Friction weakening by mechanical vibrations: A velocity-controlled process

Abstract

Frictional weakening by vibrations was first invoked in the 70's to explain unusual fault slips and earthquakes, low viscosity during the collapse of impact craters or the extraordinary mobility of sturzstroms, peculiar rock avalanches which travels large horizontal distances. This mechanism was further invoked to explain the remote triggering of earthquakes or abnormally large landslides or pyroclastic flows runout. Recent experimental and theoretical work pointed out the velocity of vibration as the key parameter which governs frictional weakening in sheared granular media. Here we show that the grains mobility is not mandatory, and that the vibration velocity governs both granular and solid frictional weakening. The velocity threshold controlling the transition from stickslip motion to continuous sliding is of the same order of magnitude, namely a hundred microns per second. It is linked to the roughness distribution of the asperities at the contact surface."It is easier to further the motion of a moving body than to move a body at rest." This sentence written by Themistius (about A.D. 320-390) is the first record of friction in history [1]. Since then, the frictional motion of a single body over a fixed substrate or of a sheared granular assembly revealed a wide variety of behaviors. At low shear velocity, the system experiences a stick-slip motion, with the alternance of loading phases (system at rest) and quick slip phases which release the energy. When increasing the shear velocity, a transition to continuous sliding motion is reported [2][3][4][5]. During catastrophic events such as earthquakes, landslides or pyroclastic flow, puzzling phenomena of frictional weakening were reported [6-9]: friction decreases with the shear velocity. Melosh [10] first proposed in 1979 that vibrations due to particle collisions could temporarily reduce the normal stress, and thus decrease the shear stress threshold to trigger sliding motion [11]. This mechanism, initially called vibrational fluidization and later on acoustic fluidization [10,11], was further sought to be at the origin of dramatic events triggered by external waves, such as earthquake remote triggering [8,12].How does endogenous noise or external mechanical disturbances drastically affect the frictional properties? Many works have attempted to tackle this issue for the last decades. They have shown that vibrations reduce or even suppress friction [13][14][15][16][17][18][19][20][21]. Interpretations were proposed based on a non-monotonic rheology curve, leading to instabilities and self-fluidization [22,23], softening effect due to non-linearity at the grains contact [24], contact opening [25,26] or sliding [27]. In single-block solid friction models, the vibration acceleration has often been stated at the parameter governing the transition between stick-slip motion and continuous sliding, with a threshold equal to the gravitational acceleration [19]. In a recent work, Lastakowski et al. [21] pointed out that the vibration velocity, and not the acceleration,...

Contributions of Rock Mass Structure to the Emplacement of Fragmenting Rockfalls and Rockslides: Insights From Laboratory Experiments

“…This low apparent friction coefficient could be caused by a variety of factors, such as the lower basal friction coefficient than that in the experiments in Manzella (2008) and the low degree of fragmentation of blocks leading to less interaction between the fragments, which decrease the energy consumption of internal friction in our experiments. High‐frequency vibration of PMMA slabs may also lead to low friction to increase the bounce of fragments (Friedmann et al, 2006; Vidal et al, 2018; Wang et al, 2015). In addition, fragmentation may be one of the important reasons for the low apparent friction coefficient (Haug et al, 2016), which is demonstrated by the finding that the normalized total runout will increase with the degree of fragmentation (Figure 10).…”

“…This result indicates that the variation of energy consumption by internal deformation does not lead to inverse relationships of the friction coefficient. Mechanical vibration may also lead to spreading of fragments or friction weakening (Vidal et al, 2018). We did observe a slight vibration of the horizontal plane when analog blocks impacted and slid on the slab.…”

Induced and endogenous acoustic oscillations in granular faults

“…Finally, let us mention that recent studies [30,52] have highlighted the role played by the velocity of the imposed mechanical vibrations on the frictional properties of sheared granular media. The friction reduction phenomenon occurring in a range of vibration frequencies in vertically shaken granular systems has been recently observed in experiments [36].…”

“…These theoretical predictions are in very good agreement with numerical simulations and indicate that friction suppression is related to the reduction of effective interface contacts in the system due to the external vibrations. Finally, let us mention that recent studies [30,52] have highlighted the role played by the velocity of the imposed mechanical vibrations on the frictional properties of sheared granular media.…”

Getting hotter by heating less: How driven granular materials dissipate energy in excess

“…In some cases, a relation K ∼ S α , with S ∼ (Af ) 2 , has been derived [26][27][28][29]. Experimental studies focused on the spe-cific role of the forcing mechanisms, such as vibration amplitude, frequency or velocity [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. In particular, an optimal frequency for energy transfer is found when the system is in a bouncing-bed state and A and f are varied keeping S fixed [30,31].…”