The onset of the frictional motion of dissimilar materials

Abstract

<jats:p>Frictional motion between contacting bodies is governed by propagating rupture fronts that are essentially earthquakes. These fronts break the contacts composing the interface separating the bodies to enable their relative motion. The most general type of frictional motion takes place when the two bodies are not identical. Within these so-called bimaterial interfaces, the onset of frictional motion is often mediated by highly localized rupture fronts, called slip pulses. Here, we show how this unique rupture mode develops, evolves, and changes the character of the interface’s behavior. Bimaterial slip pulses initiate as “subshear” cracks (slower than shear waves) that transition to developed slip pulses where normal stresses almost vanish at their leading edge. The observed slip pulses propagate solely within a narrow range of “transonic” velocities, bounded between the shear wave velocity of the softer material and a limiting velocity. We derive analytic solutions for both subshear cracks and the leading edge of slip pulses. These solutions both provide an excellent description of our experimental measurements and quantitatively explain slip pulses’ limiting velocities. We furthermore find that frictional coupling between local normal stress variations and frictional resistance actually promotes the interface separation that is critical for slip-pulse localization. These results provide a full picture of slip-pulse formation and structure that is important for our fundamental understanding of both earthquake motion and the most general types of frictional processes.</jats:p>

Supershear Frictional Ruptures Along Bimaterial Interfaces

“…The slip pulses composing positive supershear ruptures are readily identified by characteristic reductions of the normal stress at their tip. This normal stress reduction results from bimaterial coupling (Shlomai & Fineberg, 2016; Shlomai et al., 2020) and is followed by rapid recovery of at slip pulse tails. This signature of slip pulses is reflected in the narrow dark blue region (of low ) at the tips of the slip pulses in Figures 3 and 4.…”

“…As a result, slip pulse trains formed during the supershear transition actually enable us to better observe the fine structure of steady‐state slip pulses. It is now apparent that the steady‐state structure of slip pulses includes a strong and rapid compression that immediately precedes the strong release of normal stress that characterizes slip pulses (Shlomai & Fineberg, 2016; Shlomai et al., 2020). This compression has a width that is approximately that of the normal stress drop (denoted by the arrow in Figure 5b).…”

“…It was found (Shlomai et al., 2020) that the above formulation for transonic ruptures predicts crack‐like ruptures whose functional form provides an excellent description of the form of the stress and strain fields in the vicinity of the rupture tips of experimentally observed subshear crack‐like ruptures and transonic slip pulses. In order to capture, however, the localization that characterizes slip pulses in this velocity regime, it was necessary to incorporate a nontrivial friction law (Amontons‐Coulomb friction law) for x < 0.…”

“…More recent work (Shlomai et al., 2020) has looked more closely at how rupture fronts in the positive direction evolveinto slip pulses. This work found that, at , rupture fronts are well described by analytic solutions for bimaterial “cracks,” which are analogous to the singular shear cracks observed within homogeneous interfaces.…”

“…Moreover, Shlomai et al. (2020) associated slip pulse localization with interface separation, which was numerically observed only when frictional coupling between σ yy and σ xy is enabled.…”