  When a frictional interface is subject to a localized shear load, it is often
(experimentally) observed that local slip events initiate at the stress
concentration and propagate over parts of the interface by arresting naturally
before reaching the edge. We develop a theoretical model based on linear
elastic fracture mechanics to describe the propagation of such precursory slip.
The model's prediction of precursor lengths as a function of external load is
in good quantitative agreement with laboratory experiments as well as with
dynamic simulations, and provides thereby evidence to recognize frictional slip
as a fracture phenomenon. We show that predicted precursor lengths depend,
within given uncertainty ranges, mainly on the kinetic friction coefficient,
and only weakly on other interface and material parameters. By simplifying the
fracture mechanics model we also reveal sources for the observed non-linearity
in the growth of precursor lengths as a function of the applied force. The
discrete nature of precursors as well as the shear tractions caused by
frustrated Poisson's expansion are found to be the dominant factors. Finally,
we apply our model to a different, symmetric set-up and provide a prediction of
the propagation distance of frictional slip for future experiments.
