  Most of the research concerting crack propagation in discrete media is
concerned with specific types of external loading: displacements on the
boundaries, or constant energy fluxes or feeding waves originating from
infinity. In this paper the action of a moving load is analysed on the simplest
lattice model: a thin strip, where the fault propagating in its middle portion
as the result of the moving force acting on the destroyed part of the
structure. We study both analytically and numerically how the load amplitude
and its velocity influence the possible solution, and specifically the way the
fracture process reaches its steady-state regime. We present the relation
between the possible steady-state crack speed and the loading parameters, as
well as the energy release rate. In particular, we show that there exists a
class of loading regime corresponding to each point on the energy-speed diagram
(and thus determine the same limiting steady-state regime). The phenomenon of
the forbidden regimes is discussed in detail, from both the points of view of
force and energy. For a sufficiently anisotropic structure, we find a stable
steady-state propagation corresponding to the slow crack. Numerical simulations
reveal various ways by which the process approaches - or fails to approach -
the steady-state regime. The results extend our understanding of fracture
processes in discrete structures, and reveal some new questions that should be
addressed.
