Brittleness does however describe non-elastic rock behaviour. Intuitively, a more brittle material should more easily develop fractures. In agreement with this CTX0294885 hypothesis, Morita et al. (1990) pointed out that nonlinear deformation (including plasticity) contributes to make the tangential stress near the borehole wall less tensile and hence increases the pressure required for fracture initiation. Papanastasiou (1999) argued on the basis of numerical modelling that plasticity shields the stress near the tip of the fracture and hence leads to an apparent increase in fracture toughness. This may explain the common observation from the field that LEFM underestimates fracture extension pressures. The effective fracture toughness increases with increasing stress anisotropy. It decreases with increasing rock (compressive and tensile) strength, and it increases with increasing viscosity and flow velocity of the pumping fluid. Sarris and Papanastasiou (2012) proceeded to incorporate effects of poroelastoplasticity, demonstrating that high stress anisotropy and high pore pressure may lead to large plastic zones and wider fractures than in absence of plasticity, and that fluid diffusion processes influence the obtained results.