  As recently suggested in an interestring and stimulating paper by Menga,
Carbone & Dini (MCD), applying fracture mechanics energy concepts for the case
of a sliding adhesive contact, imposing also the shear stress is constant at
the interface and equal to a material constant (as it seems in experiments),
leads to a increase of contact area which instead is never observed. We add
that the rigorous MCD theory also predict a size effect and hence a distortion
of the JKR curve during sliding which is also not observed in experiments.
Finally, a simpler example with the pure mode I contact case, leads in the MCD
theory to an unbounded contact area, which is difficult to interpret, rather
than a perhaps more correct limit of the Maugis-Dugdale solution for the
adhesive sphere when Tabor parameter is zero, that is DMT's solution. We
discuss therefore the implications of the MCD theory, although they may be
rather academic: recent semi-empirical models, with an appropriate choice of
the empirical parameters, seem more promising and robust in modelling actual
experiments.
