ELASTIC-WAVES IN RANDOM-FIBER NETWORKS

The propagation of the first displacement maximum of a semi-infinite w avetrain in a two-dimensional random-fibre network is analysed. Model calculations and numerical simulations are used for demonstrating that two qualitatively different wavefront velocities appear in the networ k. A transient wave, which travels fast and whose amplitude decreases exponentially, dominates the short-time behaviour when the bending sti ffness of the fibres is small and the driving frequency is high. This mode can be described by a one-dimensional model. The transient-wave m ode propagates even if the bending stiffness of the fibres vanishes, i n which case the normal sound velocity is zero. The usual, and slower, effective medium mode always dominates at late times. It also dominat es at short times if the driving frequency is low and/or the bending s tiffness of the fibres is relatively high.