows us to study grain rearrangements on the surface during the tilting and the latter enables to selectively probe the modifications of the weak-contact fraction in the material bulk. Gradual ageing of both the surface activity and the weak-contact reconfigurations is observed as a result of repeated tilt cycles up to a given angle smaller than the angle of avalanche. For an aged configuration reached after several consecutive tilt cycles, abrupt resumption of the on-surface activity and of the weak-contact rearrangements occurs when the packing is subsequently inclined beyond the previous maximal tilting angle. This behavior is compared with literature results from numerical simulations of inclined 2D packings. It is also found that the aged weak-contact configurations exhibit spontaneous restoration towards the initial state if the packing remains at rest for tens of minutes. When the packing is titled forth and back between zero and near-critical angles, instead of ageing, the weak-contact configuration exhibits "internal weak-contact avalanches" in the vicinity of both the near-critical and zero angles. By contrast, the stronger-contact skeleton remains stable.