  We present a model for disordered 3D fiber networks to study their linear and
nonlinear elasticity over a wide range of network densities and fiber lengths.
In contrast to previous 2D models, these 3D networks with binary cross-links
are under-constrained with respect to fiber stretching elasticity, suggesting
that bending may dominate their response. We find that such networks exhibit a
fiber length-controlled bending regime and a crossover to a stretch-dominated
regime for lengths beyond a characteristic scale that depends on the fiber's
elastic properties. Finally, by extending the model to the nonlinear regime, we
show that these networks become intrinsically nonlinear with a vanishing linear
response regime in the limit of floppy or long filaments.
