  We explore the Mode I fracture toughness of a polymer gel containing a
semi-infinite, growing crack. First, an expression is derived for the energy
release rate within the linearized, small-strain setting. This expression
reveals a crack tip velocity-independent toughening that stems from the
poroelastic nature of polymer gels. Then, we establish a poroelastic cohesive
zone model that allows us to describe the micromechanics of fracture in gels by
identifying the role of solvent pressure in promoting poroelastic toughening.
We evaluate the enhancement in the effective fracture toughness through
asymptotic analysis. We confirm our theoretical findings by means of numerical
simulations concerning the case of a steadily propagating crack. In broad
terms, our results explain the role of poroelasticity and of the processes
occurring in the fracturing region in promoting toughening of polymer gels.
