Poromechanical toughening in tensile fracture of saturated solids.

Fracture in fluid-saturated porous solids is governed not only by solid mechanics but also by fluid flow within the pore space. In systems ranging from geomaterials to biological tissues, deformation in the near-tip region perturbs pore pressure and thereby alters crack growth. Yet these pore-fluid effects have largely been inferred rather than directly observed. Here, we perform in situ pore-pressure measurements during controlled tensile rupture of a water-saturated microporous cement. We demonstrate that a propagating crack generates a localized underpressure ahead of its tip, providing a direct experimental signature of poromechanical coupling. The amplitude of this transient pressure drop scales with the square root of crack velocity, in agreement with poroelastic theory. This underpressure reduces the effective stress near the crack tip, leading to a coupled mechanical response: the apparent fracture energy doubles across the tested velocity range while the fracture process zone shrinks by half. Our findings reveal poromechanical coupling as a rate-dependent control on fracture in saturated porous materials, governed by effective stress changes induced by competing fluid diffusion and crack advance.