  We study the quasi-static penetration of a flexible beam in a two-dimensional
dense granular medium lying on an horizontal plate. Rather than a buckling-like
behavior we observe a transition between a regime of crack-like penetration in
which the fiber only shows small fluctuations around a stable straight geometry
and a bending regime in which the fiber fully bends and advances through series
of loading/unloading steps. We show that the shape reconfiguration of the fiber
is controlled by a single non dimensional parameter: L/Lc, the ratio of the
length of the flexible beam L to Lc, a bending elasto-granular length scale
that depends on the rigidity of the fiber and on the departure from the jamming
packing fraction of the granular medium. We show moreover that the dynamics of
the bending transition in the course of the penetration experiment is gradual
and is accompanied by a symmetry breaking of the granular packing fraction in
the vicinity of the fiber. Together with the progressive bending of the fiber,
a cavity grows downstream of the fiber and the accumulation of grains upstream
of the fiber leads to the development of a jammed cluster of grains. We discuss
our experimental results in the framework of a simple model of bending-induced
compaction and we show that the rate of the bending transition only depends on
the control parameter L/Lc.
