  We study the response of F-actin bundles to driving forces through a simple
analytical model. We consider two filaments connected by reversibly bound
crosslinks and driven by an external force. Two failure modes under load can be
defined. \textit{Brittle failure} is observed when crosslinks suddenly and
collectively unbind, leading to catastrophic loss of bundle integrity. During
\textit{ductile failure}, on the other hand, bundle integrity is maintained,
however at the cost of crosslink reorganization and defect formation. We
present phase diagrams for the onset of failure, highlighting the importance of
the crosslink stiffness for these processes. Crossing the phase boundaries,
force-deflection curves display (frequency-dependent) hysteresis loops,
reflecting the first-order character of the failure processes. We evidence how
the introduction of defects can lead to complex elasto-plastic relaxation
processes, once the force is switched off. Depending on, both, the time-scale
for defect motion as well as the crosslink stiffness, bundles can remain in a
quasi-permanent plastically deformed state for a very long time.
