Abstract
Flexible fibers, such as biomass particles and glass fibers, are critical raw materials in the energy and composites industries. Assemblies of the fibers show strong interlocking, non-Newtonian and compressible flows, intermittent avalanches, and high energy dissipation rates due to their elongation and flexibility. Conventional mechanical theories developed for regular granular materials, such as dry sands and pharmaceutical powders, are often unsuitable for modeling flexible fibers, which exhibit more complex mechanical behaviors. This article provides a comprehensive review of the current state of research on the mechanics of flexible fiber assemblies, focusing on their behavior under compression, shear flow, and gas-fiber two-phase flow processes. Finally, the paper discusses open issues and future directions, highlighting the need for advancements in granular theories to better accommodate the unique characteristics of flexible fibers, and suggesting potential strategies for improving their handling in industrial applications.