Pore pressure at the crack tip: fluid-induced toughening during tensile fracture of fluid-saturated porous solids

Fracture mechanics has been mostly developed for elastic, brittle, dry materials, yet many subsurface geoenergy applications involve crack growth in porous rocks that are fluid-saturated. While the role of injected fluids in fracture propagation has been studied extensively, the contribution of pore fluids is still commonly treated indirectly. In most cases, pore fluid effects are ignored or absorbed into an apparent fracture resistance.

In fluid-saturated conditions, tensile fracture generates a localized dilation of the pore space ahead of the crack tip, which draws fluid inward from the surrounding pores. This results in a drop of pore pressure in this region, which reduces the effective stress applied to the skeleton, modifies the near-tip failure micromechanisms, and ultimately leads to a change of apparent fracture energy at the macroscopic scale. Up to now, these phenomena have been mostly predicted rather than observed, largely because pore-pressure transients are difficult to resolve at the spatial and temporal scales of crack propagation in the laboratory.

Here, we present a custom experimental platform designed to resolve pore-pressure transients during stable quasi-static crack propagation at prescribed velocity. We use a wedge splitting test (WST) in a water-filled triaxial pressurized cell. A miniature pressure sensor is embedded in the specimen provides real-time internal pore pressure measurements. Combining crack opening displacement, via digital image correlation, and applied force, using a force sensor, we infer the crack velocity and fracture energy throughout the test. We selected a class G oil well cement, with a homogeneous pore structure, low viscosity and well-controlled mechanical and hydraulic properties, as an analog material. In water-saturated conditions, its low permeability and high stiffness bring the characteristic poroelastic length scale ℓ_pe of pore-pressure variations into the micrometer-millimeter range for laboratory-accessible quasi-static crack speeds, for which spatiotemporal coupling becomes measurable.

Our measurements reveal a transient pressure drop that develops ahead of the crack front, whose magnitude scales with the square root of crack speed, consistent with poroelastic predictions. This underpressure reduces the near-tip effective stress, producing a fluid-induced toughening effect: the apparent fracture energy increases by up to a factor ~2 at the highest tested velocities. This toughening can be attributed to the contraction of the process zone, which leaves a measurable imprint on the fracture surface, revealed by white-light interferometry. Together, these findings identify ℓ_pe as a governing parameter for tensile rupture in saturated porous materials and motivate rate- and drainage-aware injection strategies for coupled hydro-mechanical processes in geo-energy settings.