  A novel hydraulic fracture (HF) formulation is introduced which accounts for
the hydraulically induced shear stress at the crack faces. It utilizes a
general form of the elasticity operator alongside a revised fracture
propagation condition based on the critical value of the energy release rate.
It is shown that the revised formulation describes the underlying physics of HF
in a more accurate way and is in agreement with the asymptotic behaviour of the
linear elastic fracture mechanics. A number of numerical simulations by means
of the universal HF algorithm previously developed in Wrobel & Mishuris (2015)
are performed in order to: i) compare the modified HF formulation with its
classic counterpart and ii) investigate the peculiarities of the former.
Computational advantages of the revised HF model are demonstrated. Asymptotic
estimations of the main solution elements are provided for the cases of small
and large toughness. The modified formulation opens new ways to analyse the
physical phenomenon of HF and also improves the reliability and efficiency of
its numerical simulations.
