  This paper gives a theoretical analysis for the fundamental problem of
anisotropy induced by shear forces onan adhesive contact, discussing the
experimental data of the companion Letter. We present a fracture mechanicsmodel
where two phenomenological mode-mixity functions are introduced to describe the
weak couplingbetween modes I and II or I and III, which changes the effective
toughness of the interface. The mode-mixityfunctions have been interpolated
using the data of a single experiment and then used to predict the behavior of
thewhole set of experimental observations. The model extends an idea by Johnson
and Greenwood, to solve purelymode I problems of adhesion in the presence of a
nonaxisymmetric Hertzian geometry, to the case of ellipticalcontacts sheared
along their major or minor axis. Equality between the stress intensity factors
and their criticalvalues is imposed solely at the major and minor axes. We
successfully validate our model against experimentaldata. The model predicts
that the punch geometry will affect both the shape and the overall decay of the
shearedcontact area.
