Unexpected Dynamics in the Propagation of Fracture Fronts

Abstract

Fractures are ubiquitous and lead to catastrophic failure of materials. While fracture in a two-dimensional plane is well understood, all fractures are, in fact, extended and propagate in a three-dimensional space and their behavior is more complex. Here we show that forward propagation of a fracture front always occurs through an initial rupture, nucleated at some localized position, followed by very rapid transverse expansion at velocities as high as the Rayleigh-wave speed. We study a circular geometry to achieve an uninterrupted extended fracture front and use fluid to control the loading conditions that determine the amplitude of the forward jump; we find this amplitude correlates with the transverse velocity. Dynamic rupture simulations capture the observations for only high transverse velocity. These results highlight the importance of transverse dynamics in the forward propagation of extended fracture.

Laboratory Hydrofractures as Analogs to Tectonic Tremors

“…The break amplitude (i.e., when the radial fracture front advances and transverse fracturing occurs) and the stick time (i.e., time between breaks) increase as the fluid viscosity increases, which were also shown by Cochard et al. (2023). Here, we focus on a single small but fast seismic fracture.…”

“…As studied by Cochard et al. (2023), a stick‐break instability occurs in all experiments. The fracture propagates slowly in the radial dimension overall (thousands of times smaller than the Rayleigh wave speed), but the slow radial fracture is accommodated by seismic transverse fractures, which propagate rapidly (close to Rayleigh wave speed).…”

“…We conduct two experiments with varying fracturing fluids: the first with water (low viscosity, μ = 1 cP) and the second with a mixture of glycerol and water (high viscosity, μ = 800 cP). As studied by Cochard et al (2023), a stick-break instability occurs in all experiments. The fracture propagates slowly in the radial dimension overall (thousands of times smaller than the Rayleigh wave speed), but the slow radial fracture is accommodated by seismic transverse fractures, which propagate rapidly (close to Rayleigh wave speed).…”

“…Despite these differences, the overall propagation of the slow radial fracture (and fluid diffusion) occurs at a similar rate and coincides with fluid propagation. Simple models accounting for the different fracture styles and fluid viscosity and with an intact, simple internal stress structure showed that the fluid viscosity impacted the stress loading conditions (i.e., the lag between the fluid and the fracture front; Cochard et al., 2023). In nature, the viscosity of geofluids may vary tremendously (Audétat & Keppler, 2004), as well as pre‐existing permeability (Kato et al., 2004).…”

“…Our laboratory experiments have demonstrated that the slow propagation of hydrofractures proceeds in a stick‐break fashion, with inter‐event times and fracture sizes driven by pressurized fluids that increase with fluid viscosity (Cochard et al., 2023). In nature, tectonic tremors also happen in bursts and exhibit temporal and spatial clustering (Frank et al., 2018; Husker et al., 2012; Jolivet & Frank, 2020).…”