Mixed-Mode Fracture Behavior of Penta-Graphene: A Molecular Dynamics Perspective on Defect Sensitivity and Crack Evolution

This study employs molecular dynamics (MD) simulations to investigate the 1 mechanical response and fracture behavior of penta-graphene, a novel two-dimensional 2 carbon allotrope composed entirely of pentagonal rings with mixed sp2–sp3 hybridization 3 and pronounced mechanical anisotropy. Atomistic simulations are carried out to eval- 4 uate the impact of structural defects on mechanical performance and to elucidate crack 5 propagation mechanisms. The results reveal that void defects involving sp3-hybridized 6 carbon atoms cause a more significant degradation in mechanical strength compared to 7 those involving sp2 atoms. During fracture, local atomic rearrangements and bond re- 8 constructions lead to the formation of energetically favorable ring structures—such as 9 hexagons and octagons—at the crack tip, promoting enhanced energy dissipation and frac- 10 ture resistance. A central focus of this work is the evaluation of the critical stress intensity 11 factor (SIF) under mixed-mode (I/II) loading conditions. The simulations demonstrate 12 that the critical SIF is influenced by the loading phase angle, with pure mode I exhibiting a 13 higher SIF than pure mode II. Notably, penta-graphene shows a critical SIF significantly 14 higher than that of graphene, indicating exceptional fracture toughness that is rare among 15 ultra-thin two-dimensional materials. This enhanced toughness is primarily attributed to 16 penta-graphene’s capacity for substantial out-of-plane deformation prior to failure, which 17 redistributes stress near the crack tip, delays crack initiation, and increases energy absorp- 18 tion. Additionally, the study examines crack growth paths as a function of loading phase 19 angle, revealing that branching and kinking can occur even under pure mode I loading.