How crack twisting in bouligand structures lead to damage delocalization and toughening

Fiber-reinforced composites with Bouligand structure exhibit remarkable mechanical properties due to the intricate arrangement of fibers. In this study, we propose a coarse-graining (CG) model specifically developed to capture the behavior of Bouligand structures. The model incorporates bonded interactions to represent the fibers and employs a double-well potential to describe the non-bonded interactions within the matrix. Using this model, we investigate the fracture mechanics properties of Bouligand structures, with a particular focus on the emergence of helicoidal cracks. Our primary objective is to validate the hypothesis that these twisting cracks, which align with the fiber orientation, contribute to local hardening mechanisms. By hindering the growth of individual cracks, these hardening mechanisms facilitate the nucleation and growth of multiple cracks, thereby promoting a delocalization effect within the material. Through extensive simulations and analysis, we confirm the validity of our hypothesis. The presence of twisting cracks indeed induces local hardening mechanisms, making it more challenging for individual cracks to propagate. This phenomenon effectively spreads the damage, dissipating energy across larger volumes of the material. Consequently, the toughness of these Bouligand structures is enhanced, as this delocalization effect effectively mitigates the concentration of damage. These findings provide valuable insights into the fracture behavior of Bouligand structures and shed light into the underlying mechanisms responsible for their exceptional mechanical performance. Moreover, our CG model offers a practical and efficient approach to studying and understanding the fracture mechanics properties of complex fiber-reinforced composites. The ability to simulate and analyze the behavior of helicoidal cracks within Bouligand structures opens up new avenues for designing and optimizing advanced materials with enhanced toughness and damage resistance.