Ruptures along a frictional interface are described by Fracture Mechanics: Experiments in a "Laboratory Earthquake" along both dry and lubricated interfaces

Abstract:

A dry frictional interface is composed of an ensemble of discrete contacts whose real contact area is orders of magnitude smaller than the nominal one. Slip is initiated when contacts are broken via propagating ruptures. Characterization of the dynamic fields that drive these ruptures and how they couple to the dissipative mechanisms on the interface are critical to our fundamental understanding of both frictional motion and earthquake dynamics.

We experimentally investigate ruptures along rough spatially extended interfaces bounded by the same type of material. We perform simultaneous high-speed measurements (at μsec time scale) of the real contact area and the strain fields in the region surrounding propagating rupture tips. These measurements enable us to uncover the breakdown process near the tip of the slipping zone for rapidly propagating ruptures ranging from slow rupture fronts (~0.01C_R) to nearly the Rayleigh wave speed, C_R~1255m/s.

These rupture fronts are quantitatively described by classical singular solutions for rapid shear cracks. These singular solutions, originally derived to describe brittle fracture, are in excellent agreement with the experiments for slow propagation, whereas some significant discrepancies arise as the rupture velocity approaches C_R. The energy dissipated by the fracture of the contacts (fracture energy) is nearly constant throughout the entire rupture velocity range, while the size of the dissipative zone undergoes a ‘Lorentz-like’ contraction as the rupture velocity approaches C_R.

We then turn to both fully lubricated and partially lubricated interfaces and compare the dynamic strain fields measured to those of dry interfaces. The results of these studies are surprising.

We will show that:

• Rapid rupture fronts still propagate in the stick-slip regime
• Although decreasing the overall friction coefficient along the interface, the addition of a lubricant significantly increases the fracture energy at the rupture tip
• In the fully lubricated case, the form of the fields surrounding the rupture tip may belong to a different class of solutions than the classical solutions given by dynamic shear fracture.

This coupling between friction and fracture is critical to our fundamental understanding of frictional motion and related processes, such as earthquake dynamics.