The role of hierarchical microstructures in rate-dependent energy absorption and fracture modes

Hierarchical microstructures, commonly found in natural biological materials, confer remarkable mechanical properties due to their multi-level organization. However, while much research has focused on their static mechanical behavior, the dynamic performance of these structures across a wide range of loading rates—particularly in terms of energy dissipation and fracture modes—remains less explored. This study systematically examines the failure behavior and energy dissipation of hierarchical structures from low-speed (v = 0.04–2 mm/min) to moderate-speed (v = 1.5–7 m/s) regimes, combining experimental investigation with numerical simulations. At low loading speeds, hierarchical structures exhibit superior energy dissipation compared to single-phase materials, as the soft phase undergoes higher strain rates, enhancing both strength and energy absorption. However, at higher loading rates, the benefits of hierarchical structures diminish, with premature brittle fracture leading to lower energy absorption compared to single-phase structures. Additionally, we show that when the aspect ratio of the hard phase in the secondary hierarchy is reduced, the stress distribution becomes more uniform, further enhancing energy absorption. This work provides valuable insights into the design of biomimetic energy-absorbing materials optimized for various loading rates.